• Volume 36,Issue 2,2014 Table of Contents
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    • Ambient Vibration Test for a Multi story Residential Building and the Vibration Mitigation Measures

      2014, 36(2):207-213.

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      Abstract:Brick-concrete buildings have been extensively used in urban and rural areas. However, this type of building, which frequently contains a shallow foundation, demonstrates poor structural integrity and is susceptible to influence from ambient vibrations. This case is especially valid for buildings that were constructed in previous eras. With the development of modern cities and the increasing demand for an enhanced quality of life, environmental interference caused by ambient vibrations has become a critical issue. In areas with soft soil, traffic is a primary source of ambient vibrations. Vehicle vibrations caused by road surface roughness or speed changes can cause vibrations throughout the foundation of a building. Therefore, the vibration of brick-concrete buildings is related not only to the excitation source of the vibration but also the soil characteristics, the foundation conditions, and the structural style. The human response to the vibrations of housing structures is closely correlated with a person's cognitive level and their current physical and psychological conditions. Thus, the potential difference in the reactions among building occupants is significant.Residents in a six story brick concrete building, which is located in an area of Shanghai with soft ground, frequently complained about the intermittent interference from vibrations, particularly residents on the upper two stories. The measures confirmed that the environmental vibrations were generated by heavy vehicles traveling on a road that was located approximately 80 m north of the building. The frequency and amplitude characteristics of the ambient vibrations and the human comfort levels for the different stories of the building were analyzed according to the "standard for allowable vibration of building engineering" (GB50868-2013). To comprehensively evaluate the impact induced by the transversely horizontal vibrations, which influences human comfort levels for the three components of building vibration, a numerical finite element model (FEM) of the building was developed and analyzed. The soil in the model was simulated as a virtual layer. According to the test results and the numerical analyses, two types of technical measures are proposed to reduce unfavorable vibrations. The paper yields the following conclusions: (1) The test results show that heavy-duty vehicles from nearby roads are the main cause of the building vibrations. The vertical component of the vibrations along the structure is almost constant; however, its lateral component is significantly amplified, which primarily influences human comfort.  (2) The measures indicate that the peak acceleration range for the top two stories is 68~70 dB, which is lower than the limit values proposed by the "standard for allowable vibration of building engineering" (GB50868-2013). The rationality of the limit values is discussed to satisfy the demand of human comfort. (3) The structural frequencies and modes are analyzed based on the assumption of a virtual layer of foundation; the simulated results correspond with the measures. The model is a reasonable tool for justifying the effect of vibration reduction measures. (4) To ensure that each story of the multi-story brick-concrete building complies with the previously recommended standard, the peak lateral acceleration should be reduced by approximately 21% and the two proposed technical measures, which are compared, should be applied.

    • Seismic Stability Analysis of High Slope at the Intake Tunnel of Hongyan River Nuclear Power

      2014, 36(2):214-219,227.

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      Abstract:Several Chinese nuclear power plants are in earthquake-prone areas.Thus,slope failures due to earthquakes can disrupt the normal operation of nuclear power plants.Seismic design safety calculations for nuclear power plants take the advantage of computation methods such as the slip surface,the static finite element,and the dynamic finite element method.The current nuclear specifications recommend using the three different methods of analysis that satisfy the regulatory requirements without giving additional guidance.Seismic slope stability analysis was used to examine the intake tunnel of the Hongyan River nuclear power plant.The model boundary conditions for horizontal and vertical seismic motion were established.When the lower boundary is fixed,there is the effect of the vertical displacement on the lateral boundary.When the lower boundary for the horizontal seismic motion is fixed,there is the effect of the horizontal displacement on the lateral boundary.The input seismic data were taken from earthquake evaluation reports.The peak horizontal acceleration of the sl-2 ground motions is 0.18 g,and the vertical acceleration peak is 0.12 g.The safety factor for the high slope and sliding surface position subjected to seismic loading is calculated first by the sliding surface method,and then by the static and dynamic finite element methods.Furthermore,the minimum safety factor and the minimum average safety factor of the slope are obtained according to the history curves of the safety factor.For vertical seismic motion,the sliding surface method and the static finite element method result in safety factor values of 1.296 and 1.326,respectively.The minimum safety factor is 1.163.The results of the calculations and the analysis suggest reasonable improvement for the side slope.The cutting slope unloading scheme is necessary because the safety factor of the natural slope cannot satisfy the specifications of the seismic design code for nuclear power plants.For vertical seismic motion,the calculated excavation slope safety factor with the sliding surface method is 1.524,whereas the safety factor calculated with the static finite element method is 1.508.The minimum safety factor calculated with the dynamic finite element method is 1.307.The calculated safety factor satisfies the code requirements after the cutting slope unloading scheme is adopted.Evidently,the calculation methods can be used in the seismic slope stability analysis of similar conditions.

    • Numerical Study on Stress Change within the Foundation of Offshore Wind Generators

      2014, 36(2):220-227.

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      Abstract:As a clean energy, offshore wind power has attracted increasing attention and investments. Offshore wind generators with pile foundations are mainly subjected to horizontal loading coursed by wind, waves, ice, ocean currents, and earthquakes. Such complex loading that acts upon the structure-foundation system leads to complex stress changes within the foundation. Moreover, extremely strict requirements for the deformation behaviors of offshore wind generator foundation structures and adjacent soils are controlled in their design and usage. However, knowledge of the actual stress changes and the deformation laws are very limited. This paper systematically investigates the stress-time histories of typical points in the foundations around a typical offshore wind generator and the amplitude distribution of the stress-changing induced by wind and waves through three-dimensional (3D) finite element method (FEM) numerical analysis. The effects of the angle between the directions of horizontal wind loading acting on the structure and the waves and frequency of the two loading types are studied. The results indicate that the amplitude and the principal stress direction of the wind generator foundations are both changing. The angle and the frequencies of the two loadings significantly affect the stress state, particularly the principal stress rotation. 3D cyclic rotation of the principal stress axes within a limited rotating angle is determined to occur always in the foundation soil adjacent to the foundation structure of an offshore wind generator. In particular, it is shown to become a two-dimensional (2D) cyclic rotation of principal stress axes when two directions of the wind load and the wave propagation remain the same. However, the stress paths become more complex when the frequencies of the two cyclic loads differ.

    • Study on the Seismic Response of Saturated Soil: Underground Structure Based on Dynamic Model of Fluid solid Coupling Media

      2014, 36(2):228-232.

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      Abstract:In China, various transportation facilities have been built on a large scale. Sea-crossing tunnel projects are constructed in many coastal cities, and many underground structures are built on the soft soil foundation in the south region. These are all underground structures on saturated soil, and China lies in the Pacific Ring of Fire, with many cities located in the high-intensity earthquake zone, so the seismic security of underground structures such as tunnels and subway stations is an important subject. The key is to make an accurate calculation of the dynamic response of the underground structure to the earthquake load. The dynamic response of an underground structure in a saturated soil site is very different from that in other types of soil sites. Therefore, the seismic response of underground structure in the saturated soil should be studied with the right calculation model and analyzing method. At present, one widely used method is numerical simulation and calculation. Depending on the soil calculation model it employs, the analytic method can be classified into three types: the total stress method, simplified effective stress method, and fluid-solid coupling dynamic model method. The fluid-solid coupling model, which takes into account the coupling of fluid-solid dynamic response, is a theoretically complete calculation model. Therefore, the method based on the fluid-solid coupling dynamic model is theoretically sound and more accurate. In general, the dynamic response of an underground structure in a saturated soil site needs further study. In this paper, the seismic response of an underground structure in saturated soil is studied based on the fluid-solid coupling dynamic model and ABAQUS in order to get a complete view of the characteristics and rules of the seismic response. We conduct a simulated calculation of the seismic response of an underground structure in saturated soil, with the site soil simulated by the pore pressure element deduced from the fluid-solid coupling dynamic model of fluid-saturated porous media. The example chooses the seismic record of the N-S components of the Ninghe earthquake (magnitude 6.9 aftershock of the Tangshan earthquake) as the seismic input. From the calculation, we obtain the distribution chart of soil pore pressure, vertical displacement of soil-tunnel system, and von Mises stress on the structure. The calculation results illustrate that by the end of the seismic input, the stress distribution is symmetrical. The stress distribution concentrates in the base area of the two sides, and the water pressure mainly distributes in the base and side areas. The pore pressure and vertical displacement of the site soil are maximal in the base area and decrease gradually in the side area. With increase of depth, the vertical displacement of the site soil decreases gradually. The pore pressure and vertical displacement of the site soil distribute symmetrically. The greatest seismic response occurs when the acceleration of the input earthquake wave reaches the maximum. The calculation results also indicate that the pore pressure element deduced from the fluidvsolid coupling dynamic model of the fluid-saturated porous model can be employed as an effective calculation model for the study of all types of underground structures in saturated soil sites.

    • Study on the Test of Tensile Strength of Remolded Loess

      2014, 36(2):233-238,248.

