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  • 1  Anti-seismic Measures for a Cross Structure between a RailwayTunnel and a Cross Passage under Strong Earthquakes
    SHEN Yusheng CHEN Zheng ZHOU Pengfa LUO Ningning ZHANG Xi YANG Jiaqi
    2020, 42(1):182-193. DOI: 10.3969/j.issn.1000-0844.2020.01.182
    [Abstract](572) [HTML](0) [PDF 25.49 M](38923)
    Abstract:
    With the orthogonal structure between the main tunnel and a cross passage as the analytic target, a finite element analysis model was established by ABAQUS based on the engineering of the Yuelongmen railway tunnel in Maoxian county, Sichuan Province, China. Selecting El-Centro seismic wave data and applying the extended finite element theory, crack damage of the cross structure between the cross passage and main tunnel under the most severe earthquake conditions was studied. Seismic resistant measures of the shock absorption layer combined with a flexible joint are presented. Research and analytical results showed that the combination of a shock absorption layer and flexible connectors reduced and restrained the crack range of the cross structure tunnel under a strong earthquake. Installation of flexible connectors prevented the extension of longitudinal cracks. A circular flexible connector in the middle of the intersection completely prevented the formation of cracks in the arch of the intersection. Installation of a shock absorption layer between the initial support and secondary lining gave more deformation space to the lining. Also, the secondary lining was not readily affected by the initial support deformation and cracking.
    2  A rapid evaluation method of earthquake intensity based on the aftershock sequence: a case study of Menyuan M6.9 earthquake in Qinghai Province
    ZHAO Huaiqun HE Shaolin CHEN Wenkai SI Hongjun YIN Xinxin ZHANG Can
    2022, 44(2):432-439. DOI: 10.20000/j.1000-0844.20220128002
    [Abstract](959) [HTML](43) [PDF 17.04 M](18836)
    Abstract:
    The rapid assessment product of earthquake intensity is an important basis for judging the disaster situation during the " black box period" of emergency response after a destructive earthquake. Based on the aftershock sequence within 2 h after the M6.9 earthquake in Menyuan, Qinghai Province, we used a ground motion attenuation model with the shortest fault distance to rapidly evaluate the earthquake intensity in this paper. The results showed that the intensity distribution obtained by using the aftershock sequence within 30 minutes after the M6.9 mainshock can preliminarily determine the hardest stricken area and the disaster areas, but the scope of disaster areas is smaller than the actual survey result. The intensity distribution obtained by using the aftershock sequence within 1.5 h is consistent with the field investigation results, and the calculation results using the aftershock sequence within 2 h have no obvious change. The seismic intensities obtained using the relocated aftershock sequence are more accurate than those obtained from the conventional aftershock sequence, but a suitable relocated method needs to be selected. In this earthquake, the intensity range evaluated by using the aftershock sequences shows a characteristic that the accuracy is higher with growing intensity, and the hardest-hit areas determined using the result are accurate. The proposed method enriches the existing system of rapid intensity assessment, but its applicability and conditions need to be further studied.
    3  Discussion on the automatic processing results for the aftershock sequence of Menyuan,Qinghai MS6.9 earthquake on 8 January,2022
    WANG Zudong YANG Xiaopeng YIN Xinxin PU Ju WANG Weihuan CHEN Xiaolong
    2022, 44(2):408-414. DOI: 10.20000/j.1000-0844.20220125002
    [Abstract](334) [HTML](0) [PDF 14.68 M](10408)
    Abstract:
    The continuous waveform data three days before and seven days after the MS6.9 earthquake in Menyuan, Qinghai Province on January 8, 2022 were recorded by the permanent stations of Gansu seismic network and the shared stations of neighboring provinces. In this paper, the aftershock sequence was detected by the automatic cataloging program based on the data, and the results were compared with the manual cataloging results. It was found that the results of automatic catalogue and manual catalogue are basically the same; the average epicenter position difference is within 5 km, and the magnitude difference is between ML-0.2~ML0.2. The earthquake occurrence time from automatic catalogue results is slightly earlier than that from manual catalogue,but the difference of the occurrence time of most aftershocks in the two catalogs is within 2 s. The output speed of automatic cataloging is fast, so it could detect small aftershocks that cannot be recognized manually, thus improving the completeness of the catalogue. On the whole, the output results of the automatic cataloging system can meet the expected objectives, and can provide data support for relevant scientific research, such as earthquake swarm trend judgment and rapid inversion of rupture process.
