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  • 1  Research on Coulomb Stress Change and Its Application in Seismology
    LIU Fang-bin WANG Ai-guo JI Zhan-bo
    2013, 35(3):647-655.
    [Abstract](3783) [HTML](0) [PDF 883.34 K](4794)
    An earthquake is a natural phenomenon caused by the sudden breaking and slipping of crustal rocks when stress accumulates to a certain extent.The coseismic dislocations of a major earthquake not only cause stress state changes in adjacent areas,but also change the Coulomb stress.Earthquakes can be triggered by stress changes induced by other earthquakes;the most common examples are aftershocks.Static stress changes can increase or decrease seismicity in the surrounding regions.According to the Coulomb failure criteria,ΔCFSwill affect the failure condition of a fault.WhenΔCFSis positive,it will accelerate the speed of the stress rupture accumulation in the surrounding areas and cause the next earthquake to occur sooner,which is called stress triggering;on the contrary,whenΔCFSis negative,it will defer the accumulation.In recent years,seismic Coulomb stress changes generated by the earthquake and subsequent temporal relationships have been valued by seismologists all over the world.Studies have shown that a minor static Coulomb stress change of 0.1bar may trigger earthquakes and make the regional seismic activity change in the future.Therefore,it is important to explore the relation between Coulomb stress changes and earthquake triggering not only for the study of long-term earthquake prediction and static stress changes triggering subsequent seismic events,but also for future regional seismic hazard analysis.In this paper,the definition and the physical meaning of Coulomb stress are explained,and then the research progress of the applications of Coulomb stress change in seismology are discussed from five aspects:1)The triggering of aftershocks by a strong earthquake.Based on Okada’s([year\〗)elastic half-space dislocation model,the Coulomb stress change images are calculated,and then the spatial distribution of aftershocks are studied.The positive and negative Coulomb stress distribution images are related to the activity of aftershocks directly and affect the regional seismic hazard assessment;2)Triggering of the subsequent earthquakes by strong earthquake.The main way to trigger subsequent earthquakes is stress transfer,in which the stress before an earthquake event can have an effect on aftershocks;3)Stress shadow.A socalled stress shadow means the negative Coulomb stress changes an area with the function of inhibiting or delaying the fault slip and rupture with aΔCFS <0,which will either decrease the seismic activity or prolong the time of the next earthquake;4)The analysis of seismic activity;and 5)The related application of software.Though limited by the inability to accurately predict the time of subsequent earthquakes,Coulomb stress changes have been proven to be very useful in predicting epicenter locations.There is still much research left to be conducted to predict the location and timing of future earthquakes accurately by Coulomb stress changes.
    2  Influence of Piling on Damping Coefficients of Soil Surrounding Piles
    WANG Yao-cun LI Sa WANG Yao-cun, LI Sa, ZHANG Pei-xian, DAI Xu DAI Xu
    2014, 36(4):946-951.
    [Abstract](3347) [HTML](0) [PDF 566.75 K](2500)
    The accuracy of drivability analysis for piles with large diameters is very important for pile driving safely.The prediction of soil resistance to driving (SRD) is the focus of this analysis.Numerous models have been developed to predict soil resistance.With many outstanding advantages,the technology of high-strain testing is the basis for a new advanced method for pile-driving analysis.The Case Pile Wave Analysis Program (CAPWAP) method is generally used to analyze dynamic tests data to obtain the SRD.CAPWAP is based on one-dimensional wave theory,which is widely used in pile foundation monitoring and bearing capacity evaluation in ocean engineering.Using CAPWAP to analyze the data from dynamic testing,the SRD can be determined during the entire pile installation procedure.When using CAPWAP,soil factors and pile parameters such as damping,quake force,and wave velocity should be determined.Among the parameters,the soil damping coefficient of friction Js and top of pile Jt,have the greatest influence on pile bearing capacity.Soil damping coefficients are the most important parameters of soil dynamics.Various models have proposed distinct Js and Jt,and each soil type has a specific damping value.For example,the Smith model proposed values of 0.16 and 0.48 for Js and Jt,respectively,and the Case model recommended Js values of 0.15 for cohesive soil and 0.65 for cohesionless soil;Jt was 0.5 regardless of soil type.Many models widely use the value of 2.54 mm for quakes.Although many scholars give different experience values,the Case damping coefficients are widely adopted in most cases. Engineering applications reveal that various soil parameter values strongly influence the SRD;therefore,the study of soil parameter values is meaningful.To obtain more accurate damping parameters and to improve the reliability of CAPWAP,this study examines the results of pile dynamic testing of an engineering application in the East China Sea.During pile installation,the pile is struck up to 1868 times to penetrate the design depth,with each blow containing a set of corresponding force and velocity curve.In this study,we analyze approximately 200 curves by CAPWAP to obtain a set of soil parameters for each layer.To obtain the changes in Js and Jt with depth,numerous data are analyzed.First,a study is conducted to determine Js and Jt with various penetration depths.The results show that the soil damping coefficients increasingly decay rather than remain constant during piling and change with penetration depth.Therefore,when using CAPWAP to predict the soil resistance to driving,using a constant for Js or Jt during the entire pile installation is not reasonable and will result in error.To further study the changes in soil damping coefficients during pile installation,the relationship of soil damping coefficients with blows is examined.It is determined that the change trend of blows is in contrast to that of the damping coefficient.Finally,this study analyzes the change trend of soil damping coefficients with depth.During the piling procedure,the factor value is shown to steadily decrease in a manner similar to a linear gradient.
