Table of Content

30 December 2014, Volume 36 Issue 4

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CONTENTS

KEY TECHNIQUES AND SEVERAL CASES ANALYSIS IN PALEOSEISMIC STUDIES IN MAINLAND CHINA(4) —SAMPLING AND EVENT ANALYSIS OF PALEOSEISMIC DATING METHODS

RAN Yong-kang, WANG Hu, YANG Hui-li, XU Liang-xin

2014, 36(4):  939-955.  DOI: 10.3969/j.issn.0253-4967.2014.04.001

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Lowering uncertainties of paleoseismic data is very important, which facilitates medium and long-term earthquake prediction and seismic risk assessment in paleoseismic studies. Among these uncertainties, paleoseismic timing is highly focused and its constraint is one of the most key factors in lowering uncertainties. To get the age as true as possible, choosing right dating, sampling methods and techniques on event ages are essential. Among the several primary dating techniques, radiocarbon dating is prior to the other methods. We should choose material that is breached completely during transportation for OSL dating, and samples that are hardly affected by kinds of factors for ¹⁰Be dating. Sampling at appropriate sites based on analysis of tectonic background and using sequential ages to constrain paleoseismic events are the primary keys of techniques on accurate event dating. Interval value is suggested for age constraint when using stratigraphic chronology. When there are multiple ages for different samples within a layer, we should choose the youngest and reliable age. These rules are recommended when constraining paleoseismic ages at a single site. Temporal and spatial correlation such as successive limit method, Z statistics, overlapping distributions likelihood approach, event window and ad hoc weighted overlap methods are the primary analysis approaches for event dating at multiple sites along faults or fault segments.

WAS THERE SIGNIFICANT VERTICAL CRUSTAL MOTION IN EPICENTRAL REGION PRIOR TO 2008 GREAT WENCHUAN EARTHQUAKE？

WANG Min, SHEN Zheng-kang

2014, 36(4):  956-963.  DOI: 10.3969/j.issn.0253-4967.2014.04.002

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The May 12, 2008 M_S 8.0 Wenchuan earthquake has been the greatest catastrophic event occurring in China since the 1976 Tangshan earthquake. Following the earthquake, GPS data observed in the epicentral region were analyzed, and one study, based on analysis of independent 30s interval's instantaneous positions, claimed that significant vertical crustal displacement occurred one hour prior to the earthquake. This result has raised community expectations to achieve short-term and imminent earthquake prediction using the technique of high-rate GPS monitoring. High-rate GPS is an important technique developed in recent years, and has been widely used for studying short term crustal deformation during an earthquake and earthquake rupture process. Its precision and reliability of data analysis result, however, is constrained by a variety of factors, in particular, the cycle-slip detection and the integer ambiguity resolution of carrier phase are strongly dependent on the distance between the observation site and reference site (for double-differencing analysis) or satellite ephemeris and clock errors (for non-differencing analysis). We propose a new method to analyze dynamic displacements of GPS sites based on carrier phase residuals obtained from static GPS data processing. Assuming that an observation station moves during a time period in which the data are processed, the carrier phase residuals of static data post-processing are mainly reflecting the position displacements and multipath effect. Taking advantage of periodic repeatability of the multipath effect in a sidereal day, we develop a model to reduce/remove the multipath component in the carrier phase residuals, and extract the observation station dynamic displacement signals relative to the station mean position. We apply this method to derive the dynamic displacement time series of 3 continuous GPS stations (CHDU, PIXI, and DUJY) located in the epicentral region of the Wenchuan earthquake. Our result shows that vertical displacements of the 3 sites within 7 days prior to the earthquake are within 2cm, and the disturbances appear to be random than progressive. The resolved dynamic displacements in two consecutive days are also shown to be highly correlated, suggesting that there was no significant disturbance caused by external error source or deformation source. This conclusion is true also for the day (and even one hour) prior to the earthquake. Combining with the analysis of the coseismic deformation pattern, we conclude that there was no significant crustal motion in the epicentral region prior to the Wenchuan earthquake.

GAS PERMEABILITY AND KLINKENBERG EFFECT OF FAULT ROCKS

DUAN Qing-bao, YANG Xiao-song, CHEN Jian-ye

2014, 36(4):  964-975.  DOI: 10.3969/j.issn.0253-4967.2014.04.003

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The nitrogen gas permeabilities of fault rocks were measured under confining pressure ranging from 4 to 40MPa. At each confining pressure step, we changed the upstream pressure several times and measured the permeability. The measured gas results were then corrected by Klinkenberg's equation. Relationship between slippage factor b and intrinsic permeability k_l can be described by a power law correlation expressed as b=0.004 6k_l^-0.476. Compared with sedimentary rocks, the gas slippage effect of fault rocks is stronger, which should not be ignored when using gas as a medium for permeability experiments. The relationship between gas permeability, intrinsic permeability and pore pressure is expressed as k_g/k_l=1+(0.009 2k_l^-0.476)/(P_u+P_d). Results show that the lower the permeability of rocks is, the stronger the Klinkenberg effect is. Additionally, the slippage effect decreases as pore pressure increases. For the relative permeable (10^-15m²～10^-18m²) samples, their gas permeabilities are almost identical to intrinsic permeabilities under relative higher pore pressure condition(higher than 4MPa). However, for the ultra-low permeable (10^-22m²～10^-20m²) samples, it's difficult to avoid the slippage effect although under high pore pressure condition. The intrinsic permeability of fault gouge samples ranges between 4.54×10^-19m² and 2.43×10^-17m², and those of the breccia samples are higher by 1～2 orders of magnitude which range between 2.25×10^-17m² and 7.94×10^-16m². These results demonstrate that the permeability structure of Wenchuan earthquake fault zone is characterized by a low permeable fault core and relative higher permeable damaged zone. The fault zone has the characteristics of low permeability for thermal pressurization.