    4  Research on Influence by the Air Disturbance in Microtremor Recordings based on the Broad-band Digital Data of Gaotai Station
    YIN Liang YIN Zhi-wen WANG Bi-xian WANG Ya-hong SUN Dian-feng
    2016, 38(S2):183-189. DOI: 10.3969/j.issn.1000-0844.2016.Supp.2.0183
    [Abstract](722) [HTML](0) [PDF 6.73 M](9615)
    Abstract:
    Through many studies on the changes of the amplitude and the spectrum of microtremors in the period of cold air process in the digital broadband date from Gaotai. We also researches on the process of cold air disturbance to the microtremors in time-domain and frequency-domain. Then we discovered that in the process of cold air, the microtremors curve will be a big periodic perturbation or jitter distortion. the process with the cold air from start to finish, the 0.001~0.005 Hz amplitude-frequency value changed greatly,and the process had the characteristic of rapid increase-reach to maximum-dicrease then return to the normal, and the maximum disturbance data will happen at the time of maximum temperature decrease.
    5  Mechanism of Gravity Anomaly before Earthquake and Its Role in Earthquake Prediction
    CHU Qing-zhong WU Ze SHAO Xian-jie LI Hui CHEN Yu
    2014, 36(1):201-206.
    [Abstract](2723) [HTML](0) [PDF 10.15 M](9528)
    Abstract:
    Earthquakes are among the major natural disasters facing the world.Earthquakes can threaten safety of people and cause serious damage to property and the economy of a region.Our country is one nation that has had serious earthquake disasters throughout history,with 4 400 large earthquakes being recorded.Seismic intensity is 7 degrees or higher for 60 % of the nation’s land,with 50% of the cities and 67% of the big cities located in these high(i.e.,≥ 7) intensity areas.Therefore,having the ability to predict earthquakes would help to reduce the risk posed to people and human made structures.This article examines the gravity anomaly,one of the earthquake forerunners,and the relation between gravity anomalies and earthquakes.This study is based on the concept that when tectonic movement occurs,the earth’s crust extrudes or extends in every direction. As a result,gravity will be in a state of imbalance and under the action of an equilibrium force,the earth’s crust will restore to a balanced stable condition.During this adjustment,the gravity field will adjust and change until balance is restored.A formula is used to briefly describe the three influencing factors that affected the gravity anomaly:the density changes,movement of the outer fluid,and changes of measurement elevation.In the second part of the study,gravity variation diagrams are drawn with respect to time and space before the earthquake of Yushu,Tangshan and Wenchuan.It can be seen from the cases that the gravity anomaly before earthquakes is obvious:gravity changes constantly near the epicenter,and the gravity gradient is higher than usual.There are strong tectonic activities during earthquake gestation.Large deformations are produced in the fault zones and the weak areas of the earth’s crust,thus forming some crustal thickness difference and density difference,and the phenomena of gravity anomalies are shown.In this part,the causes of the gravity anomaly are also conjectured:tectonic movement and the invasion of subsurface fluid. Finally,two prediction methods of gravity are introduced:mobile gravity observation and GRACE gravity observation.The gravity changes because of crustal deformation and material changes in the fault zones,which result from crustal movement during earthquake gestation.We believe that evolution characteristics of the gravitational field can help us to explain current tectonic activity.The whole process from gestation to eruption of the mid-strong earthquake can be tied to gravity anomaly,thus providing reliable basis for earthquake prediction.Through the study of the gravity anomaly,the characteristics and change can be understood including the quality and density of underground medium,change of elastic modulus of the crust,and relationships between gravity anomalies and earthquakes.The related indicators of the short to medium term earthquake prediction in mobile gravity observation are summarized,and the case of Wenchuan earthquake is used to summarize the method of long term earthquake prediction using the GRACE satellite observation.
    6  A Study on the Scientific Value of Historical Records for the Huaxian MS8¼ Earthquake in 1556
    GUO Zeng-jian GUO An-ning ZHANG Wei-chao REN Dong ZHAO Cheng-cheng
    2014, 36(2):281-285.
    [Abstract](2023) [HTML](0) [PDF 11.33 M](6759)
    Abstract:
    Historical records of the Huaxian MS8? earthquake in 1556 are one of the most abundant in China.In this paper,we assess the seismological scientific value of the Huaxian earthquake historical records.The study includes the following:(1)an investigation of ground rotational movement often felt before large earthquakes,which can be used as an index of immediate earthquake and an early warning;(2)we determine the time span of seismic changes,from small to large,prior to sizeable earthquakes based on historical records;(3)elaborate on the medium-term precursor of the relationship between drought and earthquake before large earthquakes;(4)strong-moderate earthquakes were found to be active in the Fenhe graben before large earthquakes;(5)the seismic cycle of Weihe basin was determined to be 600 years;(6)we use the distribution index of triplet method for the occurrence time of historical earthquakes;and(7)verification of intensity zoning that the Huaxian earthquake points out and asymmetry of intensity that the Huaxian earthquake reveals.Furthermore, we also study and discuss the scientific significance and historical status of Earthquake Records written by Qin Ke-da.