    3  Influence of Different Damping Matrices on Seismic Response in Deep Soil Sites
    ZHANG Ru-li CHENG Xu-dong GUAN You-hai
    2014, 36(4):868-873.
    [Abstract](3344) [HTML](0) [PDF 411.56 K](1995)
    Seismic response analysis of soil plays a major role in the seismic safety assessment of engineering sites;it reflects the influence of the characteristics of engineering sites on the input ground motion parameters of a structure.Seismic analysis can either be performed in the time or frequency domains.The dynamic analysis of soil is based on the principle of soil dynamics and the viscous damping model,and the Rayleigh proportional damping matrix is built on the assumption that the vibration modes are orthogonal about the damping matrix;therefore,two structural vibration modes can be chosen to obtain the proportion coefficient of the damping matrix.The selection of viable damping models is important for seismic response analysis of deep soil layers in the time domain,and in this paper,the influence is discussed using six different damping matrices formed by the hysteretic damping ratio and modal damping ratio. The paper first introduces two damping matrices based on the hysteretic damping ratio and Rayleigh damping,which is based on the viscous damping ratio.Then,six different damping matrices are constructed for the seismic response analysis of a deep soil site: the first and second frequencies,the first frequency and the frequency closest to the predominant frequency of the seismic wave,the first frequency and third frequencies,a common model based on the first frequency used in soil dynamics,and other forms,such as equivalent damping matrices based on the first frequency and the translation frequency.The damping frequency can be considered as independent in the frequency domain and an accurate solution can be obtained.The time domain solution is obtained using ANSYS,which uses a one dimensional soil column for the simulation.Using a deep soil site as an example,the influence of the different damping matrices on the seismic response is analyzed under an input of synthetic bedrock seismic waves and a natural seismic wave,the Shanghai Sheshan bedrock seismic wave,recorded in the Wenchuan earthquake.The seismic response results at different depths of the soil are compared with the frequency domain solution. The results show that the influence of the damping matrix is greater on the accelerations than on the displacements. The peak ground acceleration is most overvalued by 47.28%,and most undervalued by 32.53% compared to the frequency domain solution.The cause for the different influences of these damping matrices is investigated.The chosen wave spectrum characteristics are different,and the degree of influence of a damping matrix also varies depending on the input wave spectrum characteristics.The spectrum characteristics of a seismic wave should be considered during the selection of a damping matrix.The frequency information of actual bedrock seismic records are generally abundant and due to the frequency correlation of the selected damping in the time domain,using only one frequency is often not sufficient to reasonably reflect the characteristics of a seismic wave.Therefore,both the spectrum of a seismic wave and the actual soil condition at the site should be considered to determine an appropriate damping matrix to obtain more accurate and reasonable results.
    4  Review of the Conversional Relationship for Different Magnitude Scales
    LI Ying-zhen YING Na LI Xiao-han
    2014, 36(1):80-87.
    [Abstract](2785) [HTML](0) [PDF 13.53 M](2186)
    Magnitude of an earthquake is an important measure used to compare earthquakes in space and time.There are some problems with the current magnitude scales,ML,MS,mB and mb,including uncertainty in the physical meaning and determination results are inconsistent.It is widely recognized that the moment magnitude is an absolute standard measurement of mechanics and may be an ideal magnitude scale because it is not a magnitude of saturation but stabilization.This article briefly demonstrates some progress achieved by Chinese scholars in comparing disparate magnitude scales used by the China Earthquake Networks Center (CENC) and major foreign organizations.In addition,conversional relationships between various earthquake magnitude scales are explored.We will present the comparison between the moment magnitudes for various earthquake magnitude scales and foreign standards,for example,when 6.5≤M≤7.6,mB≈MW;when 5.2≤M≤8.0,MS≈MW;when 4.2≤M≤7.0,ML≈MW;when 4.0≤M≤6.0,and mb≈MW. In each range above,the deviation between MW and mB,and between ML andMS is less than or equal to 0.2. After derivation,the comparison of moment magnitudes and earthquake magnitude scales in our regular standards are as follows:when 4.5≤M≤5.8,mB≈MW;when 4.5≤M≤7.5,mb≈MW;when 4.5≤M≤7.0,ML≈MW;And MS≈MW;MS7=MW.In the magnitude ranges above,MW deviated from mb and MS by less than or equal to 0.2,was equal to MS7;and deviated from ML by 0.22. It is suitable for all kinds of magnitude scale ranges,whereby generally:ML is applied to magnitude 1.0~5.9 earthquakes;mB is applied to magnitude 3.5~6.5 earthquakes;mB is applied to magnitude 4.5~7.5 earthquakes;MS is applied to magnitude 6.0~8.0 earthquakes; and MW is applied to all earthquake magnitudes.It is also considered a priority to catalogue conversions from the different magnitude scales.The process of earthquake rupture is complicated,between the same rupture scales (the same M0),and the altitude source spectra of some frequency are very different because of the different stress field background,rupture velocity,and rupture pattern.It is shown that there is a big difference between magnitude scales because of the limit used by different countries,different geographical conditions, instrumentation,and formula for computing magnitude.The conversion relationship between different magnitude scales is difficult to define.This article showed that the conversion formula of moment magnitude was inferred further using empirical formulas based on prior studies and not direct conversion between moment magnitude and other magnitude scales. The approach used here could be applied to earthquake forecasting.The next step to carry out the work is that establishing direct experience relation according to the thought from Mr.Chen Peisheng: establishing direct relation formula between ML,MS,mB,mb,and MW so the results may be more reliable.In addition,it would be necessary to investigate a number of parameters such as seismic moment of great earthquake,small earthquake,shallow (-focus) earthquake,and bathyseism.Seeking empirical relationships between logs M0,ML,MS, and mb is a good approach.