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      Abstract:The mechanical characteristic of loess is the most important factor that influences the stability of buildings,slopes,tunnels,underground structures,etc.Research on its tensile strength and related properties is less compared with the research on shear strength,collapsibility,and other loess properties.The tensile strength of soils is seldom considered in engineering exploration,design,and stability analysis,because its tensile capacity is too weak and thus seems meaningless to common engineering.However,increasing infrastructure,such as railways,highways,airports,and high earth-rock dams,are being constructed in loess areas and natural hazards like landslides and slope avalanches keep occurring,so on one hand,the tensile strength and anti-cracking capacity of loess has become much more significant to the stability of earth structures in loess.On the other hand,the very weak tensile ability of loess often has an adverse effect on research into its tensile characteristics.Because the tensile strength of loess is so small,any small change caused by sampling or testing might lead to serious deviation of test results.Therefore,rigorous test parameters and correct testing procedures are gravely needed.Using the unconfined penetration test,tensile strength characteristics of the remolded loess from the Lanzhou Jiuzhou Development Zone are thoroughly studied.The advantage of the unconfined penetration test is its simplicity and short duration.For testing,a strain-controlled triaxial apparatus is applied with different loading rate,specimen height,specimen preparation method,and penetration loading cylinder diameter,but the moisture content and dry density of the loess samples remains constant.Because the tensile strength of loess is very small,test error due to manipulation is inevitable,so variations in tensile strength of remolded loess with different factors are investigated with three sets of parallel tests for each factor.The test results indicate that tensile strength is affected mainly by the loading rate,specimen height,specimen preparation method,and diameter of loading cylinder.However,the influencing degree is much different per factor.For instance,the variation of tensile strength from different tests was the smallest if the loading rate was 1.0 mm/min.The tensile strength of a specimen with the ratio of height to diameter of 1∶1 was the middle one in tensile strength from different tests; meanwhile,the relative value of these tensile strengths was the minimum.The tensile strength of specimens measured by the static pressing method is higher than that by the compaction method.The tensile strength of specimens decreases linearly with the increasing of the different diameter of loading cylinders.For other loess samples with different moisture content and density,more research is needed into other factors that might affect the tensile strength of the remolded loess and related soils.The test method and study results can be used for valuable reference for the testing of tensile strength of loess and similar soils and also for providing fundamental data and theoretical evidence.

    • Triaxial Testing of the Compacted Loess Strength

      2014, 36(2):239-242.

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      Abstract:Strain-controlled triaxial unconsolidated and undrained shear tests were used to study the strength of compacted loess from the Qinghai region.The tests were conducted at moisture contents of 14% and 17% at 97% compaction,and 95% and 96% compaction at moisture content of 17%. The specimen diameter was 3.91 cm,and the height was 8 cm.To analyze the stress-strain behavior,the Duncan-Chang model was used.The test results suggest that the deviator stress increases as the axial strain increases at different confining pressures regardless of initial moisture and compaction.The relationship between deviator stress and axial strain is close to hyperbolic.The deviator stress and the initial tangent modulus increase as the confining pressure increases.At constant confining pressure,the deviator stress at failure decreases with increasing moisture and increases with increasing compaction.With increasing moisture content,the cohesion,friction angle,K,and Rf decrease,whereas porosity n increases.With increasing compaction,the cohesion,friction angle,K,and Rf increase,whereas n decreases.

    • Effects of Beam column Joint Stiffness on Seismic Behavior of SteelFrames Based on Multiple Point Constraints

      2014, 36(2):243-248.

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      Abstract:The behavior of beam-to-column connections with angles falls between fully rigid and ideal pin connections and belongs to the category of semi-rigid connections.In this paper,a series of steel plan frames with different degrees of connection stiffness are established in ABAQUS by modeling the beam-to-column connections with the SPRING2 element and using different moment-rotation curves as the node constraint.Using multiple point constraints,the lateral force distribution is maintained and displacement-controlled pushover analysis is achieved.The model is verified by comparing the simulated curve with the experimental curve of the load displacement of a two-floor-span semi-rigid frame.Based on the verification,a six-floor-two-span plane frame is modeled with different joint stiffness influence factors α.In the finite element analysis procedure,connective,geometric,and material nonlinearities are considered.The effect under different α factors on the seismic performance is discussed,such as frame bearing capacity,lateral stiffness,ductility,energy dissipation,and stiffness deterioration.Analytic results show that frame bearing capacity,lateral stiffness,ductility,and energy dissipation increase gradually with the reduction of the α factor,but the increase in amplitude slows down.The curve of stiffness degradation show that the degeneration of rigidity is obvious as the α factor reduces.In conclusion,when the joint stiffness influence factor α is between 1/15 and 1/20,the joint has higher bearing capacity and better ductility performance to meet the requirement of earthquake resistance.The above mentioned conclusion can serve as guidance for engineering.

    • Study on the Relationship between Soil Depth and Shear Wave Velocity of Rock and Soil Mass in Xi'an City

      2014, 36(2):249-255,280.

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      Abstract:The shear wave velocity of soil is an important physical quantity in soil dynamics, which is indispensable for the determination of site classification and the site soil layer thickness. In recent years, wide application has been achieved in engineering survey, geological engineering, and earthquake engineering by testing the shear wave velocity of soil layers. Previous studies have shown that the value of shear wave velocity changes within a very small range and has certain statistical rules in certain areas with the same sedimentary environment and regional geological background. Moreover, using the statistical relationship between soil depth and the parameters of soil character for exploration is allowed by relevant specification and can thus be viewed on a theoretical and practical basis. By using various statistical methods for fitting, scholars worldwide have determined that the regular empirical relationship of shear wave velocity changes with depth in some areas. Xi'an is one of the oldest cities in the world and has a vivid and rich history and culture. During its 3,100 years of development, it served as the capital for 13 dynasties such as Qin, Han, and Tang. The three main geomorphic units of Xi'an city include the alluvial plain of the Weihe River (area I), the alluvial plain of the Chan-Ba River (area II), and the proluvial-lacustrine platform before the loess tableland (area III). In this paper, the measured data of shear wave velocity of soils from 309 boreholes obtained during seismic safety evaluation of the engineering site in Xi'an city are used to qualitatively discuss the empirical relationship between soil shear wave velocity and soil depth in Xi'an. Shear velocity variation that changes with depth is revealed by using a linear fitting model, a quadratic polynomial fitting model, and an exponential fitting model. The results show that the exponential and linear models can produce effective results within a depth of 20 m and depths of more than 20 m, respectively. Thus, a reliable method includes a combination of both. We adopt the exponential model to fit the former and the linear model to fit the latter regions. Moreover, we analyze the relationships among the shear wave velocities of various sedimentary types, including fine sand, medium sand, coarse sand, loess, silty clay, ancient soil, and pebble soil, with the soil layer depth by using the statistical regression method. Subsequently, we obtain the empirical formula of the soil shear wave velocity (VS) with soil depth (H) in different geomorphic units. Our results show that the value of correlation higher than 0.9 accounts for 29.4%, which indicates a very close relationship between VS and H; that between 0.8 to 0.9 accounts for 55.9%, which indicates a close relationship between VS and H; and that between 0.7 to 0.8 accounts for 14.7%, which indicates that VS is associated with H. The best correlation value was 0.9 475 (function correlation coefficient is 1), and the worst correlation coefficient value was 0.704 5 (0.7 is the threshold value of correlation). Only five of the correlation coefficients were less than 0.8, which indicates that the statistical relationship is relative, and good correlation accounts for the majority. A comparison of the measured and predicted values indicates that the discrepancy is very slight. The results show that the empirical formula we have determined between the VS of various types of soil layers in different geomorphic units and the H is reliable and therefore fits the characteristics of localized soil and rock layers. Therefore, our formula can be used for general construction engineering survey and seismic safety evaluation in Xi'an city.

    • A Study on Hazards of Background Seismicity in Mongolia

      2014, 36(2):256-260.

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      Abstract:In this article,we calculate seismic hazards from background seismicity by using a spatially smoothed seismicity model for Mongolia,and we obtain a peak ground acceleration (PGA)map for 10% probability of exceedance in 50 y.We propose two seismicity models to calculate seismic hazard maps for 10% probability of exceedance in 50 y.The results show that the seismic activity model and the seismic hazard results,both calculated by using the instrumental seismic catalog (M≥3.0),can reflect contemporary seismic activity levels and seismic hazard levels of the Mongolia area.In addition,the seismic activity model and the seismic hazard results,both of which are calculated by using historical earthquake data (M≥5.0),can quite well represent the seismic hazard level of moderate earthquakes in Mongolia.The two seismic hazard maps both show that the background seismic hazard level is 0.05 g in most areas of Mongolia and that the values of this parameter are high,at 0.1~0.15 g,in other areas,which indicates a background of high seismic hazards in Mongolia.Therefore,it is highly necessary to completely consider the impacts of background seismicity when implementing seismic hazard analysis and seismic hazard mapping for Mongolia. Moreover,we compare two seismicity models.The results also illustrate significant spatial differences between the seismicity models with different minimum complete magnitudes.Therefore,when adopting a spatially smoothed seismicity model to analyze seismic hazards for Mongolia,the weighted average of several different models should be considered to balance the impacts of earthquake occurrences and magnitudes.

    • A New Discovery by Seismological Archaeology in the Jianghuai Area

      2014, 36(2):261-268.

