• Volume 35,Issue 4,2013 Table of Contents
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    • Disaster Loss Assessment of the Minxian-Zhangxian MS6.6 Earthquake, 2013

      2013, 35(4):717-723.

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      Abstract:An MS6.6 earthquake occurred on the junction of Minxian County and Zhangxian County on July 22,2013.The earthquake has caused 95 persons dead,2 414 persons injured,and different degrees of damage to rural buildings,lifeline engineerings,education system and health system in seismic areas.According to investigation of characteristics of earthquake damage,the seismic intensity scale was given in time.In order to determine the ratios of building damage of different structural buildings,a wide range of sample surveys,individual surveys and other methods were applied.At last,a reasonable assessment of the total direct economic losses caused by the earthquake and post-disaster reconstruction funds were provided.The recommendations proposed for disaster recovery and reconstruction could provide a scientific basis for earthquake relief and scientific reconstruction in disaster areas.

    • Surface Wave Investigation and Dynamic Stability Analysis for Earthquake-Induced Loess Landslides

      2013, 35(4):724-729.

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      Abstract:The sliding and collapse of a slope during an earthquake are the main types of slope-related geological disasters,and the dynamic stability of slopes has been one of the most important topics in geotechnical engineering and seismic engineering.About 600 instances of landslides,collapses,and other geological disasters such as collapses,debris flow,and shattered mountains were induced by the Minxian and Zhangxian MS6.6 earthquake on July 22,2013,in Dingxi area in Gansu province; this earthquake damaged many towns and killed 95 people.Furthermore,it resulted in a very high death toll and economic loss in China.The largest and most serious and damaging landslides post this earthquake were the two neighboring landslides in Yongguang village in Meichuan town,Minxian.The western landslide had a length of 1 500 m,average width of 28 m,and area of about 42 000 m2.The landslide occurred from an elevation of 2 700 m to 2 545 m,i.e.,a distance of 155 m.It can be considered as being a crushed-debris flow,remote landslide,resulting in 12 deaths.The area of the eastern landslide was about 33 000 m2.A field survey on geological disasters is very helpful to gain a better understanding of mechanisms of earthquake-induced disasters,particularly of dynamic slope stability.Such geological disasters associated with earthquakes (e.g.,landslides,collapses,debris flow,and shattered mountain) can be subsequent potential geological hazards with long-term,hidden,and unexpected features.According to a variety of survey methods,especially Rayleigh-wave exploration conducted on both of the top and bottom of the landslide in detail,the status of the loess landslide and the topography,stratigraphy and other factors are investigated and the sliding mechanism is analyzed.Based on it,a dynamic numerical analysis method was used to evaluate the dynamic response of the slide,and the factors influencing the stability of the slide and the relationship between instability failure and earthquake motions were identified.The dynamic stability of the slide was analyzed by the dynamic finite element method and a strength reduction method.As a result,it was found that the shear strength of the loess on the slope surface reduced because of heavy rain before the earthquake,and tensile stress occurred,which led to the collapse of seabed slides.

    • Crustal Deformation before the Minxian-Zhangxian MS6.6 Earthquake in Gansu

      2013, 35(4):730-736.

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      Abstract:Using GPS observation data since 1999 in the Lintan-Tanchang fault and the surrounding regions,the deformation characteristics of the region before the Minxian-Zhangxian MS6.6 earthquake are obtained through analysis of the GPS profile,the block strain rate,and the baseline time series.The results from the block strain rate show that the strain accumulation level of the northern margin of the western Qinling fault zone and its nearby faults were promoted by the Wenchuan MS8.0 earthquake.The response of the Wenchuan earthquake on the Lintan-Tanchang fault is obvious in the southern block and not obvious in the northern block,indicating that there is a strong earthquake risk in the boundary area between the two blocks.The results of the GPS profile across the LintanTanchang fault show that the parallel component response of the Wenchuan earthquake is obvious;however,the vertical component is not obvious.This indicates that there might be a higher level of compressive strain and degree of fault locking in the area.In general,the GPS observation results show that the pre-earthquake deformation characteristics are consistent with the coseismic rupture characteristics of the Minxian-Zhangxian earthquake.

    • The Activity and Trends of MS≥6.0 Earthquakes at the Northeastern Margin of the Qinghai-Tibetan Plateau

      2013, 35(4):737-742.

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      Abstract:The northeastern margin of the Qinghai-Tibetan plateau is an earthquake-prone zone,where some MS≥8.0 strong earthquakes have had serious consequences,for example,the 1920 Haiyuan earthquake of magnitude 8.5 resulted in more than 220 000 deaths.The Minxian-Zhangxian MS6.6 earthquake of 2013,which occurred in the northeastern margin of the Qinghai-Tibetan plateau,broke about ten years of seismic quiescence (MS≥6.0) since the Minle earthquake of 2003.The significance of the MS6.6 earthquake and the earthquake tendency has attracted a great deal of attention,because many seismologists believe that the seismic risk in this area increased after the Wenchuan earthquake of 2008.The Benioff strain energy is a physical quantity that can be used to describe seismic activity,and it contains information on the frequency and strength of earthquakes in a given region.Some research on Benioff strain energy has been done to obtain a law for seismic activity.In this article,we calculate the Benioff strain energy of MS≥6.0 earthquakes in the northeastern margin of the Qinghai-Tibetan plateau,and analyze the periodic characteristics of MS≥6.0 strong earthquakes from 1875 to 2013 using the Morlet wavelet method.The results show that there were several main periods of activity,which are about 2 to 3 years,8 to 10 years,and 25 to 30 years,and the first two periods were determined after the significance test was made,From the Benioff strain energy,the frequency of strong earthquakes in the northeastern margin of the Qinghai-Tibetan plateau has been normal since 1977,but the strength is relatively low.Using the effective earthquake case,this paper also discusses the background of the Minxian-Zhangxian MS6.6 earthquake and estimates the postearthquake trend by analyzing the relationship between the probability of earthquake occurrence and the leaving time.The results show that there is a fairly large risk of a MS ≥ 7.0 earthquake in this region.The probability of earthquake occurrence has significantly increased since the Gonghe MS7.0 earthquake of 1990,and it will reach 1.00 by the end of 2015.In summary,the Minxian-Zhangxian MS6.6 earthquake displays the risk of a strong earthquake in the northeastern margin of the QinghaiTibetan plateau and may be an inflection point for the strength of seismic activity in this region.The primary results in this paper have a reference value for seismic research.

    • Determination of Fault Plane Parameter of Lintan-Tanchang Fault Using Distribution of Small Earthquakes and Regional Stress Field

      2013, 35(4):743-750.

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      Abstract:First,we precisely relocated small earthquakes near the Lintan-Tanchang fault using a combination of the Hypo2000 single-event absolute positioning method and the double difference location method.The biggest advantage of the first method is that we can use a multi-velocity model,which is in line with complex underground structures and crustal heterogeneity.Relative to using only one velocity model in the traditional positioning method,the use of the multi-velocity model can improve the near-field positioning precision in a large part.Further,the biggest advantage of the second method is that it can eliminate the influence of the underground velocity structure on the relative position of the earthquake; subsequently,we can obtain an accurate relative position between the two earthquakes.Then,we divided the small earthquake distributed in the northern part of the Lintan-Tanchang fault into two segments,which are the east and west segments,respectively,according to the distribution of the relocated small earthquake,location of the fault,and distribution map of a medium-strong earthquake' s intensity.Further,in consideration of three similar focal mechanisms of the Minxian M5.2 earthquake on November 13,2003,Minxian M5.0 earthquake on September 7,2004,and Minxian M6.6 earthquake on July 22,2013,we combined the two segments together into one segment.Then,we separately estimated the strikes,dips,location,and corresponding errors of fault planes at different locations by simulated annealing and Gauss-Newtonian nonlinear inversion algorithms.It can not only search the global optimal solution but also estimate the parameter errors.Further,we can estimate the slip angle on the fault by using the parameters of the regional tectonic stress field after a large earthquake fault plane.Further,we inverted the rakes of the different fault planes by using local stress field parameters.The strike,trend,dip angle,and rake angle of the inverted fault plane were found to be NW-NWW,NE-NEE,71°~86°,and 102°~132°,respectively.Because of the small number of earthquakes in the study area,the error values of the strike angle,dip angle,and distance from the origin of coordinates for the inverted fault plane are too large.The error of the inverted dip angle is larger than that of the inverted strike angle owing to greater dispersion of small earthquakes on the fault vertical.Further,the error of the slip angle is much larger.The strike,trend,and dip of the fault plane inverted in this study are similar to those in previous studies and also similar to the fault parameters displayed on focal mechanism of the three medium-strong earthquakes.However,the fault types differ between this study and previous studies,which are separately dextral and sinistral strike slip.Further,the fault-type considered in this study differs from that displayed on the focal mechanism of the three medium-strong earthquakes.It was primarily concluded that an inversion error of the regional stress field parameters exists because of focal-mechanism solutions of small earthquakes,and this error influences the inversion error of the rake angle on the fault plane.However,the inverted slip angle in this study is comparable to that reported in previous studies.

    • New Activities of Lintan-Dangchang Fault and its Relations to Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):751-760.