DISCOVERY OF THE LATE-QUATERNARY ACTIVITY ALONG THE EASTERN SEGMENT OF MAIDAN FAULT IN SOUTHWEST TIANSHAN AND ITS TECTONIC IMPLICATION

WU Chuan-yong, Alimujiang, DAI Xun-ye, WU Guo-dong, CHEN Jan-bo

2014, 36(4):  976-990.  DOI: 10.3969/j.issn.0253-4967.2014.04.004

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The late-Quaternary deformation characteristics of the boundary fault zones are critical to understanding the crustal deformation of the Tianshan Mountains. Based on remote sensing image interpretation, field surveys, trenching and optically stimulated luminescence dating methods, we obtain the reliable activity evidences of the Maidan Fault in late-Quaternary.
The Maidan Fault is the boundary fault of the Tianshan Mountains and Tarim Basin. The fault, with a total length of 400km and the maximum width about 15～17km, comprises a series of secondary faults. During the late Quaternary, the fault was still very active. The fault dislocated the late-Quaternary landform surfaces, forming obvious scarps on the surfaces. The height of the scarps range several to hundred meters. Trench excavation shows that paleoearthquakes occurred on the faults during late Holocene. The vertical displacement caused by the last paleoearthquake event is above 2m. The different late Quaternary landforms with different vertical displacement heights indicate that several strong earthquake events have occurred on the Maidan Fault since the late Quaternary.
The discovery of activity on the Maidan Fault shows that the deformation does not focus solely on the newly born reverse fault and fold belt. Faults at the root of Kalpin nappe system have also participated in absorbing and partitioning some of the tectonic deformation. This phenomenon may explain why the shortening rate got by geology method of the Kalpin nappe structure is much less than that obtained by GPS. This deformation mode of the Tianshan orogenic belt is obviously different from the piggyback propagation as considered previously. The activities of the Tianshan root faults migrated to the frontal faults of the piedmont nappe, and the root fault activity weakened gradually. But the activity in Kalpin nappe structure does not accord with this mode. The root faults and the frontal faults of the Kalpin nappe structure are all obviously active, which indicates the nappe structure in the southwestern Tianshan is an out-of-sequence, or a non-sequence thrust system. This kind of structure mode brings new challenges to us in constructing seismogenic tectonic models and assessing seismic risk.

THE MAGMA SOURCE PROPERTIES AND EVOLUTION OF HOLOCENE VOLCANOES IN TENGCHONG, YUNNAN PROVINCE, SW CHINA

LI Xin, LIU Jia-qi, SUN Chun-qing, DU De-dao, Wang Shi

2014, 36(4):  991-1008.  DOI: 10.3969/j.issn.0253-4967.2014.04.005

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This paper focuses on the characteristics of petrology, geochemistry, magma source and magma evolution characteristics of three Holocene volcanoes (Heikongshan, Dayingshan, Maanshan) from the Tengchong volcanic eruption field, with the aim of ascertaining the characteristics of the volcanic eruption and the relationship between these three magma chambers. Studies of the major element and trace element compositions of the Holocene volcanic rocks from the Tengchong volcanic eruptive field indicate that they are high potassium calc-alkaline series rocks including trachybasalt, basaltic trachyandesite, trachyandensite, and dacite. These rocks are rich in LILE and LREE, and depleted of Nb-Ta-Ti in mantle-normalized incompatible trace element patterns which show the chemical characteristic of arc volcanics. The volcano eruptions since Cenozoic are not controlled by plate subducting and volcanic arc but by faults within the basin. Trace elements' partial melting modeling shows that the Holocene volcanic rocks in Tengchong volcanic eruption field have been derived by large-scale partial melting of a garnet lherzolite in low pressure condition. They are from intra-plate background but with island arc volcanics' geochemical characteristics which suggest tectonic settings of island arcs or active continental margins. The magma source of these rocks is from the mantle which was enriched by palaeo-oceanic crust subduction. The enrichment happened before partial melting. The K-rich mineral in the source is rutile not amphibole. The magma of Heikongshan, Dayingshan and Maanshan experienced crust contamination and crystal fractionation of phosphorite, pyroxene, olivine, limenite, and plagioclase at the chamber stage.

APATITE ⁴He/³He THERMOCHRONOMETRY —A NEW METHOD OF LOW TEMPERATURE THERMOCHRONOMETRY

YANG Jing, ZHENG De-wen, CHEN Wen, ZHANG Zhu-qi

2014, 36(4):  1009-1019.  DOI: 10.3969/j.issn.0253-4967.2014.04.006

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The apatite has the lowest available He closure temperature (about 70℃); the ⁴He concentration profile is a very rich source of information on an apatite's low temperature (30～90℃) cooling history. According to classical diffusion theory, the ⁴He/³He thermochronometry technique in which the natural spatial distribution of ⁴He is constrained using a mathematical technique in a sample containing synthetic, proton-induced ³He. The approach is useful for constraining the low temperature cooling histories of individual samples. The review gives the fundamental theory, simulation method, the example application of this thermochronometry technique and the potential applications. Although this technique is in the exploration, but the sensitivity of the low-temperature cooling history makes this method have broad prospects.

A NEW METHOD FOR DETERMINING SLIP RATES OF STRIKE-SLIP FAULT ASSOCIATED WITH LATERAL EROSION OF ACCUMULATED OFFSET

REN Zhi-kun, ZHANG Zhu-qi, CHEN Tao, WANG Wei-tao

2014, 36(4):  1020-1028.  DOI: 10.3969/j.issn.0253-4967.2014.04.007

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Fault slip rate is one of the most important subjects in active tectonics research, which reveals the activity and seismic potential of a fault. Due to the improvement of dating precision with the development of dating methods, Holocene geological markers, even the young markers of thousands or hundreds years old, are widely used in fault slip rate calculation. Usually, in strike-slip fault slip rate calculation, there are two types of uncertainties. The first is correspondence of the offset and accumulation time; the second is the lateral erosion of the accumulated offset. In this paper, we suggest that the effect of lateral erosion of the accumulated offset should be removed. We also propose a new method for determining slip rate of strike-slip fault—the differential method. According to analyses of river terrace evolution and displacements accumulation, terrace heights (relative height above river), corresponding ages and measured offsets on the terraces are correlated to each other. We could use the terrace height, corresponding ages and the measured offsets to calculate the offsets that could be used to obtain the fault slip rate. Usually, the heights, ages and offsets of at least three terrace levels are needed in this method. If the terrace height is graded in order, the lateral erosion to each terrace is almost the same. Consequently, direct difference of offset and corresponding ages of the terraces could be used to calculate the fault slip rate. This kind of differential method could avoid the uncertainties from the lateral erosion in fault slip rate determination. By applying the differential method, we got the revised slip rates of 4.7～8.8mm/a on the Altyn Tagh and Kunlun Faults. These low slip rates could fit previous geodetic and geological fault slip rates, shortening rates as well as the millennial recurrence intervals of strong earthquakes along the major segments of these faults.