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      Abstract:Seismological archeology is an interdisciplinary study of archeology and seismology that studies the ancient earthquakes of archeological sites using seismic,geological, and archeological methods.It enriches the understanding of the laws of seismic activities,and thereby provides information for seismic deformation phenomenon analysis,earthquake prediction,and seismic hazard assessment.According to historical records,eastern China underwent moderate and strong seismic activity,but there is a lack of historical records and even less information on its prehistoric seismological archeology.The eastern China area,across the Qinling-Dabie collision zone,falls mostly in the North China plate and Southern China plate boundary zones,and its prehistoric earthquake activity is worthy of study.Based on the above considerations and with the support of the China Earthquake Administration industry-scientific special project “Recognition and study of prehistoric earthquakes in the North China and Southern China border belt,” we conducted cooperative research in seismic archeology with the departments of cultural relics and archeology.The Jianghuai area is located at the intersection of the Qinling-Dabie collision zone and the Tan-lu fault zone,and it is a highly significant area for the study of its prehistoric earthquake events.We conducted earthquake archaeological research in four archeological excavations sites in the Jianghuai region and found abundant tectonic deformation phenomena,including newly discovered ancient seismic events.The four archeological sites are the Guzhen Xiaolijia site,Hefei Luandunzi site,Hexian Dachengzi site,and Maanshan Shendong site.The Guzhen Xiaolijia and Hefei Luandunzi sites revealed continuous tectonic deformation characteristics of ancient ruins,pointing to their relationship with earthquakes.The results show that most earth-pit tombs of the Hefei Luandunzi site are in irregular deformation,but the deformation of M66 is obviously affected by the Tan-lu fault dextral slip traction formation,which also has nothing to do with the seismic loads.The excavation exposed some ancient earthquake ruins in the Hexian Dachengzi and Maanshan Shendong sites,which mainly include earthquake fissures,cultural layer dislocation,and fold phenomenon.Many earthquake fissures were found in the Hexian Dachengzi site,with the fracture filled with black yellow powder sand,the sand source for the underlying silty strata;the crack length is 0.5~3 m and width is 2~5 cm.Based on the stratigraphic horizontal relationship,these cracks were determined to have developed in the (10) and(5)layers.In order to establish the time the seismic event occurred,we collected six 14C samples in the(11),(10),and (5)layers.Three samples were sent to BETA laboratory in the USA for testing.Sample test results show that the ages for the(11),(10),and(5)layers are (2 990±30) BP,(2 760±30) BP,and (2 580±30) BP,respectively.Thus,the earthquake occurred between(2 580±30) BP and (2 760±30) BP,with a calendar age of 760~980 BC,during the middle to late Western Zhou Dynasty.At the Maanshan Shendong site,an earthquake fissure was found with a bottomless crack terminating in the middle part of the cultural layer.By archeologically dating the cultural layer and the cutting relationship between the earthquake fissure and the cultural layer,we identified a paleoearthquake event which also occurred in the middle to late Western Zhou Dynasty.Comprehensive research results of this ancient earthquake with the previous research in the Jianghuai region of the Western Zhou Dynasty suggest that the Jianghuai area in the middle to late Western Zhou experienced a moderate strong earthquake activity period.

    • Study on the Influence of Ground Motion Field of Scenario Earthquake in Mining Area of Huating,Gansu

      2014, 36(2):269-273.

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      Abstract:The study of distribution of strong ground motion is one of the crucial tasks in earthquake engineering.Near-fault strong ground motion is the basis for establishing building codes for the field of earthquake engineering and it has significant applicability in the field of earthquake engineering.It is important to estimate a suitable building code for an industrial area,and even for a mining area,which may be located in or near potential sources of earthquakes.Several studies have focused on the physics of strong ground motion during an earthquake. Results of such studies have demonstrated a strengthening phenomenon of earthquake activity and the outputs of cool mines in Huating region,Pingliang City,Gansu province,from 2001 onward.The acceptable conclusion is that this strengthening phenomenon is caused by mining in Huating region.In this study,we analyze the earthquake activity of Huating region in Gansu province on the basis of earthquake data from 2001 onward that were downloaded from earthquake networks centers of Gansu province and the China earthquake networks center.Specifically,the mining-induced earthquake data from 2001 to 2008 are based on the catalog and study results of earthquake networks centers of Gansu province,and the mining-induced earthquake catalog from 2009 to 2013 is based on the results of the China earthquake networks center.As an example,the distribution of strong ground motion around the Liupanshan southern segment fault was predicted under simulated earthquake activity that included recent earthquake trends in the northern segment of the north-south seismic belt.A special finite fault source model was constructed for this simulated earthquake located on the Liupanshan southern segment fault.Further, the strong ground motion, including acceleration and velocity,near the causative fault was calculated using the finite fault source model.Next,the characteristics of these simulated accelerations and velocities were studied and their respective peak values were calculated.As another useful and essential example,the near-fault ground motion distribution of Changchun City was predicted under a setting earthquake.After the occurrence of an MS6.0 earthquake on the Jianshanzi-Kalun fault,the influence range of potentially destructive ground motion lied near the fault,and the distribution of this influence range was along the fault.The peak acceleration reached 200 cm/s2.Based on these results,we considered an M7 earthquake to occur on the Liupanshan southern segment fault,whose influence range was along the fault;the variation in the peak acceleration along the fault of the Liupanshan eastern segment of the west Qinling seismic gap was up to 1 500 cm/s2.The study area included three mining regions:the Huating region,Ningxian south mining region,and Ningxian middle mining region.The Huating region is located near the eastern boundary of the Liupanshan eastern segment of the west Qinling seismic gap,almost 30 km.The peak ground acceleration in Huating region is 100 cm/s2,and those in the Ningxian south and Ningxian middle mining regions are 50 cm/s2 each.The numerical modeling developed in this study has high potential for applicability in the estimation or prediction of ground motion for earthquake engineering.Furthermore,the numerical algorithm could also be used to generate a near-real-time shaking map at the implementation level if the current finite fault inversion technique is applied.

    • Preliminary Analysis of Mobile Observation Records for theAftershocks in the Yushu M7.1 Earthquake

      2014, 36(2):274-280.

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      Abstract:The Yushu M7.1 earthquake occurred on April 14,2010 in Yushu,the Tibetan autonomous prefecture of the Qinghai province (E 96.7°, N 33.1°). After the earthquake,we set up seven strong motion mobile observation instruments around the epicenter.As of October 15,2010,we had captured 71 seismic events and a total of 213 acceleration records,with the maximum magnitude of M4.6. In these records,there were 10 records whose peak ground acceleration was greater than 30 gal,and the maximum peak ground acceleration was 122 gal.The waveform of records was clear and complete,which makes up for the lack of fixed stations and local records near the earthquake zone.It not only offered the quantitative evidence for analysis of the earthquake damage,but also provided important data for the study of the relationship among the peak ground motion,holding time,the spectrum,and macro-seismic intensity.In this paper,we collected and preliminarily processed these records and obtained the speed schedule,Fourier spectrum,and the power spectrum,then analyzed and discussed the recording features and related issues.The results showed that (1)the maximum peak ground acceleration in this mobile observation was 122 gal,and the holding time of vibration was about 5 seconds,with a frequency of 7.2 Hz. This record came from a M3.6 aftershock, and it indicated that a small earthquake may also obtain high peak acceleration.This would appear to be the opposite phenomenon of a high vibration peak with low seismic intensity. In fact, there were many major projects located in the area which had small seismicity background.With the increasing coverage of observation stations,this opposite phenomenon would become more apparent. We argue,therefore,that the study of this phenomenon should be conducted.(2) In the M4.6 aftershock,the maximum peak ground acceleration was 35.5 gal,and the holding time of the vibration was about 6 seconds,with a frequency of 4.0 Hz.With the expansion of the epicentral distance,the peak ground acceleration of each mobile station gradually decreased.But by the effect of the anisotropic medium and site conditions,the same epicentral distance (or essentially the same epicentral distance)at different locations may experience unequal peak ground acceleration.(3)The factors that determined or influenced the severity of the earthquake damage were complex and diverse,and included the acceleration intensity,spectral distribution,site conditions,and structure type,among other factors.Strong ground motion records truly reflected the intensity distribution of the seismic elastic wave in the propagation space.Although the relationship between strong motion records and the degree of structural damage was not a simple linear correlation,it may be very necessary to use appropriate methods to establish an appropriate model of the relationship between the ground motion parameters and the degree of damage for the typical building,which could be widely used in rapid damage assessment,intensity quick reports,and other emergency work. In this paper,these analyses and the discussion of the results are preliminary,pending further study after accumulating data supplements.

    • A Study on the Scientific Value of Historical Records for the Huaxian MS8¼ Earthquake in 1556

      2014, 36(2):281-285.

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      Abstract:Historical records of the Huaxian MS8? earthquake in 1556 are one of the most abundant in China.In this paper,we assess the seismological scientific value of the Huaxian earthquake historical records.The study includes the following:(1)an investigation of ground rotational movement often felt before large earthquakes,which can be used as an index of immediate earthquake and an early warning;(2)we determine the time span of seismic changes,from small to large,prior to sizeable earthquakes based on historical records;(3)elaborate on the medium-term precursor of the relationship between drought and earthquake before large earthquakes;(4)strong-moderate earthquakes were found to be active in the Fenhe graben before large earthquakes;(5)the seismic cycle of Weihe basin was determined to be 600 years;(6)we use the distribution index of triplet method for the occurrence time of historical earthquakes;and(7)verification of intensity zoning that the Huaxian earthquake points out and asymmetry of intensity that the Huaxian earthquake reveals.Furthermore, we also study and discuss the scientific significance and historical status of Earthquake Records written by Qin Ke-da.

