Table of Content

20 March 2015, Volume 37 Issue 1

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Research Paper

HOLOCENE PALEOSEISMOLOGY OF THE MIDDLE AND SOUTH SEGMENTS OF THE NORTH ZHONGTIAOSHAN FAULT ZONE, SHANXI

WANG Yi-ran, LI You-li, YAN Dong-dong, LÜ|Sheng-hua, SI Su-pei

2015, 37(1):  1-12.  DOI: 10.3969/j.issn.0253-4967.2015.01

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Shanxi Graben System(SGS)is a NNE-trending tectonic system located in the eastern and southern edges of Ordos Block and between Yinshan tectonic zone and Qinling tectonic zone. Composited with a series of NNE-, NE-trending graben basins, SGS is an important seismic zone in North China. Yuncheng Basin locates in the southern end of SGS, and is the deepest basin of the system. It is a half-graben with a shallow northern part and a deep southern part. As the south and east boundary of Yuncheng Basin, the North Zhongtiaoshan Fault Zone(NZFZ)played a key role in the development of the topology and tectonics setting of Yuncheng Basin. NZFZ is a normal fault zone, NE- to NEE-trending and 130km long. This fault zone can be divided into 3 segments: the south, the middle, and the north segment, and there are no historical M≥7, but only 3 M=6 have taken place. Previous study suggests that the tectonic activity in this area is low and the earthquake interval is relatively long; seismic activity in the north and south segments is the weakest: the earthquake recurrence interval in the north segment is >50ka and it has not been active for >50ka, the interval of the south segment is ～12ka and its last movement is about 7ka ago. Our work tells a different story: based on thorough fieldwork, satellite imagery analysis, and trench investigation, we found that there have been multiple earthquake events taking place in all of the 3 segments during Holocene. This study includes two trenches that locate in the south and middle segments of NZFZ, each of them renders 3 paleo-earthquake events: a 10 570～8 010a BP event, an around 6 670～6 550a BP event, and a 2 110～320a BP event in TC-1; a 7 930～11 300a BP event, a 4 800～6 010a BP event, and a 2 680a BP event in TC-2. Through comparing these events with other 2 trenches in the middle segment of NZFZ(a XTC trench that renders a 19 980a BP event and a 8 570～7 780a BP event with >2.2m vertical displacement; a XLC trench that renders a 8 980～7 340a BP event with ～1.8m vertical displacement), we identified: a 8 570～8 010a BP event which broke through both middle and south segment of NZFZ with a largest known vertical displacement of >2.2m and 95km surface rupture, a 4 800～6 010a BP and a 2 110～320a BP event with 7～30km surface ruptures and largest known vertical displacements of 0.7m and 1.2m, respectively. The recurrence interval of these events is about 3 500a. Calculated with empirical formula of normal fault in North China, the magnitude of the 8 570～8 010a BP earthquake should be at least 8.0, and the magnitudes of the 4 800～6 010a BP and 2 110～320a BP events are both ≈7.0. These findings suggest that, as a Holocene active fault zone which made up with multiple faults, the middle and south segments of NZFZ have shown strong seismic activity and have triggered multiple earthquake events during the Holocene. Contrary to previous study, the activity of south segment might not be the weakest, and it is possible that the south segment is more seismic active than the middle segment.

THE GEOMETRIC CHARACTERISTICS AND TECTONIC DEFORMATION MECHANISM OF INVERSION STRUCTURES IN NORTHERN SONGLIAO BASIN AND THEIR SEISMO- GEOLOGICAL SIGNIFICANCE: A CASE FROM DA'AN-DEDU FAULT

YU Zhong-yuan, MIN Wei, WEI Qing-hai, ZHAO Bin, MA Yan-chun

2015, 37(1):  13-32.  DOI: 10.3969/j.issn.0253-4967.2015.02

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Meso-Cenozoic inversion structures developed in northern Songliao Basin represent the third tectonic evolution stage of the basin following the rifting stage and the depression stage of thermal cooling. The paper collects systematically 30 regional seismic reflection profiles laid out to cover the Da'an-Dedu Fault and its adjacent14 3-D seismic projects, and on this basis, accurately tracks and interprets a total of seven seismic reflection horizons of T₀₆, T₁, T₁₁, T₂, T₃, T₄ and T₅. At the same time, geological age of key reflection horizons is re-determined based on 52 boreholes data, the distribution characteristics and pattern of different periods are analyzed, and the geometric configuration and deformation characteristics and mechanisms of Meso-Cenozoic inversion structures developed in the northern Songliao Basin are discussed in particular. The study concludes that the inversion structure is the main deformation model of Da'an-Dedu Fault in the late Mesozoic and early Cenozoic, which coincides to the geometric shape and deformation mechanism of the "thrust-fold" structures.
Based on the experimental shallow seismic exploration results of section line No.10 and No.11 in the research area, we find that the small faults, which are widely developed on the top of anticline and disconnect the reflection interface of T₀₆, represent the latest activity of Da'an-Dedu Fault, and the deformed layer formed in mid-Pleistocene may indicate the latest activity time of Da'an-Dedu Fault, which is mid-Pleistocene.
In addition, according to the study on shallow seismic exploration results, modern earthquake activity and focal mechanism solutions, we strongly believe that the latest activity of "thrust-fold" inversion structures of the Da'an-Dedu Fault has an obvious effect on the near surface reflection layers, which offsets the lower-middle Pliocene and controls the moderate-strong earthquake activity of the research area and the adjacent areas. These structures should be regarded as one of the most important and typical seismogenic structures in the interior of Songliao Basin and in the Northeast China region. Based on this, we can understand scientifically the deformation process of the interior of Songliao tectonic block through the research of structural deformation of Da'an-Dedu Fault developed inside the block after the late Cenozoic era, and the future earthquake activities controlled by the Da'an-Dedu Fault.