A CLUSTERING APPROACH FOR INCORPORATING SPATIAL DEPENDENCE INTO THE AUTOMATIC FLUVIAL TERRACE EXTRACTION FROM DIGITAL ELEVATION MODEL: A CASE STUDY FROM THE ZHANGLA BASIN ALONG THE UPSTREAM OF THE MINJIANG RIVER

SONG Zhuo-qin, ZHANG Jun-long, LI Jian-jun

2014, 36(4):  1029-1042.  DOI: 10.3969/j.issn.0253-4967.2014.04.008

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The terraces along the upper reaches of the Minjiang River record rapid uplift around the Tibetan plateau since the Quaternary. However, the common method to obtain the terraces elevation data always relies on single point or line to represent the whole landform. The available results and further analyses are usually not continuous and systematic. All of these, therefore, restrict the development of further study. A large number of qualitative information and the digital terrace models are relied on the improvement of DEM processing technology. Our present paper applies fuzzy C-means algorithm to a bunch of the Minjiang River terrace cross sections in different dimensions to get the integrated distribution characteristics of the terraces in the Zhangla Basin. SPOT5 image is adopted when generating the DEM. Then we consider both the spatial correlation and distribution characteristics of terrace cross sections into analysis and we consequently find a successful way to extract different levels of terraces based on both whole indicator and internal correlation. The precision evaluation suggests that result of terrace extraction is highly consistent with the field survey data. Based on these, we discuss relationship between characteristics of terraces, the incision rates and the regional uplift patterns. We suggest that a clustering approach for incorporating spatial dependence into the automatic fluvial terrace extraction can be used to study and understand the regional tectonic and geomorphic features in the layered landscapes.

RESEARCH ON METHOD OF ESTIMATING GROUND MOTION PARAMETERS IN WESTERN CHINA USING INTENSITY DATA

CHEN Kun, YU Yan-xiang, GAO Meng-tan, KANG Chuan-chuan

2014, 36(4):  1043-1052.  DOI: 10.3969/j.issn.0253-4967.2014.04.009

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Probability distributions P(I|GM) for Bayesian methods are established from an intensity/ground motion dataset for 6 earthquakes in western China. We tested the method using intensity to estimate the ground motions and found it is feasible for two earthquakes: Luhuo Sichuan earthquake (M5.3) and Jiashi Xingjian earthquake (M5.8) in 2011. This study also shows that the estimation accuracy of Bayesian method is significantly correlated with the priori probability (attenuation relationship). Comparison of root-mean-squares (rms) of PGAs estimated using the ground motion parameter means of each intensity level, the ground motion parameter from attenuation relationship and the Bayesian method with the observations shows that the accuracy of the parameters estimated by Bayesian method is much better than that by the other two methods.

OPTICALLY STIMULATED LUMINESCENCE DATING OF LATE QUATERNARY SEDIMENTS AT DAHUANGSHAN TRENCH ALONG FUKANG FAULT ZONE, XINJIANG

DAI Xun-ye, SHEN Jun, WU Chuan-yong, YANG Hui-li

2014, 36(4):  1053-1063.  DOI: 10.3969/j.issn.0253-4967.2014.04.010

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In the study of active faults, obtaining the exact age of the strata is an extremely important step. The optically stimulated luminescence (OSL) dating method, a technique closely related to thermoluminescence (TL), is developing extensively on dating for Quaternary sediments in recent years. Fukang Faults, located in the eastern Tianshan arc nappe tectonic zone, are typical arc thrusting faults. The dating samples collected from Dahuangshan trench of Fukang Fault zone are used to determine the activity of the fault. 23 OSL samples were obtained from the trench. We selected 4～11μm fine-grained quartz through pre-treatment process and analysed them by using sensitivity-corrected multiple aliquot regenerative-dose (SMAR) protocol. Equivalent dose (De) preheat plateau test is an often used approach to determine the appropriate preheat temperature in OSL dating. The preheat plateau test of sample LED12-297 shows that 220～260℃ are the appropriate preheat plateau temperature regions to get fundamental De. The dating results show that the OSL stratigraphic ages of the samples are consistent with stratigraphic sequence and that Fukang Fault is a Holocene active fault. It is found that the last event of Fukang Fault occurred (1.90±0.14) ka to (3.47±0.17) ka ago. The OSL ages and their related stratigraphic vertical displacement are used to calculate the vertical slip rate of the fault, which is 0.17mm/a.

LATE QUATERNARY ACTIVITY RESEARCH OF THE NORTHERN MARGINAL FAULT OF EMEI PLATFORM, SHANXI PROVINCE

XU Wei, GAO Zhan-wu, YANG Yuan-yuan

2014, 36(4):  1064-1076.  DOI: 10.3969/j.issn.0253-4967.2014.04.011

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Based on the 1: 50 000 geological mapping of active fault, the paper investigates the stratum, topography and faulted landforms of the northern marginal fault of Emei Platform, and preliminarily divides the northern marginal fault of Emei Platform into three sections by two stepovers near Tanjiazhuang Village and Nanliu Village according to different fault activity of each section.
At west of Tanjiazhuang Village is a loess platform, and the high terrain scarp can be seen from the northern margin. The height of scarp decreases progressively and the slope becomes gentle westwards at the place between Nanchi Village and Xikang Village, and to the place near Xiaoliang town, we cannot see obvious terrain scarps. The faulted sections can only be seen in the gullies which cross the terrain scarp at the south of Guozhuang Village and Tanjiazhuang Village. The fault dislocates the Pliocene red clay and the middle Pleistocene Lishi loess and covered by Malan loess; continuous paleosoil can be seen across the terrain scarp in some gullies. These indicate that in this section the fault was active in the early middle Pleistocene and its activity becomes weaker or no longer active after that.
The fault in the section between Tanjiazhuang Village and Nanliu Village can be divided into three parts by Shidian Village and Jinming Village, which are named, from west to east in sequence according to each faulted landform, the northern marginal fault of lacustrine terrace, the piedmont fault of Zijin Mountain and the northern marginal fault of loess platform. The fault transition area between each part is continuous and the fault is in linear distribution, so we see the whole fault section as having the same activity. In this section the Holocene diluvial fan is faulted. At least two plaeoearthquake events happened since Holocene, and the latest activity is in (2.00～1.29) ka BP according to Renzhuang trench and Jinsha trench, which can be well compared with former researches. The fault slip rate is over 0.33mm/a in the section south of Maguduo Village and is more than 0.36mm/a according to Renzhuang trench since the later period of the late Pleistocene.
In the section between Nanliu Village and Xizhangpo Village, the fault distributes along the frontal edge of the diluvial platform and is covered by thick loess. A 50～200m high linear terrain scarp formed due to the activity of fault can be seen along the frontal edge especially in the part between Xunwang Village and Xulu Village. At north of Wuzhai Village, the height of scarp decreases progressively and to the place near Xizhangpo Village, the terrain scarp cannot be seen clearly. In this section, Malan loess is faulted, which indicates that this fault section has been active since the late Pleistocene, but the evidence of Holocene fault activity has not been obtained yet due to the non-development of Holocene stratum. The fault slip rate is no less than 0.1mm/a since the late Pleistocene according to the faulted section at south of Xunwang Village.