    • Analyzing Changes of Focal Mechanism of Small-moderate Earthquakes before a Moderate strong Earthquake

      2014, 36(2):286-291.

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      Abstract:In this paper a parameter called “concentration ratio of distribution of P axis” was defined to analyze the variation of focal mechanisms of small earthquakes before a moderate-strong earthquake.This parameter shows the dynamic change of the direction of the P axis in the seismogenic process. In this paper,we use two moderate strong earthquake examples,namely the Jingtai earthquake of on Jun. 6,2000 and the Sunan earthquake of on March 30,2008. We invert the focal mechanism solutions and the seismogenic stress field using the joint inversion method of first motion and amplitude ratio of P and S wave for small events before the two moderate strong earthquake examples.The data used are from the networks set by China and France and the Gansu digital monitoring network.We study time-varying characteristics (prior to the two earthquake examples)using the parameter “concentration ratio of distribution of P axis”. The results show that the P axis of the focal mechanisms for events begin to concentrate one or two years before the main earthquakes and in accordance with the main earthquakes’ source mechanism just prior to the main earthquake.The concentration ratio of distribution of P axis prior to the Jingtai earthquake varies regularly;moreover,the parameter prior to the Sunan earthquake began to show concentration from the beginning of 2006,whereas the concentration ratio decreases from the beginning of 2007 and increases a greater degree by the end of 2007 until the main shock occurred. This could potentially be due to the singularity of the seismogenic fault structure of the Jingtai earthquake or the complexity in the epicenter region of the Sunan earthquake.For the Jingtai earthquake,the location stress field would be regionally adjusted during the concentration process and followed by strengthening of the local stress field,which makes the source mechanisms of medium and small earthquakes uniform.

    • Analysis of Weihe Basin′s Crustal Deformation Observations

      2014, 36(2):292-300.

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      Abstract:We selected the gps data since we have the GPS mobile observation network in Shaanxi province, and compared with the mobile gravity and cross-fault deformation observation data, we are aimed to investigate the variation of Weihe basin topography changes and their relationships.With the data of GPS mobile observation network in Shaanxi province during 2001-2010 and 2001-2012, the high-precision horizontal velocity field of Weihe basin processed by the software of GAMIT / GLOBK was obtained. The regional background strain characteristic parameters were calculated according to the velocity and strain theory by least squares collocation method.We have used the software,which is updated by China Institute of Seismology,to process the mobile gravity data of Guanzhong area,we also use some points nearby Weinan to do adjustment calculation to be stable.We have used the cross-fault deformation observation data,which is observed by high precision balance level NI002,to do the feet long correction.Based on fine processing on GPS observation data, mobile gravity and cross fault deformation observation data in Weihe basin, temporal and spatial variation maps are drawn correspondingly and further analyzed comprehensively combined with related deep detecting data and other research results. Results are shown mainly in 5 aspects as follows: ① Longxian-Qishan-Zhouzhi-Huxian area is a surface strain compressed zone, a gravity change gradient belt and a Bouguer gravity anomaly gradient belt, which indicates that the abnormal changes in stress field in the area is related to the deep tectonics. The Moho mutants to uplift over the Erdos block and the high-speed substances in the upper mantle intrude into the lower crust is one of the deep tectonic backgrounds of moderate and strong earthquakes in the region. ② Longxian-Qishan-Mazhao fault, passing through the above mentioned area, is of significant inverse tectonic movement with fault movement characterized in extrusion deformation since 2010. What should been paid more attention to is that the decrease amplitude of Dayuan 2-3, 2010.7~2012.3 (a test point) over the fault is as high as 15.19 mm. ③ In the surface strain compressed area, the crustal medium density increases and the change of gravity is positive, fault activity is of extrusion deformation characteristics. Whereas, in the surface strain stretched area, all of the former mentioned items are of the opposite situations. ④ Two reverse tectonic movements occurred in Qishan-Mazhao fault since July 2004 are probably the far field effect in the pregnancy and occurrence process of Diebu M4.9 and Minxian M6.6 earthquakes in Gansu Province. Having been observed for more than 20 years, related measuring sections of the Dayuan deformation site display significant reverse tectonic movement characteristics once a relatively big earthquake activity happens. ⑤ GPS surface strain rate, gravity field variation and cross fault deformation are greatly uniform and correlated in ground deformation changes in western Weihe basin, which is rare and valuable geographic phenomena that may inspire and promote geoscience research in a certain degree.

    • The Retrospective Test of The Earthquake Great Situation in Northeast China

      2014, 36(2):301-308.

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      Abstract:According to the objective summary and assessment analysis of the prediction effects and criteria of the seismic situation in northeast China in 1998-2010,the risk regions of M5~6 earthquakes have been concentrated mostly in the area near the Bohai Sea-Liao-Meng juncture and the juncture of the Heilongjiang,Jilin,and Inner Mongolia provinces since 1998.Geological tectonic backgrounds and live seismic activities also show that the three regions and its adjacent regions are the main areas of moderate and strong seismic activity in northeast China. Reports during recent years offer a better understanding of the northeast risk regions.The prediction accuracy of a great northeastern earthquake situation is 57% for the first year and 71% for the second to third years.Eight main prediction criteria are used for seismic situations,including the enhancement of small and moderate earthquakes;gaps and belts;and seismic activity parameter scanning,which are three seismological indicators of higher reliability.In addition,the R value evaluation is more than 0.22.Statistics show that the number of abnormal indicators given in the annual prediction report and the maximum magnitude predicted for the subsequent year are not clearly connected.However,the enhancement of small and moderate seismic activities before moderate and strong earthquakes is one of the important features of northeastern regional seismic activities.Although the active-calm periods,tectonic backgrounds,and historical earthquake activities easily give false prediction rates,they may be used to draw lessons,particularly in long term tendency forecasting.Correct judgment of the present seismic state in northeast China,whether active or calm,and definite indices of active-calm period conversion can appropriately increase the accuracy of prediction in risk regions and reduce the rate false results.It has also been determined during the process of reviewing and summarizing that current forecast analysis of 1~3 years focuses mostly on statistical and qualitative judgment basis,and is mainly based on comparison and analysis of earthquake case statistics and qualitative analysis.Information reflecting great earthquake gestation and physical processes is very limited.In the future,we will attempt to improve the statistical analysis,obtain and utilize multidisciplinary observation data as much as possible,and analyze the development of earthquake trends from the perspective of geophysical field dynamic change.

    • Analysis of Crust and Upper Mantle Structure in Ningxia and its Adjacent Area

      2014, 36(2):309-313,337.

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      Abstract:Ningxia and its neighboring area are located in three blocks including the Erdos block,Alashan block and the Qinghai-Xizang block.There are many active faults in this area,such as the Haiyuan fault,the eastern piedmont fault of Helanshan,the Huanghe fault,and the Liupanshan fault.Because of the special stress environment in this area,all earthquakes of magnitude greater than 8 and most of those 5 to 7.9 occurred along the boundaries of the three active tectonics blocks.Researchers began exploration in this district in the 1960s and have used many methods such as wide-angle reflection and refraction seismic exploration,electrical prospecting,shallow seismic exploration,the gravity method,Global Positioning System (GPS),geodetic deformation survey,seismic array,and surveying and prospecting of active faults.Many valuable results have been obtained by using the various types of study methods.This paper summarizes the results of 40 years of such studies in this area by geoscientific researchers.The active block hypothesis states that strong earthquakes occur in such areas that the position wave velocity changes from high to low or the placement of the upper low-velocity body changes.Distribution of the geotemperature field in this area shows that many strong earthquakes occur in the position of high temperature and anomalous temperature gradient.The differential movement of block interaction is very strong.The results of wide-angle reflection and refraction seismic exploration also indicate the occurrence of the upper Moho uplift phenomenon in the strong seismic focus,in which superheated materials likely rise and melt rock as the hot mantle rock ascends in this area.By examining approximately 40 years of research in this area,including the deep seismic sounding profile,high gravity and magnetic anomalies,and inversion of seismic array data,we conclude that the Alashan and Erdos blocks are relatively stable and that the earthquake activity is weak,whereas the earthquake activity of the northeastern margin of the Qingzang-Tibet Plateau and Yinchuan Basin is stronger.The result of the probe of the deep seismic sounding profile shows that the upper Moho in the Yinchuan Basin is an uplift that resembles a concave bowl from east to west.Geothermal data support this theory.A low-velocity body exists in the crust of Yinchuan Basin,which also upholds this theory;many strong earthquakes that occurred in this area also offer direct evidence.The northeastern margin of the Qingzang-Tibet Plateau spatially and is the boundary of the active block,exhibiting comprehensive anomalous gradient zones with gravity,aeromagnetism,and crust thickness.Natural and artificial earthquake research reveals significant changes in crust thickness and several low-velocity bodies in the crust.There is obvious underplating in the crust and upper mantle transition zone.Geothermal data shows the same result.In short,strong earthquakes that occurred in this area are connected with a deep fault in the crust,a low-velocity body in the middle-lower crust,uplift of the upper mantle,high gravity-magnetic anomalies and a complex crust and upper-mantle transition zone.