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      Abstract:The Minxian-Zhangxian M6.6 earthquake,with a focal depth of 20 km,occurred at the boundary of Minxian and Zhangxian counties (34.5°N,104.2°E),Dingxi city,Gansu Province,at 7:45 CST on July 22,2013.By 8:00 on July 29,1034 aftershocks occurred.Among them,nine above M3.0 include one M5.0~5.9,one M4.0~4.9,and seven M3.0~3.9 earthquakes.The strongest aftershock,which occurred at 9:12 on the date of the mainshock,attained MS5.6.Catastrophic damages caused by this earthquake are mainly attributed to poor residential structures erected by the government during a weak economy.The affected area was more than 10 000 km2 and involved in 6 cities and 13 counties ineluding Dingxi,Baiyin,Tianshui,Longnan,and Gannan Zang and Linxia Hui autonomous prefectures.Residents in Baoji,Hanzhong,Xianyang,Xi' an,Chengdu,and Mianyang felt this earthquake.The earthquake occurred at southeastern region of Gansu province,which is covered by very thick loess layer and exhibits complex topography such as steep,highly sloped mountains and crisscrossed ravines.It killed 95 people and injured 1 366.Secondary disasters caused by this earthquake include loess landslides,collapses,and pitfalls.Immediately following the earthquake,the China Earthquake Administration quickly organized field survey teams for seismicity monitoring,intensity studies,earthquake damage assessment,and scientific research,through which the first intensity distribution map of the Minxian-Zhangxian M6.6 earthquake was developed for conditions at 14:00 on July 24.The macroscopic epicenter of this earthquake,with an intensity of Ⅷ degrees,was located at Yongguang and Yongxing villages of Meichuan and Lalu Village of Hetuo.The long axes of intensity circled with an NW strike,in accordance with regional structures.This study discusses the basic parameters of this earthquake such as basic intensity distribution,damage features and seismogenic structure.This earthquake occurred at the eastern margin of the Qinghai-Tibetan Plateau at the intersection of the N-S seismic zone and the Kunlun-Qinling fault zone,which is the tectonic transformation zone between the NWW east-striking Kunlun fault and the northern margin of the western Qinling fault zone.We determined that the Hetuo fault,active since the late Pleistocene,passes through the extreme seismic area.Surface rupture phenomena such as landslides,ground fissures,earthquake pitfalls,sandblasting,and water oozing all exhibited zonal distribution along the Hetuo fault.The focal mechanism of this earthquake is consistent with the Hetuo fault.The strike of the nodal plane Ⅱ is 320° and 301.3°,as reported respectively by the Institute of Geophysics and the Institute of Geology,China Earthquake Administration,which also corresponds to the strike of the Hetuo fault.Moreover,the NE-dipping slip plane reported by the two agencies,51° and 57.3°,respectively,corresponds with the dipping direction of the Hetuo fault.Further,we determined that the Anjia Shan landslide slips laterally along a fault valley striking NW that resembles a long tongue with length and width of 310 m and 30~35 m,respectively.The plunge angel of fault striations on the slip plane is 12°~23°,and the main slip plane corresponds with the fault strike.Therefore,the Hetuo fault is seismogenic fault of the Minxian-Zhangxian M6.6 earthquake,which is a secondary fault,belongs to the Lintan-Tanchang fault zone.Three strong earthquakes above M6.0 have occurred near the epicenter of the Minxian-Zhangxian M6.6 earthquake on the Lintan fault,and two moderate earthquakes have occurred at this site in recent years,which include the Minxian M5.2 on November 13,2003,and the Minxian-Zhuoni M5.0 on September 7,2004.This earthquake represents the most recent activities of the Lintan-Tanchang fault zone.

    • Analysis of Parameters of Landslides Triggered by the Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):761-767.

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      Abstract:On July 22,2013 at 07:45 (Beijing time),an earthquake of MS6.6 occurred at the boundary between Minxian County and Zhangxian County in the mountain area of Western Qinling,southern Gansu Province.A strong aftershock of MS5.6 occurred at 34.6°N,104.2°E at a focal depth of 14 km.As of 13:00 24 July,the earthquake had affected 780 100 000 people in 491 towns of 33 counties in 6 cities (prefectures):Dingxi,Longnan,Tianshui,Baiyin,Linxia,and Gannan.The earthquake caused 95 deaths and 1 366 injuries.The earthquake triggered many landslides for several reasons,including that the earthquake-struck area in the western Qinling area of southern Gansu Province was covered by loose thick loess and has steep topography,and there was strong rainfall before the earthquake.Strong aftershocks caused more landslides to occur.The landslides triggered by the Lushan earthquake were of various types,such as falls,slides,and topples on loess scarp,soil deepseated landslides,large soil avalanches,and slope cracks.From visual interpretation of post-earthquake remote sensing images and selected field checks,at least 2 330 landslides were triggered by this earthquake,and these landslides occurred in a rectangular area of about 330 km2.Based on the geographic information system (GIS),we analyzed six topographic,geological,and seismic parameters:elevation,slope angle,slope aspect,slope curvature,formation lithology,and PGA.The elevation of the study area ranges from 2 207 m to 3 340 m.The study area was divided into 11 classes based on 200 m intervals for landslide statistics.The slope angles in the study area range from 0° to 64.6°,and were divided into nine classes:0°~5°,5°~10°,10°~15°,15°~20°,20°~25°,25°~30°,30° ~35°,35°~40°,and 40°~64.6°.The slope aspect was divided into nine classes:flat,north,northeast,east,southeast,south,southwest,west,and northwest.Based on the slope curvature value,the study area was divided into six classes,and the ranges of the slope curvature value were <-1,-1 to-0.1,-0.1 to 0,0 to 0.1,0.1 to 1,and >1.The six categories of strata in the study area are:(1) Q4,Qcxh,Q3;(2) Nb; (3) Eb; (4) P11-c; (5) P11-b,P11-a,(6) D3dc,and (7) D22-e.The PGA values for the study area were divided into eight classes:0.34 g,0.32 g,0.28 g,0.24 g,0.2 g,0.16 g,0.12 g,and 0.08 g.Analysis of the topographic parameters showed that the Minxian-Zhangxian earthquake-triggered landslides mostly occurred in areas with these characteristics:an elevation between 2 400 m and 2 600 m,a slope angle between 10° and 20°,a W or NW slope aspect,and a slope curvature between-1and0.1.The Paleogene sandstones and conglomerates (Eb) suffered more landslides.The statistics of the peak ground acceleration (PGA) showed that 0.16 g is the PGA for most of the earthquake-triggered landslides.This paper can show which topographic,geological,and earthquake factors were linked with landslide occurrence.The point-based inventory of landslides triggered by the Minxian-Zhangxian earthquake can provide basic data for subsequent studies of earthquake-triggered landslides,such as spatial distribution analysis,hazard assessment,and mitigation of landslides and debris flows in earthquake-struck areas.

    • Influence of Aftershocks of the Minxian-Zhangxian MS6.6 Earthquake in a Loess-rich, Hilly Region

      2013, 35(4):768-773.

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      Abstract:Minxian-Zhangxian M6.6 earthquake occurred on July 22,2013 in Dingxi,Gansu province.Shortly after the earthquake,we placed eight digital flow strong motion observation recording instruments in the surrounding area which were produced by Kinemetrics Company in the USA.We set up three K2 strong motion recording instruments in Yongxing village,Yongxing primary school and Dalong village.A large number of seismic records that can be used to better understand the effects of earthquakes on the loess layer were obtained.Up to August 11,172 (516) seismic records were acquired,including 48 earthquakes of magnitude ML ≥ 3.The maximum magnitude was ML4.4,and the maximum acceleration value was 65.9 gal.From the observed data of strong aftershocks from the Yongxing array,we selected records of large magnitude aftershocks for analysis.The Minxian ML 3.8 earthquake record from August 8th was the largest,and we focused on those results.The results showed that the maximum acceleration value decayed rapidly with increasing distance from the epicenter,and the largest acceleration value was from Dalong village station,which was the nearest station from the epicenter.Because of the presence of a thin and soft loess layer in the observation area,the acceleration records from the Yongxing village station and the Yongxing primary school station could not be directly compared with those from Dalong village,but we conducted a further spectrum analysis on each record,and the effect of absorption by loess on the high-frequency components was significant.To further explore the impact of topography on the observed values,we further expanded the scope of the data filtering,and used the seismic data of four ML ≥2.5 earthquakes as the basis for measuring the peak ground acceleration value of each station.We conducted Fourier analysis on the records,obtained the dominant frequencies,and illustrated the Fourier spectrum of some typical records.The results show that the dominant frequency of Dalong village is 5.2 Hz,that of Yongxing village is 4.1 Hz and that of Yongxing primary school is 5.3 Hz.Noted that Dalong village was in foothills and its observation point was bedrock,the dominant frequency of acceleration was higher.The survey also showed that low frequency and high intensity events had greater destructive power for buildings.The seismic observation of ML3.8 earthquake indicated that the acceleration records of NS direction of bedrock point are larger than vertical direction,which is related to the focal mechanism of the earthquake.The observation results are slightly different than the previous observation in isolated peaks,because the magnitudes of earthquakes are small,and the observation distance is near,the influence of epicentral distance may exceed the influence of topographic factors,which is too complex to be further studied.

    • Strong Motion Records and Attenuation Analysis of July 22, 2013,Minxian MS6.6 Earthquake

      2013, 35(4):774-779.