A REVIEW OF THE THINKING AND PROCESS ABOUT PREDICTION OF LUSHAN M7.0 EARTHQUAKE ON APR. 20, 2013

SU Qin, YANG Yong-lin, ZHENG Bing, WANG Shuang-hong, LI Fei-fei, LIU Guan-zhong

2014, 36(4):  1077-1093.  DOI: 10.3969/j.issn.0253-4967.2014.04.012

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Before the Lushan M7.0 earthquake of April 20, 2013, fault displacement time histories inferred from near-field observation in Xianshuihe, Anninghe and Zemuhe Fault zones in Sichuan Province showed long-, mid-, and short-term anomalies which deviated from the normal background. The long-term anomalies were mainly represented by extensional faulting, the mid-term anomalies were marked mainly by turning of anomaly trends, and the short-term anomalies appeared mainly as compressional movement of the central-northern segment of Xianshuihe Fault. And based on these anomalies, a relatively accurate short-term prediction to this earthquake was proposed by the Survey Engineering Institute of Earthquake Administration of Sichuan Province. In this article, we summarize the thoughts and process of the prediction to Lushan earthquake, and present the bases and process for the long-, mid-, and short-term prediction. This experience can be useful for short-term prediction of strong earthquake in the future. Our preliminary results show that: 1)The sudden acceleration of variation in anomaly of short-baseline and short-leveling has short-term predictive significance to the origin time of strong earthquake. 2)Combined with analysis on the characteristics of time series of the occurrence time of cross-fault deformation anomalies, and the high gradient zones and their temporal evolutions revealed by mobile gravity surveys, we chose the intersection of the anomalous zones as the hazardous area and found it has a better predictive effect for determining the location of strong earthquake. 3)Statistical analysis shows that the duration of cross-fault deformation anomalies has certain significance for predicting the magnitude of strong earthquake. According to the characteristics of fault activity, the southeastward movement rate of the Bayar Har block may be greater than that of the eastern boundary of the Sichuan-Yunnan block before the Lushan earthquake, which aggravated the compressional and extensional movement of the northern segment of Longmenshan Fault, the middle-northern segment of Xianshuihe Fault and the Zemuhe Fault, and made the southern segment of Longmenshan Fault and the northern segment of Anninghe Fault locked. The faults differed remarkably in their activity patterns, which were mutually restrained and adjusted, as well as convertible. Thus, brittle rocks in the transition zone between plateau and basin ruptured and earthquake occurred. Currently, short-term earthquake prediction is still based on empirical results, and its scientificity is insufficient and pending further discussion.

A RAPID ASSESSMENT OF THE FELT RANGE OF WENCHUAN EARTHQUAKE BASED ON INTERNET INFORMATION

SHUAI Xiang-hua, HU Su-ping, ZHENG Xiang-xiang

2014, 36(4):  1094-1105.  DOI: 10.3969/j.issn.0253-4967.2014.04.013

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Based on the felt information from the Internet after the earthquake, this paper studied the methods of earthquake damage information acquisition and analysis. We carried out the research based on 4515 pieces of felt information after the Wenchuan earthquake acquired from the Internet. After the analysis on the publishing time, location and intensity of the feeling of the earthquake in the felt information, we found that the release of felt information was concentrated within the 3 hours after the Wenchuan earthquake happened. And we initially identified 2 289 felt data released in the subsequent three hours after Wenchuan earthquake, which distributed in 199 areas, as the data sources for spatial interpolation. In order to do spatial interpolation for these datasets, we classified the datasets into three categories, which are "unawareness", "felt" and "strongly felt", and assigned the value of 0, 1 and 2 to "unawareness", "felt" and "strongly felt" respectively. Before doing the spatial interpolation for the datasets, we preprocessed the datasets distributed in 193 different areas to remove the redundant and abnormal value data and to finally extract 193 felt data distributed in different areas. At the same time, this paper did exploratory analysis for the datasets, by which we knew the law of the datasets and determined the parameters and model for the spatial interpolation. After a series of analysis, this paper adopted IDW, RBF and Kriging methods to interpolate the felt datasets, and get the results of the felt range of Wenchuan earthquake. Then, the paper compared the results from different spatial interpolation ways by interpolation accuracy and the goodness of fit with the actual earthquake disaster. After the comparison, we found that the result of Kriging method, with a certain interpolation model and parameters, was consistent with actual disaster distribution of the Wenchuan earthquake to a certain extent, especially for the fitting with the strongly felt area (Ⅵ degrees). This study provides an effective way for us to know quickly the earthquake felt range and correct for the disaster information of the earthquake. And it will play an important role in making scientific and rational decisions during the earthquake emergency.

A FAST CLASSIFICATION FOR LUSHAN EARTHQUAKE-INDUCED LANDSLIDE HAZARD ZONES BASED ON COMPREHENSIVE INDEXES METHOD

YU Lu, SHAN Xin-jian, CHEN Xiao-li

2014, 36(4):  1106-1115.  DOI: 10.3969/j.issn.0253-4967.2014.04.014

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Strong earthquake-induced landslides in mountainous region often cause serious damages to buildings, transportation route, lifeline engineering etc. It is one of the major reasons causing significant casualties and economic losses of property. For such serious and wide range disasters, doing classification for earthquake-induced landslide hazard zones is one of the effective methods for reducing losses. This paper, based on former studies, takes Lushan earthquake on April 20, 2013 as a sample and selects Lushan County, Baoxing County and the surrounding area, which were influenced seriously by landslide, as a study area. This paper takes comprehensive indexes method to classify earthquake-induced landslide hazard zones for this area. There are 5 impact factors, including lithology, slope, seismic intensity, distances from faults and distances from drainages, are selected and weight is determined for each factor by AHP (the Analytic Hierarchy Process). The study area then is classified into 4 levels of hazards zones by comprehensive indexes method to indicate possibilities of triggering landslide in region under the suffering from given seismic intensities. The paper makes a comparison between the existing landslide sites map and the earthquake-induced landslide hazard zoning map. The results show a relatively high match between the two maps (about 77% landslide sites are in the higher hazard zones and highest hazard zones). This research will provide a reference for emergency response of earthquake-induced landslide disaster, prediction of earthquake-induced landslides in mountainous area and prevention of landslide disaster.