    7  Analysis of Kinematic Mechanism of Active Fold-Thrust Fault in front of the Southern Longmenshan Mountain: A Case Study of the Lushan Earthquake
    LI Jing-bo LI Yong ZHOU Rong-jun YAN Zhao-kun YAN Liang ZHENG Li-long YANG Yu-qiang REN Cong
    2015, 37(1):202-213. DOI: 10.3969/j.issn.1000-0844.2015.01.0202
    [Abstract](1963) [HTML](0) [PDF 11.18 M](6479)
    Abstract:
    The Lushan earthquake (MS7.0, April 20, 2013)that occurred in the southern segment of the Longmenshan Fault was another destructive earthquake that followed the Wenchuan (MS8.0) earthquake that occurred in the middle-northern section of the Longmenshan Fault.The epicenter was 30.3° N, 103.0° E at a depth of about 13 km.The Longmenshan Fault is located on the eastern side of the Qinghai-Tibetan Plateau, which marks an area of abrupt geological, geomorphological, and climatological changes in western China.There are three main faults in the southern section of the Longmenshan Fault, i.e., the Gengda-Longdong, Yanjing-Wulong, and Shuangshi-Dachuan faults.The research area encompassed three levels of geomorphic units that from west to east comprised landforms of the Qinghai-Tibetan Plateau, the alpine zone of the southern section of the Longmenshan Fault, and a piedmont alluvial plain area (Southwest of Chengdu Basin).The three main faults are located in the alpine zone in the southern section of the Longmenshan Fault, which crosses the Qingyi River.In this paper, a preliminary analysis of the seismogenic tectonic mechanism of the Lushan earthquake is presented based on data of post-earthquake surface deformational characteristics, aftershocks, focal mechanism solutions, and oil seismic exploration profiles.The results indicate that the Lushan earthquake was a typical folding earthquake, the seismogenic fault of which might have been the buried fault in front of the Longmenshan Mountain.The shortening produced by tectonic compression can be absorbed by folding.Considering the Lushan earthquake as an example, analysts suggest that the southern segment of the Longmenshan frontal region has the kinematic characteristics of an active fold-thrust fault, which is indicated by the enlarged Longmenshan thrusting towards the inner Sichuan Basin.Young, rapidly growing active folding is related not only to the earthquake location and seismic structure but also to the product of continuous earthquakes.In the southern Longmenshan thrust belt, at the same time as the late-Cenozoic faulting, the thrust belt entered the western Sichuan Basin, and the Longquanshan, Xiongpo, and Qiongxi faults were formed. From the Shuangshi fracture eastward to the Longquanshan tectonic belt, there are several rows of fault-related folds over one decollement at the bottom of the middle and lower Triassic Plaster gypsum stratum.Research has shown that the Lushan earthquake was an active fault-folding earthquake that differed from the Wenchuan earthquake.These active folds with stick-slip mechanisms are called "earthquake drapes". Traditionally, it has been considered that fold structures are products of homogeneous stress states, and therefore, earthquakes that occur in active folds are considered non-dangerous.However, the the Lushan folding earthquake caused great damage.In the southern segment of the Longmenshan Fault and its front areas, there is less chance of a great earthquake like the Wenchaun earthquake occurring;however, stress will concentrate in the Sichuan Basin by the detachment fault, and therefore, a watchful eye must be maintained on the faults in that basin, especially the Pujing-Xinjin fault.
    8  Seismic Response of Subway Stations in Soft Soil Area
    LIU Rui LI Yan-tao YANG De-jiani LI Yai ZHANG Hai
    2014, 36(1):16-21.
    [Abstract](2215) [HTML](0) [PDF 11.57 M](6133)
    Abstract:
    With the rapid growth of urban population in recent years,traffic problems have become increasingly serious.Subways offer an effective means for solve such issues.It is worth noting that many underground structures are located in seismically active areas;therefore,seismic design and reliable assessment of underground structures must be conducted. However,many studies on earthquakes have indicated that underground structures are not as safe once believed and can collapse under dynamic loadings such as earthquake loading.Therefore,seismic design and reliable assessment of underground structures has been an important subject;however,research on seismic areas of underground structure in our country is insufficient.Particularly in areas with soft soil,where the situation is more complex,it must be resolved that the underground structures affected by seismic wave are maintained as safe and reliable.Thus,it is necessary to study the analysis theory and design method of underground structures in the soft soil areas in Tianjin by combining by including underground engineering structure characteristics and soil conditions.According to the actual engineering conditions of the typical subway station structure in Tianjin metro line 3,a model for soft soil station structures has been developed by using ANSYS software.The numerical analysis on the dynamic responses of this model under dynamic loading,Tianjin seismic wave,Taft seismic wave,and artificial seismic wave were conducted.Under different loading conditions,the internal force and deformation of main component were analyzed,particularly for of the center pillar. The displacement amplitudes of this station structure under various seismic waves are approximate and can conform to the standard requirements.The maximums of amplitudes were also determined.The internal force under the Tianjin seismic wave is the maximum,and the seismic vulnerability is located in the bottom of middle column.Therefore,these research results can be used for the seismic design of subway station structures in the soft soil area of Tianjin.
    9  Ambient Vibration Test for a Multi story Residential Building and the Vibration Mitigation Measures
    ZHU Bin CHEN Long-zhu DING Li
    2014, 36(2):207-213.