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      Abstract:The Minxian MS6.6 earthquake,the largest earthquake to occur in Gansu province during the past decade,devastated areas in Minxian and Zhangxian and affected counties in Longnan and the Gannan Tibetan Autonomous Prefecture more than 100 km from the epicenter.During the tenth five-year plan,20 strong-motion stations have been erected in southeastern Gansu,most of which recorded the main shock.In addition,several stations in Lanzhou also recorded strong motion.After preliminary processing of the strong motion data,we determined that the majority of stations that recorded the main shock are broadly distributed with same strike; therefore,a simplified mathematical model was developed to analyze the attenuation of these strong motion records,The epicentral distances of the strong motion stations recording the main shock are 18 km to more than 200 km.The largest peak ground acceleration was recorded by the Minxian strong motion station at 172.5 cm/s2 with an epicentral distance of 18 km.For the free-field soil observation conditions,these values essentially reflect the local ground acceleration; therefore,the extent of damage reached Ⅷ°.Stations near Lanzhou with epicentral distances close to 200 km also recorded the motion,with peak acceleration between 1 cm/s2 and 8 cm/s2.Chinese ground motion attenuation data originally used ground motion data observed in the United States as a reference before such information was recorded in China.In recent years,a Chinese digital strong motion observation network was been created to provide important data in earthquakeprone regions for in-depth research of Chinese ground motion attenuation.Therefore,the study of Minxian MS6.6 earthquake ground motion attenuation also has practical significance in areas outside of southeast Gansu.In the study of peak ground acceleration attenuation,the following formula is often used:lg y =c1 + c2 M+ c3M2+ c4lg [R + c5exp (c6 M)],…………………… (1)where y is the peak ground acceleration,M is the magnitude,R is the epicentral distance,and c1,c2,c3,c4,c5,and c6 are regression coefficients.When the magnitude is determined,equation (1)can be simplified to:lg y=A + B*lg (R + R0).…………………………………… (2)The correspondence of the parameters is as follows:A =c1 + c2 M + c3 M2; B =c4; R0 =c5exp (c6 M).We fit the peak ground acceleration attenuation curve by using formula (2).Because the peak ground acceleration from the station on a thick layer (bedrock) would be significantly greater (smaller),data from five stations were removed during the attenuation curve fitting.We took R0 as 30 for lg y and lg (R + R0) to obtain the best linearity.The results show that the peak ground acceleration attenuation curves of the three components had the same shape.However,the value of limit acceleration,or the value of A + B*lg (R0) differed.The formulas of the peak ground acceleration attenuation of three components are:EW:lg y=-2.847 7*lg(R + 30) + 7.056 9; R2=0.846 6NS:lg y =-2.793 5*lg(R + 30) + 6.984 4; R2 =0.860 3UD:lg y =-2.796 9*lg(R + 30) + 6.751 8;R2 =0.849 7.We compared the results with the disaster assessment intensity map to determine that the ranges of Ⅷ and Ⅶ degrees given by our results are slightly larger because of fewer near-field strong motion records were used.

    • Application of Surface Wave Surveys and Array Microtremor Observations for Earthquake Disaster Mitigation in Western China

      2013, 35(4):780-788.

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      Abstract:Since a simple,fast,and low-cost method for investigating subsurface structures to prevent seismic disasters is needed in the western part of China,the authors have taken the opportunity to study the Gansu Mingxian Zhangxian MS6.6 earthquake and have conducted field tests for the effectiveness of surface wave surveys (SWS) and small size array microtremor observations.Sites having different geological conditions and having undergone different types of earthquake disasters,such as collapse of houses and landslides,were selected for the test.A portable seismic recorder and 4 Hz vertical component geophones were used for the acquisition of data for both surface wave surveys and small size array microtremor observations.Surface wave survey was done using a 24-receiver spread with a 2 m receiver interval for 5 m,10 m,15 m,and 20 m source offsets.Small size array microtremor observations were done using a triangular array of 16 m for observation duration of 30 minutes.The results have shown that the SWS can become an effective method for the geological investigation of the low western area covered by loess and the low groundwater level.The thickness of the loess is large and the inner structure is simple,which provides favorable conditions for surface wave surveys.The depth of the survey is 30 m more due to the seismic energy sources from artificial hammering.High-frequency microtremors of more than 2 Hz can also be used for subsurface structure surveys in the mountainous rural areas for rare mechanical vibrations.Small size array microtremor observations need a small site and euvironmental requirements are not strict.The depth of survey is 30 m more using small size array microtremor observation,which is seen to be highly effective.The same types of equipments were employed for surface wave surveys and small size array microtremor observations,which improved the usage efficiency of the equipments.Small size array microtremor observations were done using a triangular array of 16 m and for observation duration of 30 minutes.The results have shown that surface wave surveys can estimate the shear wave velocity for structures down to a depth of 30~40 m,while the small size array microtremor observations can estimate down to a depth of 40~50 m.Small size array microtremor observations can explore slightly deeper but the resolution of shallow layers is lower.As the instrument is potable,the method is both simple and lowcost; hence,surface wave surveys and small size array microtremor observations are very suitable for the prevention of seismic disasters in the western region of China.

    • Analysis of Satellite Infrared Anomaly in Middle and Eastern Parts of Qaidam Block prior to Minxian MS6.6 Earthquake

      2013, 35(4):789-793.

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      Abstract:Remote sensing of rock mechanics has been used to determine that temperature changes in rocks are related to changes in rock volume.During elastic deformation,compression (expansion) of volume causes an increase (decrease) in temperature.Therefore,research of temperature abnormalities on the surface prior to earthquakes should consider areas of ground deformation by earthquakes.Qaidam block is closely connected to earthquakes such as the July 22,2013,Minxian MS6.6 earthquake in Gansu province,and Global Positioning System (GPS) data indicate that the middle and eastern parts of Qaidam block underwent compression.Therefore,research of the anomalous increase in infrared radiation by remote sensing prior to the earthquake should include data of infrared levels in the middle and eastern parts of Qaidam block.In the present study,infrared abnormalities are determined by remote sensing in the middle and eastern parts of Qaidam block prior to the July 22,2013,Minxian MS6.6 earthquake.On the basis of Moderate Resolution Imaging Spectroradiometer (MODIS) data of infrared bright temperature determined through remote sensing,we analyzed lowfrequency information of average bright temperature extracted from wavelet decomposition in the middle and eastern parts of Qaidam block.The following results were obtained:In the middle and eastern parts of Qaidam block,infrared bright temperature showed a significant anomalous increase twice in four months prior to the earthquake,the duration of which was approximately one month.No significant anomalous increase was noted between March 1,2004,and February 28,2013.Therefore,we conclude that the anomalous increase in bright temperature recorded prior to the Minxian earthquake was related to that earthquake.Moreover,among all 5 M ≥ 5.0 earthquakes occurring in the middle and eastern parts of Qaidam block between January 1,2004,and September 15,2013,only one showed an anomalous increase of bright temperature prior to the Minxian earthquake,which may indicate that such temperature increases did not appear on the surface prior to M < 5.6 earthquakes in the middle and eastern parts of Qaidam block.Further,such increases were short-term,which indicates the significance of anomalous increases in bright temperature for the occurrence of M6.0 earthquakes in the middle and eastern parts of Qaidam block.

    • Research on the Anomaly of Outgoing Long-wave Radiation prior to the Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):794-799.

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      Abstract:Outgoing Long-wave Radiation (OLR),also known as thermal radiant flux density,refers to the energy density of electromagnetic waves emitted by the earth-atmosphere system into outer space.OLR is the broadband (5~50 μm) information that forms after reprocessing of the telemetry data of a single infrared band.The dataset adopted in this study is the evening data of the National Oceanic and Atmospheric Administration (NOAA) OLR satellite networking with a resolution of 1 ° × 1 ° in which the unit is W/m2.Such information appears on the NOAA Website at ftp://ftp.cpc.noaa.gov.In this study,the eddy method and time-frequency analysis are used to examine the temporal and spatial distribution characteristics of Outgoing Longwave Radiation (OLR) in the epicenter area of the Minxian-Zhangxian MS6.6 earthquake (102.50° E-105.50° E,33.50°N-37.50° N) determined by infrared remote sensing.Results indicate that an enhancement of the monthly average of the eddy field of the OLR did not appear in the corresponding period between 2009 and 2012.The time series curve of the OLR eddy monthly average,in the epicenter grid points (103.50°E,35.50°N),(104.50°E,34.50°N) of the earthquake,showed was no significant seasonal variation in the OLR eddy value.Generally,the eddy value fluctuated randomly near the average value line.Prior to the earthquake,however,the value showed abnormal and distinct short-term changes.To examine these change characteristics,the short-time Fourier transform method was used to determine the time-frequency analysis of the daily eddy value of the grid points (103.50°E,35.50°N).The results indicate that prior to the earthquake,an unusual increase in energy density occurred.During the abnormal period,the characteristics of the spectral structure changed.The low-frequency components were principal in the early stage,whereas the radio-frequency component became the principal part during the later stage.Previous research identified an abnormal increase in OLR information of the nearby epicenter prior to the Wenchuan MS8.0 earthquake.The Lushan MS7.0 earthquake occurred near the Guan County-An County fault zone in the southern region of the Longmenshan Mountains following the Wenchuan earthquake.Prior to the former earthquake,an abnormal two-center high-value area was present in the epicenter area that was similar to that occurring prior to the latter earthquake.The present study identified an additional abnormal increase in the OLR eddy in southeastern Gansu,which was the epicenter area of the Minxian-Zhangxian MS6.6 earthquake.This example proves that prior to some earthquakes,the OLR eddy increases abnormally.Therefore,OLR has very broad application prospects in earthquake prediction research.

    • Reliability Study on Water Radon Concentration Anomaly of Huaniu Spring in Tianshui before Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):800-807.