DISCUSSION ON BLOCK KINEMATIC MODEL AND FUTURE THEMED AREAS FOR EARTHQUAKE OCCURRENCE IN THE TIBETAN PLATEAU: INSPIRATION FROM THE LUDIAN AND JINGGU EARTHQUAKES

XU Xi-wei, CHENG Jia, XU Cong, LI Xi, YU Gui-hua, CHEN Gui-hua, TAN Xi-bin, WU Xi-yan

2014, 36(4):  1116-1134.  DOI: 10.3969/j.issn.0253-4967.2014.04.015

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By use of existing active faults, GPS monitoring data and other information and supposing a block-like motion, the Tibetan Plateau can be divided into multiple first-order and second-order blocks in accordance with basic definition of active block, and the block kinematic model with slip vector is given. Then by analysis of the relationship between the earthquake series, including the 2014 Ludian and Jinggu earthquakes, occurring in the Tibetan Plateau in the past more than a decade and the block motion, the themed areas for future surface rupturing earthquakes are determined to be related to the eastward or southeastward motion of the Bayan Har and Qiangtang blocks. The risk fault segments for the future earthquakes may include the southeastern segment of the Xianshuihe Fault, the Anninghe Fault, the Daliangshan Fault, the southern segment of the Xiaojiang Fault and the southeastern segment of the Red River Fault as well as the Maqin-Maqu segment of the eastern Kunlun Fault. In the earthquake monitoring and prediction tests, attention should be paid to the different structural strain patterns around the specific block boundary faults.

STUDY ON FAULT LOCKING CHARACTERISTIC AROUND THE DALIANGSHAN SUB-BLOCK

ZHAO Jing, LIU Jie, NIU An-fu, YAN Wei

2014, 36(4):  1135-1144.  DOI: 10.3969/j.issn.0253-4967.2014.04.016

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GPS campaign observations can monitor dynamic characteristic of crustal deformation near the fault zone effectively. Dynamic characteristic of crustal deformation is the manifestation of the dynamic action of the faults in deep and shallow structures. The locking and movement state of faults in deep and shallow structures can be an objective characterization of strain accumulation in seismogenic fault. So we can use dynamic GPS observations to invert fault locking and fault slip deficit rate by some models, and then judge the mid- to long-term seismic potential of the faults. Research about the faults around the Daliangshan sub-block is relatively poor, and the moderate-strong earthquakes increased significantly around the sub-block over the past decade, which makes fault locking and seismic potential around the sub-block be the problem to be urgently studied. Therefore, by using the GPS horizontal velocity field of 1999-2007, 2009-2013 and the negative dislocation model of DEFNODE, we inverted for spatial fault locking and fault slip deficit rate in the Daliangshan sub-block which contains three major fault zones, named Daliangshan, Mabian-Yanjin and Huize-Yiliang, before and after the Wenchuan earthquake. We analyzed the seismic potential characteristic of the three faults combining with the seismic gap and the spatial distribution of b value. The results show that the locking state of the three faults was basically same before and after the Wenchuan earthquake, which indicates that the earthquake probably has a very weak influence on the faults. The inversion results of two periods show that the southern segment of Daliangshan, Mabian-Yanjin and Huize-Yiliang Faults are basically completely locked except the southwestern segment of Huize-Yiliang Fault. The slip deficit rates of the three faults are not huge before and after the earthquake. Daliangshan Fault is mainly of a sinistral strike-slip deficit, Mabian-Yanjin Fault has a little amount of sinistral strike-slip and compressional deficit which was slightly enhanced after the earthquake, and Huize-Yiliang Fault is characteristic of compressional deficit with a small amount of dextral strike-slip deficit, which reduced to about zero after the earthquake. Combining with some other results, we conclude that current seismic potential for strong or major earthquakes exists on the three faults.

CRUSTAL DENSITY STRUCTURE OF YUNNAN LUDIAN M_S6.5 EARTHQUAKE AREA

YANG Guang-liang, SHEN Chong-yang, TAN Hong-bo, WANG Jia-pei, WU Gui-ju

2014, 36(4):  1145-1156.  DOI: 10.3969/j.issn.0253-4967.2014.04.017

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In this paper, based on three gravity profiles in Yunnan Ludian M_S6.5 earthquake and adjacent area, we obtained Bouguer gravity anomaly, residual density correlation image and crustal stratification structure along the profiles. The study shows a saddle-shaped distribution of Bouguer gravity anomalies along the Huili-Ludian-Zhaotong, Panzhihua-Menggu-Dajing and Shekuai-Tangdan-Huize profile, with the values ranging -278～-197×10～5ms^-2, -273～-200×10～5ms^-2, -280～-254×10～5ms^-2, respectively; the local low values locate in the Xiaojiang fault zone, the amplitude difference decreases gradually from the north to the south; the density in the Xiaojiang fault zone is lower than that of the sides, the low density zone extends to the middle and lower crust, and the material density in the east is lower than that in the west; positive and negative density anomalies overlap, indicating a poor stability of the lower crust. The Ludian earthquake occurred in this region. Layered crustal structure shows the undulation of Moho surface, with uplift beneath the Xiaojiang fault zone as the center and change of the maximum depth of Moho from 50km up to 41km from north to south. This reflects the position of Xiaojiang Fault in the regional geological structure as block boundary of Sichuan-Yunnan block and South China block.