    [Abstract](2747) [HTML](0) [PDF 14.70 M](6103)
    Abstract:
    Brick-concrete buildings have been extensively used in urban and rural areas. However, this type of building, which frequently contains a shallow foundation, demonstrates poor structural integrity and is susceptible to influence from ambient vibrations. This case is especially valid for buildings that were constructed in previous eras. With the development of modern cities and the increasing demand for an enhanced quality of life, environmental interference caused by ambient vibrations has become a critical issue. In areas with soft soil, traffic is a primary source of ambient vibrations. Vehicle vibrations caused by road surface roughness or speed changes can cause vibrations throughout the foundation of a building. Therefore, the vibration of brick-concrete buildings is related not only to the excitation source of the vibration but also the soil characteristics, the foundation conditions, and the structural style. The human response to the vibrations of housing structures is closely correlated with a person's cognitive level and their current physical and psychological conditions. Thus, the potential difference in the reactions among building occupants is significant.Residents in a six story brick concrete building, which is located in an area of Shanghai with soft ground, frequently complained about the intermittent interference from vibrations, particularly residents on the upper two stories. The measures confirmed that the environmental vibrations were generated by heavy vehicles traveling on a road that was located approximately 80 m north of the building. The frequency and amplitude characteristics of the ambient vibrations and the human comfort levels for the different stories of the building were analyzed according to the "standard for allowable vibration of building engineering" (GB50868-2013). To comprehensively evaluate the impact induced by the transversely horizontal vibrations, which influences human comfort levels for the three components of building vibration, a numerical finite element model (FEM) of the building was developed and analyzed. The soil in the model was simulated as a virtual layer. According to the test results and the numerical analyses, two types of technical measures are proposed to reduce unfavorable vibrations. The paper yields the following conclusions: (1) The test results show that heavy-duty vehicles from nearby roads are the main cause of the building vibrations. The vertical component of the vibrations along the structure is almost constant; however, its lateral component is significantly amplified, which primarily influences human comfort.  (2) The measures indicate that the peak acceleration range for the top two stories is 68~70 dB, which is lower than the limit values proposed by the "standard for allowable vibration of building engineering" (GB50868-2013). The rationality of the limit values is discussed to satisfy the demand of human comfort. (3) The structural frequencies and modes are analyzed based on the assumption of a virtual layer of foundation; the simulated results correspond with the measures. The model is a reasonable tool for justifying the effect of vibration reduction measures. (4) To ensure that each story of the multi-story brick-concrete building complies with the previously recommended standard, the peak lateral acceleration should be reduced by approximately 21% and the two proposed technical measures, which are compared, should be applied.
    10  A Study on Hazards of Background Seismicity in Mongolia
    XU Wei-jin GAO Meng-tan
    2014, 36(2):256-260.
    [Abstract](2449) [HTML](0) [PDF 11.06 M](6060)
    Abstract:
    In this article,we calculate seismic hazards from background seismicity by using a spatially smoothed seismicity model for Mongolia,and we obtain a peak ground acceleration (PGA)map for 10% probability of exceedance in 50 y.We propose two seismicity models to calculate seismic hazard maps for 10% probability of exceedance in 50 y.The results show that the seismic activity model and the seismic hazard results,both calculated by using the instrumental seismic catalog (M≥3.0),can reflect contemporary seismic activity levels and seismic hazard levels of the Mongolia area.In addition,the seismic activity model and the seismic hazard results,both of which are calculated by using historical earthquake data (M≥5.0),can quite well represent the seismic hazard level of moderate earthquakes in Mongolia.The two seismic hazard maps both show that the background seismic hazard level is 0.05 g in most areas of Mongolia and that the values of this parameter are high,at 0.1~0.15 g,in other areas,which indicates a background of high seismic hazards in Mongolia.Therefore,it is highly necessary to completely consider the impacts of background seismicity when implementing seismic hazard analysis and seismic hazard mapping for Mongolia. Moreover,we compare two seismicity models.The results also illustrate significant spatial differences between the seismicity models with different minimum complete magnitudes.Therefore,when adopting a spatially smoothed seismicity model to analyze seismic hazards for Mongolia,the weighted average of several different models should be considered to balance the impacts of earthquake occurrences and magnitudes.
    11  Contrastive Experimental Analysis of Characteristics of Single-and Double-row Pile Retaining Structures under Earthquakes
    KANG Jingwen LIU Changqing DENG Yiming WANG Junhong HU Jianzong
    2021, 43(1):195-204. DOI: 10.3969/j.issn.1000-0844.2021.01.195
    [Abstract](259) [HTML](0) [PDF 18.26 M](5852)
    Abstract:
    Pile-sheet retaining wall developed from anti-slide piles could be used as the retaining structure for slope engineering in normal, inundated, and seismic areas. However, the retaining effect of single-or double-row pile-sheet structures in earthquake areas has not been well researched yet. In this study, the practical application of single-and double-row pile-sheet retaining structures is analyzed based on shaking table model test. Soil pressure, soil peak acceleration, and pile displacement are then compared between the two types of piles at different levels of loads. The deformation characteristics of the retained slope under static and dynamic situations are summarized. Results may provide reference for seismic design of pile-sheet retaining structures.