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      Abstract:Since April 2013,the water radon concentration of Huaniu Spring has shown abnormal changes that are very similar to the precursory anomaly of the Wenchuan MS8.0 earthquake.From the beginning,it was tracked as important anomaly.Although this change began three months prior to the Minxian-Zhangxian MS6.6 earthquake,we could not conclude that it was a precursor to this earthquake it.Further research is needed to determine a reasonable relationship between the anomaly of water radon concentration and the Minxian-Zhangxian MS6.6 earthquake.In the present study,we analyze the tendency of annual changes in radon concentration of Huaniu Spring,Tianshui,China,which began in April 2013 and terminated after the Minxian-Zhangxian MS6.6 earthquake.The results show a decline tendency with a fixed rate.The annual change is high in summer and low in winter,and the radon concentration is higher than that recorded during the corresponding period of the previous year.In addition,we analyze the main factors that influence the radon concentration,and we determine that the annual change is caused by atmospheric temperature.Abnormal changes in radon concentration occur when the rainfall reaches a particular value; however,the duration is short.Analysis of the reliability of the radon concentration anomaly indicates that although the result is shown to be dependable,it is not consistent with the influencing factors.Moreover,we analyze the relationship of the radon concentration in Huaniu Spring and the earth resistivity and water temperature in Tianshui.The results show that their anomalies began at the same time; however,their change directions differed.The water temperature and earth resistivity showed high anomalies,and the radon concentration anomaly was low.We also analyze the reproducibility of the radon concentration anomaly.The anomaly characteristics of the radon concentration beginning in April 2013 were very similar to the precursory anomaly of the Wenchuan MS8.0 earthquake.Finally,we examine the temporal and spatial relationship between the radon concentration anomaly and the Minxian-Zhangxian earthquake.We conclude that the radon concentration anomaly of Huaniu Spring beginning in April 2013 is a credible and reliable precursory anomaly of the Minxian-Zhangxian MS6.6 earthquake.

    • The Relationship between Underground Fluid Anomalies in Southeastern Gansu and the Minxian MS6.6 Earthquake

      2013, 35(4):808-815.

      Abstract (698) HTML (0) PDF 484.49 K (986) Comment (0) Favorites

      Abstract:Six major earthquakes have occurred in the Tibetan Plateau during the past few decades:the 1997 Mani earthquake of magnitude 7.5 in Tibet,the 2001 Kunlun Mountain Pass earthquake of magnitude 8.1 in Qinghai,the 2008 Yutian earthquake of magnitude 7.3 in Xinjiang,the 2008 Wenchuan earthquake of magnitude 8.0 in Sichuan,the 2010 Yushu earthquake of magnitude 7.1 in Qinghai,and the 2013 Lushan earthquake of magnitude 7.0 in Sichuan.All of these occurred around the boundaries of the Bayan Har block of the Tibetan block.A series of strong earthquakes have occurred at the southwestern,northwestern,western,eastern,and southeastern boundaries of the Bayan Har block,which have attracted a great deal of attention from geologists.Some research has indicated that the adjacent zone and the boundary of the Bayan Har block are earthquake-prone areas,where strong earthquakes might occur in future.Earthquakes frequently occur in the north-south seismic belt,which has also been the focus of research in recent years.The southeastern area of Gansu is located in the northern section of the north-south seismic belt,and adjacent to the northeastern boundary of the Bayan Har block.We therefore believe that the southeastern part of Gansu Province is an area at risk of disastrous earthquakes in future.The Minxian earthquake of magnitude Ms 6.6 occurred on July 22,2013 in this area.In this study,we discussed the effect of strong earthquakes on the seismic situation and studied data on underground fluids in southeast Gansu Province.From analysis of underground fluid data from the southeastern part of Gansu Province,we concluded that underground fluid data from 300 km from the epicenter showed obvious mid-term anomalies,shortterm anomalies,impending earthquake anomalies,and co-seismic response.We analyzed and discussed data on the water radon level,water temperature,water level,and outflow data to elucidate the medium-term anomalies,short-term anomalies,impending earthquake anomalies,and co-seismic responses.The medium-term anomalies before the Minxian earthquake were extracted by combining qualitative and quantitative methods.The data on water radon and water level changes in the southeast of Gansu provinces were analyzed according to the idea of "identity" and the method of subordinate function method.The preliminary conclusion is that it is obvious that the anomaly trend in the underground fluid appeared before the Minxian earthquake,and the subordinate function method can be successfully used to extract the medium-term anomaly of water radon.Based on the data for water radon (from Fujianchang spring,No.1 spring,and No.2 spring at Pingliang station,Huaniu spring at Tianshui station,No.1 spring at Wushan station,Diangou spring at Longnan station),water level (from Wenquan Well at Qingshui station),water temperature (from Wenquan Well and Ligou Well at Qingshui station),flow data (from Ligou Well at Qingshui station) for the Minxian earthquake,there were marked short-term precursor anomalies in southeastern Gansu.From one record of water radon,four records of water temperature (Liuhu Well at Pingliang station,Liangshui Well at Longnan station,No.1 spring at Wushan station,and Diangou Well and Chengxian Well at Longnan station),and two records of water levels (Huating Well and Liuhu Well at Pingliang station),we detected obvious coseismic effects caused by the earthquake.From the statistical analysis,water temperature and water level data sampled at a high rate showed more coseismic response,and flow data and water radon data sampled at a low rate did not show coseismic response.

    • The Anomalies of Earth Resistivity Recorded by the Deep Well Observing System of Tianshui Station before Minxian M6.6 Earthquake and Lushan M7.0 Earthquake

      2013, 35(4):816-818.

      Abstract (764) HTML (0) PDF 177.47 K (1146) Comment (0) Favorites

      Abstract:The earth resistivity observation station in Tianshui,China,is located in the northern region of the Qinling latitudinal tectonic belt,which is the western settlement zone of the Neocathaysian structural system and includes the southward-insertion portion of Qilianshan-Luliangshan-Helanshan front arc structural system and the southern edge of the Weihe fault.This area exhibits a clear diamond structure with an N-E,N-W,and E-W configuration,in which seismic activity is frequent and epicenter location varies.The station has an altitude of 1 150 m,average humidity of approximately 63%,and annual average temperature of 18 ℃.The polar region of this area includes Ⅰ-and Ⅱ-grade terraces of the Yongchuan River.During the past five years,two larger earthquakes have occurred in this region including the Lushan M7.0 earthquake on April 20,2013,and the Minxian-Zhangxian M6.6 earthquake on July 22,2013.The epicenters of these two earthquakes were 550 km and 200 km,respectively,from Tianshui station.Prior to their occurrences,the earth resistivity observation data of Tianshui station showed obvious short-term anomalies that differed in morphology and amplitude.Eleven days prior to the Lushan earthquake,the N-S,E-W,and N45°W channels of the deep well observing system showed clear synchronous abnormal changes,which remained for 11 days following the earthquake.Forty days prior to the Minxian-Zhangxin earthquake,the 7.05±0.01 normal value of the E-W channel showed intermittent fluctuation anomalies of approximately ± ±0.02.Although both earthquakes occurred in the S-N seismic belt,the Lushan earthquake occurred in the Longmenshan fault zone,and the Minxian-Zhangxian earthquake occurred in the Lintan-Tanchang fault zone.

    • The Geo-electrical Resistivity Anomaly before the Minxian-Zhangxian MS6.6 Earthquake in Gansu

      2013, 35(4):819-826.

      Abstract (684) HTML (0) PDF 780.79 K (1024) Comment (0) Favorites

      Abstract:The MS6.6 earthquake occurred at the junction of Minxian County and Zhangxian County.The earthquake focal depth was 20 km.It was the largest event since the Shandan MS7.25 earthquake of 1954.The epicentral area was already known to be earthquake-prone,as there have been 25 earthquakes with magnitudes greater than five in recorded history within about 200 km of the epicenter.This earthquake occurred on the northeastern edge of the Qinghai-Tibet Plateau,which is formed by the subduction of the Indian plate in the north of China.Therefore,the earthquake was caused by the push from the Indian plate.The north-south seismic belt in which this earthquake happened has entered a new active period since May 12,2008,after the Wenchuan earthquake.The Wenchuan earthquake,the Yushu earthquake,and the Lushan earthquake show that its activity has not decreased.There are nine apparent resistivity stations within 400 km of from the epicenter.These are at Tongwei,Hanwang,Tianshui,Lanzhou,and Linxia in Gansu Province,Guyuan in the Ningxia Hui Autonomous Region,and Baoji,Zhouzhi,and Qianli in Shanxi Province.The Tianshui seismic station in Gansu uses the deep bore-hole technique,and the electrode depth is 100 m.This station is 156 km from the epicenter of the earthquake.This paper analyzed apparent resistivity observation data from these stations for 2002 to 2013 using the Fourier slide method and the normalized variation rate method.The Fourier slide method is used to exclude annual variation in geo-electrical resistivity observation data.The normalized variation rate method avoids the difficulties and arbitrariness of using human judgment to determine the abnormal changes in the base value.At the same time,this method retains the abnormal change patterns of the original curve.The results show,(1) for the geo-electrical resistivity anomaly phenomena before the earthquake,the relation between the abnormity and the earthquake is significant in time.The normalized variation rate method is better for the extraction of mid-term and short-term anomaly effects,and there were obvious abnormal changes.(2) The mediumterm anomalies mainly decreased,and the amplitude of the anomalies increased with distance from the epicenter.The Tongwei,Lanzhou,Tianshui,and Zhouzhi stations recorded sustained mediumterm anomalies for months or about a year.However,examination of the observation environment of the stations with abnormal changes showed that the recorded anomalies at some stations require further study.(3) Although the geo-electrical resistivity stations had environmental interference and other effects in the area surrounding the epicenter,abnormal changes in the geo-electrical resistivity were quite clear before the earthquake.There were large-amplitude seismic anomalies before the earthquake,as well as a decline in the main anomaly.Finally,we discussed the preliminary causes of the apparent resistivity anomalies.During the gestation of the earthquake,geo-electrical resistivity anomaly changes in the source area were associated with the accumulation of stress and ultimately result in progressive failure of rock.When the stress had accumulated up to the critical strength of the medium,micro-fractures started to form,this time because of the stress load in the later part of the preparation period.The accelerated change in this medium produced prominent geo-electrical resistivity anomalies.Decreasing anomalies are credible precursors in earthquake preparation processes.