    5  Ambient Vibration Test for a Multi story Residential Building and the Vibration Mitigation Measures
    ZHU Bin CHEN Long-zhu DING Li
    2014, 36(2):207-213.
    [Abstract](2747) [HTML](0) [PDF 14.70 M](6103)
    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.
    6  Mechanism of Gravity Anomaly before Earthquake and Its Role in Earthquake Prediction
    CHU Qing-zhong WU Ze SHAO Xian-jie LI Hui CHEN Yu
    2014, 36(1):201-206.
    [Abstract](2723) [HTML](0) [PDF 10.15 M](9528)
    Earthquakes are among the major natural disasters facing the world.Earthquakes can threaten safety of people and cause serious damage to property and the economy of a region.Our country is one nation that has had serious earthquake disasters throughout history,with 4 400 large earthquakes being recorded.Seismic intensity is 7 degrees or higher for 60 % of the nation’s land,with 50% of the cities and 67% of the big cities located in these high(i.e.,≥ 7) intensity areas.Therefore,having the ability to predict earthquakes would help to reduce the risk posed to people and human made structures.This article examines the gravity anomaly,one of the earthquake forerunners,and the relation between gravity anomalies and earthquakes.This study is based on the concept that when tectonic movement occurs,the earth’s crust extrudes or extends in every direction. As a result,gravity will be in a state of imbalance and under the action of an equilibrium force,the earth’s crust will restore to a balanced stable condition.During this adjustment,the gravity field will adjust and change until balance is restored.A formula is used to briefly describe the three influencing factors that affected the gravity anomaly:the density changes,movement of the outer fluid,and changes of measurement elevation.In the second part of the study,gravity variation diagrams are drawn with respect to time and space before the earthquake of Yushu,Tangshan and Wenchuan.It can be seen from the cases that the gravity anomaly before earthquakes is obvious:gravity changes constantly near the epicenter,and the gravity gradient is higher than usual.There are strong tectonic activities during earthquake gestation.Large deformations are produced in the fault zones and the weak areas of the earth’s crust,thus forming some crustal thickness difference and density difference,and the phenomena of gravity anomalies are shown.In this part,the causes of the gravity anomaly are also conjectured:tectonic movement and the invasion of subsurface fluid. Finally,two prediction methods of gravity are introduced:mobile gravity observation and GRACE gravity observation.The gravity changes because of crustal deformation and material changes in the fault zones,which result from crustal movement during earthquake gestation.We believe that evolution characteristics of the gravitational field can help us to explain current tectonic activity.The whole process from gestation to eruption of the mid-strong earthquake can be tied to gravity anomaly,thus providing reliable basis for earthquake prediction.Through the study of the gravity anomaly,the characteristics and change can be understood including the quality and density of underground medium,change of elastic modulus of the crust,and relationships between gravity anomalies and earthquakes.The related indicators of the short to medium term earthquake prediction in mobile gravity observation are summarized,and the case of Wenchuan earthquake is used to summarize the method of long term earthquake prediction using the GRACE satellite observation.
    7  Seismic Fragility Analysis of Steel Structure Considering Steel Corrosion
    ZHENG Shan-suo TIAN Jin HAN Yan-zhao XU Qiang SUN Le-bin
    2014, 36(1):1-6.