    12  Present-day Crustal Tectonic Deformation Characteristics of China Inferred from Large-scale GPS
    WANG Dong-zhen ZHAO Bin YU Jian-sheng TAN Kai
    2017, 39(3):521-526,544. DOI: 10.3969/j.issn.1013-0019.2017.03.0521
    [Abstract](830) [HTML](0) [PDF 9.24 M](5829)
    Abstract:
    This paper calculated the tectonic deformation characteristics of mainland China and the characteristics of the strain field in the plate space distribution, using more than 260 continuous CMONOC sites and more than 2,000 area stations, that are the latest observation data. According to the intensive, wide range of GPS velocity field shows strong east and west weak trends from the entire velocity field in mainland China. The maximum velocity appeared in the Himalayan region, with a general rate between 35 and 42 mm/a. The Sichuan-Yunnan region dextral shear zone formed by the deformation characteristics of the most striking that the southwest of the minimum rate is between 3 to 9 mm/a, north of maximum rate in 17~23 mm/a. It can be seen from the spatial distribution that the largest strain-stress are mainly from the following regions: Himalayan, Kunlun Mountains, Sichuan and the Yunnan areas of fresh water river fault zone, Tianshan region, and Beijing-Tianjin area. The strain rate of the east coast shows tension that is mainly due to the earthquake in Japan in 2011, although the influence of the region has not completely subsided.
    13  Research on Stress Field Characteristics and Normal Fault SmallEarthquakes of Southeastern Qinghai
    MA Hui-qing YAO Jia-jun MA Jian-xin LI Wei-jie YUAN Fu-quan
    2014, 36(2):372-379.
    [Abstract](2059) [HTML](0) [PDF 13.13 M](5287)
    Abstract:
    Digital seismic waves recorded by Qinghai and Gansu digital networks from January 2008 to August 2013 are used to obtain 92 focal mechanism solutions of 4.9 ≥ML≥2.5 moderate and small earthquakes in southeastern Qinghai by using the methods of P-wave and S-wave maximum amplitude ratio and cut and paste (CAP).The focal mechanism solutions of 3.9≥ML≥2.5 are retrieved by the former method,and the other larger earthquakes are retrieved by the latter.Moreover,analysis of these focal mechanism solutions by means of statistics and systematic cluster analysis reveals that the predominant distributions of the plunge of T- and P-axes are less than 45° and that a few have a high plunge of P.These results indicate that the moderate and small earthquakes in this region are mostly strike-slip and that a certain number of normal fault earthquakes are present;however,small thrust fault earthquakes are rarely seen.The distributions of all of the azimuths of the P-axis are widely scattered,which may be attributed to the fact that the small earthquakes are more happenstance than tectonic earthquakes;however,the distributions of the P-axis azimuth of ML≥3.0 are so regular that it is easy to see the predominant distributions.By using focal mechanism solutions of ML≥3.0 by the grid test method,the mean regional stress field is obtained.The tectonic stress field is primarily represented by NE-SW horizontal compressive stress and NW-SW horizontal tensile stress in the southeastern Qinghai province,and the mean contradictory ratio is 0.32.These results are the same as those in previous research.Furthermore,it is determined that the small normal fault earthquakes are mainly distributed in the eastern part of the Kunlun active fault(Maqin-Maqu)and in the southern part of the Elashan Mountain fault.This paper analyses the mechanism in which small normal fault earthquakes are produced.The southeastern region of Qinghai province still belongs to the inner Qinghai-Tibet block,and the significant differences in elevation could create many small pull-apart basins in some area,such as the eastern part of the Kunlun active fault(Maqin-Maqu)In this region,the small normal fault earthquakes have a certain proportion;therefore,it is suggested that these small normal fault earthquakes may be attributed to gravity slumping because of the high altitude.
    14  A Preliminary Study on Sea-bottom Seismograph Observation in Bohai Sea Area, China
    Feng Deyi Masaji Ichikawa Hideteru Matsumoto Chen Jinlin Wu Guoyou
    1988, 10(3):98-123.
    [Abstract](538) [HTML](0) [PDF 1.66 M](5071)
    Abstract:
    The present paper is a preliminary technical report of the development of ocean-bottom seismograph observation in the Bohai Sea area of China, conducted under financial support from Japan Foundation for Shipbuilding Advancement.
    This report mainly includes the following contents:
    (1) The seismicity and seismic risk assessment of the Bohai Sea area and its vicinity.
    (2) Conditions for sea-bottom seismograph observation in the Bohai Sea area.
    (3) Some results of preliminary study on instruments and techniques for ocean-bottom seismograph observation in the Bohai Sea area.