THE KINEMATIC CHARACTERISTICS OF THE M_S6.5 LUDIAN YUNNAN EARTHQUAKE IN 2014

ZHAO Xu, LIU Jie, FENG Wei

2014, 36(4):  1157-1172.  DOI: 10.3969/j.issn.0253-4967.2014.04.018

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We employ multistep inversions in the frequency or time domain to infer the kinematic characteristics of the M_S6.5 Ludian, Yunnan earthquake in 2014, mainly using regional broadband waves recorded by the China Digital Seismic Network. In this paper, we firstly invert the focal mechanism solution and the centroid depth of the Ludian earthquake, and then determine the best-fitting finite-fault model and the dominant rupture direction. According to the above results, we further analyze and discuss the kinematic characteristics of the Ludian earthquake, and explore preliminarily the reason for the serious disaster caused by this event.
We take into account some factors which could have effects on the inversion results, e.g. the use of different waves and simplified 1-D velocity models. Several test results indicate that the misfit between observed waves and synthetics is better, if we use the full waves and the 1-D velocity model (Model M2) in this study area. According to the point-source model (focal mechanism solution), this event occurs on a true rupture plane (strike=342°, dip=83°, and rake=-34°), which shows a left-lateral strike-slip faulting with a minor normal oblique component. The centroid in the horizontal direction is located at nearly 5.4km southeast of the epicenter(27.109°N/103.354°E), and the best-fitting centroid depth is around 4.4km. The total scalar moment, M₀, is retrieved with an average value of 2.1×10¹⁸N·m (or moment magnitude M_W6.1).
The rupture history indicates the event can be considered to have an asymmetric bilateral rupture source with a radius of 10km. The total rupture area is about 227.6 km² with an average slip of nearly 0.16m. Most of the energy releases within about 6s. From 0s to 2s, the energetic rupture starts at the nucleation center, then propagates bilaterally along the fault plane. After 2s, the rupture mainly extends south-east, showing an obvious rupture directivity. Finally, the rupture ends at nearly 6s.
In order to investigate rupture directivity of the Ludian earthquake, we retrieve the apparent source duration at different stations, using the method developed by Cesca et al.(2011). Rupture directivity of the Ludian earthquake is detected on the basis of a frequency domain inversion of the apparent duration at each station and the further interpretation of its azimuthal variation. The result indicates that it is obvious that the apparent source durations at majority of stations located southeast of the epicenter of the Ludian earthquake are relatively shorter, and the shortest one is round 2s. However, the apparent source durations of majority of stations distributed northwest of the epicenter are much longer, and the longest one is up to 9s. The mean value of apparent source durations of all stations is about 4.96s. Based on the principle of the Doppler effect, this result provides a clear indication for the rupture propagating towards southeast, and thus can be used to discriminate the true fault plane (NW-SE).
In the end, we speculate that one of the most important reasons why the Ludian earthquake caused the devastating damage is that most of the energy is instantly released within relatively short duration in the shallow layer of the upper crust.

RELOCATION OF THE AFTERSHOCK SEQUENCE OF THE M_S6.5 LUDIAN EARTHQUAKE AND ITS SEISMOGENIC STRUCTURE

FANG Li-hua, WU Jian-ping, WANG Wei-lai, LÜ|Zuo-yong, WANG Chang-zai, YANG Ting, ZHONG Shi-jun

2014, 36(4):  1173-1185.  DOI: 10.3969/j.issn.0253-4967.2014.04.019

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We integrated two-month phase data recorded by Yunnan Seismic Network, Zhaotong Seismic Network, Qiaojia Seismic Array and temporal stations deployed around the Ludian earthquake source region and relocated the aftershock sequence of the Ludian earthquake. The locations of 1 750 aftershocks were determined using double-difference location algorithm. The relocation result shows that the aftershock distribution has two predominant directions, to the southeast and southwest, and shows itself as an asymmetric conjugate shape. The lengths of the two aftershock strips are about 16km. The angle between the two strips is about 100°. Aftershock distribution shows that the seismogenic fault of the Ludian earthquake is a high-angle strike-slip fault. The mainshock is located at the middle at southwest of the two aftershock strips. Early aftershocks are distributed mainly along the NW-SE direction, perpendicular to the Zhaotong-Ludian Fault. The aftershocks located to the southwest of the mainshock may be triggered by the mainshock. According to the aftershock distribution and its relations with neighboring faults, focal mechanism of the mainshock, the long axis orientation of seismic intensity map, and distribution of landslides, we speculate that the seismogenic fault is the Baogunao-Xiaohe Fault. There are significant differences not only in seismic activity, deep velocity structure, but also the block movement direction and rate on both sides of the Baogunao-Xiaohe Fault. The northward expansion of aftershock activity may be blocked by the high-velocity anomaly zone located on the north side of the Baogunao-Xiaohe Fault.

INVENTORY OF LANDSLIDES TRIGGERED BY THE 2014 M_S6.5 LUDIAN EARTHQUAKE AND ITS IMPLICATIONS ON SEVERAL EARTHQUAKE PARAMETERS

XU Chong, XU Xi-wei, SHEN Ling-ling, DOU Shuai, WU Sai-er, TIAN Ying-ying, LI Xi

2014, 36(4):  1186-1203.  DOI: 10.3969/j.issn.0253-4967.2014.04.020

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On August 3, 2014, an earthquake with M_S6.5 occurred at Ludian County, Yunnan Province of China. The earthquake caused 617 deaths, 112 missing, and 3, 143 injured. Thousands of landslides were triggered by the earthquake and some of the landslides buried a few settlements, which resulted in significant casualties and property losses. In this study, we compiled an inventory of landslides triggered by the Ludian earthquake based on visual interpretation of high resolution satellite images, which are TH01-02 and SJ9A satellite images photoed post-earthquake and GF1 images shot pre-earthquake. Furthermore, some of the landslides were verified by field photos and/or very high resolution aerial photographs. The result shows the Ludian earthquake triggered at least 1024 landslides with an area equal to or larger than 100 m². The landslides are distributed in a 250km² area, with a total landslide area of 5.19 km² and a total volume of 2.2×10⁷m³. In the landslide-distributing area, the landslide number density is 4.03km^-2, the landslide area density is 2.04%, and the landslide erosion thickness is about 86.7mm. The statistical result of landslide number and area in different directions of the epicenter shows that the main spread direction of the landslides is northwest-southeast and most of the landslides occurred southeast of the epicenter. This suggests that the seismogenic fault of the earthquake probably trends to northwest and the rupture direction is from northwest to southeast, which is consistent with evidences from seismic, geological, geophysical, as well as other aspects. Comparing the landslide distribution area, landslide number, landslide area, and landslide volume related to the Ludian earthquake with those of other earthquakes worldwide, the result shows that the earthquake registered a smaller landslide distribution area but a larger landslide area and a much larger landslide volume. It suggests that the hypocenter of the Ludian earthquake is shallow and seismic energy attenuation of the event is quite rapid.

THREE-DIMENSIONAL CRUST STRUCTURE BENEATH SE TIBETAN PLATEAU AND ITS SEISMOTECTONIC IMPLICATIONS FOR THE LUDIAN AND JINGGU EARTHQUAKES

LI Yong-hua, XU Xiao-ming, ZHANG En-hui, GAO Jia-yi

2014, 36(4):  1204-1216.  DOI: 10.3969/j.issn.0253-4967.2014.04.021

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A detailed 3D crust S-wave velocity model is derived from joint analysis of Rayleigh wave group velocity and teleseismic P-wave receiver functions at permanent stations on the southeast margin of Tibet plateau and its surrounding area. Our new models show the velocity structure in the crust beneath SE Tibet is strongly heterogeneous. There are strong lateral variations in crustal thickness, which increases gradually from 30km in the south and east of Yunan to～65km in the SE Tibetan Plateau. Two obvious low velocity zones (LVZs) are revealed at various depths in the crust. The shallower LVZ in the middle crust (15～20km depth) are limited in the Tengchong volcano and Sichuan-Yunnan (Chuan-Dian) rhombus block. Another LVZ in the middle-to-lower crust varies between 25 and 40km and it shallows toward the east and southeast and is absent in the Sichuan Basin and the southern part of this study area. Our shear velocity model clearly shows an upper crustal high-velocity body and two LVZs in the middle crust and middle-to-lower crust (30～40km depth) across the source area of the 2014 Ludian earthquake. Ludian earthquake sequences and the neighbouring Yongshan-Daguan seismic zone are distributed in the upper crustal high-velocity body. In contrast, no obvious intra-crustal low velocity zones (IC-LVZs) appear beneath the Jinggu earthquake and its adjacent areas. But low velocity anomalies are found in the upper crust beneath the Jinggu earthquake and its neighbouring Simao-Pu'er seismic zone, which may be due to a highly fractured and fluid-filled rock matrix that may have initiated the nucleation of the Jinggu earthquake.