    [Abstract](2533) [HTML](0) [PDF 11.48 M](3682)
    Steel frames not only have the advantages of being light weight,and high strength but also are corrosion resistant and fireproof,so steel is widely used for industrial construction.Unfortunately,research on corrosion mechanisms and measures of corrosion protection were mainly aimed at the timber level and based on experiments.Due to differences in test conditions,test methods and other factors,conclusions from these experiments were highly variable and empirical formulas were difficult to utilize.In addition,research aimed at component-level corrosion was also not yet clear,so uniform corrosion is often assumed for convenience.For example,some simply reported weakened cross-section members and considered material deterioration to analyze structural performance. In this paper,we consider the seismic performance of overall structure in different service times.Material deterioration and non-uniform corrosion were accounted for in column and beam cross-sections.Steel corrosion caused the deterioration of seismic performance and stiffness for structures.When steel frame structures survive earthquakes,it does not guarantee that the structures will survive one that reaches the design capacity at a later service time.Therefore,when evaluating the seismic performance of steel frame structures in different service times,we must account for the increased seismic risks caused by steel corrosion over time.Changes in seismic performance of structures over different service times were examined in this paper. The deterioration of elastic modulus and steel yield strength were evaluated using existing empirical formulas. At the same time,the thickness variation of the column and beam cross sections in steel frame structures was accounted for by using findings from related research.Different methods including IDA,pushover,and Monte Carlo were combined together to estimate values for different limit states in a probabilistic seismic capacity model. In this paper,the maximum drift angle was taken as the damage index. The statistical parameters of the probabilistic seismic capacity model were estimated through pushover analysis,while the relationship of probabilistic seismic demand was obtained by IDA analysis for random structure samples and different service times.Seismic fragility surfaces of steel-frame structures were obtained in different limit state and different service time (0 year,15 years, 30 years,45 years,and 60 years). Finally, a case study of a 15-floor steel-frame structure was adopted to illustrate the change in structure fragility that may occur as a result of corrosion. Seismo-structure software and finite-element models were used to test different service times of the structure. Pushover and IDA analysis were then used to calculate values of different limit states in a probabilistic seismic capacity model and the relationships of probabilistic seismic demand models,respectively,over different service times. These achievements would provide theoretical support for the life-cycle design of steel frame structures,operation and management,and especially seismic hazard assessment of existing steel frame structures.
    8  A Study on Hazards of Background Seismicity in Mongolia
    XU Wei-jin GAO Meng-tan
    2014, 36(2):256-260.
    [Abstract](2449) [HTML](0) [PDF 11.06 M](6060)
    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.
    9  Preliminary Analysis of Poisson’s Ratio of Shallow Stratum in Tianjin
    GAO Wu-ping CHEN Yu-kun LIU Fang
    2014, 36(1):47-53.
    [Abstract](2437) [HTML](0) [PDF 15.73 M](4811)
    Poisson’s ratio was characterized for shallow stratum in Tianjin from more than ten borehole logs collected during the period of the“10th five-year-plan”.Associated borehole histograms and laboratory test documents were also compiled for discussion of influencing factors such as the soil moisture,wet density,and porosity ratio. Based on the borehole logging data,we developed a scatter graph of Poisson’s ratio versus depth and fit a curve to the data to represent shear wave velocity and compression wave velocity. The graph shows that the Poisson’s ratio changes by three stages as a function of depth.In the first stage,in the range of 0 to 5 m,it increases rapidly from 0.420 near the surface to 0.495 at a depth of 5 m. Fluctuation in Poisson’s ratio may result from stiff artificial soil cover of different thicknesses that formed for thousands of years of natural processes and human activities near earth surface. In the second stage,in the range of 5 to 20 m,it tends to be stable and the value mostly remains at the level of 0.495,where the stratum mainly consists of soft,saturated,and plastic mucky stratum. In the last stage,in the range of 20 to 200 m,it slowly decreases but shows considerable discreteness and some kind of linear feature is revealed by the fitting correlation coefficient.This decrease in the last stage may be the result of changes in soil lithology,soil sedimentary environment,and soil provenance at greater depths.Many cycles of transgression and regression occurred in Tianjin during the Quaternary period,so the shallow stratum mainly consists of continental facies,marine facies,and marine continental mixed facies.These varying depositional settings caused great changes of lithology,and sedimentary environment in Tianjin. Moreover,the migrations of the Yellow River and Haihe River also influenced the sediment mineral compositions greatly:In summary,the sedimentary environment and evolution mechanism strongly affected Poisson’s ratio of the shallow stratum.Factors influencing Poisson’s ratio in the sediment were examining by selecting two typical boreholes and developing a Poisson’s ratio curve and associated histogram.Additional data including curves of soil moisture,porosity ratio,and wet density were all displayed in one figure together.It shows that the Poisson’s ratio is positively correlated with soil moisture and porosity ratio and negatively correlated with soil wet density.The form of Poisson’s curve is strikingly similar with that of soil moisture and porosity ratio curves and is opposite to that of a wet density curve.By comparing soil moisture,wet density or solid mineral components,it is easy to see that soil moisture has greater effect than wet density.In other words,soil moisture is may be the key parameter influencing the soil’s Poisson ratio.Further,comparing the curve of Poisson’s ratio and related borehole histogram, we can see that the changes of soil lithology at greater depths indicate a change of Poisson’s ratio. However,it is difficult to obtain Poisson’s ratio accurately based on soil lithology because the characteristics of stratum are very complicated and strongly regional.Because of the great number of factors contributing to Poisson’s ratio,further examination of the factors is necessary especially for application in a different geologic setting.
    10  Seismic Hazard Assessment for Active Faults of Longnan City
    SHAO Yan-xiu YUAN Dao-yang HE Wen-gui LIU Xing-wang ZHENG Wen-jun LIU Hong-chun ZHANG You-long WANG Ai-guo
    2014, 36(3):645-655.