    (4) The results of experimental modelling researches on the noises of ocean-bottom seismograph observation in the case of shallow sea.
    The historical and present seismicity, the long-term seismic risk, and the relation between large earthquakes which occurred in North China and Japan have been studied. The problems on influences of a strong earthquake and tsunami upon the Bohai Sea navigation routes and near populated areas were discussed. Based on the result the necessity of sea-bottom seismograph observation in the Bohai Sea area was explained.
    The conditions in the Bohai Sea area, introduced in this paper, include the sea-bottom geomorphology, strong waves and meteorological conditions, and the sea navigation and Sea work condition in this sea area.
    Some problems on the instruments and techniques for sea-bottom seismograph observation in the Bohai Sea area were studied and discussed. The preliminary seismograph observation test at the platform of an oil well in the Bohai Sea area was made by the Seismological Bureau of Tianjin, Some new techniques, and successes in noise reduction and signal transmission etc. of sea-bottom seismograph observation advanced by the Meteorological Research Institute of Japan Meteorological Agency (JMA) may be applied to the sea-bottom seismograph observation in the Bohai Sea area. On the basis of these results, a preliminary project to develop the sea-bottom seismograph observation in the Bohai Sea area of China was proposed.
    To study the noises of sea-bottom seismograph observation in the case of shallow sea, a new experimental modelling research has been completed. The noises caused by sea surface source and sea interior source were studied separately. Some important experimental results have been presented.
    The preliminary results, obtained in this paper, may be useful to develop the sea-bottom seismograph observation in the Bohai Sea area and its vicinity.
    15  Preliminary Analysis of Poisson’s Ratio of Shallow Stratum in Tianjin
    GAO Wu-ping CHEN Yu-kun LIU Fang
    2014, 36(1):47-53.
    [Abstract](2437) [HTML](0) [PDF 15.73 M](4811)
    Abstract:
    Poisson’s ratio was characterized for shallow stratum in Tianjin from more than ten borehole logs collected during the period of the“10th five-year-plan”.Associated borehole histograms and laboratory test documents were also compiled for discussion of influencing factors such as the soil moisture,wet density,and porosity ratio. Based on the borehole logging data,we developed a scatter graph of Poisson’s ratio versus depth and fit a curve to the data to represent shear wave velocity and compression wave velocity. The graph shows that the Poisson’s ratio changes by three stages as a function of depth.In the first stage,in the range of 0 to 5 m,it increases rapidly from 0.420 near the surface to 0.495 at a depth of 5 m. Fluctuation in Poisson’s ratio may result from stiff artificial soil cover of different thicknesses that formed for thousands of years of natural processes and human activities near earth surface. In the second stage,in the range of 5 to 20 m,it tends to be stable and the value mostly remains at the level of 0.495,where the stratum mainly consists of soft,saturated,and plastic mucky stratum. In the last stage,in the range of 20 to 200 m,it slowly decreases but shows considerable discreteness and some kind of linear feature is revealed by the fitting correlation coefficient.This decrease in the last stage may be the result of changes in soil lithology,soil sedimentary environment,and soil provenance at greater depths.Many cycles of transgression and regression occurred in Tianjin during the Quaternary period,so the shallow stratum mainly consists of continental facies,marine facies,and marine continental mixed facies.These varying depositional settings caused great changes of lithology,and sedimentary environment in Tianjin. Moreover,the migrations of the Yellow River and Haihe River also influenced the sediment mineral compositions greatly:In summary,the sedimentary environment and evolution mechanism strongly affected Poisson’s ratio of the shallow stratum.Factors influencing Poisson’s ratio in the sediment were examining by selecting two typical boreholes and developing a Poisson’s ratio curve and associated histogram.Additional data including curves of soil moisture,porosity ratio,and wet density were all displayed in one figure together.It shows that the Poisson’s ratio is positively correlated with soil moisture and porosity ratio and negatively correlated with soil wet density.The form of Poisson’s curve is strikingly similar with that of soil moisture and porosity ratio curves and is opposite to that of a wet density curve.By comparing soil moisture,wet density or solid mineral components,it is easy to see that soil moisture has greater effect than wet density.In other words,soil moisture is may be the key parameter influencing the soil’s Poisson ratio.Further,comparing the curve of Poisson’s ratio and related borehole histogram, we can see that the changes of soil lithology at greater depths indicate a change of Poisson’s ratio. However,it is difficult to obtain Poisson’s ratio accurately based on soil lithology because the characteristics of stratum are very complicated and strongly regional.Because of the great number of factors contributing to Poisson’s ratio,further examination of the factors is necessary especially for application in a different geologic setting.
    16  Research on Coulomb Stress Change and Its Application in Seismology
    LIU Fang-bin WANG Ai-guo JI Zhan-bo
    2013, 35(3):647-655.