    • The Variation of Geoelectrical Field at Pingliang Station, Gansu Province,before the Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):827-834.

      Abstract (733) HTML (0) PDF 1.56 M (1059) Comment (0) Favorites

      Abstract:On July 22,2013,an MS6.6 earthquake occurred at a depth of 20 km in the border region of Minxian and Zhangxian (34.5°N,104.2°E).The earthquake was on the Lintan-Tanchang dislocation of the north-south seismic belt.The north-south seismic belt became active after the 2008 Wenchuan MS8.0 earthquake in Sichuan Province.The 2010 Yushu MS7.1 earthquake in Qinghai Province and the 2013 Lushan MS7.0 earthquake in Sichuan Province show that activity has continued unabated.Pingliang Station (35.55°N,106.68°E) was built in December 2007,and is located in the eastern part of the north Qilianshan fold belt and the southern part of the Helan-Liupan Mountain inland organic belt.The system for observing the geo-electric field runs well,the electromagnetic environment is good,and there is no remarkable interference around the station.The observation device makes observations in three directions and has a total of six channels,with a sampling interval of one minute.This paper studied changes in the geo-electric field for the Minxian-Zhangxian MS6.6 earthquake in Gansu Province in 2013 using data from Pingliang Station.The data used were from November 1,2012 to July 31,2013,which covers a period of 163 days before and nine days after the Minxian-Zhangxian earthquake.The results showed:(1) significant self-potential changes were recorded at Pingliang Station,situated on the northeastern edge of the Qingzang-Tibet Plateau,before the Minxian-Zhangxian MS6.6 earthquake of 2013 in Gansu Province and the Lushan MS7.0 earthquake of 2013 in Sichuan Province.These changes were more prominent and lasted longer before the Minxian-Zhangxian MS6.6 earthquake than before the Lushan MS7.0 earthquake.The anomalies began on April 7,2013,about 100 days before the Minxian-Zhangxian MS6.6 earthquake.(2) An increased power spectrum density (PSD) of the geo-electric field (in particular,PSD anomalies in the low-frequency components,under 5× 10-3 Hz) was recorded at Pingliang Station before the Lushan MS7.0 earthquake,and lasted until the end of the Minxian-Zhangxian MS6.6 earthquake.Finally,the mechanism of the extremely low frequency phenomenon was discussed.The observed electric field at Pingliang station is in the southern part of the Haiyuan-Liupan Mountain dislocation,and so the medium under the station is weak.The station is close to the epicenter of the Minxian-Zhangxian earthquake,and it is easy to change the physical environment of the underground medium and thus cause this significant electromagnetic phenomenon.From our preliminary analysis,we consider that tectonic movement associated with the source region caused changes in the physical conditions of the medium under the station because of the unusual tectonic setting.During non-linear development of numerous micro-cracks and their directional arrangement in the media of the focal region before the earthquake,influencing factors such as underground water migration and electro-mechanical transformation function induced geo-electromagnetic anomalies.The focal process of the earthquake and the local media action under these geo-electric stations led to the spatially and temporally complicated evolution of geo-electric anomalies.

    • Study on the Mechanism of the Macroscopic Anomaly of a Cellar in Shendu following the Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):835-839.

      Abstract (717) HTML (0) PDF 868.01 K (959) Comment (0) Favorites

      Abstract:Precursory and coseismic anomalies of fault gas often appear prior to and following an earthquake.Following the Minxian-Zhangxian MS6.6 earthquake,an abnormal phenomenon occurred in Shendu,Minxian,and China,in which a large amount of white smoke was observed in a cellar.Physical evidence obtained recently demonstrates that deep fluid has a significant influence on earthquake gestation and that earthquakes involve a large number of chemical processes and may not be results of purely mechanical or physical processes.These deep fluids,which migrate to the ground surface,originate from the dehydration of hydrous minerals and contain a substantial amount of information about the crust and mantle.Thus,gases present in the soil offer the same information.Convenient,low-cost methods of measuring soil gases such as Hg and Rn that have been developed for wide application to active fault detection have also proven to be highly efficient for hazard assessment.The anomaly site is located in the southeastern Gansu area,which is at the northeast edge of the Tibetan Plateau in the middle of the North-South-trending seismic belt.Because this belt contains a group of active faults that produce intense tectonic activity in its boundary and interior,it is often used to study seismic hazards and the background field of southeastern Gansu.In this study,Rn and Hg soil gases collected near a cellar in Shendu,Minxian,following the Minxian-Zhangxian MS6.6 earthquake were tested on site.The results were combined with those of test samples examined by gas chromatography and a biological microscope to analyze the source,storage,transport,and physical and chemical mechanisms of the underground gases.The results reveal that following the earthquake,Rn and Hg values,in addition to those of CO2 and He,were higher;therefore,the deep gases migrated upward along ruptures and fractures during the earthquake.In addition,white smoke observed in the cellar and examined by a biological microscope was found to contain a large amount of sporopollen.We conclude that changes in the cellar condition led to the rapid reproduction and maturation of certain fungi growing on rotting wood at the top of cellar,and the resultant the white sporopollen floated in the escaping deep gas to resemble white smoke.Thus,the white smoke was related to the underground fault gas escaping from the deep strata.Therefore,because this cellar is located near a fault,a large amount of fault gases were released after the earthquake.The line between the epicenter and the anomaly site is the same as the long axis of intensity distribution,which indicates that the site is on the seismic fault.Therefore,it appears that this region is sensitive to fault activities and can thus be used as a long-term fault-gas monitoring site to study the aftershock sequence,fault activities,and earthquake prediction trend of southeastern Gansu.This method can lead to earlier warnings of large earthquakes,which may save thousands of lives.

    • Discussion on the Relationship between Regional Landform and Seismogenic Structure of the Minxian-Zhangxian MS6.6 Earthquake

      2013, 35(4):840-847.

      Abstract (1355) HTML (0) PDF 1.53 M (1324) Comment (0) Favorites

      Abstract:To clearly recognize and understand the geomorphologic characteristics and seismogenic structure of the Minxian-Zhangxian MS6.6 earthquake,we use DEM (Digital Elevation Model) data to analyze the landform characteristics of southeastern Gausu.Regional tectonic environment and crustal kinematic characteristics at northeastern margin of Tibet Plateau are discussed combined with the distribution of regional active faults and their kinematic features around the hypocentral region of Minxian-Zhangxian MS6.6 earthquake.The seismogenic conditions including tectonic environment,activity of seismogenic fault and seismogenic process are also studied.In this paper,we use terrain profile method to analyze the geomorphologic characteristics of source region of this earthquake to reflect the relation between regional geomorphologic characteristic and seismogenic structure.Deformation characteristic of regional middle and upper crust and seismogenic process are explored from the geomorphologic point of view.At last,regional seismogenic structure and geodynamics background are discussed.We suggest that the Minxian-Zhangxian MS6.6 Earthquake is related with coseismic and postseismic deformation process of the Wenchuan MSS.0 earthquake.

    • Sequence Type Estimation of the Minxian-Zhangxian Ms6.6 Earthquake and a Review of the Prediction Process

      2013, 35(4):848-850.

      Abstract (679) HTML (0) PDF 187.88 K (915) Comment (0) Favorites

      Abstract:After the Ms6.6 earthquake occurred in the border region between Min and Zhang counties of Gansu Province on July 22, 2013, we preliminarily estimated the earthquake sequence to be a main shock-aftershock type based on the history of moderate-strong earthquake sequences in this area. As time went on, there were more aftershock events. These could be used for further analysis, and then for further decision on the earthquake sequence type. Finally, we determined the Ms6.6 earthquake sequence that occurred in the border region between Min and Zhang counties, Gansu Province as having been a main shock-aftershock type, with the largest Ms5.6 aftershock having occurred on the same day as the main Ms6.6 shock, from a comprehensive analysis of the historical characteristics of moderatety strong earthquakes of the earthquake zone, and the space-time evolution characteristics and parameters of the earthquake sequence. These provided a correct basis for anti- earthquake relief work and played an important role in mitigating the earthquake disaster and stabilizing the disturbed soci- ety after the earthquake in the earthquake zone and its neighboring areas. Reviewing the forecasting process and the re- sults, we found that we had successfully predicted the Ms6.6 earthquake that occurred in the border region between Min and Zhang counties on July 22, 2013 several years before it occurred. The magnitude and location of the earthquake had been predicted accurately, and the accuracy of the prediction was much higher than any other example in Chinese earth- quake prediction history. Forecasting on a monthly scale, we had indicated at the monthly meeting on earthquake prediction at the end of February, 2013 that there would be a risk of a moderately strong earthquake in Gansu Province from the change in moderately strong earthquake activity on the Chinese mainland. Even for short and impending earthquake prediction from several days to several dozens of days, we had proposed the likelihood of a moderate-strong earthquake happening in Gansu Province and the adjacent areas from the results of previous studies and the cases of earthquakes with MI ≥ 4.0 from the time before the Ms6.6 earthquake occurred. In a meeting about earthquake prediction held several days before the occurrence of the Ms6.6 event, we made the prediction that there would be an earthquake of M≥ 5.0 happening somewhere in Gansu Province and the surrounding area within dozens of days. The fact we had successfully predicted the Ms6.6 earth-quake on a several-year scale, as well as over a short time period to some extent, reinforces our belief that earthquakes can be forecast. Even with our present level of understanding, we can still capture some information on the gestation and occurrence of earthquakes before the arrival of a disaster. However, in order to achieve the goal of earthquake prediction in China, earthquake scientists still need to make arduous efforts. As long as earthquake scientists use the correct approach, and government supplies the necessary manpower and material resources to predict earthquakes, we believe that there will be a hope to achieve the aim of earthquake prediction with a relief effect. It is promising that we have achieved at least one or two earthquake forecasts.