    [Abstract](2354) [HTML](0) [PDF 4.28 M](1056)
    Based on active fault mapping in Longnan City,Gansu Province,we determine in this study parameters used in seismic hazard assessment for active faults.Five main fault zones are identified.The first is the Diebu-Bailongjiang fault zone,which ruptured during the M7 earthquake in 186 BC.The second is the Guanggaishan-Dieshan fault zone,which is separated into three main strands.The north strand had been found to cause a paleo earthquake,although the Pingding-Huama strand had a significantly faster slip rate.The Hanan-Qingshanwan-Daoxizi fault zone is the third.A surface rupture of approximately 3.5 km long was found in the Zhuyuanba strand.The fourth is the Lianddang-Jiangluo fault zone,which had strong activity in the Holocene.The fifth is the Wudu-Kangxian fault zone,in which four paleo-earthquakes have occurred since the Late Pleistocene.We estimated potential maximum earthquake magnitude and repeat interval time with magnitude rupture length and magnitude-frequency relationships for every fault or fault sub-area.The probability of destructive earthquake occurrence in the future 50,100,and 200 years was computed with a time-dependent seismic potential probability model and a Poisson distribution model.The methods for estimating potential maximum earthquake magnitude include four principles:(1)maximum magnitude can be an upper limit magnitude in the same seismic zone and (2)should not be smaller than the largest historical earthquake.(3)It can also be estimated by a relationship,such as M-L and G-R.Moreover,(4) it may have a reference to the maximum magnitude in a seismic zone with intensity of VIII degrees during the 200 years since last big earthquake because the lapsed time is longer than the interval time.The article gives mean maximum magnitudes,which are 7.5,7,7,7,and 6 for the Diebu-Bailongjiang,Guanggaishan-Dieshan,Liangdang-Jiangluo,Wudu-Kangxian,and Hanan-Qingshanwan-Daoxizi fault zones,respectively,with a combination of the four principles.We also used the method of b value spatial mapping to reveal high stress or high seismic hazard zones.On the basis of the seismic gaps identified,we used the relationship between magnitude and number or frequency to compute the b value of the study area with small earthquakes of the most recent 40 years.The small earthquakes were relocated with seismological methods.If the area had a lower b value,lower strain was accumulated in the area.We determined higher strain areas from the b value map;such areas are future high seismic hazard zones.We used the maximum curvature method to determine minimum magnitudes of completeness,which are minimum magnitudes of earthquakes for computing the b value.We used a 1° × 1° grid for the study area.Every node had a b value that was calculated with at least 50 selected small events.The events for computing the b value were selected by a circle with a particular radius and a node center point.The results indicate that the west Guanggaishan-Dieshan fault zone has a higher potential for seismic hazards in the future; therefore,more attention should be paid to this area.Although many methods were used to prevent error in the results,many unknown parameters remained,which caused our results to have uncertainties.
    11  Relocation of Mainshock and Aftershocks of the 2013 Minxian-Zhangxian M_S6.6 Earthquake in Gansu
    FENG Hong-wu; ZHANG Yuan-sheng; LIU Xu-zhou; CHEN Ji-feng; YAO Hai-dong; ZHANG Xuan;
    2013, 35(3):443-447.
    [Abstract](2347) [HTML](0) [PDF 756.00 K](2126)
    A 3-D velocity model of the southeast Gansu Province region,including the 3-D grid searching algorithm and double difference method,was used to examine the relocation the Minxian-Zhangxian MS6.6 earthquake occurring on July 22,2013,and its aftershocks from July 22to July 25.The grid search method was used to determine earthquake location and the S-P arrival time of 400seismic events,which agreed with data detected by at least three stations.Of these events,336were used to calculate double-difference location,and 261were used to obtain the relocation results.After relocation,the average estimation errors of the source location were 1498m in the E-W direction,1385.2min the N-S direction,and 2 492.3min the vertical direction.On the basis of these calculation results,the seismogenic structure was compared with the data of geological structure,and the results showed that the mainshock was relocated to 34.54°N,104.189°E,with a focal depth of 13.5km.The aftershocks were distributed mainly between the Lintan-Tanchang fault and the northern edge of the West Qinling fault;several were distributed in the southwestern region of the Lintan fault.The distribution of the aftershocks was in the NW or NWW direction,which is essentially consistent with the strike of Lintan-Tanchang fault,although the dominant distribution was not obvious.Therefore,we examined the A-A′profile perpendicular to the direction to research the relationship between the distribution of earthquakes and fracture structure and determined that this profile crosscuts the Lintan-Tanchang fault.We obtained similar results using the 3-D grid searching algorithm and the double difference method, which revealed that the focus depths are mainly distributed in 0~10km before relocation,and the average depth was 6km.In contrast,the focus depths of the aftershocks were distributed mainly in 5~20km after relocation,which indicates that the aftershock sequence occurred mainly in the upper and lower crust.The distribution of the earthquake was in the SW direction,thus,we speculate that the dip direction of the seismogenic fault was SW,and the deep distribution is deeper.The focus depths determined by the 3-D grid-searching algorithm can reach to 25km,whereas those determined by the double difference method were deeper at a maximum of 30km.The Lintan-Tanchang fault,with a NW-NWW strike and a SW dip of 50°-70°,is in closest proximity to the earthquake swarm and is located between the East Kunlun fault and the northern edge of West Qinling fault.The transition fracture of the structural change and the geologic structure of the transition zone are complex and provide the regional geological background.The seismic profile A-A′shows that the basic characteristics of the seismogenic fault,with a SW dip direction,differ from those of the Lintan-Tanchang fault such that their exposures at the surface are 20km apart.Therefore,we believe that Fault F3is a seismogenic fault.However,other characteristics of Fault F3are unclear and need to be developed through active fault detection research.