CONTENTS

RESEARCH ON SOURCE RUPTURE PROCESS OF M_S6.6 EARTHQUAKE OF JUNE 30, 2012, IN THE BORDER AREA OF XINYUAN AND HEJING COUNTY, XINJIANG

WANG Qiong, JI Zhan-bo, ZHAO Cui-ping, WANG Hai-tao, NIE Xiao-hong, LI Zhi-hai

2015, 37(1):  33-43.  DOI: 10.3969/j.issn.0253-4967.2015.03

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By using the digital teleseismic seismograms recorded by the global network(IRIS), we studied the rupture process of the M_S6.6 earthquake of Jun.30, 2012, in the border area between Xinyuan and Hejing Counties. In addition, based on the tectonic feature of the eastern segment of Kashi River Fault and regional stress state in the central segment of Tienshan region before the M_S6.6 earthquake, we discussed the possible seismogenic mechanism of the M_S6.6 earthquake. The result indicates that the lasting time of the M_S6.6 earthquake's rupture process is about 35s and the main energy release appears in the early 16s. The total rupture process consists of two sub-events, strength of the first rupture is higher than the second one. The rupture process is relatively simple and has bilateral rupture characteristics; the maximum slip amount is 45.0cm. The initial rupture point of the M_S6.6 earthquake locates on the verge of high slip amount area, the dislocation mode of main rupture area in the depth is reverse and right-lateral strike-slip type, and the one in the shallow is mainly right-lateral strike-slip type; M_S≥3.0 aftershocks are mainly distributed on the verge of high slip amount area or high gradient area of slip amount change; the feature of focal mechanisms of those aftershocks occurring within the short time after the mainshock is consistent with the slip vector distribution on the fault surface; The mid-strong earthquakes with reverse fault type taking place before the M_S6.6 earthquake had increased obviously in Xinjiang area, especially in the central segment of Tienshan area, and the azimuth and plunge of P axis of these earthquakes are consistent with the regional stress field. This suggests to a certain extent that the central segment of Tienshan area is more under the control of the tectonic stress field, which may promote the right-lateral strike-slip motion on the right-lateral wrench faulting eastern segment of the Kashi River Fault. And this may be the generating mechanism of the M_S6.6 earthquake.

QUANTITATIVE ANALYSIS OF SITE EFFECT ON SEISMIC GROUND MOTION PEAK ACCELERATION IN SHANDONG AREA

WANG Hong-wei, FENG Zhi-jun, LIU Xi-qiang, CHEN Shi-jun

2015, 37(1):  44-52.  DOI: 10.3969/j.issn.0253-4967.2015.04

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Site effect on seismic peak ground acceleration is widely considered in such fields as seismic ground motion parameter zonation, earthquake early warning and quick assessment of seismic damage losses. Documents on seismic safety evaluation of engineering sites, including site type, overburden thickness, shear wave velocity, calculated seismic peak ground accelerations on both rock and soil surface for 2 630 engineering sites in Shandong Province have been collected in this paper. On the basis of these data, site amplification factor and site index for each site have been calculated. The site index is an integrated index to characterize the site condition and is computed from the shear wave velocity, density and thickness of the cover layers. Its values may change from 0 to 1. With the method of regression analysis, the relations among the site amplification factor, peak acceleration of bedrock and the site index are studied. A quantitative statistical relation between site amplification factor and other parameters is achieved. The site amplification factor in this paper is expressed as functions of peak acceleration of bedrock and site index, thus, its value may change continuously with the changing of the other parameters. The statistical relation for the site amplification factor in this paper is acquired on the basis of numerous, widely distributed data in Shandong Province, especially, site conditions can be represented in detail with site index. Therefore, the acquired statistical relation could characterize the changing scenarios of the site amplification factor with both the peak acceleration of the rock surface and the site condition details. The results show that the site amplification factor is negatively linearly related with the rock surface seismic peak ground acceleration and nonlinearly related with the site index.

SIMULATION STUDY OF THE INFLUENCE OF MEDIUM VISCOSITY ON AFTERSHOCK ACTIVITIES

QU Jun-hao, JIANG Hai-kun, SONG Jin, LI Jin

2015, 37(1):  53-67.  DOI: 10.3969/j.issn.0253-4967.2015.05

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After a large earthquake, more seismic activities are observed in the focal region and its adjacent area. The obvious increased earthquakes are called the aftershocks. Generally speaking, aftershock sequence gradually weakens and sometimes has ups and downs. The time when the aftershock activity begins to be confused with background seismic activity is known as the aftershock activity duration. Aftershock sequence is one of the enduring research fields in seismology. Aftershocks accord with two important statistical relationships, one is the G-R relationship describing the relation between the magnitude and frequency, the other is the modified Omori formula describing the characteristics of aftershock decay with time. On this basis, a number of studies from different angles explain the mechanism of aftershock activity. From the perspective of the medium heterogeneity, it is universally accepted that aftershock is a result of further rupture of residual asperities. From the perspective of stress, these models, e.g. rate-state dependence, subcritical crack growth, creep or afterslip and so on, think that the fault stress change caused by mainshock is the main cause for aftershock. But other researchers, by studying real aftershock observations, think that the fault stress change caused by mainshock is not the main cause or has very weak control over the aftershocks. Pore pressure diffusion caused by mainshock fault slip is also considered as an important incentive for aftershocks. There is a relationship between the frequency of aftershocks and pore pressure changes. Dry rock pressurized in physical experiment can produce acoustic emission sequence similar to mainshock-aftershock sequence type earthquake. Though fluid plays an important role in aftershock activities, it is not the essential element for aftershock. Overall, there is no single model which can fully explain the phenomenon of aftershock activity.
Assuming the rupture of the residual asperities inside the mainshock rupture plane randomly leads to the aftershocks, the size of the residual asperities conforms to fractal distribution, and the rupture or instability strength of the residual asperities accords with the lognormal distribution. Taking the postseismic stress relaxation as the mechanical load, the loading stress attenuates according to negative exponential law. Taking the Coulomb failure as the judgment criterion of the instability, combining the mechanical interactions among the residual asperities, the artificial aftershock sequence, including occurring time, location and magnitude, is simulated under different conditions. The agreement between output and the actual statistical characteristics of aftershock activities is detected by G-R relationship and modified Omori formula as a basis for further adjustments to the model parameters. On this basis, the influences of the medium viscosity properties on aftershock activities have been discussed.
The results show that viscosity coefficient of rheological properties of the lower part of the lithosphere has an important effect on the duration of aftershock activity. The viscosity coefficient of the lower part of the lithosphere controls the duration of the aftershock activity, the lower the viscosity coefficient, the sooner the stress relaxation of the lower lithosphere, and the faster the loading rate to the upper part of the lithosphere, the shorter the duration of the aftershock activity. On the contrary, the higher the viscosity coefficient, the slower the loading rate to the upper part of the lithosphere, and the longer the duration of the aftershock activity. This simulation conclusion is consistent with the observed result. The viscosity coefficient as one of the important lithosphere physical parameters controls the decay rate of aftershock activity. Under this model conditions, p value, the decay rate of modified Omori law, changes with the viscosity coefficients in a negative exponential function. The relationship that the viscosity coefficient is lower and the decay of aftershock sequence is faster provides a reference for the study of the main influence factors of aftershock decay. The relationship corresponds to the observation that the decay rate of the aftershock sequence shows a good positive correlation. The b value of the G-R relationship of aftershock sequence characterizes the ratio relationship of large to small earthquakes. The modeling studies suggest that the G-R relationship of the aftershock sequence is irrelevant with the viscosity coefficient, but mainly controlled by the size distribution of the residual asperities. In another word, it is mostly correlative to the heterogeneity of tectonics and medium.