    [Abstract](3783) [HTML](0) [PDF 883.34 K](4794)
    Abstract:
    An earthquake is a natural phenomenon caused by the sudden breaking and slipping of crustal rocks when stress accumulates to a certain extent.The coseismic dislocations of a major earthquake not only cause stress state changes in adjacent areas,but also change the Coulomb stress.Earthquakes can be triggered by stress changes induced by other earthquakes;the most common examples are aftershocks.Static stress changes can increase or decrease seismicity in the surrounding regions.According to the Coulomb failure criteria,ΔCFSwill affect the failure condition of a fault.WhenΔCFSis positive,it will accelerate the speed of the stress rupture accumulation in the surrounding areas and cause the next earthquake to occur sooner,which is called stress triggering;on the contrary,whenΔCFSis negative,it will defer the accumulation.In recent years,seismic Coulomb stress changes generated by the earthquake and subsequent temporal relationships have been valued by seismologists all over the world.Studies have shown that a minor static Coulomb stress change of 0.1bar may trigger earthquakes and make the regional seismic activity change in the future.Therefore,it is important to explore the relation between Coulomb stress changes and earthquake triggering not only for the study of long-term earthquake prediction and static stress changes triggering subsequent seismic events,but also for future regional seismic hazard analysis.In this paper,the definition and the physical meaning of Coulomb stress are explained,and then the research progress of the applications of Coulomb stress change in seismology are discussed from five aspects:1)The triggering of aftershocks by a strong earthquake.Based on Okada’s([year\〗)elastic half-space dislocation model,the Coulomb stress change images are calculated,and then the spatial distribution of aftershocks are studied.The positive and negative Coulomb stress distribution images are related to the activity of aftershocks directly and affect the regional seismic hazard assessment;2)Triggering of the subsequent earthquakes by strong earthquake.The main way to trigger subsequent earthquakes is stress transfer,in which the stress before an earthquake event can have an effect on aftershocks;3)Stress shadow.A socalled stress shadow means the negative Coulomb stress changes an area with the function of inhibiting or delaying the fault slip and rupture with aΔCFS <0,which will either decrease the seismic activity or prolong the time of the next earthquake;4)The analysis of seismic activity;and 5)The related application of software.Though limited by the inability to accurately predict the time of subsequent earthquakes,Coulomb stress changes have been proven to be very useful in predicting epicenter locations.There is still much research left to be conducted to predict the location and timing of future earthquakes accurately by Coulomb stress changes.
    17  Abnormal variation of dominant azimuth and load/unload response ratio of geoelectric field before the Menyuan MS6.9 earthquake
    XIN Jiancun SUN Junsong YU Chen FANG Wei ZHAO Jie YANG Yihai
    2022, 44(2):415-424. DOI: 10.20000/j.1000-0844.20220124002
    [Abstract](244) [HTML](0) [PDF 11.45 M](4759)
    Abstract:
    An MS6.9 earthquake occurred in Menyuan County, Qinghai Province on January 8, 2022. Based on the observation data of the electric field within 300 km of the epicenter in recent 5 years, 9 observation stations were analyzed and selected. According to the seepage (movement) model of rock fissure water (charge), the dominant azimuth of the geoelectric field was calculated. The loading and unloading response ratio (LURR) value of the geoelectric field was calculated through the LURR calculation method of Coulomb stress-triggered model. The results showed that: (1) The stations with geoelectric field anomalies calculated by the two different methods are consistent in spatial distribution. Among them, the dominant azimuth and LURR of the geoelectric field at Gufeng, Huangyangchuan, Sitan, and Lanzhou stations showed abnormal changes before the earthquake, while those at Shandan and other stations did not show obvious anomalies. (2) The time series changes of Lanzhou and Sitan stations using the two calculation methods are consistent, showing quasi synchronization. Combined with the focal mechanism solution, the correlation between the dominant azimuth of abnormal stations and the principal compressive stress P-axis direction was analyzed, and the results basically conform to the petrophysics theory. The two calculation methods of geoelectric field dominant azimuth and LURR value are related in mechanism, and a comprehensive analysis of their abnormal evolution characteristics may help to further understand the physical process of earthquake preparation.
    18  Static Coulomb Stress Changes and Triggering Interaction amongStrong Earthquakes in the Eastern Segment of North Qilian Mountain
    LIU Fang-bin WANG Ai-guo YUAN Dao-yang
    2014, 36(2):360-371,379.