    • Analysis of Energy Field of Seismic Activity before theMinxian-Zhangxian MS6.6 Earthquake

      2014, 36(2):314-319.

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      Abstract:The Minxian-Zhangxian MS6.6 earthquake that occurred on Jul.22,2013 is of great interest to the earthquake research community.In this paper,we describe the data we obtained for energy released by earthquakes and use the data to represent the basic elements of the seismic energy field.Then,we discuss the spatiotemporal features of the regional seismic activity by the random function theory and the method of natural perpendicular function before 2013.The time weight factors of the typical energy field indicate that the energy field was increasing in this region.The factors corresponded to the four key eigenvalues of the energy field before the Minxian-Zhangxian MS6.6 earthquake.The spatial image of the key typical fields showed a danger zone having an abnormally high value relative to local energy,which may be the medium-short-time abnormal. After analyzing the mean spatial image of the energy field in recent years,indications are that the Minxian-Zhangxian MS6.6 earthquake occurred in a high release zone of energy field,which may be the medium-short-space abnormal. Finally,after analyzing the mean energy field of the image of different time periods and different study areas,we found that high values of the long-time energy field existed before the earthquake.Several conclusions can be drawn including that the zone of high values may be the danger zone,mean value of short-time energy field is significantly abnormal, and,furthermore,there is along-time high value danger region of seismic energy field in the south of the Ningxia area.Although the mean abnormal of short-time seismic energy field is non-notable,our analysis suggests that there may be a strong earthquake in the region the next few years. Additionally,we analyzed the seismic energy field in the epicenter region of the Minxian-Zhangxian MS6.6 earthquake. Our analysis indicates that there were different distinct changes of four time factors of typical seismic energy fields from Sep. to Nov. 2011,Aug. to Nov. 2012,and Apr. to July 2013. In other words,a medium-short abnormal characteristic of the seismic energy field existed two years before the earthquake on July 22,2013. Secondly,by analyzing the spatial contours of the two main seismic energy fields of the July 22,2013 earthquake our research indicates that a danger area of high value abnormal of seismic energy field existed near the Minxian and Zhangxian in the preceding five years.The danger area may have coincided with the spatial anomaly of the July 22,2013 earthquake. Thirdly,by analyzing distribution characteristics of spatial contours of the seismic energy field over different time periods,we concluded that there were spatial medium-short high value abnormal regions of earthquake energy at different times and for different epicenter areas before the July 22,2013 earthquake. Furthermore,strong earthquake danger may exist near several faults including the Lixian-Luojiapu fault and Diebu-Bailongjiang fault in the region of south-east of Gansu province,Xiangshan-Tianjingshan fault,Haiyuan fault,and Liupan mountain fault in the south of Ningxia. Fourthly,we can analyze time-space information for abnormal earthquake activity using the method of natural orthogonal function.This type of analysis may help to forecast a strong earthquake or medium-strong earthquake,in a strong earthquake danger area.

    • Activity Characteristics of Primary Active Faults inYunnan-Sichuan Area and their Seismic Activity in the Past

      2014, 36(2):320-330.

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      Abstract:Vast active faults and their seismic activity in Yunnan-Sichuan area had been developed since the Cenozoic. Based on the earthquake catalogue and existing data of active faults, we compared the actual seismic activity of major fault zones in the past ten years in Yunnan-Sichuan area with the forecast results made by GSHAP (Global Seismic Hazard Assessment Program), and then concluded the similarities and differences between them.  The comparison showed that seismic activities in Longmenshan fault zone were greatly underestimated in GSHAP, for most of the earthquakes along Longmenshan fault zone were aftershocks of the 2008 Wenchuan earthquake. Coulomb stress change caused by the mainshock of Wenchuan earthquake triggered the 2013 Lushan earthquake, and then reduced the seismic hazard of the southeastern segment of Xianshuihe fault. In the past decade, low seismic activities showed in outer arc belt of Yunnan-Sichuan area, mainly including Xianshuihe fault, Anninghe fault, Xiaojiang fault and other minor faults, whose seismic hazard were greatly overestimated in GSHAP. The similar situation appeared on Red River fault, Lancangjiang fault and Litang fault, part of the inner arc belt, which had the highest seismic hazard assessed by GSHAP. Otherwise, seismic activities of Yingjiang area in southwestern Yunnan started to enhance since 2008, while the seismic hazard in that area was neglected in GSHAP. Along Nujiang fault, only in the south segment and Baoshan area existed higher seismic activity, which was consistent with GSHAP. On the other side, eastern piedmont fault of the Haba-Yulong Snow Mountains, belonging to the middle part of the inner arc belt, showed higher seismic activities in recent years, which conformed to the assessment of GSHAP. From the Zhaotong area to the east of Xiaojiang fault showed high seismic activities, consistent with GSHAP as well. The above comparisons were based on the earthquake catalogue in the past ten years, while the time horizon of GSHAP would be fifty years in future, and cycle of one devastating earthquake could be millennium, therefore large difference doesn't mean complete distortion of the estimates of GSHAP. 

    • Variation Characteristics of a Recent Gravity Field in Xinjiang and its Adjacent Regions

      2014, 36(2):331-337.

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      Abstract:By calculating six stages of gravity data from 1998 to 2010,we obtained images of the differences and the cumulative changes in the gravity field within 2~3 year periods in Xinjiang and its adjacent areas (Tibet,Gansu,Qinghai,and Ningxia).We then analyzed the corresponding relationship between the gravity changes and strong earthquake preparation through the dynamic structural environment and earthquake activity in Xinjiang.The results showed that the gravity field in Xinjiang changed obviously with a maximum of about -146×10-8 ms-2 within two to three years;earthquakes usually happened in a high gradient belt and in the main structural section;and the changing mechanism and differential of the gravity field were strongly related to the structural environment.In order to obtain dynamic changing images of the gravity field in Xinjiang and its adjacent region and the corresponding relationship between gravity change and strong earthquake preparation,this paper selected 100 gravity points (four to six absolute points and more than ninety solute points)which formed a monitoring network spread throughout Xinjiang,Tibet,Gansu,Qinghai and Ningxia via the China Seismological Bureau,the Zongcan Bureau,the Chinese Academy of Sciences and the state bureau of surveying and mapping unit.The data was processed with LGADJ software from the China Earthquake Administration.First,we selected relative and stable points to adjust the calculation process and address a few errors arising from a more significant and gross error,then proceeded with processing to reasonably determine the prior variance of each instrument.When we used the absolute gravity data to adjust the calculation process,we calibrated the weight in the 5×10-8 ms-2 precision relative gravity measurement data with the instruments used,by iterative computation.The results showed the accuracy to be within(8~12)× 10-8 ms-2 and the gravity change accuracy to be 15×10-8 ms-2.We found the anomaly region of the gravity field to usually have a high spatial gradient.Through our study on the evolution process in Xinjiang and its adjacent region,study results also revealed that an earthquake with an MS of 6.0 happened in this region.Due to an accumulation of stress, strain,and various physical features during earthquake preparation,Tianshan was pressed by the Tibetan plateau and Pamir with gravity decreasing in negative areas.The gravity value shows that the Tarim basin descended in the northwest,being pressed and accumulating in this region.

    • Evolution of Late Cenozoic Geography and Environment in Shanxi Province and the Neotectonic Response

      2014, 36(2):338-346,352.

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      Abstract:The Late Cenozoic paleogeography of Shanxi was affected by geomorphology, rivers,sedimentary environment,animals,plants, climate,loess,and ancient human activities during the Pliocene and Pleistocene epochs,from about 5.5 million to 12 thousand years ago.The evolution of the paleogeography formed the basis of the modern geography and environment,particularly the modern landscape and water systems.Understanding the ancient environment is also of great importance for effective environmental protection and for predicting future climate.Using information from our recent field investigations conducted throughout Shanxi Province,this paper systematically gives explanatory notes on the paleogeographical and paleoenvironmental characteristics for all epochs of the late Cenozoic in Shanxi Province.During tectonic movements in the early Pliocene,the Shanxi rift zone began to subside. In the Datong,Xinding,Taiyuan,and Linfen areas a rift basin began to form,with the corresponding emergence of small lakes.In the middle and late Pliocene,these basins formed large-scale rift lake because of the intense tectonic movement.The neighboring mountains were uplifted significantly.From then on,the basic geomorphological pattern was consistent with that of the current basin.In the late Pliocene,the rift basin subsided significantly,the climate became cooler,and large lakes began to appear in the rift basin.The early Pleistocene is characterized by wide-spread lakes in rift basins.In general,the climate at this time was a temperate forest and grassland monsoon climate alternating between wet and dry.It was moister and cooler than the Pliocene period,warm and humid in the earlier part,and cool and humid later.Loess deposition began,but mainly in the west of the Luliang Mountain watershed;the Yellow River developed in the early Pleistocene,during which five terraces formed.In the middle Pleistocene the lakes in the first basins began to recede,and loess deposits up to 200 m thick were deposited throughout the province,indicating a fairly arid climate.During this time,large-scale basalt flooding and eruptions occurred in the Datong Basin,caused by Himalayan orogenic movements.In the middle Pleistocene the climate was temperate humid and semi-arid forest steppe,warm and humid in the early part of this interval,cold and drought-prone later,and in general colder and drier than the early Pleistocene.Middle Pleistocene hominid cultural sites have been found at the He River in Ruicheng,containing mainly stone and also burned bones. In the Middle Pleistocene,loess was deposited almost throughout the province which formed the thickest deposits are still preserved today.The most prominent event in the late Pleistocene was the demise of the lakes,and the geography became much closer to its current form.During this time,the mountains were being uplifted and undergoing erosion and sedimentation occurred in the rift basin,with accumulation of thick lacustrine,fluvial-lacustrine and fluvial-alluvial deposits,which formed the present-day plain and became the main sites of ancient and modern human activity.During this interval, the humid,semi-arid forest steppe climate gradually became a steppe climate.Finally,we predict that the Shanxi basin will continuously subside,and,ultimately,the rivers in the basin will not drain,forming a great-lake landscape,similar to modern Lake Baikal.