QUANTITIVE INFLUENCE OF WEAK PHASE LAYER ON STRENGTH OF FAULT GOUGE

LU Zhen, HE Chang-rong

2015, 37(1):  68-80.  DOI: 10.3969/j.issn.0253-4967.2015.06

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We investigated frictional sliding behavior of mixture gouges of quartz with various weight proportions of biotite and their structured equivalents with same weight proportions of biotite as layers embedded in quartz gouge. Our experiments were performed under effective confining pressure of 200MPa, pore pressure of 30MPa, temperature of 100℃ and the shear displacement rate of 1.22μm/s. The results show that for structured gouges with biotite layers embedded in quartz gouge as a weak structure, the strength has a power law decreasing trend with increasing weight proportions of biotite. The fault gouges can be weakened significantly by as little as 5wt% biotite, and 30wt% biotite corresponds to a beginning point of less sensitive strength change in response to increasing biotite proportion. On the other hand, the strength of mixed gouges shows a linear decreasing trend with increasing biotite proportion. Microstructures of deformed samples show that in mixed gouges, biotite and quartz are both sheared and grain size extremely reduced, and their contributions to overall strength have a close relation with their respective contents. However, in structured gouges, the shear deformation mainly occurred in the weak biotite layers with no shears crossing the quartz gouge. These results confirm the importance of the weak fabric in its effect on frictional strength. If the weak minerals form foliations and interconnected arrangements, it will lead to weakness of fault zones.

THE BRITTLE-PLASTIC TRANSITION IN EXPERIMENTALLY DEFORMED HORNBLENDE UNDER HIGH TEMPERATURE AND HIGH PRESSURE

SONG Juan, ZHOU Yong-sheng, ZHONG Ke, LIU Gui, LIU Zhao-xing

2015, 37(1):  81-93.  DOI: 10.3969/j.issn.0253-4967.2015.07

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In this paper, rheological experiments are carried out on natural hornblende under high temperature and high pressure. We used polarizing microscope and scanning electron microscope to analyze the experimental samples' microstructure, investigating the mechanisms of hornblende under the condition of different temperature, pressure and strain rate. The experimental results reveal the following features of the stress-strain curves of deformed samples: As the temperature increases, the stress-strain curve of the samples changes gradually from strengthening to yielding and weakening, sample strength reduces significantly; with the increase of confining pressure, the sample strength increases; and with the decrease of strain rate, the sample strength reduces, and it significantly reduces in the samples with the compression direction heterotropic to foliation. Plenty of transgranular fractures as well as a small amount of cataclastic deformation occur in hornblendite at temperature of 500℃, and the deformed sample is dominated by brittle deformation. At temperature of 600℃, porphyroclast system consisting of residual plaques and cataclastic series grows in the samples, wavy extinction appears in part of hornblende crystals, the deformation is characterized mainly by cataclastic deformation with ductile deformation, locally. At temperature of 700℃, the deformation is mainly dominated by intragranular kink, and dehydration and fine-grained microcrystalline appear locally, containing microcracks. The deformation of the sample is in the brittle-plastic transition phase; At temperature of 800℃, almost no obvious brittle deformation is observed in the deformed samples, the samples are dominated by dynamic recrystallization, and dehydration appears. Therefore, at the temperature conditions of 500℃, 600℃, 700℃ and 800℃, the deformation of hornblende is characterized by brittle fracture, cataclastic flow, crystal kink, dynamic recrystallization and dehydration, which shows the deformation mechanism varying from brittle to brittle-ductile, and to ductile deformation.

DELINEATING POTENTIAL RISK AREAS OF VOLCANIC ERUPTION IN TENGCHONG REGION BASED ON ARCGIS MODELING ANALYSIS

GONG Li-wen, LI Ni, FAN Qi-cheng, ZHANG Liu-yi, CAO Yuan-yuan, CHEN Li-juan

2015, 37(1):  94-109.  DOI: 10.3969/j.issn.0253-4967.2015.08

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Abundant magma sources, open magma conduit and high magma temperature are the three major factors that affect volcanic eruption. Meanwhile, topographic condition, lithology of overlying strata and condition of historical volcanic eruptions also restrict the formation of a volcano. Combining the previous researches on Tengchong volcanic cluster and considering the depth of magma chamber, location of fault and magma temperature as the main factors, as well as lithology, strain, stress, volcano distribution density as accessory factors, we set up a model with ArcGis Modelbuilder. Through a series of modeling, analyzing and weighted stack calculating on the basis of classifying all the factors concerned as nine grades according to the risk degree of each factor to the eruption, we finally work out a grading map indicating the potential risk of volcanic eruption in Tengchong region. According to the different colors shown in the map, we can see clearly that the most dangerous area is near the Hehua-Ma'anshan-Shangzhuang Fault. In addition, Qushi-Jiaoshan-Dapotou, Tengchong-Qingshui-Hehua and Wuhe-Tuantian faults are also worth paying attention to. The article provides the theoretical basis for forecast of future volcanic eruption hazard.

THE LATEST SURFACE RUPTURE EVENTS ON THE MAJOR ACTIVE FAULTS AND GREAT HISTORICAL EARTHQUAKES IN HETAO FAULT-DEPRESSION ZONE

LI Yan-bao, RAN Yong-kang, CHEN Li-chun, WU Fu-yao, LEI Sheng-xue

2015, 37(1):  110-125.  DOI: 10.3969/j.issn.0253-4967.2015.09

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Hetao fault-depression zone, the largest one of 4 fault-depression zones around the Ordos block, is characterized by intense tectonic activities. According to historical records, 2 large earthquakes, occurring in 849AD and 7BC respectively, were recognized to be located at this zone. However, there is still some dispute about the seismogenic structure of the 849AD earthquake, and there is no tangible geological evidence to support the view that the 7BC event occurred in Hetao fault-depression. In this paper, based on the image interpretation(from Google Earth), field investigation, trench excavation, and ¹⁴C and single grain OSL dating, we analyzed the tectonic landform and paleoseismic events on the Daqingshan piedmont fault, Wulashan piedmont fault and Langshan piedmont fault in the Hetao fault-depression zone. Furthermore, a comparative study of the latest rupture events on the 3 active faults was carried out. In order to lower the uncertainty of paleoseismic event dating, several effective measures, such as sampling according to the stratigraphic sequence, collecting multi samples in important strata, were adopted. Combining the previous achievements, the seismogenic structures of the 849AD earthquake and the 7BC earthquake were discussed. The results support that the Daqingshan piedmont fault is the seismogenic structure of the 849AD earthquake, and the latest surface rupture event of the Langshan piedmont fault may be related to the 7BC earthquake.

