Table of Content

20 December 2018, Volume 40 Issue 6

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RUPTURE CHARACTERISTICS OF LATE QUATERNARY STRONG EARTHQUAKES ON THE WESTERN BRANCH OF THE XIAOJIANG FAULT ZONE

LI Xi, RAN Yong-kang, WU Fu-yao, MA Xing-quan, ZHANG Yan-qi, CAO Jun

2018, 40(6):  1179-1203.  DOI: 10.3969/j.issn.0253-4967.2018.06.001

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The Xiaojiang fault zone is located in the southeastern margin of the Tibetan plateau, the boundary faults of Sichuan-Yunnan block and South China block. The largest historical earthquake in Yunnan Province, with magnitude 8 occurred on the western branch of the Xiaojiang Fault in Songming County, 1833. Research on the Late Quaternary surface deformation and strong earthquake rupture behavior on the Xiaojiang Fault is crucial to understand the future seismic risk of the fault zone and the Sichuan-Yunnan region, even crucial for the study of tectonic evolution of the southeastern margin of Tibetan plateau. We have some new understanding through several large trenches excavated on the western branch of the Xiaojiang fault zone. We excavated a large trench at Caohaizi and identified six paleoseismic events, named U through Z from the oldest to the youngest. Ages of these six events are constrained at 40000-36300BC, 35400-24800BC, 9500BC-500AD, 390-720AD, 1120-1620AD and 1750AD-present. The Ganhaizi trench revealed three paleoearthquakes, named GHZ-E1 to GHZ-E3 from the oldest to the youngest. Ages of the three events are constrained at 3300BC-400AD, 770-1120AD, 1460AD-present. The Dafendi trench revealed three paleoearthquakes, named E1 to E3 from the oldest to the youngest, and their ages are constrained at 22300-19600BC, 18820-18400BC, and 18250-present. Caohaizi and Ganhaizi trenches are excavated on the western branch of the Xiaojiang Fault, the distance between them is 400m. We constrained four late Holocene paleoearthquakes with progressive constraining method, which are respectively at 500-720AD, 770-1120AD, AD 1460-1620 and 1833AD, with an average recurrence interval of 370~440a. Large earthquake recurrence in the late Holocene is less than the recurrence interval of~900a as proposed in the previous studies. Thus, the seismic hazard on the Xiaojiang Fault should be reevaluated. We excavated a large trench at Dafendi, about 30km away south of Caohaizi trench. Combining with previous paleoseismological research, it is found that the western branch of Xiaojiang Fault was likely to be dominated by segmented rupturing in the period from late of Late Pleistocene to early and middle Holocene, while it was characterized by large earthquakes clustering and whole segment rupturing since late Holocene.

HOLOCENE SLIP RATE ALONG THE NE-TRENDING QIXIANG CO FAULT IN THE CENTRAL TIBETAN PLATEAU AND ITS TECTONIC IMPLICATIONS

LI Kang, WANG Duo, SHAO Qing-feng, XU Xi-wei

2018, 40(6):  1204-1215.  DOI: 10.3969/j.issn.0253-4967.2018.06.002

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The two mainstream deformation models of the Tibet plateau are continental escape model and crustal thickening model, the former suggests that the NW-trending Karakoram Fault, Gyaring Co Fault, Beng Co Fault and the Jiali Fault as the Karakoram-Jiali fault zone is the southern border belt and that the dextral strike-slip rate is estimated as up to 10~20mm/yr. However, research results in recent years show that the slip rates along those faults are significantly less than earlier estimates. Taylor et al. (2003)suggest that the conjugate strike-slip faults control the active deformation in the central Tibet.
The lack of research on the slip behavior of the NE-trending faults in the central Tibet Plateau constrains our understanding of the central Tibet deformation model. Thus, we choose the NE-direction Qixiang Co Fault located at the north of the Gyaring Co Fault as research object. Based on the interpretation of satellite images, we found several faulted geomorphic sites. Using RTK-GPS ground control point and unmanned aerial vehicle (UAV)topographic surveying, we obtained less than 10cm/pix-resolution digital elevation model (DEM)in the Yaqu town site. We used the LaDiCaoz_v2.1 software to automatically extract the left-lateral offset of the largest gully on the terrace T₂ surface, which is (21.3±7.1)m, and the vertical dislocation of the scarp on the terrace T₂ surface, which is (0.9±0.1)m. The age of both U-series dating samples on the terrace T₂ is (4.98±0.17)ka and (5.98±0.07)ka, respectively. The Holocene left-lateral slip rate along Qixiang Co Fault is (3.56±1.19)mm/a and the vertical slip rate is (0.15±0.02)mm/a. The kinematic characteristics of the sinistral strike-slip with normal slip coincide with the eastward motion of the central Tibet plateau, and its magnitude is in agreement with its conjugate Gyaring Co Fault, suggesting that the deformation pattern of the central Tibetan plateau complies with the conjugate strike-slip faults mode.