    12  Moment Tensor Inversion and Seismogenic Tectonics of the 2013 Minxian M_S6.6 Earthquake in Gansu
    CHEN Ji-feng; LIN Xiang-dong; HE Xin-she;
    2013, 35(3):425-431.
    [Abstract](2256) [HTML](0) [PDF 849.23 K](2371)
    On July 22,2013,the Minxian magnitude 6.6 earthquake occurred beneath the northeast margin of the Tibetan plateau,Gansu,China.The earthquake and its aftershocks caused considerable damages,including more than 95 casualties,2414injuries,and approximately 17.5billion yuan in property damage.More information on the source properties of such events can help to characterize fault the structures,which can contribute to an improved understanding of the hazards associated with future great earthquakes.As of August 28,13:35,aftershocks have been recorded by the Gansu seismology network,including a magnitude 5.6event following the mainshock.We analyzed the broadband waveforms for the mainshock with sufficient signal-tonoise levels to invert for seismic moment tensors.All stations involved in this study are broadband stations situated near the July 22mainshock;data showing such problems as misorientation of horizontal components,mislabeling,and polarity reversal in one or more components(Niu et al.,2010)were removed.During the data processing,we removed the mean value and instrument response,detrended the waveforms,integrated the three-component velocity waveforms to displacement,and rotated the horizontal components to the radial and tangential directions.Before performing moment tensor inversion,waveforms were filtered with a Butterworth filter with frequencies between 0.016Hz and 0.05Hz.On the basis of the Crust 2.0global crustal model,and considering the heterogeneity of the crust and the distances to the recording stations,various velocity models for each station were employed to account for variabilities in the crustal structure.To insure the quality of the results of the inversion process,we retained only the station with variance reduction(VR)is≥80,and got 6stations for the inversion.After depth iteration,the best solutions obtained at a depth of 8km were 87,13,and 93for double couple(DC),the compensated linear vector dipole(CLVD)component,and VR,respectively,which indicates highquality results.The two panel's strike,dip,and rake of the solution were 195°,54°,and 149°and 305°,65°,and 40°respectively.The moment tensor solutions showed considerable reverse features and contained part of the strike-slip component.Geologic survey indicated no surface rupture.Regarding tectonic framework and the aftershock distribution features,our inversion results were essentially consistent with the strike of the Lintan-Tanchang fault,and the aftershock distribution occurred near the fault.This phenomenon indicates that the Lintan-Tanchang fault was the main tectonic trigger for the Minxian magnitude 6.6earthquake and that the seismogenic fault of the earthquake was a subfault of the Lintan-Tanchang fault.
    13  Mechanism of Changes in Microstructure of CompactedLoess Based on Triaxial Test
    CHEN Wei ZHANG Wu-yu MA Yan-xia CHANG Li-jun WANG Meng
    2014, 36(3):753-758.
    [Abstract](2239) [HTML](0) [PDF 2.39 M](1244)
    Loess is widely distributed in China, and it’s an example of regional soil with uncommon characteristics.Because of its special formation process and environmental conditions,loess has unique structural characteristics,which are closely related to its physical and mechanical properties.Studying the microstructure of loess can speak to its possible uses in macroscopic engineering and explain the essential factors of its deformation and strength.At present,the study of the quantitative parameters of the microscopic structure of soils has achieved many results.These results provide a convenient method for the quantitative analysis of how the microstructure of loess changes under triaxial shear tests, which can provide a more reliable basis for the interpretation and validation of macroscopic test results.The macroscopic tests are unconsolidated and undrained triaxial shear tests.The confining pressures for the samples are 100,200, and 300 kPa,and the shear rate of all samples is 0.8 mm/min. Performing macroscopic tests to simulate the actual projected path of building loads allows shear strength parameters to be obtained.The samples for the microstructure test are taken after macroscopic triaxial shear test,with a sample size of 3.91 cm (diameter) × 8 cm (height).All samples are sliced from the major principal stress direction,which is the vertical plane, as well as the horizontal plane,and the cut positions are in the center of the specimens.A JSM-6610LV scanning electron microscope (SEM) from Qinghai University was used to observe samples.During the scanning,2~3 areas of each sample were selected to take photographs.The magnification of all samples was 150 times.Quantitative analysis of the changes in the microstructure of loess was then made by reviewing the SEM images.The SEM was used to observe the microstructure of the compacted loess in the horizontal and vertical planes before and after shearing under triaxial shear tests,and the observed planes are 8.This paper analyzes the microscopic nature of soil strength in three respects:the changing characteristics of pore arrangement,pore patterns, and pore scale.Test results reveal that the probable entropy is near 1 before and after shearing and the pore arrangement is disordered at different confining pressures after shearing,which minimally influences the macroscopic strength.The average shape factor and fractal dimension of the pore morphology at different confining pressures show a decreasing trend after shearing,and the changing characteristics of the pore pattern contribute significantly to the strength of the compacted loess.The aggregate level of soil particles is positively related to the strength of the soil samples.Increases or decreases in the void ratio result in the opposite changes in strength,and the changes in pore scale are the control factor of strength.Changes in the microstructure before and after shearing could be explained by macroscopic experimental phenomena.