EFFECT OF FAULT STEPS ON PROPAGATION AND TERMINATION BEHAVIOR OF STRIKE-SLIP EARTHQUAKE SURFACE RUPTURES

LI Zheng-fang, XIAO Hai-bo, ZHOU Ben-gang

2015, 37(1):  126-138.  DOI: 10.3969/j.issn.0253-4967.2015.10

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Based on the data of 28 strike-slip fault steps and the surface rupture traces at home and abroad, the paper analyzes the relations between the step type, size and earthquake rupture by using the mathematical statistical method, and obtains the barriers yardsticks that stop rupture propagation of earthquakes with different magnitude intervals by using the method of statistical analysis. The results show that the limiting dimensions of strike-slip fault step are different for different magnitude intervals. The limiting dimension of step width is about 3km for magnitude between 6.5 to 6.9, 4km for magnitude between 7.0 to 7.5, 6km for magnitude between 7.5 to 8.0, and about 8km for magnitude between 8.0 to 8.5. The result implies that releasing steps should be easier to rupture through than restraining steps. The limiting dimension of step width determined in this paper is basis for rupture segmentation and is of practical importance to seismic hazard analysis.

THE RESEARCH ON ACTIVITY BEHAVIOR OF CHONGGANGSHAN-WANGQIAN SEGMENT OF TAN-LU FAULT ZONE SINCE LATE PLEISTOCENE

SHEN Xiao-qi, YAO Da-quan, ZHENG Hai-gang, YANG Yuan-yuan, ZHAI Hong-tao, ZHENG Ying-ping, WANG Jun, ZHAO Peng, XIAO Wei-peng

2015, 37(1):  139-148.  DOI: 10.3969/j.issn.0253-4967.2015.11

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The relationship between the latest displacement of active fault and seismic events is an important basic problem. Taking the Sihong segment of the Tan-Lu Fault as the main research area, we selected the Chonggangshan-Wangqian segment as the study section by Google map terrain analysis, excavated 6 large scale trenches, identified and cataloged the Late Quaternary deformation events and prehistoric earthquake relics, analyzed activity stages and behavior of the section. The result indicates that the Chonggangshan-Wangqian segment of the Tan-Lu Fault has undergone intense compression and thrust movement since the Late Quaternary. It is displayed that the brick-red sandstone of Upper Cretaceous thrust at high angle over the yellowish-brown clay of the Late Pleistocene in Chonggangshan segment, while the white or yellow sandstone of Oligocene in Wangqian segment thrust westward over the Late Quaternary sediments, accompanied by rifting activity. By the ¹⁴C dating, we get two paleoearthquake events, and their age is (11 755±45)～(10 525±45)a BP and about(10 135± 50)a BP, respectively. This suggests that the Chonggangshan-Wangqian segment of Tan-Lu Fault zone has undergone strong thrust movement since the late Pleistocene, and this activity had continued to the early Holocene.

POSSIBLE THERMAL BRIGHTNESS TEMPERATURE ANOMALIES ASSOCIATED WITH THE LUSHAN(CHINA) M7.0 EARTHQUAKE ON 20 APRIL 2013

XIE Tao, ZHENG Xiao-dong, KANG Chun-li, MA Wei-yu, LU Jun

2015, 37(1):  149-161.  DOI: 10.3969/j.issn.0253-4967.2015.12

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Lushan M7.0 earthquake occurred in Lushan County, Ya'an City, Sichuan Province of China, on 20 April 2013, causing 196 deaths, 23 people missing and more than 12 thousands of people injured. In order to analyze the possible seismic brightness temperature anomalies which might be associated with Lushan earthquake, daily brightness temperature data are collected from Chinese geostationary meteorological satellite FY-2E, for the period from 20 April 2011 to 19 April 2013 and the geographical extent of 25°～35°N latitude and 98°～108°E longitude. Continuous wavelet transform method is used to analyze the power spectrum of brightness temperature data, for its good resolution both in time and frequency domains. The results show that relative wavelet power spectrum(RWPS)anomalies appeared since 15 January 2013 and still lasted on 19 April. Anomalies firstly appeared at the middle part of Longmenshan Fault zone. Then, they gradually spread towards southwestern part of Longmenshan Fault. Anomalies also appeared along the Xianshuihe Fault since about 1 March. Eventually, anomalies gathered at the intersection zone of Longmenshan and Xianshuihe Faults. The anomalous area and RWPS amplitude increased since the appearance of anomalies and reached maximum in late March. Anomalies attenuated with earthquake approaching, and eventually the earthquake occurred at the southeastern edge of anomalous area. Lushan earthquake was the only obvious geological event within the anomalous area during the time period, so the anomalous changes of RWPS are possibly associated to the earthquake.

ELECTRICAL STRUCTURE OF THE CRUST BENEATH THE CENTRAL-NORTHERN SEGMENT OF THE LONGMEN SHAN FAULT ZONE AND ITS GEODYNAMIC MODEL

PENG Miao, JIANG Mei, TAN Han-dong, LI Qing-qing, ZHANG Li-shu, XU Le-hong, ZHANG Fu-bin, TANG Lu-te

2015, 37(1):  162-175.  DOI: 10.3969/j.issn.0253-4967.2015.13

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Magnetotelluric (MT) exploration with station spacing of about 3km in 2012 was undertaken across the Longmenshan region from Maoxian to Mianzhu in Sichuan, China to investigate the crustal structure of this region, and a 2-D resistivity model with 70km in length and 50km in depth is acquired. By jointly interpreting and comparing with a seismic reflection profile at the same sites, we obtained the crustal structure of central-northern segment of Longmenshan Fault zone as follows: 1)Sichuan Basin has a very stable electrical structure, beneath which is the rugged Yangtze middle and lower crust; the three main faults in Longmenshan show low resistivity or middle to low-resistivity, and its structural features are tilted to the northwest by a steep to gentle slowing downward, with the steepest part greater than 60° at the shallow, and the gentlest part less than 30° at the deep. 2)the upper crust beneath Longmenshan Fault zone is thrusting over the rigid basement of Yangtze block, forming not only the three main faults, but also numerous blind secondary faults as well due to frequent seismicity; there is an is obvious electrical gradient zone beneath the Yingxiu-Beichuan Fault, which is steep and inclined to the northwest, and where Wenchuan earthquake aftershocks are concentrated. A large concealed basement fault may be developed on the footwall of the Anxian-Guanxian Fault, which might be the seismogenic fault of Wenchuan earthquake. Seismic energy transmitted along the secondary faults, leading to severe damage to Yingxiu-Beichuan Fault. 3)The subduction of the mid and lower crust beneath the eastern margin of Tibetan plateau causes the high-resistivity mid-lower crust of the Yangtze block embedded in the mid and lower crust between the Longmenshan thrust zone and the eastern margin of Tibetan, forming a "crocodile mouth structural frame". The uplift of Longmenshan is a result of brittle deformation under overthrusting in the upper crust and high-conductivity crustal mass flow in the middle and lower crust. We propose a "conveyor-like" geodynamic model that the crustal mass flow is not only blocked by rigid Yangtze block, but also flows downward into the upper mantle beneath Yangtze block. The "conveyer" drives the brittle thrusting deformation in the hanging wall of the fault zone.