    • Analysis of Strength Variation of Different Kinds of Structural Loess and Adaptability of Traditional Strength Criterion

      2013, 35(4):851-857.

      Abstract (786) HTML (0) PDF 465.06 K (1142) Comment (0) Favorites

      Abstract:Loess is distributed extensively and is considered a special kind of soil; its strength and deformation are often different from those of other soil materials.At present,studies on the strength of structural loess are insufficient and the destruction form of loess and its destructive mechanism are also far from perfect.Owing to a lack of awareness about the structure of soil and test conditions,previous theories pertaining to structural strength criteria that describe the structural strength of soil are inadequate for establishing a law for the structural strength of loess.In particular,the adaptability of these criteria theories remains to be studied in the case of loess strength under complex stress variation.Among the many available strength criteria,the ones used commonly for soil are the generalized Tresca criterion,generalized von Mises criterion,Mohr-Coulomb criterion,and Matsuoka-Nakai criterion,among others.Testing of a large number of rock and soil materials has shown that the generalized von Mises criterion and the generalized Tresca criterion have fairly good applicability despite the introduction of the influence of ball stress on the rock and soil materials; however,these criteria give larger errors.The Mohr-Coulomb criterion has been widely applied in the field of soil engineering on account of its clear physical meaning,ease of setting of parameters,and the rationality and applicability in describing the destruction of rock and soil materials.However,this criterion also has an obvious drawback in that it does not consider the impact of intermediate principal stress on soil strength.Although the Matsuoka-Nakai criterion,which is based on the Mohr-Coulomb criterion,considers the impact of the intermediate principal stress,it is associated with true triaxial tests of sand and its physical meaning is not sufficiently clear.Natural loess is a unique structural and collapsible soil.In this study,based on true triaxial tests of different kinds of structurally intact loess,the influence of structural changes on the strength of the intact loess is analyzed.The Mohr-Coulomb criterion and Matsuoka-Nakai criterion,which are commonly used in geotechnical engineering,are used to analyze the adaptability of different kinds of undisturbed structural loess.The results show that the strength of natural structural loess is closely related to its structure and its property index.When the moisture content is low,the influence of change in the dry density on the structural strength of undisturbed loess is greater than the influence of change in moisture content; in contrast,the influence of the change in moisture content on the structural strength of undisturbed loess is greater than that of change in dry density.In terms of the failure line of structural loess in the PI plane,the greater the strength of structural loess,the closer it is to a rounded crooked triangle.The failure line is close to the Mises circle with structural infinity.Conversely,the smaller the strength of structural loess,the closer it is to a rounded triangle.It follows that with an increase in the structural strength of loess,its failure line in the PI plane gradually changes from a rounded triangle to a rounded crooked triangle,eventually tending toward the Mises circle.For different kinds of structurally intact loess,the use of the Mohr-Coulomb criterion and Matsuoka-Nakai criterion in describing the strength of structurally undisturbed loess has certain drawbacks.Overall,the strength described by low-strength criteria is smaller than the actual strength of structurally intact loess.The Mohr-Coulomb criterion has a larger error than the Matsuoka-Nakai criterion; despite the fact that these criteria are traditionally used for describing weak structural soil,their error increases with an enhancement of structural strength?"

    • Analysis of Seismic Performance Characteristics of Bridge with High Piers under Strong Earthquake Motion

      2013, 35(4):858-865.

      Abstract (753) HTML (0) PDF 491.76 K (1011) Comment (0) Favorites

      Abstract:With the implementation of Western Development Program in recent years,many highways and railways have been planned and erected in West China.For bridges in mountainous areas with high piers,a nonlinear numerical analysis model is constructed on the basis of the fiber.Finite element analysis is simulated by code OpenSees developed by Pacific Earthquake Engineering Research (PEER).The Mander model is used to imitate the constitutive model of concrete,and the Giuffré-Menegotto-Pinto model is used to imitate the constitutive model of reinforcement.Incremental dynamic analysis (IDA) is used to research the distribution features of pier section curvature under strong earthquake motions.As a result of the effects of higher modal contribution,the seismic response and performance of bridges with high piers are complicated.The formation and development of the plastic range in the pier upper position,as well as the damage process and failure mechanism of high piers,are studied under various earthquake motions.Results indicate that high pier base section curvature and its top displacement are not simultaneous; therefore,the latter is not inappropriate for representing the performance index.Therefore,seismic performance characteristics of bridges with high piers in the plastic stage are discussed in which pier section curvature is used as the performance index.In addition,the seismic performances of such bridges are more complex due to higher mode contribution,and are strongly influenced by the spectral characteristics of ground motion.In this study,six earthquake waves are selected from a database of PEER Ⅱ class sites.Because of space constraints,this study involves the seismic performances of only bridges with high piers under transversal earthquake action.Careful analysis reveals that the base section curvature of one pier is greater than that in other piers when all piers are plastic,and the location of the pier depends on the degree of higher mode contribution.Damage to higher piers could be more substantial than that to low piers,and the envelopes of section curvature above the pier base plastic region do not exhibit a linear trend.The occurrence of plastic hinges at middle and upper parts of piers is attributed to the contribution of higher modal shape,and the extent and size of this plastic region are affected by Therefore,bridges with high piers are not safe in earthquakes when designed according to existing specifications.

    • A Seismic Analysis of Using Thermal-insulating Roof as TMD in Small High-rise Buildings

      2013, 35(4):866-871.

      Abstract (786) HTML (0) PDF 317.93 K (979) Comment (0) Favorites

      Abstract:China is an earthquake prone country; earthquakes endanger people's lives and cause destruction of property.Therefore,researchers have raised serious concern on the seismic performance of buildings and how to improve them.Since the last century,the structural vibration control technology has been widely studied both within the country and abroad.Tuned mass dampers (TMDs) are one of the applications of passive control techniques that have good shock absorbing functions in earthquake engineering and can effectively protect buildings and internal facilities.With the development of the housing industry,small high-rise buildings have become common.In order to improve the seismic performance of high-rise structures and achieve the desired reliability and validity of the damping design,the concept of a roof insulation layer and lead rubber beating as a TMD device is proposed.Compared to the traditional TMD,a roof insulation layer TMD has advantages in terms of structuring and installation.In order to study the damping effect of the new TMD control device,a small high-rise building having a roof insulation layer and lead rubber bearing as the TMD device,is studied.In this paper,the damping effect of the TMD device is analyzed using a dynamic time history analysis method with the SAP2000 finite element software.The results show that the roof insulation TMD device can effectively reduce the dynamic response of structures.The displacement and acceleration of the floor decreased by about 20 %,the inter storey displacement; decreased by about 30 %,and the interlaminar shear force decreased by 20 %~30 % on an average.Therefore,using a roof insulation layer and lead rubber beating,a feasible and practical damping system can be obtained for small high-rise buildings.

    • Repairing and Strengthening of Reinforced Concrete Members and Study of its Seismic Performance

      2013, 35(4):872-876.

      Abstract (753) HTML (0) PDF 799.57 K (817) Comment (0) Favorites

      Abstract:Reinforced concrete frame structures should meet the design concepts of strong columns,weak beams,and so on,in order to survive during earthquakes.Due to various reasons of design and construction,the vertical load-bearing component suffered various degrees of damage and destruction in previous earthquakes and the ideal broken mechanism of "strong columns weak beams" which was required by the specification did not appear.In order to avoid massive destruction and collapse of reinforced concrete frame structures during strong earthquakes,vertical load-bearing members of reinforced concrete must undergo damage after the horizontal ones.Therefore,it is very important to repair and strengthen the reinforced concrete members at some important positions.Strengthening treatments such as increasing the section,the carbon fiber reinforced method,reinforced method of shotcrete,the steel-encasing strengthening method,and the method of sticking steel reinforcement of outsourcing were taken as examples in this paper to discuss the repair and reinforcement of concrete members,which could provide effective engineering measures for the strengthening and repairing of this type of concrete member.The research has a certain practical value for the repair and reinforcement of the concrete member.

    • Thermal Stability of Fabricated Foundations with Different Warming Patterns in Permafrost Regions

      2013, 35(4):877-884.

      Abstract (663) HTML (0) PDF 559.21 K (1015) Comment (0) Favorites

      Abstract:Climate warming and engineering activities could cause changes in the thermal conditions of permafrost,mainly in two ways:warming of the permafrost and increasing the thickness of the active layer,which will lead to a marked change in the thermal stability of foundations in permafrost regions.Generally based on the IPCC SRES A2,A1B,and B1 climatic changes,this study uses numerical methods to research the freezing and thawing process,the changes in the active layer,the thaw depth and the soil temperature of fabricated foundations in permafrost regions along the ± 400 kV Qinghai-Tibet power transmission line.The results show that engineering activities and climate warming change the thermal conditions of permafrost and promote degradation; these are important factors affecting the long-term stability of foundations.Enhanced heat conduction from concrete pile foundations aggravates the freezing and thawing process,and climatic warming leads to a decrease in the thickness of the active layer and a rise in soil temperature,With increasing depth,the permafrost response decreases,and the rate of change of temperature becomes small.The permafrost response increases and the rate of change of temperature is greatly altered closer to the active layer.Under three warming patterns,after 50 years,the soil temperature at 1 m depth increases to 2.80 ℃,4.66 ℃,and 5.60 ℃ from 1.90 ℃,the soil temperature at 2 m depth increases to 1.22 ℃,3.66 ℃,and 4.82 ℃ from 0.50 ℃,and at 10 m depth,the permafrost temperature will rise from-0.70 ℃ to -0.42 ℃,-0.35 ℃,and-0.30 ℃.With the three warming patterns,IPCC SRES B1,A1B,and A2,the maximum thaw depths under the centers of foundations are 3.12 m,5.07 m,and 6.02 m after 50 years,while the thicknesses of the active layers of natural ground are 2.07 m,4.37 m,and 5.62 m.The maximum thaw depth of natural ground is much shallow than the maximum thaw depth under the centers of foundations.This shows that permafrost has different responses to different warming patterns,and the degradation of permafrost under the centers of pile foundations is faster under the twin effects of climatic warming and engineering activities.There will be hidden troubles with foundation bases after 50 years if steps to prevent the degradation of permafrost under warming patterns A2 and A1B are not taken.The thawing rates of the permafrost under the foundation centers are 1.5,6.2,and 8.6 with the three warming patterns B1,A1B,and A2 from the tenth year to the fiftieth year.The thaw depth increases with a rise in the rate of change of temperature,and the speed of degradation accelerates with increasing rate of change of temperature.The thaw depth of permafrost under the centers of foundations is three times deeper as that of the permafrost of natural ground.This is because engineering activities promote permafrost degradation with a low rate of temperature increase.At this point,permafrost mainly is changed by engineering activities,while permafrost degradation is mainly influenced by climate warming with a higher heating rate.