    14  Seismic Response of Subway Stations in Soft Soil Area
    LIU Rui LI Yan-tao YANG De-jiani LI Yai ZHANG Hai
    2014, 36(1):16-21.
    [Abstract](2215) [HTML](0) [PDF 11.57 M](6133)
    With the rapid growth of urban population in recent years,traffic problems have become increasingly serious.Subways offer an effective means for solve such issues.It is worth noting that many underground structures are located in seismically active areas;therefore,seismic design and reliable assessment of underground structures must be conducted. However,many studies on earthquakes have indicated that underground structures are not as safe once believed and can collapse under dynamic loadings such as earthquake loading.Therefore,seismic design and reliable assessment of underground structures has been an important subject;however,research on seismic areas of underground structure in our country is insufficient.Particularly in areas with soft soil,where the situation is more complex,it must be resolved that the underground structures affected by seismic wave are maintained as safe and reliable.Thus,it is necessary to study the analysis theory and design method of underground structures in the soft soil areas in Tianjin by combining by including underground engineering structure characteristics and soil conditions.According to the actual engineering conditions of the typical subway station structure in Tianjin metro line 3,a model for soft soil station structures has been developed by using ANSYS software.The numerical analysis on the dynamic responses of this model under dynamic loading,Tianjin seismic wave,Taft seismic wave,and artificial seismic wave were conducted.Under different loading conditions,the internal force and deformation of main component were analyzed,particularly for of the center pillar. The displacement amplitudes of this station structure under various seismic waves are approximate and can conform to the standard requirements.The maximums of amplitudes were also determined.The internal force under the Tianjin seismic wave is the maximum,and the seismic vulnerability is located in the bottom of middle column.Therefore,these research results can be used for the seismic design of subway station structures in the soft soil area of Tianjin.
    15  Analysis of Designed Velocity Response Spectra Subjected to Near-fault Ground Motion
    DU Yong-feng XU Tian-ni WANG Ya-nan BAO Chao
    2014, 36(4):997-1002, 1013.
    [Abstract](2170) [HTML](0) [PDF 3.28 M](1753)
    In recent years,several disastrous near fault earthquakes,such as the Wenchuan earthquake in China,the Chi Chi earthquake in Taiwan,the Kobe earthquake in Japan,and the Northridge earthquake in the United States,have occurred.A large amount of near-fault pulse-type earthquake data from all these earthquakes has been recorded,and has attracted close scrutiny and extensive research.This seismic record contains obvious long period velocity pulses.Near-fault ground motions not only cause serious damage to buildings,but also cause slope instability and damage to infrastructure,These disasters have a serious impact on the economic infrastructure and safety of people.Therefore,understanding the response of structures to near-fault ground motions is necessary.This allows for more reliable and efficient seismic design.Different ground motion records contain different types of information such as peak acceleration,peak velocity,peak displacement,and duration of ground motions.Near-fault ground motions have prominent features and destructive force and their velocity response spectra are significantly different from the velocity response spectra of far-fault ground motions.This study used 30 near-fault earthquake records from the Pacific Earthquake Engineering Research Center’s strong motion database;the records contain magnitude,fault mechanism,soil layer shear wave velocity,and fault distance information.First,a motion equation for a single degree-of-freedom system was established.Then,a state-space method and MATLAB was used to change the motion equation to a state equation.Next the equation was solved to obtain the velocity response of a single degree-of-freedom system and based on the velocity response,the velocity response spectra of the single degree-of-freedom system was obtained.Because the velocity response is related to the acceleration response and the displacement response,the velocity response of structures are analyzed in the present study.The features of the velocity response spectra were obtained by normalizing and averaging,and then the 30 near-fault earthquake records selected from the PEER strong motion database were divided into different groups based on the period corresponding to the peak velocity.A velocity design spectra was established using a piecewise linear fitting method.The reasonableness of the velocity design spectra was verified by comparing the velocity design spectra and the velocity average spectra of different groups.The results show that the velocity response spectra of near-fault ground motions consist of four stages:ascent segment,peak segment,depression segment,and horizontal segment.The damping ratio of the structure has no influence on the spectral shape of the velocity response spectra,but the damping ratio of the structure can affect the spectra peak of the velocity response spectra.When the damping ratio of the structure is different,the maximum relative error of the velocity response spectra peak is 16.68% after pairwise comparison.
    16  Static Coulomb Stress Changes and Triggering Interaction amongStrong Earthquakes in the Eastern Segment of North Qilian Mountain
    LIU Fang-bin WANG Ai-guo YUAN Dao-yang
    2014, 36(2):360-371,379.
    [Abstract](2162) [HTML](0) [PDF 18.64 M](4687)
    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.
    17  Calculation of the Focal Depth of M>4.0 Earthquakes in Minxian,Gansu,Based on sPn Seismic Phases
    WEI Ya-ling; CAI Yi-chuan; SU Jin-rong;
    2013, 35(3):438-442.