GEOMETRIC FEATURES AND LATEST ACTIVITIES OF THE NORTH SEGMENT OF THE ANQIU-JUXIAN FAULT

WANG Zhi-cai, WANG Dong-lei, XU Hong-tai, GE Fu-gang, YANG Chuan-cheng, LI Jian-hu

2015, 37(1):  176-191.  DOI: 10.3969/j.issn.0253-4967.2015.14

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The Anqiu-Juxian Fault is a major branch fault and an active prominent fault of the Yishu Fault belt. The spatial distribution, geometric features and the latest activities of the Anqiu-Juxian Fault are studied by field survey and mapping in this study. The northern segment of the Anqiu-Juxian Fault between Juxian and Changyi can be divided into four segments, namely from north to south, the Changyi-Nanliu segment, the Anqiu-Mengtuan segment, the Qingfengling segment and the Mengyan segment. These segments are left-step en echelon arranged, and each of the fault segments consists of right-step en echelon arranged sub-segments. The Changyi-Nanliu segment is about 31km long and composed of 4 sub-segments in right-step en-echelon arrangement, namely, Wenshan sub-segment, Zhuli sub-segment, Shuangguan-Meicun sub-segment and Nanliu sub-segment, from north to south. The length of these sub-segments is 5km, 7km, 10km and 9km, respectively. The width of the stepover between them is about 2～3km. The Changyi-Nanliu segment generally strikes～15°, and the fault plane dips both west and east with dip angle 70°～80°. This segment offsets the widely distributed eolian yellow or orange fine sand and silt that were formed in the latest late Pleistocene, and it also offsets the mid-Holocene grey-yellow clay. The latest active age of the Changyi-Nanliu segment is the middle and late Holocene. This segment is characterized by right-lateral strike-slip motion with thrust and normal fault component, and the normal faulting activity is usually younger than the reverse faulting activity. The Anqiu-Mengtuan segment is about 50km long and exposes～21km. It strikes 15°～20°with the major fault plane dipping NWW with dip angle 70°～80°. This fault segment is characterized by right-lateral strike-slip motion with west-to-east thrust component. The segment can also be divided into two sub-segments, namely, the 13km long Anqiu-Guangong sub-segment and the 8km long Anshang-Mengtuan sub-segment, as in right-step en echelon arrangement, with a stepover of about 3km in width. The youngest offset stratum along the Anqiu-Mengtuan segment is the late Pleistocene, so, its latest active age is the late Paleocene and early Holocene. The Qingfengling segment is about 32km long, striking 15°～20°, dipping mainly southeast and partly west with dip angles more than 60° generally. This segment is characterized by right-lateral strike-slip motion with minor thrust component. It is composed of 4 sub-segments, which are the Xiaodianzi-Henhushan sub-segment, Kushan-Chezhuang, Maobu and Wangtaizi sub-segment, respectively from north to south. The length of these sub-segments is 6km, 8km, 14km and 4km, respectively. The former three sub-segments are aligned right-laterally. The Qingfengling segment offsets the upper late Pleistocene and the early Holocene strata; its latest active age is the early Holocene. The Mengyan segment exposes about 20km, striking 20° and dipping northwest with dip angle ～70°. It is also characterized by right-lateral strike-slip motion with thrust component, and its latest active age is the early Holocene.
The only historical earthquake that occurred on the north segment of the Anqiu-Juxian Fault between Juxian and Changyi is the 70BC Anqiu M7 earthquake. However, paleo-earthquake researches show that several strong earthquakes occurred along the Qingfengling segment and the Mengyan segment between the latest late Pleistocene to early Holocene. The time of the latest strong earthquake is ～3 500a BP, 2 084a BP (-70BC), ～10 000a BP, ～10 000a BP on the Changyi-Nanliu segment, Anqiu-Mengtan segment, Qingfengling segment, and Mengyan segment, respectively. Since the strong earthquake recurrence interval is still not known for each segment, the exact time for the next strong earthquake can't be predicted. However, according to the geometric features, latest active age, latest activity features, historic earthquake data and paleoearthquake documents of this active fault, the 4 segments do have seismotectonic conditions for generating M≥7 earthquake, and the potential earthquake risk does exist and may be rather high and imperative. Thus, the fault activities and the potential earthquake hazard should be considered during future earthquake hazard prevention and prediction.