    • S-wave Velocity Structure of the Eastern Margin of the Tibetan Plateau

      2013, 35(4):885-892.

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      Abstract:To study the S-wave velocity (VS) structure of the middle and lower crust beneath the eastern margin of the Tibetan Plateau and viscous flow in the deep crust,we adopted data from a regional network of approximately 50 temporary broadband seismometers and 10 permanent seismic stations of the Chinese Digital Seismological Network (CDSN) to invert the detailed structure of the crust and the upper mantle in the eastern margin of the plateau and in Sichuan Basin by using the linear inversion method.We determined that from 0 km to 10 km,VS is lower in the basin because it contains a huge thickness of sediments characterized by low velocity.In contrast,the regions in the eastern margin of the plateau show a higher velocity.From 10 km to 40 km,the basin shows higher VS in the middle and lower crust.In contrast,the eastern margin of the plateau shows lower velocity; the average velocity of the middle crust is 3.0~3.4 km/s.From 50 km to 60 km,the basin extends to the upper mantle,which shows high velocity.Low velocity still occurs in this depth range in the lower crustal portion of the eastern margin of the plateau at an average velocity of 4.0~4.5 km/s.The prominent features of the VS profiles indicate that the majority of low-velocity zones appear in the middle crust at 20 km to 40 km and that small low-velocity zones occur in the upper crust at 10 km to 20 km and in the lower crust at 40 km to 60 km.In addition,only a portion of the low-velocity zone occurs in the southern region of eastern margin of the plateau.The crust varies in thickness from 50 km to 66 km in the eastern margin of the plateau and from 40 km to 44 km in the basin.The gradient of the Moho discontinuity is highest in the boundary along the eastern margin of the plateau toward the basin.The crustal flow does not increase,and most of the flow direction occurs along the main active faults.The effective middle and lower crustal thicknesses have been estimated to be between the 20 km and 40 km; the crustal viscous flow is generally heterogeneously dispersed at a depth of 20 km but extends to 60 km in some areas.The dominant flow direction is along the main active faults.When the viscous flows in the middle and lower crust were obstructed by the strong,rigid Sichuan Basin,they divided into two or more branches with different directions.The upper part of viscous flow upwelled to produce sufficient pressure for intrusion into the upper crust,thereby uplifting the mountain ranges and high peaks along the eastern margin of the Tibetan Plateau.In contrast,the lower downwelling part of viscous flow produced sufficient pressure for intrusion into the lower crust and upper mantle to deepen the Moho discontinuity,which caused the crust to thicken.

    • Love Wave Tomography in the Tarim Basin and Its Adjacent Regions

      2013, 35(4):893-899.

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      Abstract:In this paper,we conduct background noise seismic tomography of the Tarim Basin and adjacent regions.The Tarim Basin of northwestern China,which is the focus of the study,includes the northern edge of the Tibetan Plateau,major mountain belts (the West Kunlun Mountains,Altyn Tagh,and Tianshan),and other important oil-and gas-bearing basins (the Junggar and Qaidam basins).Due to its complex topography and geological structure,the Tarim Basin and adjacent regions have become a prime place to understand the processes of continental collision,the mechanisms of mountain building and the interaction of tectonic blocks.The data used for the study include 9 months (January to September 2009) of horizontal component continuous data recorded at 74 seismic stations of the newly updated China Provincial Digital Seismic Networks and the regional Kyrgyzstan and Kazakhstan networks.Empirical Love wave Green' s functions were obtained from interstation cross-correlations.Phase velocity dispersion curves for Love waves for periods between 8 and 30 S were measured for each interstation path by applying the multiple-filter analysis method with phase-matched processing.The high-resolution phase velocity maps show clear lateral variations that correlate well with major geological structures and tectonic units in the study area.Because of the thick sedimentary layers in the basins,we can clearly see that the phase velocity for Love waves with periods between 8 and 30 s is relatively low,while the phase velocity for mountain areas is high.This significant difference in the phase velocity may be related to the sharp changes in the surface topography and terrain elevation from the basin at low altitude to the high mountain areas.It is also possible that the phase velocity is quite different because of the different seismic wave transmission mediums in the study area.The basin is filled with sedimentary rock,and the Tibetan Plateau and the mountain areas are mostly metamorphic rock.The seismic wave velocity of sediments is far lower than that of the metamorphic rock.In addition,our results reveal obvious lateral phase velocity changes in the crust beneath the Tarim Basin.In the upper crust,the eastern basin has a lower velocity than the western basin; however,the northern basin has a lower velocity than the southern basin.The Manjaer depression in the northeast has a very thick sedimentary layer (nearly 20 km),however,the Bachu uplift has a relative thin sedimentary cover,therefore the Manjaer depression in the eastern Tarim Basin has a very low velocity and the Bachu uplift in the Tarim Basin has a high velocity.These areas have been shown to be places with hydrocarbon enrichment.This is mainly affected by the terrain differences,the sedimentary differences,the thickness of sediments and the sedimentary cover,and it is possible that some velocity variations are connected with the central suture of Presinian bedrock blocks in the northern Tarim Basin.In the Tarim Basin and adjacent regions,exploring the relationship between the different phase velocities for different times and geological formations is helpful for our understanding of the deep structural characteristics and dynamics in the Tarim Basin and its adjacent areas.

    • Research on the Dynamic Change of Acceleration and Seismic Intensity Distribution of the Lushan MS7.0 Earthquake

      2013, 35(4):900-907.

      Abstract (1045) HTML (0) PDF 1.25 M (935) Comment (0) Favorites

      Abstract:In order to acquire the disaster loss assessment data accurately and quickly when a strong earthquake occurs,we need both the detailed information about the structural features of the earthquake occurring region and reliable seismic observation data.After the huge earthquake of Wenchan MS8.0 on May 12th,2008,many observation stations have been deployed and some important research results have been obtained in the Longmenshan fault zone by the China Earthquake Administration (CEA),which also provided valuable data for investigating the Lushan MS7.0 earthquake of April 20th,2013.By analyzing the seismic waveform data recorded by 70 strong-motion seismographs,we reveal the variation characteristics of the peak ground acceleration (PGA) for different epicentral distances,discuss the spatial acceleration dynamic change and the related waveform propagation features,and calculate the seismic intensity distribution of the Lushan MS7.0 earthquake based on the obtained PGA in this study and empirical formulae.Our results indicate that the highest seismic intensity is about Ⅸ degree and both the Lushan and Baoxing regions are among the severely afflicted areas.Areas in which the seismic intensities are above Ⅷ are mainly located in the center of Lushan and jointly in the regions of Baoxing,Tianquan,Ya'an,Minshan,etc.Furthermore,seismic intensities above Ⅴ have a NE long axis distribution along both sides of the Guanxian-Anxian fault in the Longmenshan fault zone.The characteristics of the seismic intensity shown in this study are consistent with the results reported by the CEA.In addition,the PGA shows that the initial rupture of the Lushan earthquake has a thrust fault,which agrees well with the focal mechanism solution of Harvard University.It is also noted that the large fault probably has an effect on the PGA distribution.As a result,our method of analysis is feasible and stable,and it can offer important references for earthquake relief work to the government.

    • Focal Mechanism Solutions and Accurate Determination of Focal Depth of Diebu MS4.0 Earthquake

      2013, 35(4):908-913.

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      Abstract:Seismic source is a main focus in seismologic research,and the focal mechanism solution and earthquake depth are important parameters.Researchers use the depth phase to enhance the precision of focal depth for M ≧ 5.0 earthquakes by using the arrival time difference between teleseismic depth sP (or pP) and reference phase P to determine the focal depth of teleseismic activity.For the small,regional earthquakes,local depth phases such as sPL,sPg,sPmP,sPn,and sSmS may be observed in the proper distance range.If the regional depth phases can be identified,the focal depth of moderate-small earthquakes may be estimated.To obtain accurate seismic parameters of the Diebu MS4.0 earthquake occurring on February 23,2010,this study uses local broadband waveforms and the cut-and-paste (CAP) method to obtain the best focal mechanism.The waveform fitting method for synthetic and observed seismograms is then used to further determine the focal depth of the earthquake.Because Minxian station (MXT) and the earthquake epicenter are 27 km apart,we use the data of that station to determine the depth phase sPL.Three component waveforms of MXT are first resolved to radial,tangential and vertical components,although the tangential component of the P wave energy is weakest for determining the best angle of rotation in obtaining the radial and tangential components.We next integrate the data of displacement and adopt a low-pass filter below 1 Hz.Finally,we use the waveform comparison method to accurately confirm focal depth.The results show that the Pg and sPL phases of time and amplitude in synthetic and observed seismograms agree stronger at 7 km,as do the three components of these seismograms.Therefore,the most accurate focal depth of the Diebu MS4.0 earthquake is 7 km,which implies that this event was a shallow earthquake in the upper crust.