    • Analysis of the TEC Anomaly prior to Two Earthquakes in Pakistan and Iran by Adopting Statistics of IGS

      2014, 36(2):347-352.

      Abstract (1430) HTML (0) PDF 14.62 M (1096) Comment (0) Favorites

      Abstract:Exploring and detecting the portents of earthquakes are common puzzles for experts and scholars at home and abroad.While studying earthquake in Alaska,Leonard and Barnes determined that the ionosphere changes prior to earthquakes,and they first recognized the causal nexus between them.In recent years,the Global Positioning System (GPS)has expanded rapidly to offer a new and more effective method for observing Turkish earthquake code (Total Electron Content (TEC))values and detecting portents of earthquakes.In 1998,the International GPS Service(IGS)began to release global ionosphere service products.Due to the broader covering range of IGS,its statistics have been adopted by an increasing number of scholars to investigate TEC values and their anomalies,aiming at detecting portents of earthquakes.On April 16 and September 24,2013,two earthquakes,both at a scale of 7.7,struck Iran and Pakistan.At such a high scale,the two quakes showed the traits of narrow space,time occurrence,and high casualties and losses;therefore,we used them as study targets.Existing research proves that when the sun cycles and geomagnetic activity show a relatively equal period,the TEC does not exhibit strong fluctuation unless there are other strong disturbance sources.Accordingly,in seismically active zones,excessive TEC fluctuation is much more likely to indicate impending earthquakes. We chose the core of the two earthquakes as our study targets,and we developed two wave figures of TEC variation with statistics of 20 days prior to earthquake occurrence.We determined that TEC varies periodically.The statistics generally showed steady movement,with substantial rising or reduction only on some days.To make our study more precise and logical,we enlarged our TEC study from the quake cores to the affected areas.We made a sliding comparison of the TEC variation in the entire area and obtained its distribution figure.To provide clarity,we illustrated these waves with various colors.After accounting,we noticed that the anomaly presented itself in the quake cores in addition to the surrounding areas.From the angle of anomaly degree,we noticed a decrease from the cores to the outer regions.With the TEC statistics issued by IGS and adoption of the sliding standard deviation from the cores to the impacting area of the quakes,we analyzed the TEC variations prior to the two earthquakes and detected the TEC anomalies.The TEC reduced unconventionally on April 14 and 16 and increased unconventionally on September 19,20,and 21.Moreover,with a plane survey,we detected TEC anomalies in some regions prior to both earthquakes,which decreased from the cores of the anomalies to their periphery regions;however,these anomaly cores did not all correspond with the cores of the two earthquakes.After analyzing the statistics form the cores of the earthquakes to affected areas,we determined the degree and increase area of these TEC anomalies.However,due to length of the thesis and our research facilities,additional work is required to determine the causes of these anomalies.Factors leading to these anomalies vary widely and include macula,geomagnetic index,cycling tracks of the Sun and the Earth,and weather anomalies,which are worth of our further study.

    • Study on Multi scale Temporal Gravity Field in the Eastern Margin of Tibetan Plateau Based on Satellite Gravity

      2014, 36(2):353-359.

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      Abstract:In this paper,we calculate annual satellite gravity changes in the eastern margin of the Tibetan Plateau from 2003 to 2012 by using the Gravity Recovery and Climate Experiment(GRACE) time-variable gravity field models with decorrelated filtering.By contraposing the three strong earthquakes that occurred recently in this region,including the Wenchuan MS8.0,Yushu MS7.1,and Ya’an MS7.0 earthquakes,we analyze the changes in satellite gravity field features related to the strong earthquakes with the annual gravity changes before and after each earthquake.For analyzing the relationship between these earthquakes and the gravity change mode in detail,we also caculate the monthly gravity change for the Wenchuan MS8.0 and Yushu MS7.1 earthquakes.Moreover, we use the recently relesed weekly GRACE gravity models to caculate the point-wise gravity change near Longmenshan fault for the Wenchuan MS8.0 earthquake.The regional annual differentiated dynamic gravity changes image indicates that all three earthquakes occurred in a period of small gravity change,which means a small migration of crust-mantle material occurred according to the mantle convection theory.The same feature is indicated in the regional annual cumulative dynamic gravity changes image.These gravity changes likely indicate a medium-short term earthquake precursor.In the monthly differentiated dynamic gravity changes image,the gravity change mode shows significant changes in both the Wenchuan MS8.0 earthquake and the Yushu MS7.1 earthquake,which indicates that the gravity gradient change direction altered from perpendicular to the plane of a fault to parallel.These change in the gravity change mode may confirm the theory of post-earthquake potential field restoration.The nine points of the weekly gravity change sequences in Longmenshan fault area that occurred during 2008 indicate that a nearly nine-week contrary gravity change trend occurred between the eastern and western areas of the fault after the occurrence of the Wenchuan MS8.0 earthquake.These gravity mode changes may be the reflection of an adjustment procedure of the crust-mantle material after the earthquake.Gravity measurement is an important subject in earthquake prediction research.Satellite temporal gravity analysis greatly enriches and complements the deficiency of the ground gravity network in time and space scales and can be quickly used for the study of crustal movement and earthquake prediction.

    • Static Coulomb Stress Changes and Triggering Interaction amongStrong Earthquakes in the Eastern Segment of North Qilian Mountain

      2014, 36(2):360-371,379.

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      Abstract:Earthquakes are a natural phenomenon caused by sudden rupture and slippage of crustal rocks after stress reaches the point of rock failure.The coseismic dislocations of a major earthquake not only cause the stress state changes in adjacent areas, but also change the Coulomb stress.Research shows that a very small static Coulomb stress changes can trigger seismic activity (i.e.,earthquakes).Therefore,it is important to explore the relationship between Coulomb stress changes and earthquake triggering.First of all,we conducted primary research for strong earthquakes by Coulomb 3.3 on the static Coulomb stress changes with the primary study region being the eastern margin of north Qilian Mountain and adjacent area from 1561 to present. Meanwhile,we used cumulative and individual methods to calculate the Coulomb stress changes between foreshocks and aftershocks.Our results show that the next events,except Menyuan earthquake in 1986,would occur in the stress triggering area.The value of changes is between 0.1 bar and 4.066 bar,and trigger rate reached a high of 87.5%.In other instances,when the source fault is closed to receive fault,the former event can trigger the next.Secondly,according to the relationship between Coulomb stress changes and stress accumulation rate,we calculated the stress accumulated before the next event,not considering foreshocks.Finally,we assessed the future strong seismic hazard for this area according to earthquake migration and stress triggering.We used Okada’s method to calculate the change of static Coulomb failure stress among faults for the study area.In 1920 and 1927,two magnitude 8 earthquakes occurred,prior to the earthquakes the Coulomb stress significantly increased in the eastern side of Yunwu Mountain and western part of Jingtai-Tianzhu-Gulang.This suggests that an increase of Coulomb stress may forecast a triggered earthquake.It is noteworthy that the Tianzhu 6.2 earthquake occurred in the western part of the danger zone after two prior earthquakes.Considering the Coulomb stress triggering and the history of seismic activity,future earthquake risk should be strong in this area as well as the eastern side of Yunwu Mountain and Jinqianghe fault zone in the western part of Jingtai-Tianzhu-Gulang.This paper attempts to combine Coulomb stress adjustment and fracture characteristics with seismic activity.Results are consistent with other approaches in the Northeastern Qilian area,indicating that Coulomb stress changes can be used as an indicator of oncoming seismic activity. Application of this method will require further improvement to assess future seismic risk.

    • Research on Stress Field Characteristics and Normal Fault SmallEarthquakes of Southeastern Qinghai

      2014, 36(2):372-379.