SEISMOTECTONIC BACKGROUND OF THE 2013 BENZILAN M5.9 EARTHQUAKE, YUNNAN PROVINCE

CHANG Zu-feng

2015, 37(1):  192-207.  DOI: 10.3969/j.issn.0253-4967.2015.15

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The epicenter region of the Aug. 31 2013 M5.9 earthquake lies near the northwest boundary of the Sichuan-Yunnan rhombic block in the junction area of the three rivers geosynclinal folded system and Songpan-Ganzi geosynclinal folded system. The geological structure of this area is quite complicated, where the near-SN trending Jinshajiang Fault zone and the NW-trending Deqin-Zhongdian-Daju Fault zone converge. The Jinshajiang Fault zone is a large-scale ultra-lithosphere fault zone and an old suture line structure after the closing of the ancient Tethys Sea, which generally consists of 5-6 faults, constituting a 50km wide fault zone, with a long history of geological evolution. Since Cenozoic, the faults are mainly dominated by intense near east-west extrusion, thus, absorb partially the energy of the movement of east Tibet block towards Sichuan and Yunnan block. Therefore, the fault zone is dominated by dextral strike-slip since Pliocene.
According to the results of field geological and geomorphic investigations, the late Quaternary activity of Jinshajiang Fault zone is mainly embodied by the Zengdatong Fault, the Lifu-Riyu Fault, the Langzhong Fault and the Guxue Fault, with straight fault troughs, linear ridges, orientated facets in landforms and clear linear features in satellite imagery. These faults are obviously active in late Quaternary, and dominated by dextral strike-slip motion and partially thrust motion, with an average rate of 3.5～4.3mm/a horizontally and 0.9～1.1mm/a vertically, respectively since Holocene. Historically, the October 20, 1923, Batang M 6 (1/2) earthquake occurred near to the fault zone.
The Deqin-Zhongdian-Daju Fault is also obviously active in late Quaternary and dominated by dextral strike-slip motion and normal motion, the latest active time is from late-Pleistocene to Holocene with an average rate of 1.7～2.0mm/a horizontally and 0.6～0.7mm/a vertically, respectively. Along the fault, several string-beaded Quaternary basins are developed. The fault has played an obvious control role in the development of the Benzilan Basin, the Zhongdian Basin, the Haba Basin and the Daju Basin, and has a strict control on the boundary of the basins. Historically, several moderate-size earthquakes occurred around the fault, e.g. the 1961 Zhongdian M6.0 earthquake, the 1966 Zhongdian M5.2 earthquake and the 2013 Benzilan M5.9 earthquake, etc. It is an important NW-treading fault developed along the northwestern boundary of the Sichuan and central Yunnan rhombic block, and together with the Jinshajiang Fault, composes up the northwestern boundary of the Sichuan and central Yunnan rhombic block. The Deqin-Zhongdian-Daju Fault is a significant dextral strike-slip fault under the east-west extension of the Qinghai-Tibetan plateau and plays a role of regulating the southeast movement of the plateau substances.
The long axis of the intensity distribution of the 2013 Benzialn M5.9 earthquake is NW-directed, consistent with the strike of Deqin-Zhongdian-Daju Fault. According to the focal mechanism solutions, the M5.9 earthquake is of dextral strike-slip with normal faulting, the attitude of the NW nodal plane is consistent with the strike of the fault too, and the seismic rupture is identical to the kinematical characteristic of the fault. The linear distribution of the earthquake-induced landslides and collapse is consistent with the strike of the fault as well. Based on all these features and phenomena, we can conclude that the Deqin-Zhongdian-Daju Fault is the seismogenic fault of the Benilan M5.9 earthquake. The earthquake is closely related to the Deqin-Zhongdian-Daju Fault, but the Jinshajiang fault zone may be also the breeding structure. Based on analysis of the earthquake intensity and frequency, there is the possibility to generate larger earthquake in this area.

RESEARCH ON THE CHARACTERISTICS OF QUATERNARY ACTIVITIES OF FEIHUANGHE FAULT IN XUZHOU AREA

ZHANG Peng, LI Li-mei, LIU Jian-da, XU Han-gang, LI Jin-liang, GU Qin-ping, JIANG Xin

2015, 37(1):  208-221.  DOI: 10.3969/j.issn.0253-4967.2015.16

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Running diagonally across the urban area of Xuzhou, the Feihuanghe(the abandoned Yellow River)Fault starts from Jiahezhai in the northwest, extending southeastwards through Sushantou, Xuzhou City and Liangtang along the abandoned Yellow River till the north of Wangji Town of Suining County, striking NWW, dipping SW, with a total length of about 70 kilometers. It is a buried fault, crosscutting Xuzhou-arc structure. There are significant topographic features indicating the existence of the fault on the earth's surface, which are clearly displayed in remote sensing images. There have been no devastating earthquakes occurring along the fault since the recorded history.
Feihuanghe Fault is one of the target faults of the project "Urban active fault exploration and seismic risk assessment in Xuzhou City". Few researches have been done on this fault in the past. The previous analysis assumes that the fault is a sinistral transtensional fault with extensional faulting in the Xuzhou-Suzhou arcuate structure at first and transtensional faulting of the Neocathaysian system later.
Based on field geological survey, shallow seismic exploration and composite drilling section method, Quaternary activities of Feihuanghe Fault are analyzed. Shallow seismic exploration results show that the Feihuanghe Fault is composed of a NE-trending south branch and a SW-trending north branch, forming a graben structure with the width of 1～2km. All breakpoints of the Feihuanghe Fault on the seismic exploration profiles show that only the bedrock surface was dislocated, not the interior strata of the Quaternary. The composite drilling profiling results show that Feihuanghe Fault has dislocated the strata of Mid Pleistocene, but not the top surface of Mid Pleistocene. Furthermore, we discovered a secondary fault of Feihuanghe Fault exposed at Fengshan Hill, and its latest activity date is the mid period of Mid-Pleistocene inferred from the cementation degree of gouge, dating results and geomorphic features. Combining the above results, we conclude that Feihuanghe Fault is of sinistral strike-slip in the early stage, and extensional faulting since the Quaternary, and the latest activity date is the middle period of Mid Pleistocene. Controlled by the tectonic setting, the activities of the NW-trending faults in Xuzhou area are significantly weaker than that of the NW-trending fault in adjacent southwest Shandong.

COSEISMIC 3D DEFORMATION FIELD ACQUISITION OF THE WENCHUAN EARTHQUAKE BASED ON INSAR AND GPS DATA

SONG Xiao-gang, SHEN Xing, JIANG Yu, WAN Jian-hua, SHAN Xin-jian, QU Chun-yan

2015, 37(1):  222-231.  DOI: 10.3969/j.issn.0253-4967.2015.17

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The variance component estimation method (VCEM) in generalized surveying adjustment theory, which realizes optimal weights allocation for different data sources, is applied to jointly invert two independent datasets, InSAR and GPS, for 3-D deformation field acquisition in this paper. Illustrated by the case of the Wenchuan earthquake, 3-D deformation field in high coherent area near the fault is achieved by using this method, which shows clearly a whole picture for the locations of right-lateral and thrust components movements generated by the earthquake. The 3-D deformation results are quantitatively consistent with GPS observations with RMS errors less than 5cm in 3-D directions, which demonstrates the feasibility to acquire precise 3-D deformation field by employing VCEM to fuse independent deformation datasets.

3-D LASER SCANNER(LIDAR): A NEW TECHNOLOGY FOR ACQUIRING HIGH PRECISION PALAEOEARTHQUAKE TRENCH INFORMATION

ZHENG Wen-jun, LEI Qi-yun, DU Peng, CHEN Tao, REN Zhi-kun, YU Jing-xing, ZHANG Ning

2015, 37(1):  232-241.  DOI: 10.3969/j.issn.0253-4967.2015.18

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LiDAR, as a newly developed surveying technology in recent decades, has been widely used in engineering survey, protection of cultural relics and topographic measurement, and it has also been gradually introduced to studies of tectonic activities. Although the digital photography technology has been used in the study of palaeoearthquake, the information would be still acquired by traditional geological sketch from trenches. Due to the limitation of photography itself, it is difficult to overcome the distortion of information. With its high information content, accuracy, convenience, safety and easy operation, LiDAR, as a new technology, broadens the access to data and information for palaeoearthquake study.