    [Abstract](2162) [HTML](0) [PDF 18.64 M](4687)
    Abstract:
    Earthquakes are a natural phenomenon caused by sudden rupture and slippage of crustal rocks after stress reaches the point of rock failure.The coseismic dislocations of a major earthquake not only cause the stress state changes in adjacent areas, but also change the Coulomb stress.Research shows that a very small static Coulomb stress changes can trigger seismic activity (i.e.,earthquakes).Therefore,it is important to explore the relationship between Coulomb stress changes and earthquake triggering.First of all,we conducted primary research for strong earthquakes by Coulomb 3.3 on the static Coulomb stress changes with the primary study region being the eastern margin of north Qilian Mountain and adjacent area from 1561 to present. Meanwhile,we used cumulative and individual methods to calculate the Coulomb stress changes between foreshocks and aftershocks.Our results show that the next events,except Menyuan earthquake in 1986,would occur in the stress triggering area.The value of changes is between 0.1 bar and 4.066 bar,and trigger rate reached a high of 87.5%.In other instances,when the source fault is closed to receive fault,the former event can trigger the next.Secondly,according to the relationship between Coulomb stress changes and stress accumulation rate,we calculated the stress accumulated before the next event,not considering foreshocks.Finally,we assessed the future strong seismic hazard for this area according to earthquake migration and stress triggering.We used Okada’s method to calculate the change of static Coulomb failure stress among faults for the study area.In 1920 and 1927,two magnitude 8 earthquakes occurred,prior to the earthquakes the Coulomb stress significantly increased in the eastern side of Yunwu Mountain and western part of Jingtai-Tianzhu-Gulang.This suggests that an increase of Coulomb stress may forecast a triggered earthquake.It is noteworthy that the Tianzhu 6.2 earthquake occurred in the western part of the danger zone after two prior earthquakes.Considering the Coulomb stress triggering and the history of seismic activity,future earthquake risk should be strong in this area as well as the eastern side of Yunwu Mountain and Jinqianghe fault zone in the western part of Jingtai-Tianzhu-Gulang.This paper attempts to combine Coulomb stress adjustment and fracture characteristics with seismic activity.Results are consistent with other approaches in the Northeastern Qilian area,indicating that Coulomb stress changes can be used as an indicator of oncoming seismic activity. Application of this method will require further improvement to assess future seismic risk.
    19  Improvement of Newmark model and prediction of seismiclandslide risk based on rainfall infiltration
    LEI Zhen LI Linrui LONG Jiaofeng CHEN Jingnan YANG Yang
    2022, 44(3):527-534. DOI: 10.20000/j.1000G0844.20210424001
    [Abstract](226) [HTML](0) [PDF 13.46 M](4612)
    Abstract:
    As a kind of destructive geological disaster, earthquake and rainfall are two key factors to induce landslide. From the perspective of earthquakes occurred during rainfall, this paper improved the Newmark model based on the Green-Ampt rainfall infiltration model, and derived the slope safety factor FS under the coupled action of two factors. The seismic landslide risk prediction and the influence of slope and infiltration depth factor on the displacement in an area of Ludian County, Yunnan Province were carried out under three conditions, i.e., no rainfall, rainfall with ponding, and rainfall without ponding. Then the Newmark cumulative displacement distribution and risk zoning in the study area were obtained through a comparison between the three cases. The results show that: compared with the case without rainfall, the area proportions of high-risk area of earthquake landslide in the latter two cases increase from 1% to 9% and 12% with the increase of rainfall time, respectively, and the area proportions of low-risk landslides reduce from 51% to 35% and 33%, respectively. With the increase of slope and infiltration depth, the displacement and the risk of landslide also increase. The improved Newmark model fully considers the promotion effect of rainfall on the occurrence of earthquake landslide and can better reflect the relative landslide risk of each site in the study area, thus having certain guiding significance for the landslide risk prediction.
    20  Seismic disaster characteristics of the surface rupture of Menyuan MS6.9 earthquake in 2022
    XUE Shanyu XIE Hong YUAN Daoyang LI Zhimin SU Ruihuan WEN Yameng
    2022, 44(2):458-467. DOI: 10.20000/j.1000-0844.20220215001
    [Abstract](223) [HTML](55) [PDF 43.15 M](4508)
    Abstract:
    An MS6.9 earthquake occurred in Menyuan County, Qinghai Province, on January 8th, 2022. The epicenter is located at the tectonic transform region of the Lenglongling fault and Tuoleshan fault along the Qilian-Haiyuan fault zone (37.77°N, 101.26°E) of the northeastern Tibetan Plateau. This paper introduced the typical earthquake disasters and characteristics of co-seismic surface ruptures. The results of field investigation after the earthquake showed that the main shock ruptured two obvious surface zones with the length up to 26 km, and the overall strike was directed along NWW. The rupture property is mainly sinistral strike-slip with thrusting slip by dipping to NE. The typical rupture landforms caused by fault dislocation mainly exhibit as en echelon combined of separation fractures, tension shear fractures, seismic bulges, and fault scarps. The surface rupture at the segment from Daohe to Liuhuanggou is the strongest, with large scale and good continuity, causing the most significant earthquake damage, and the scale of the surface rupture gradually decreases to the east and west ends. The main shock ruptured the ice surface of the Daohe and the Liuhuanggou, with typical deformation of a series of fractures and bulges as well as small scale of geological disasters, such as slope collapse and rolling stones formed along the river bank. Based on the comprehensive analysis of post-earthquake damage and surface rupture distribution, the region between Liuhuanggou and Daohe can be determined as the macro epicenter of the Menyuan MS6.9 earthquake.

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