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      Abstract:Digital seismic waves recorded by Qinghai and Gansu digital networks from January 2008 to August 2013 are used to obtain 92 focal mechanism solutions of 4.9 ≥ML≥2.5 moderate and small earthquakes in southeastern Qinghai by using the methods of P-wave and S-wave maximum amplitude ratio and cut and paste (CAP).The focal mechanism solutions of 3.9≥ML≥2.5 are retrieved by the former method,and the other larger earthquakes are retrieved by the latter.Moreover,analysis of these focal mechanism solutions by means of statistics and systematic cluster analysis reveals that the predominant distributions of the plunge of T- and P-axes are less than 45° and that a few have a high plunge of P.These results indicate that the moderate and small earthquakes in this region are mostly strike-slip and that a certain number of normal fault earthquakes are present;however,small thrust fault earthquakes are rarely seen.The distributions of all of the azimuths of the P-axis are widely scattered,which may be attributed to the fact that the small earthquakes are more happenstance than tectonic earthquakes;however,the distributions of the P-axis azimuth of ML≥3.0 are so regular that it is easy to see the predominant distributions.By using focal mechanism solutions of ML≥3.0 by the grid test method,the mean regional stress field is obtained.The tectonic stress field is primarily represented by NE-SW horizontal compressive stress and NW-SW horizontal tensile stress in the southeastern Qinghai province,and the mean contradictory ratio is 0.32.These results are the same as those in previous research.Furthermore,it is determined that the small normal fault earthquakes are mainly distributed in the eastern part of the Kunlun active fault(Maqin-Maqu)and in the southern part of the Elashan Mountain fault.This paper analyses the mechanism in which small normal fault earthquakes are produced.The southeastern region of Qinghai province still belongs to the inner Qinghai-Tibet block,and the significant differences in elevation could create many small pull-apart basins in some area,such as the eastern part of the Kunlun active fault(Maqin-Maqu)In this region,the small normal fault earthquakes have a certain proportion;therefore,it is suggested that these small normal fault earthquakes may be attributed to gravity slumping because of the high altitude.

    • Research Progress and Thoughts on Reservoir-induced Seismicity

      2014, 36(2):380-386,412.

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      Abstract:Reservoir-Induced Seismicity (RIS) refers to seismic activity(i.e.,earthquakes) that occurs in the vicinity of artificial water reservoirs as a consequence of impoundment.RIS often is characterized as repeatable,clustered,shallow,and as having abundant high-frequency ground motion.A low magnitude RIS earthquake can cause major damage to not only human-made structures (e.g.,buildings and equipment),but may also cause geological disasters such as landslides and surges.Knowledge of RIS is immature because documented cases of RIS are limited in number and complex in nature.In the absence of well documented cases,seismic events occurring near a reservoir are often attributed to the impoundment of water.In numerous parts of the world today,including some of the most highly developed countries,many dam designers and operators tend to close their eyes to the engineering problems posed by RIS.There are some kinds of defensive arguments including:(1) no convincing correlation has yet been demonstrated between earthquakes and reservoirs;(2) since the natural seismicity at a given site is low,the danger of reservoir induced events is therefore also low;(3) the geology at a given site is different from that at localities where major reservoir-induced events have occurred;(4) only three or four out of some 11,000 large dams worldwide have experienced significant induced earthquakes,and one should, therefore,not worry about a given site;and(5) no dam has yet failed disastrously because of a reservoir-induced earthquake so the danger is thus grossly exaggerated.However,detractors of RIS are essentially evading the primary issues:Virtually every careful study has concluded that there is indeed a cause-and-effect relationship between some reservoirs and some earthquakes.Two dams (Koyna,India,and Hsinfengkiang,China) have,in fact,come uncomfortably close to disastrous failure during such events.Development of hydropower resources in southwestern China is one of the basic state policies;however,high dams built in high intensity areas may be accompanied by some risk of RIS.Therefore,the possibility of RIS and tendency of a reservoir to cause earthquakes needs to be scientifically investigated so that capacity for disaster prevention and mitigation can improve.At present,earthquake monitoring near reservoirs and related research are of great significance.Development of hydropower resources in southwestern China urgently needs to build an earthquake monitoring system associated with reservoirs and conduct in-depth studies of the monitoring data.Comprehensive analysis of reservoir seismic monitoring data is an important way to reveal the mechanism(s) of RIS and determine the factor(s) that induced an earthquake.This type of research is of great significance for scientifically forecasting earthquakes;and is also the foundation for proper identification of RIS and scientific guidance to aid in disaster relief.Therefore,it is urgent and essential to build up seismic monitoring systems for reservoirs using modern methods.It is also important to investigate the relationship among monitoring data,seismic activity,geological structure and properties of rock to reveal the mechanism(s) of RIS and the factor(s) related to reservoir induced earthquakes.The research results will be helpful in predicting the possibility of RIS,understanding the behavior of induced earthquakes,and preparing for disaster relief.

    • The Design and Realization of a Type of Monitoring Equipment forSeismic Stations Based on 3G Wireless Data Transmission

      2014, 36(2):387-392.

      Abstract (1408) HTML (0) PDF 9.70 M (1005) Comment (0) Favorites

      Abstract:Quickly recovering the seismic signals channel or solving problems with seismic station equipment is a major challenge once they are damaged or experience sudden failure.In our study,we designed and constructed wireless data transmitting and monitoring equipment for seismic stations based on the global system for mobile communication(GSM),third generation mobile communication technology(3G) and short message service(SMS).This equipment can control(shut down,switch on,restart,and enable the backup device)remote equipment easily,monitor the earthquake data acquisition system (EDAS) by the serial port and network,and transmit the seismic data by 3G.The hardware is composed of a core interfacing circuit board and a driving circuit board.The core circuit board includes one LAN port,two COM ports,and a number of I/O interfaces.The working status of the EDAS at a local seismic station is real time monitored through the LAN port and COM port. When abnormal data is outputted from the EDAS,the data output stops or is unable to be recognized.Abnormal information of the EDAS is sent to the earthquake monitoring center as short messages,and the center staff is able to make a judgment and take the correct action according to the messages.We designed two CPUs on the core circuit board,one a P89c668 and the other an STC12C5A60S2. The device can make data exchanges with the EDAS-24IP through the LAN chip.Meanwhile,P89c668 can monitor the real-time data flow of the EDAS-24IP,and analyze the correctness of the seismic data flow of the EDAS-24IP as well. In order to store parameter data,we used an electrically erasable programmable read-only memory (EEPROM)chip known as AT24c08.The driving circuit board is composed of an isolated driving circuit(such as HC4066 and 6N173)and various relays.A double AC-DC switching power supply,designed for use as backup,begins to work when the main supply is in failure.The design and realization of this equipment can solve some common problems in seismic stations.First,it can solve the problem of seismic data signal failure caused by cable channel breakdown.When there is something wrong with the cable transmission channel,the earthquake monitoring center can start 3G wireless data transfer through the remote controlling function,and the seismic data can be transmitted to the earthquake monitoring center through the 3G wireless transmission channel.Thus we can ensure continuous transmission of the seismic data signal, and reduce the rate of loss of seismic data.Second,the equipment can remotely control the switches in the seismic station network transmission between the master and backup equipment. When there is damage,such as following an earthquake,a rapid replacement of the damaged equipment with a backup system is crucial to ensure a continuous transmission of seismic data.Third,the equipment can control the seismic data acquisition server and the equipment to shut down,switch on and restart.When the seismic data acquisition server system or a related equipment system crashes,this equipment can help them to restart.Fourth,the equipment can monitor the operating status of the seismic data acquisition server through the LAN port or a serial port,and related information can be sent to the earthquake monitoring center to help center staff monitor the equipment's working status.Finally,the double AC-DC switching power supply will assure a continuous transmission of seismic data.This equipment can enhance the working stability of the seismic network,decrease the frequency of field maintenance,and greatly shorten the duration of seismic data transmission interruption caused by the instrument system failure.

    • The Spectrum Centroid Variation in Geomagnetic Z Component Data before and after the Wenchuan Earthquake

      2014, 36(2):393-397.