THE ACCURATE LOCATION METHODS FOR BURIED ACTIVE FAULT EXPLORATION: AN EXAMPLE OF LUHUATAI FAULTS IN YINCHUAN GRABEN

WANG Yin, MENG Guang-kui, CHAI Chi-zhang, LEI Qi-yun, DU Peng, XIE Xiao-feng

2015, 37(1):  256-268.  DOI: 10.3969/j.issn.0253-4967.2015.20

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Based on the discussions on the basic ideas, methods and procedures for detecting buried faults and taking the example of Luhuatai buried faults in Yinchuan Basin, the paper introduces in detail the multi-means, multi-level detection methods for gradually determining the accurate location of faults. Multi-means refer to the technical methods such as shallow seismic exploration, composite drilling section, trenching, dating of sedimentary strata samples and calculation of upward continuation of fault's upper breakpoints, etc. Multi-levels refer to gradually determining accurate location of fault at different levels with the above means.
Results of shallow seismic exploration reveal that the Luhuatai buried fault has a strike of NNE in general, dip SEE, with the dip angle between 73° to 78°. Geometrically, the fault consists of a main fault and a small north-segment fault in plane. The main fault runs along the NNE direction from Xixia District of Yinchuan City, passing through Jinshan Township to Chonggang Township, and there is a 4km or so intermittent zone between the main fault and the small north-segment fault. The small north-segment fault is 9km long, distributed between the north of Chonggang Township to the south of Shizuishan City. According to dating of sediments sampled from drill holes, the main fault can be further divided into the southern segment and the northern segment. The southern segment of Luhuatai buried fault is active in Pleistocene, while the northern segment is active in Holocene.
Shallow seismic exploration can detect the upper breakpoint of fault deeper than drilling or trenching does. If simply connecting the vertical projections of these breakpoints on the surface, there is a certain bias of fault strike. To this end, we did accurate location for the Holocene active northern segment of Luhuatai buried fault, in which upward continuation calculation is done based on the fault dip to match the upper breakpoint of fault obtained from shallow seismic exploration with the depth of the upper breakpoints obtained from drilling. Through the accurate location of the fault, we get the geometric distribution, occurrence and segmentation of activity of Luhuatai buried fault at the near-surface. Our results provide reliable basis for the safety distance from active faults for engineering construction projects in the Luhuatai buried fault area of Shizuishan City. The methods discussed in this paper for accurate location of buried active faults are of reference value for buried fault exploration in other similar cities or regions.

SEISMIC LANDSLIDES AND SEISMOGENIC STRUCTURE OF THE 2014 LUDIAN M_S6.5 EARTHQUAKE

ZHOU Qing, WU Guo

2015, 37(1):  269-278.  DOI: 10.3969/j.issn.0253-4967.2015.21

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Focal mechanism solutions, distribution of aftershocks and field investigation suggest that the 3 August 2014 Ludian, Yunnan M_S6.5 earthquake was spawned by the NW-trending steep left-slip fault that stretches along Baogunao-Xiaohe. The landsides, dominated by avalanches, induced by the shock are distributed in a rectangular area(15km×12km)with a NW-orientated long axis. The bedrock avalanches at many sites indicate that the primary direction of ground motion changes from SE in the north to SN in the south. The distribution of landslides triggered by the shock can be explained by the following two models of seismogenic structure: 1)Left-lateral strike-slip faulting occurred on a generally NW-trending arc-like fault, which caused seismic ground motion with the direction changing gradually from SE in the north to near-SN in the south; and 2)in addition to the left-lateral slip of the NW-trending fault, the NE-trending fault has also contributed to the hazard, which thrust from NW toward SE. So the Ludian M_S6.5 event was the result of the joint action of both the NW and NE directed faults, of which the NW left-lateral slip fault is the dominant and the NE thrust fault is the secondary. Another evidence is that most aftershocks are distributed linearly in NW direction, meanwhile some are clustered in nearly NE around the epicenter, which also implies the possibility that both faults moved simultaneously during the event. Besides, focal mechanism solutions of a lot of earthquakes in the Yongshan-Zhaotong area, northeastern Yunnan Province, where the epicenter of the 2014 Ludian M_S6.5 event is located, indicate that the seismogenic faults are primarily dominated by thrust faulting, associated with strike-slip faulting.

NUMERICAL SIMULATION STUDY OF HONGSHIYAN LANDSLIDE TRIGGERED BY THE M_S6.5 LUDIAN EARTHQUAKE

CHEN Xiao-li, CHANG Zu-feng, WANG Kun

2015, 37(1):  279-290.  DOI: 10.3969/j.issn.0253-4967.2015.22

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Although the landslides triggered during earthquake events are common phenomena in the southwest China, the occurrence of the Hongshiyan landslide triggered by the M_S6.5 Ludian earthquake in 2014 is attractive for its giant volume which exceeds ten million cubic meters. The Hongshiyan landslide formed a quake lake and inundated a village. Based on the geological and geomophological data obtained through the immediate field investigation after the earthquake, we build the Hongshiyan slope model and at the same time, we apply numerical simulation to study the landslide formation. Result indicates that the Hongshiyan slope was at safe conditions with the Factor of safety (Fs) value greater than 1, but the ground seismic motion during the Ludian earthquake lowered its Fs to a value smaller than 1, which resulted in the occurrence of the landslide. Moreover, this study shows that an existing slip surface is important for generating a giant landslide, and steep slopes without existing slip surfaces are likely to generate shallow landslides with normal volumes.