    [Abstract](2112) [HTML](0) [PDF 957.82 K](1875)
    Earthquake focal depth is an important parameter for research on seismology,the structural study of earthquakes,seismic hazard assessment,and earthquake event recognition.As a basic seismic parameter of time and space,earthquake focal depth affecting the size of the earthquake disaster and is one of the most difficult parameters to measure accurately because its determination is related to source process fault structures and stress fields.Stations layouts are sparse in our country;therefore,the velocity model generally used to determine earthquake focal depth.The China Earthquake Networks Center detected a magnitude 6.6earthquake in Dingxi City,Gansu Province,on July 22,2013,at 7:45Beijing time in the border area between Minxian and Zhangxian with a focal depth of approximately 20km.The epicenter was approximately 15km from Minxian,45km from Zhangxian,120km from Dingxi City,and 170km from Lanzhou City.As of July 30,2013,1066aftershocks were recorded,nine of which were greater than magnitude 3.0.According to the analysis,the seismic phases of three M>4.0earthquakes occurred in Gansu Minxian in July 2013.The Sichan digital seismograph network recorded relatively clear sPn seismic phases of more than 250km in the area near the epicenter.In this study,sPn and Pn seismic phase methods were used to calculate the seismic depth.The focal depths of the three Minxian earthquakes were recalculated to obtain a more accurate earthquake focal depth.The earthquake focal depth is an important focal shock parameter.Because the method of using more than one pair of sPn and Pn seismic phases to calculate the seismic depth was adopted in this study,the main error source is the seismic phase and the Earth's crust model error;therefore,identification of sPn and Pn is crucial.By recalculating the focal depth of the three Minxian M >4.0earthquakes,it is concluded that focal depth of the M6.6principal earthquake was 15.4km,the focal depth of the M 5.6aftershock was 9.6km,and the focal depth of the M4.1aftershock was 9.2km.The focal depth of the two aftershocks is obvious shallow;these results show that the rupture process of the M 6.6principal earthquake was from deep to shallow regions.An accurate focal depth was obtained by deep seismic phases to provide basic data for earthquake prediction and earthquake hazard analysis.
    18  Analysis of the Focal Mechanism and Tectonic Stress of Minxian M_S6.6 Earthquake
    LIU Xu-zhou; ZHANG Yuan-sheng; QIN Man-zhong;
    2013, 35(3):432-437.
    [Abstract](2110) [HTML](0) [PDF 645.26 K](1594)
    The Minxian MS 6.6 earthquake,the largest destructive earthquake to affect Gansu province,occurred on July 22,2013.Its epicenter at N34.54°,E104.21°was located between the northern fringe fault of western segment of Qinling Mountain and the Lintan-Tanchang fault. After the earthquake occurred,we solved the focal mechanism of the main shock with P wave polarity from the waveforms obtained from the Gansu province seismic network.We then compared the solution with the focal mechanisms published by Harvard and the United States Geological Survey(USGS).During the 10years prior to the Minxian earthquake,several earthquakes stronger than Mb 4.5occurred in this region.We solved the focal mechanisms of these earthquakes with P wave polarity data,and we compared the focal mechanisms with those of the Minxian earthquake.Similar morphologies were revealed,and each contained a larger thrust component.The focal mechanism of a small earthquake is more difficult to solve than that of a large earthquake.However,because small earthquakes occur more often than large earthquakes,it is desirable to refer to the tectonic stress by the focal mechanisms of multiple small earthquakes to decrease the effects of local structures and to highlight the features of the region tectonic stress.Previous methods used to determine tectonic stress from multiple focal mechanisms include grid search.However,because the smallest search grid is 5°,the search grid was too large to meet the exact solution of the tectonic stress.Other methods ignore the data accuracy or cannot give the error of parameters.The newest method obtains the optimal solution of tectonic stress using finer grid parameters of 1°×1°and gives the error of parameters.In this paper,we solve the tectonic stress using this method.A broadband mobile seismometer observation array was conducted in southeastern Gansu in 2010-2011,in which abundant near shock waveforms were obtained.We examined small earthquakes in close proximity to the Lintan-Tanchang fault,which was the triggering seismic fault of the Minxian earthquake,at a distance of less than 50km.To maintain the reasonableness of the focal mechanism,after identifying the P wave polarity of selected earthquakes,we identified 31earthquakes in which the P wave polarity was more than 9and had a good envelope to the source.We solved their focal mechanisms,which we used to solve the tectonic stress in this region.The results show that the direction of the main pressure was 70.12°,which is very close to the results of previous studies.The main pressure was NEE in this region because of the combined action of the Qinghai-Tibetan Plateau,Ordos,and South China blocks.After the Minxian earthquake,a series of aftershocks occurred,from which and we expect to obtain additional data for analyzing the changes of tectonic stress.
    19  Research on Stress Field Characteristics and Normal Fault SmallEarthquakes of Southeastern Qinghai
    MA Hui-qing YAO Jia-jun MA Jian-xin LI Wei-jie YUAN Fu-quan
    2014, 36(2):372-379.
    [Abstract](2059) [HTML](0) [PDF 13.13 M](5287)
    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.
    20  A Study on the Scientific Value of Historical Records for the Huaxian MS8¼ Earthquake in 1556
    GUO Zeng-jian GUO An-ning ZHANG Wei-chao REN Dong ZHAO Cheng-cheng
    2014, 36(2):281-285.
    [Abstract](2023) [HTML](0) [PDF 11.33 M](6759)
    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.

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