      Abstract (1396) HTML (0) PDF 7.81 M (1058) Comment (0) Favorites

      Abstract:Research on the seismo-geomagnetic effect goes back more than 100 years,but the use of modern methods and experimentation began in the middle of the last century.Some geomagnetically abnormal changes have been reported before large earthquakes,e.g.,the Alaska earthquake (March 27,1964),the Loma Prieta earthquake (M7.1,Oct,17,1989),and the Iwateken Nairku Hokubu earthquake(M6.1,Sep. 3,1998).The Wenchuan earthquake was the largest one after the Tangshan earthquake in China.It is important to explore the mechanism of great earthquakes when studying the Wenchuan earthquake.In this paper,we study vertical component (component) geomagnetic data observed by three stations(Chengdu station (CDP),MuLi station(MUL),and XiChang station(XIC))in Sichuan Province,with a data sampling rate of one point per second.The maximum spectral centroid,the average spectral centroid,and the spectral centroid standard deviation were calculated hourly over three time segments (May 1,2008 to May 12,2008,May 1,2009 to May 12,2009,and May 1,2010 to May 12,2010).The purpose of this time segment division is,first,to reduce seasonal disturbance,and,second,to avoid interference in the calculated results due to the data length.The results show that among the maximum spectral centroid,the average spectral centroid,and the centroid standard deviation,the spectral centroid variability of CDP is greater than the others in the same time period before the Wenchuan earthquake,and it rises gradually before the earthquake.Other stations did not show these characteristics in the same time segments.So we believe there are significant changes in the geomagnetic Z component spectral centroid at CDP,which is information characteristically associated with a great earthquake.This characteristic is also related to the distance from the station to the earthquake position.The distance is 35 km from the epicenter to CDP,357 km from the epicenter to XIC and 400 km from the epicenter to MUL.Our results suggest that these geomagnetically abnormal changes occur in the preseismic period,and can only be measured by a nearby station.Many theories or hypotheses have been put forth to explain the seismo-magnetic phenomena.Generally,earthquake preparation is considered to be a process of the accumulation of stress.When the stress accumulation is more than the elastic limitations of the rock,the rock suddenly dislocates or ruptures and an earthquake occurs.When media is in the process of stress accumulation,the stress redistributes spatially,so the media’s elastic deformation changes into nonelastic deformation.Fracture morphology and occurrence in space is also changed.A new crack generates,expands and experiences fluid infiltration.Rock swelling may be produced that has pressure-magnetic effects,induced magnetic effects,rheological-magnetic effects,electric-magnetic effects,thermal-magnetic effects,etc.The existence of abnormal activity before an earthquake is an important issue in the study of great earthquakes.Our intent is not to focus on a discussion of the occurrence mechanism of seismo-magnetic phenomena,but on how to extract anomalous information about an earthquake from the geomagnetic data from a nearby station.Our results show that this type of study can provide important reference data for research on seismo-magnetic effects,earthquake mechanisms,earthquake predictions,and position estimations of future earthquakes.Furthermore,our results indicate that the spectrum centroid may provide a reasonable and suitable source of information prior to an earthquake.

    • Adaptive Polarization Filtering for Improving the S/Nof Station Seismic Data

      2014, 36(2):398-404.

      Abstract (1504) HTML (0) PDF 16.37 M (841) Comment (0) Favorites

      Abstract:Polarization properties differ among various types of seismic waves.The seismic wave actually collected is the result of interference and is superimposed by vibrations with different types and different polarization properties.Polarization analysis is a signal processing method based on polarization characteristic of seismic waves and can simplify the extraction of information by measuring the polarization properties of the various types of seismic waves.It has a good effect on the identification and separation of specific wave type,the suppression of noise,shear-wave splitting analysis,multi-wave seismic phase identification,and wave arrival time determination.The polarization filtering method based on the covariance matrix plays an important role in multi-component seismogram processing due to its explicit physical meaning,easy implementation,and high efficiency.This type of polarization filtering calculates the polarization parameters in a given time window;thus,the choice of time is very critical.The window length in polarization analysis method based on the standard covariance matrix(SCM)is fixed in the time domain;the computed polarization attribution is an average value in the time window.In practical application,the selection of time window length of the SCM is entirely dependent on the experience,and the polarization attributions in a given length window do not have time-varying characteristics.The filtering results of the SCM are relatively stable,insensitive to disturbance,and unable to determine the polarization parameters in the beginning and end of the seismic record.Thus,the filtering effect is not ideal and will inevitably appear glossy in interpretation.For this reason,the present study introduces a new polarization method based on the adaptive covariance matrix(ACM).We use an approximate formula to compute the adaptive window function,in which the length is adapted to the instantaneous frequency of three-component seismic data.In particular,the window length of the covariance matrix adjusts to the minimum cycle of the desired signal,which reduces the factitious impact of the window length selection.In addition,there is no need to interpolate processing because the polarization parameters are computed in every time sampling point of the three-component seismic records,except for one-half of the time window of the start and end points.Due to the above advantages,ACM greatly improves the filtering accuracy.The processing results of the model and actual three-component station seismic data show that the SCM represents a smoothed version of the instantaneous attributes from the ACM.Furthermore,because the time window is fixed for the standard method,it is not possible to characterize the polarization attributes of a seismic event with a period lower than that of the time window used for the analysis.The polarization curves computed by SCM and ACM agree quite well in the region in which the period of the dominant signal is close to the time window selected for the covariance analysis,which greatly reduces the effective signal waveform differences before and after filtering.With ACM,a comparison of the original signal and that after filtering reveals that almost no high-frequency interference exists near the effective signal.In general,the significantly different regions in which the two curves from the SCM and ACM indicate that the selected window for the ACM is optimal.

    • Study on Long term Observation Data of Earth Resistivity at BaochangStation in Inner Mongolia

      2014, 36(2):405-412.

      Abstract (1239) HTML (0) PDF 15.38 M (972) Comment (0) Favorites

      Abstract:Since the Xingtai earthquake in 1966,the geo-electrical resistivity observation has been implemented in a large scale in China.At present,approximately 90 geo-electrical resistivity stations are in operation in mainland China as a result of network construction in the period from the seventh five-year plan to the tenth five-year plan.In the long-term observation,significant anomalies in geo-electrical resistivity have been recorded before the MS7.8 Tangshan earthquake in 1976,the MS7.2 Songpan-Pingwu earthquake in 1976,the MS7.6/7.2 Lancang-Gengma earthquakes in 1976,and the MS8.0 Wenchuan earthquake in 2008. Moreover,several moderate-strong earthquakes have been accurately forecasted on the basis of geo-electrical resistivity observation.However,many people still have different understandings on the anomalies in geo-electrical resistivity before earthquakes due to lack of knowledge of earthquake preparation and occurrence processes,crustal structure,and medium conditions from the epicentral area to observation station and dynamical processes of underground media.Such confusion may be attributed to the small detecting depth/range of current DC geo-electrical resistivity stations and the complicated behaviors of anomalies in geo-electrical resistivity that include the anomalous changes caused by observational environments.The Baochang geo-electrical resistivity station in Inner Mongolia has continually observed for more than 30 years and has met the technical requirement of geo-electrical resistivity observation in China. Many moderate and great earthquakes have occurred near this station.This study evaluates the reliability of long-term geo-electrical resistivity observation data of the station by processing the data recorded since 1985 by using the normalized variation rate method and the Fourier sliding method for identifying earthquake-related anomalies in geo-electrical resistivity and studies the monitoring ability in the medium-term and imminent stages before an earthquake occurs. The research results are as follows:Baochang station has not been affected by the observational environment in the long-term. Its observation data are valuable in earthquake scientific research and earthquake prediction,and the anomalies identified by the aforementioned two methods are credible.We believe that the geo-electrical resistivity observation of station Baochang has certain monitoring ability for moderate-great earthquakes near the station.The geo-electrical resistivity of two orthogonal channels of the station has shown continuous medium-term and short-term anomalies around the station related to the earthquakes.Furthermore,the decreasing or increasing anomalies which lasted for one year,were also recorded before the long-distance MS8.0 or above earthquakes that occurred near mainland China and its surrounding area,which were historically well corresponded to the occurrence of the great earthquakes.

    • The Application of Time Frequency Analysis Methods in Deformation Data

      2014, 36(2):413-420.

      Abstract (1278) HTML (0) PDF 24.92 M (899) Comment (0) Favorites

      Abstract:The time-frequency method is a useful tool for analyzing nonlinear and non-stationary signals.The earthquake data we obtained from the deformation instrument are both nonlinear and non stationary;therefore,this method is suitable for earthquake data analysis.Fast Fourier Transform(FFT)has resulted in significant improvements in signal analysis and is used to determine the frequency resolution of the signals.The obvious advantage of the time-frequency method over the traditional FFT transform is that it offers a frequency trend that varies with time,which means that it has both time resolution and frequency resolution.In recent years,this method was gathered increasing attention in earthquake data processing.Its applications include seismic signal analysis,geological exploration,and mechanical vibration signal analysis.However,its application in terrain data analysis is still in its infancy.Many different methods are used in time-frequency analysis,including linear and nonlinear analysis methods.In this paper,we introduce the theories of wavelet transform,Stransform,and Hilbert-Huang transform,which are all nonlinear analysis methods,and design signals that are similar to the deformation signals.Moreover,we list terrain data to compare the respective merits,scope of application,and areas for improvement of these three methods.Through the analysis of the signals designed in this paper,we determine that the time-frequency methods can demonstrate the frequency variation over time,and all of the methods we mention in this paper can display the characteristics of the data in the time-frequency domain.By using various time-frequency analyses methods,we analyze wide broadband data and sampling data hours before and after the Wenchuan earthquake to obtain different results of impending earthquake events in these methods and to propose a proper method for analyzing the terrain deformation data.The analysis of impending earthquake events offers a method of forecasting earthquakes.In this paper,we also determine that the wavelet transform has its own advantages of frequency resolution in the high-frequency area,although the low-frequency analysis results are insufficient;the S transform has its own advantages of low-frequency resolution;and the Hilbert-Huang transform is not suitable for terrain deformation data analysis,which is needed to display the change trends of the data.Further,we obtained the scope of application of these three methods.Wavelet transform can be used to analyze a large number of data and to obtain the high-frequency characteristics of the signals;the S transform can analyze data that includes many low-frequency components;and the Hilbert-Huang transform can analyze the data that requires information of the component in detail because the analysis uses empirical mode decomposition (EMD).Recently,a new method based on the Hilbert-Huang transform,known as ensemble empirical mode decomposition(EEMD),has been proposed for signal analysis.However,its improvement in the Hilbert-Huang transform method has not been determined and requires extensive research.

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