SPATIAL DISTRIBUTION ANALYSIS OF COSEISMIC AND PRE-EARTHQUAKE LANDSLIDES TRIGGERED BY THE 2014 LUDIAN M_S6.5 EARTHQUAKE

TIAN Ying-ying, XU Chong, XU Xi-wei, WU Sai-er, CHEN Jian

2015, 37(1):  291-306.  DOI: 10.3969/j.issn.0253-4967.2015.23

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On August 3, 2014, an M_S6.5 earthquake happened in the Ludian County, Zhaotong City of Yunnan Province. This earthquake caused a large number of landslides. In order to study the spatial distribution of the coseismic and pre-earthquake landslides, a 44.13km² area at the junction of Ludian County, Qiaojia County and Huize County along the Niulanjiang River is selected in this study. By visual interpretation of the Google Earth pre-earthquake high resolution images and the coseismic aerial data of 0.2m resolution of this area, the landslide databases of pre-earthquake and coseismic are established. The result shows that there are 284 pre-earthquake landslides, and 1053 earthquake-induced landslides. Then with the help of 10m×10m resolution DEM data and the GIS, the extracted factors of elevation, slope angle, slope aspect, curvature, lithology, earthquake intensity and drainages are used to analyze the spatial distribution of the coseismic and pre-earthquake landslides by adopting LAP(Landslide Areas Percentage)and LND(Landslide Number Density). The results show that areas with elevation <1 200m and 1 200～1 300m are prone to landslides whatever pre-earthquake or coseismic. With the slope gradient increasing, it is much more prone to landslide, and the area of <10°, close to the rivers, is also much susceptible. The advantage slope aspect is almost near S direction. Concave slope(when the curvature is negative)is much susceptible to landslides, and with the curvature decreasing, the landslide susceptibility gets higher. The region of limestone with dolomitic limestone is sensitive to landslide; in the areas consisting of basalt and volcanic breccia, the slope stability is greatly reduced under the effect of seismic force. The larger the intensity is, the more landslides happened. For either pre-earthquake or coseismic landslides, there is a positive correlation between landslide spatial distribution and the distance to rivers. The large pre-earthquake landslides have effective influence on LAP.

UNDERGROUND FLUID ANOMALIES AND THE PRECURSOR MECHANISMS OF THE LUDIAN M_S6.5 EARTHQUAKE

LIU Yao-wei, REN Hong-wei, ZHANG Lei, FU Hong, SUN Xiao-long, HE De-qiang, YU Huai-zhong, ZHOU Zhi-hua, ZHANG Guo-meng

2015, 37(1):  307-318.  DOI: 10.3969/j.issn.0253-4967.2015.24

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We summarized the fluid anomalies associated with the Ludian M_S6.5 earthquake based on the Sichuan and Yunnan earthquake network observations and field survey. The fluid anomalies were divided into long-term, medium-term, short-term, imminent and macroscopic anomalies according to the basic principles of earthquake forecasting. The long-term and medium-term anomalies distributed mainly in the range 300～500km away from the epicenter. By contrast, the short term, imminent and macroscopic anomalies clustered in an epicentral distance less than 100km. The underground fluid anomalies in the higher station density area reflect the enhancement of fluid movement, which are conducive to determine the seismic risk area and trace the short-term precursor of earthquake. The regional stress variations may cause the fractures in a fault zone open and close, leading to the change of water level and temperature in boreholes or spring and emission of deep-sourced gases. It may also lead to intense water-rock reaction and groundwater intrusion, resulting in the change of ion contents in groundwater, or sometimes, the occurrence of significant macroscopic anomalies. Therefore, it is highly possible to obtain reliable earthquake precursor information for predicting the forthcoming earthquake risk zone in the region with dense observation stations.

EARTHQUAKE PREDICTIONS: SPATIAL-TEMPORAL GRAVITY CHANGES BEFORE THE LUDIAN M_S6.5, LUSHAN M_S7.0 AND WENCHUAN M_S8.0 EARTHQUAKES

ZHU Yi-qing, FU Guang-yu, LIANG Wei-feng, XU Yun-ma

2015, 37(1):  319-330.  DOI: 10.3969/j.issn.0253-4967.2015.25

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Using the multiple-scale campaign gravity data in Sichuan-Yunnan area, we systematically analyzed the relationships between the spatial-temporal gravity changes and the 2014 Ludian, Yunnan Province M_S6.5 earthquake and the 2013 Lushan M_S7.0 and the 2008 Wenchuan M_S8.0 earthquakes in Sichuan Province. Our main results are as follows. 1)Before the occurrence of great earthquakes, gravity anomalies occurred in large area around the epicenters. The directions of gravity change gradient belts usually agree roughly with the directions of the main fault zones of the study area. Such gravity changes might reflect the increase of crustal stress, as well as the significant active tectonic movements and surface deformations along fault zones, during the period of gestation of great earthquakes; 2)The continuous significant changes of the multiple-scale gravity fields, as well as the greater gravity changes with larger time scales, can be regarded as the medium-range precursors of great earthquakes. The subsequent great earthquakes always occur at the area where the gravities change greatly; 3)The spatial-temporal gravity changes are much useful in determining the epicenter of the coming great earthquakes. The large gravity networks are useful in determining the major areas of the coming great earthquakes. However, the local gravity networks with high spatial-temporal resolution are suitable for determining the location of epicenters. Therefore, denser gravity observation networks are necessary for better forecasting the epicenters of great earthquakes. 4)Using gravity changes from campaign observation data, the first author of this paper had made medium-range predictions to the 2004 Ludian M_S6.5, 2013 Lushan M_S7.0 and 2008 Wenchuan M_S8.0 earthquakes, specially, the successful predictions of the location of epicenters. Based on above discussions, we emphasize that the medium/long-term potential of great earthquakes might exist nowadays in some areas with significant gravity anomalies in the study region. Because of this reason the monitoring should be strengthened.

A STUDY ON DIRECTION OF THE SEISMOGENIC FAULT OF LUDIAN M6.5 EARTHQUAKE BASED ON STRONG MOTION AND SEISMOGRAPHIC DATA

FENG Wei, LIU Jie, LUO Jia-hong, HOU Jian-sheng, XU Wan-zhen

2015, 37(1):  331-341.  DOI: 10.3969/j.issn.0253-4967.2015.26

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Ludian M6.5 earthquake on August 3, 2014 occurred in Ludian country, Zhaotong City, Yunnan Province. The epicenter is located at longitude 103.3°E and latitude 27.1°N with focal depth of 12km. Macro epicenter is located in Longtoushan Town, Ludian Country. The seismogenic fault is the NW-striking Baogunao-Xiaohe Fault, which is a second-order strike-slip fault of the NE-striking Zhaotong-Ludian Fault system. In this paper, we use observation records of 50 strong motion stations in the Ludian earthquake to calculate the distributions of peak ground acceleration and horizontal peak ground acceleration. According to the records, we find that the strong motion records decay slowly along the northwestern direction, thus we infer that the seismogenic fault is in NW direction. Meanwhile, the horizontal peak acceleration distribution map is similar with the synthetic three-component peak acceleration distribution map, which illustrates the horizontal component is the main part. Normalized S-wave radiation patterns are drawn using the observation records of 135 seismic stations. In contrast to single couple source radiation pattern, we infer that the seismogenic fault is in NW direction. The amplitude in the southeast direction is slightly larger than the amplitude in the northwest and the difference is not big, showing that the rupture trends slightly to the southeast. The above results show that, the conclusion of Ludian M6.5 earthquake rupture along the northwest direction is in consistency with the observation results of the strong motion stations and the seismic stations. In addition, the distribution of earthquake disaster also supports this conclusion.