    • An Application of the Co-seismic Displacement Intensity Method to the Heze 5.9 Earthquake

      2013, 35(4):914-920.

      Abstract (687) HTML (0) PDF 731.61 K (1066) Comment (0) Favorites

      Abstract:In this paper,the crust velocity model of Shandong was selected and Shandong virtual station' s Green's function database was established.The surface co-seismic displacement around the earthquake zone was calculated using the Heze 5.9 earthquake source parameters,based on the earthquake time,location,magnitude,fault strike,dip,and slip.At first,a generalized empirical relationship between the co-seismic displacement and earthquake intensity is established and then the intensity distribution of the strong earthquake zone is quickly obtained.By comparing with the Heze 5.9 earthquake macroseismic intensity data,it is seen that the co-seismic displacement intensity is similar to it.These data have a very important guiding role in the early warning of strong earthquakes,rapid disaster assessment after the earthquake,and guiding emergency rescue.Quick reporting of seismic intensity especially big earthquake intensity data can provide a new earthquake monitoring technology and a new means of reducing earthquake related disasters.Within a few minutes after a devastating earthquake,a quick determination of the spatial distribution of the seismic intensity is required which estimates the extent of damage in different regions and enables the government to carry out emergency rescue and relief operations.Then rescue workers can timely and accurately reach the epicenter area,search and rescue,thereby reducing loss of life and property.Currently,many earthquake-prone countries and regions have paid more attention to quick reports of earthquake intensities which have been widely used.An earthquake structure is very complex,but for most earthquakes,it may be approximately inferred that they are caused by sudden dislocations along fault planes.Calculation of the co-seismic displacement field was considered to change over a very short time scale and mainly included elastic dislocation effects which were simulated ground motion changes at the time of occurrence of the earthquake.Through this change we can approximate the extent of the earthquake damage.Intensity and co-seismic displacements have a direct relationship in strong earthquake zones.Therefore,we can use the co-seismic displacement intensity to approximate the seismic intensity.Through comparisons and global experiments on several earthquakes,the strong earthquake zone was found to have a significant effect.Although this theoretical calculation had some errors that need to be corrected using follow-up information,this method does not depend on the distribution of stations.They play a very important guiding role in giving early warnings of big earthquakes,particularly in regions that have less number of stations.Based on the Shandong velocity model,this paper established the Shandong virtual station' s Green's function database and the co-seismic displacement theory was applied to the Heze 5.9 earthquake to validate that application of the method was also possible for the Shandong moderately strong earthquake epicenter area.

    • The Variation of Wave Velocity Ratio at the Eastern Region of Inner Mongolia

      2013, 35(4):921-927.

      Abstract (868) HTML (0) PDF 715.23 K (1007) Comment (0) Favorites

      Abstract:From 2008 to 2012,the integrity of observation data of earthquake monitoring network in Inner Mongolia was above 96.3%,and the monitoring ability and data integrity at eastern Inner Mongolia (41 ° ~ 51 ° N,114.5 ° ~ 124.6 ° E) were more outstanding than other regions.Combined with the actual geological structure and seismic activity characteristics of eastern Inner Mongolia,we select the earthquakes of ML≥2.0 at study area (41 ° ~ 51 ° N,114.5 °~124.6 ° E) from 2008 to 2012 to calculate the average wave velocity ratio there and its variation.As the comprehensive analysis on existing research data may impact the uncertainty of calculation,we use Multi-station method to make the results more accurate and reliable.Calculation results show that the average wave velocity ratio at study area from 2008 to 2012 is 1.709 4,and basic range is between 1.690 5~1.732 8.Through the analysis of two moderate earthquakes at research area,we find that the temporal change of wave velocity ratio at the eastern region of Inner Mongolia show some significant anomaly,which satisfies the law of "decline-low value-recover-seismogenic" and there is a change in the abnormal period after the earthquake,the change rate is about 3%.After drawing the spatial contour of wave velocity ratio at that region,we found that the high values within the area are mainly distributed along Daxinganling,showing the spatial distribution of a trend of low West high East and low North high South.In terms of the geological structure,high values of wave velocity ratio in a range of 1.73 ~ 1.78 are mainly concentrated in Balihan fault,Beipiao-Chaoyang fault and Chifeng-Kaiyuan fault.Combined with the seism-geological background of study area and the change of wave velocity ratio in recent one or two years,it could be concluded that the stress variation of underground medium in eastern Inner Mongolia is more significant,with certain geological conditions of the occurrence of moderate and strong earthquake.Because the data we use here do not contain the analog seismic record before 2008,it does not show the complete seismic record and the whole change trend of wave velocity at eastern Inner Mongolia,moreover,there are few seismic stations at eastern Inner Mongolia region and they are distributed very unevenly,the results are just preliminary.With the enrichment of digital observation data in the future,we can do more detailed studies in this area.

    • Configuration Technology of Chinese Character Library of Generic Mapping Tool Drawing in Windows System

      2013, 35(4):928-935.

      Abstract (1370) HTML (0) PDF 749.60 K (3069) Comment (0) Favorites

      Abstract:The Generic Mapping Tool (GMT) system was initiated in late 1987 at Lamont-Doherty Earth Observatory,Columbia University,New York,by graduate students Paul Wessel and Walter H.F.Smith.GMT is written in ANSI C programming language.With the characteristics of beautiful graphics,intense color,flexible organization,and easy adaptation,GMT is widely used in geographical or geophysical research and is considered to be the best drawing tool for publishing high-quality documents.However,because GMT was originally written for command-line usage rather than a Windows environment,its application is difficult for ordinary users.In particular,the absence of a Chinese character display function poses constraints for the vast majority of Chinese users.This issue has been solved in recent years with successful application to the Linux system.However,GMT with a Chinese character display function remains elusive for general users.Although several research-related papers appear in Internet blogs at http://blog.sina.com.cn/s/blog_5e16f1770100o7gc.html and http://xxqhome.blog.163.com/blog/static/1967330202011112810120598/,for example,no systematic papers on the GMT Chinese character display technology for Windows are available for public access.Therefore,on the basis of a large number of related references,GMT text annotation commands,and detaile dparameter-setting instructions,this paper discusses the revelation principle and technology approach of GMT in the Windows system by using the relational structure among GMT,Ghostscript,and postscript.We address the flexible character labeling problem and show examples of title,axis,and text labels during the GMT drawing process.Thus,this paper is an important reference for Chinese users.

    • Analysis of Typical Cases of Earthquake Damage in Rural Areas of China and Earthquake Safety Projects for Rural Houses in Recent Years

      2013, 35(4):936-944.

      Abstract (968) HTML (0) PDF 834.83 K (1184) Comment (0) Favorites

      Abstract:From statistical analysis of information on disaster losses caused by more than 30 5.0≤MS≤7.0 earthquakes and some MS <5.0 earthquakes in China since 2000,we summarize the typical characteristics of seismic damage and experience in rural areas.(1) The epicenters of these earthquakes are mostly located in active tectonic belts and seismically active belts,so casualties and economic losses are great.The direct economic losses caused by an earthquake swarm of about magnitude 5.5 are equivalent to 3.6 % of the provincial revenue budget.(2) Destruction and collapse of houses and public facilities are major factors in direct losses from earthquake disasters.Damage to and loss of rural houses in MS<7.0 earthquakes accounts for over 60 % of total direct losses from earthquake disasters,and can be as much as 96.7 %.(3) Earthquake damage to rural houses with different structures in different regions can vary markedly:damage to rural houses in the western region is greater than in the middle and eastern regions; damage to rural houses with traditional,non-formal civil,brick,stone and wood structures is significantly greater than for formally designed and constructed rural houses of brick,with frame structures.(4) Some small earthquakes of magnitude<5.0 may cause great disasters,and in these earthquakes the loss of houses is substantially equal to all other losses from the disaster.(5) The implementation of requirements for seismic fortification of rural houses and public buildings has a tremendous impact on the earthquake damage level,especially in western areas inhabited by minority nationalities.If rural houses and public facilities without scientific fortification are hit by an earthquake,the damage will be serious.(6) Because rural houses and many public buildings are built without formal design and construction guidance,their seismic performance is extremely poor.Scientific siting,specific design,and good construction are the most direct,effective,and reliable ways of mitigating casualties and economic losses caused by earthquakes at present.(7) In earthquake-prone areas and high-seismic-intensity areas,a seismic threat to the safety of rural schools and hospitals (clinics) still exists.The seismic safety of rural schools and hospitals (clinics) is still a matter of great importance.(8) In the western poverty-stricken regions,seismic damage is an important factor that causes rural people to become poor or return to poverty,which also affects social stability.Disasters caused by small earthquakes are relatively common,so to some extent earthquake disasters widen the gap in economic and social development between rural and urban areas.(9) Because most farmers and herdsmen in rural areas,especially in areas where the culture and economy are relatively backward,lack knowledge of earthquake prevention and disaster reduction,and their awareness of disaster prevention and mitigation is still very weak,popularization of the science and technology of earthquake prevention and seismic fortification of rural buildings is extremely urgent.From summarizing typical cases and successful experiences of earthquake safety projects for rural houses in Xinjiang,Gansu and Hainan provinces in recent years,we point out that vigorous promotion of implementation of earthquake safety projects for rural houses is an important measure for mitigation of losses and casualties caused by earthquakes,and can completely improve the weak seismic capacity in rural areas.We also propose some suggestions for further promoting earthquake safety projects for rural houses and improving the seismic fortification levels of rural houses.

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