GRAVITY CHANGES BEFORE THE LUDIAN, YUNNAN, M_S6.5 EARTHQUAKE OF 2014 AND THE THREE-DIMENSIONAL INVERSION RESEARCH

SHI Lei, JIA Xiao-dong, CHEN Shi, XU Wei-min, LU Hong-yan

2014, 36(4):  1217-1227.  DOI: 10.3969/j.issn.0253-4967.2014.04.022

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Based on the 6 campaigns relative gravity observation data of the Xichang gravity survey network from 2012 to 2014, we analyze the gravity change patterns between two adjacent observation campaigns, the dynamic patterns of cumulated gravity change relative to the first campaign and the gravity time-variation at some stations near the epicenter of Ludian earthquake in the study area. We find that there was no obvious cumulative trend anomaly before the Ludian M_S6.5 earthquake 2 years ago, and the possible precursor signals are related to the gravity difference changes of the stations located at the two sides of the fault slip surface during the period of 2014-03—2014-06. Using equivalent source model to inverse these gravity anomalies, we deduce the density changes in 10^-5g/cm³ orders of magnitude to the depth of epicenter. It is inferred that the change of the mass source in the short time may be related with the filling or migration of pore fluid in the crust medium. Under the equal dynamic condition of short-time tectonic movement, fault slip in the seismogenic zone may be triggered due to fluid migration and filling, thus, resulting in generation of earthquake.

TECTONIC CHARACTERISTICS OF STRONG EARTHQUAKES IN DALIANGSHAN SUB-BLOCK AND IMPACT OF THE M_S6.5 LUDIAN EARTHQUAKE IN 2014 ON THE SURROUNDING FAULTS

CHENG Jia, LIU Jie, XU Xi-wei, GAN Wei-jun

2014, 36(4):  1228-1243.  DOI: 10.3969/j.issn.0253-4967.2014.04.023

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The M_S6.5 Ludian earthquake occurred on 3 August 2014 in Yunnan, China. The epicenter of this earthquake is located in the Dalingshan sub-block, a boundary region among the Bayan Har block, the Sichuan-Yunnan block and the South China block, which is dominated by the left-lateral and thrust-slip faults. The studies on the characteristics of the crustal deformation, the mechanism of strong earthquakes and stress changes after the M_S6.5 Ludian earthquake in the Daliangshan sub-block will help us understand the tectonic implication of the earthquake and facilitate further in-depth studies in the region.
This article introduces the slip behavior around the Daliangshan sub-block and strong earthquake distribution on the faults. Using the GPS data in the southeastern Tibetan Plateau, this study analyzes the motions of sub-blocks bordering the Daliangshan sub-block and the slip behavior of the boundary faults in the block model, and estimates the crustal motion in the Daliangshan sub-block after removing the whole block motion of the sub-block. It can be shown clearly that the Daliangshan sub-block has absorbed the compression via the Xianshuihe Fault, the Anninghe Fault and the Longmenshan Fault. Due to the compression from the boundary faults, the crustal motion in the sub-block is mainly characterized by the north-northwest trending left-lateral strike-slip faults. The boundary faults of the Daliangshan sub-block are mainly characterized by strong earthquakes with magnitude larger than M7 in the history. But in the interior of the Daliangshan sub-block, there are mainly the moderate earthquakes. The M_S6.5 Ludian earthquake ruptured the Baogunao-Xiaohe Fault, a left-lateral strike-slip fault associated with the main thrust Ludian-Zhaotong Fault and the Lianfeng Fault, where the whole thrust slip rate of the two faults is about 2.4mm/a.
Around the epicenter of the Ludian earthquake, there are strong earthquakes larger than M7 recorded in the nearby region, which might have impact on the occurrence of the Ludian earthquake. Also, the Ludian earthquake may further affect the occurrence of subsequent earthquakes. In this paper, we have calculated the static Coulomb failure stress changes (ΔCFS) on the fault plane of the Ludian earthquake induced by the 3 nearby big earthquakes, including the M7 1/2 earthquake in 1850 on the Zemuhe Fault and the M7 3/4 earthquake in 1733 on the Xiaojiang Fault. The M_S6.5 Ludian earthquake ruptured the north-northwest trending Baogunao-Xiaohe Fault, which is associated with the main thrust fault system of the Zhaotong-Ludian Fault and Lianfeng Fault. In our results, the Ludian earthquake was promoted by the M7 3/4 in 1733 on the Xiaojiang Fault and the M7 1/2 in 1850 on the Zemuhe Fault earthquakes. From the Coulomb failure stress change calculation, the ΔCFS value is about 0.03MPa, which may advance the occurrence of the M_S6.5 Ludian earthquake obviously. The calculations also showed opposite results of ΔCFS from the Ludian earthquake on the Yongshan M_S5.0 earthquake of 17 August and on the Yuexi M_S5.0 earthquake of 1 October in 2014. The former one is not much related to Ludian earthquake, but to the normal seismicity in the reservoir area as to the minus value of ΔCFS, while the Yuexi M_S5.0 earthquake was promoted by the M_S6.5 Ludian earthquake. Moreover, the M_S6.5 Ludian earthquake has advanced, to a certain extent, the enhancement of Coulomb failure stress on the northern segment of the Daliangshan Fault, Ebian Fault, eastern segment of the Zhaotong-Ludian Fault and the southern segment of the Zemuhe Fault, and has enhanced the earthquake energy accumulation of these faults.

CAN THE DD-RELOCATED EARTHQUAKE CATALOGUE BE USED FOR THE STATISTICAL PARAMETERS OF AN EARTHQUAKE SEQUENCE？—A CASE STUDY OF THE SPATIAL DISTRIBUTION OF B-VALUES FOR THE AFTERSHOCKS OF THE 2014 LUDIAN M_S6.5 EARTHQUAKE

ZHANG Sheng-feng, WU Zhong-liang, FANG Li-hua

2014, 36(4):  1244-1259.  DOI: 10.3969/j.issn.0253-4967.2014.04.024

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Relocated earthquake catalogue by double difference(DD)algorithm, while significantly improving the location precision, suffers from the degeneration of catalogue completeness. One of the questions subject to discussion is whether such DD-relocated catalogue is really an improvement, or otherwise a drawback, when calculating the spatially dependent statistical parameters of seismicity. In such statistical calculation, catalogue completeness is one of the key issues determining the quality of the result. Investigating this problem, this paper carries out a case study on the August 3, 2014, Ludian, Yunnan, M_S6.5 earthquake sequence. Aftershocks within 40 days since the mainshock are analyzed using the routine catalogue provided by the national seismic network and a DD-relocated catalogue. The Gutenberg-Richter b-value, as well as its spatial distribution, from both catalogues, are calculated and compared. Results show that the degeneration of catalogue completeness of the DD-relocated catalogue depends on the clustering property of the earthquakes to much extent. Degeneration of catalogue completeness occurs at the margin of an earthquake cluster. Distribution of b-values based on the DD-catalogue provides clues to the source properties of the Ludian earthquake.

LATE-QUATERNARY ACTIVITY OF THE ZHAOTONG-LUDIAN FAULT ZONE AND ITS TECTONIC IMPLICATION

CHANG Zu-feng, ZHOU Rong-jun, AN Xiao-wen, CHEN Yu-jun, ZHOU Qing-yun, LI Jian-lin

2014, 36(4):  1260-1279.  DOI: 10.3969/j.issn.0253-4967.2014.04.025

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The Zhaotong-Ludian Fault zone, composed mainly of three right-step en echelon faults, namely, the Zhaotong-Ludian Fault, the Sayuhe Fault and the Longshu Fault, strikes 40°～60° on the whole, with the Sayuhe Fault and the Longshu Fault dipping SE and the Zhaotong-Ludian Fault dipping NW, and they all together constitute a complicated thrust fault system. Based on years of field investigation results of geology and geomorphography, we elaborate the late Quaternary active features, the geological and geomorphic evidences of the latest activity of the Zhaotong-Ludian Faults. Our observation shows that: the late Cenozoic basins along the Zhaotong-Luian Fault zone are obviously dominated by the fault; there are many neo-active fault landforms, such as, flat and straight fault troughs, directional aligned fault facets and fault scarps, and the upper Pleistocene to Holocene strata are offset by the fault. The fault zone has been active since the late Quaternary. For example, the fault at Daqiaobian dislocated a set of strata of the Pliocene, and middle to upper Pleistocene, with an apparently reverse character. The fault trending NE is developed in the Holocene diluvium with oblique striation on the fault plane at Guangming Village. Deposits with an OSL age of(23.4±1.8)ka BP on T2 terrace of a small river near Beizha town was offset by the fault. There is a fault scarp trending NE 40°, 0.5～2.0m in height, on the first terrace of the Longshu River near the Longshu Village. Several Quaternary faults are revealed by the trench which offset the late Pleistocene to Holocene strata and there are three poleo-earthquake events discovered in the trench. At Yanjiao Village the gravel layer has risen steeply and is aligned in a line because of squeezing effect of the fault; the rivers and ridges nearby are synchronously offset dextrally up to 30～40m. The fault zone is dominated by reverse faulting with a small amount of right-lateral motion. Besides, there are some NW-trending faults interweaving with the NE-trending fault zone, some of which are active since late Quaternary as well, and they are the conjugate structures with the NE-trending faults. Surface deformation, such as NE- and NW-trending ground fissures and reverse scarp landforms, has been generated in the epicenter area of the 2014 Ludian M6.5 earthquake, the distribution of which is in consistence with the NE- and NW-trending faults. Because of far-field deformation response and energy exchange and transfer between blocks, the Liangshan active sub-block formed on the east of the Sichuan-Yunnan block, and the Zhaotong-Ludian Fault zone lies in the forefront of the SE movement of this sub-block. On account of its distinct location and its complicated geometric structure, the Zhaotong-Ludian Fault zone is one of main carriers of the tectonic deformation of the Liangshan active sub-block to absorb and accommodate the strains produced by the block's SE movement, and is the southern boundary of the Liangshan sub-block. From the point of view of the regional tectonic positions and the kinematic characteristics, the relation of Zhaotong-Ludian Fault zone to the Liangshan active sub-block is exactly as the relation of the Longmanshan Faults to Bayan Har block. Consequently, the Zhaotong-Ludian Fault zone has an important significance in the division of active block boundaries and the regional tectonic framework, and meanwhile, it is also an important seismogenic structure in the northeastern Yunnan.

LUDIAN M_S 6.5 EARTHQUAKE SURFACE DAMAGE AND ITS RELATIONSHIP WITH STRUCTURE

LI Xi, ZHANG Jian-guo, XIE Ying-qing, MIAO Qing-wen

2014, 36(4):  1280-1291.  DOI: 10.3969/j.issn.0253-4967.2014.04.026

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The 3^rd Aug 2014 Ludian M_S 6.5 earthquake caused heavy casualties and damages to houses, which is one of the typical phenomena that a small earthquake causing heavy disasters in recent years in Yunnan Province. After the earthquake, a team (including the author) carried out a number of field investigations to the hardest-hit Longtoushan Township. The surveys involved various earthquake disasters, including landslides, ground fissures, fault profiles, building (construction) disasters, and so on. It is found that Longtoushan Township is the most seriously hit area, which locates on the intersection of the NW- and NE-trending faults. The earthquake caused a number of landslides with a predominant distribution in NE direction. The landslides distribution has a good relationship with the NE-trending Zhaotong-Ludian Fault. The ground fissures in the hardest-hit Longtoushan township showed a predominant distribution of NE direction, while the ground fissures in Guangming Village, which locates on the south of Longtoushan township about 6.5km away (Fig. 2), showed a predominant distribution of NW direction.
Meanwhile, a series of NE-, NW-trending fault outcrops, suspected fault scarps and troughs were found, and fissures and small dislocations were generated along these structures. On the basis of the field investigations, and considering the distribution characteristics of aftershock sequences and earthquake isoseismic lines, we infer that the seismogenic structure is the NW-trending Baogunao-Xiaohe Fault. Meanwhile, we could not exclude the tectonic effect of the NE-trending fault, for a large number of geological disasters such as tectonic fissures and landslides were observed along the NE-trending fault. The ground fissures and small dislocations are closely related to the distribution of the faults, which are likely to be a reflection of coseismic surface rupture of this earthquake.