PALEOSEISMOLOGICAL RESEARCH ON THE LANGSHAN RANGE-FRONT FAULT, HETAO AREA, CHINA: INSIGHTS FOR STUDIES ON TECTONICS AND SEISMIC RISK

DONG Shao-peng, ZHANG Pei-zhen, ZHENG Wen-jun, YU Zhong-yuan, LEI Qi-yun, YANG Hui-li, LIU Jin-feng, GONG Hui-ling

2018, 40(6):  1216-1239.  DOI: 10.3969/j.issn.0253-4967.2018.06.003

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The Langshan range-front fault (LRF)is a Holocene active normal fault that bounds the Langshan Mountain and Hetao Basin at the northwest corner of the Ordos Plateau. Paleoseismic trenching research at three sites, Dongshen Village trench (TC1), Qingshan trench (TC2)and Wulanhashao trench (TC3)from north to south was performed in this study to reveal the seismic hazard risk in Hetao Basin. The paleoevents ED1, ED2, ED3 from TC1 can be constrained to have occurred (6±1.3)ka, (9.6±2)ka and (19.7±4.2)ka respectively, while the paleoevent EQ1 from TC2 occurred about (6.7±0.1)ka and the paleoevents EW1, EW2, EW3 at TC3 took place about (2.3±0.4)ka, (6±1)ka and before 7ka respectively. In combination with paleoseismic results of previous researchers, the Holocene earthquake sequence of the LRF could be established as 2.3~2.43ka BP (E1), 4.41~3.06ka BP (E2), 6.71~6.8ka BP (E3), 7.6~9.81ka BP (E4), and (19.7±4.2)ka BP (E5). Although the possibility of missing events cannot totally be ruled out, based on the analysis on faulted geomorphology at Wulanhashao site, we argue the paleoearthquake history of the LRF during Holocene may be complete with an average recurrent interval about 2500 yrs. The apparent displacements associated with events E1, E3 and E4 are significantly larger than that of event, E2, that suggests that they might be great events with magnitudes 7.5 to even over 8 that ruptured the entire LRF, while the event E2 may be a smaller event that only ruptured a segment of the fault. The magnitude of event E2 might be about M7. This poses a significant seismic hazard to the area of the Linhe depression in the western Hetao graben region. With the further limitation of previous radiocarbon dating result near our trench site at Wulanhashao, the slip rate at Wulanhashao should be not smaller than, but close to 0.66mm/a since 15ka BP. And the slip rate at Qingshan site is supposed to be about 1.4~1.6mm/a since 6.8ka BP. Both our combined most recent paleoseismic cognition and current tectonic geomorphologic research results supports to reveal that the Langshan range-front fault now is an unsegmented fault, preferring to rupture the whole fault in a surface-rupture event. Considering the most recent event E1 and fault slip rate obtained above, the accumulated strain on the LRF could be estimated as about 1.52~3.94m. Given the ~2500a recurrent interval, we argue that the elapsed time since last major quake, E1, is approaching or even over the recurrence, and the seismic risk for another major quake is imminent, at least cannot be ignored.

GROWTH STRATA REVEAL THE QUATERNARY TECTONIC FEATURE OF GUDIAN FAULT IN SONGYUAN, SONGLIAO BASIN, CHINA

YU Xiao-hui, SHEN Jun, DAI Xun-ye, WANG Chang-sheng

2018, 40(6):  1240-1253.  DOI: 10.3969/j.issn.0253-4967.2018.06.004

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Because of the frequent seismic activity in Songyuan in recent years, the modes of tectonic movement in this area since the Quaternary have attracted increasing consideration. This paper selects the Gudian Fault which locates between the southeast uplift and central depression of Songliao Basin as the research object. We discussed the Quaternary structural characteristics of the Gudian Fault using growth strata. Using the data of deep seismic reflection prospecting for oil, we determined the location, geometry and kinematics characteristics of the Gudian Fault. And using the shallow seismic reflection prospecting data, the combined drilling exploration data and TL data, we determined precisely the inversion tectonics feature of the fault since late Cenozoic. Based on the above data, we believe that the Gudian Fault is dominated mainly by thrust-folding since Quaternary. A set of growth strata is recognized by shallow seismic reflection exploration data. According to the overlap of growth strata and the relationship between deposition rate and uplift rate, we confirm that the uplift rate of Gudian Fault in the early of Early Pleistocene is less than 0.15mm/a. And according to the offlap of growth strata and the relationship between deposition rate and uplift rate, the uplift rate of the Gudian Fault is more than 0.091mm/a in the late of Early Pleistocene and more than 0.052mm/a in middle Pleistocene. According to the chronological data, it is determined that the uplift rate of the Gudian Fault is 0.046mm/y since 205ka.

USE OF SEISMIC WAVEFORMS AND INSAR DATA FOR DETERMINATION OF THE SEISMOTECTONICS OF THE MAINLING M_S6.9 EARTHQUAKE ON NOV.18, 2017

LIU Yun-hua, SHAN Xin-jian, ZHANG Ying-feng, ZHAO De-zheng, QU Chun-yan

2018, 40(6):  1254-1275.  DOI: 10.3969/j.issn.0253-4967.2018.06.005

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On November 18, 2017, a M_S6.9 earthquake struck Mainling County, Tibet, with a depth of 10km. The earthquake occurred at the eastern Himalaya syntaxis. The Namche Barwan moved northward relative to the Himalayan terrane and was subducted deeply beneath the Lhasa terrane, forming the eastern syntaxis after the collision of the Indian plate and Asian plates. Firstly, this paper uses the far and near field broadband seismic waveform for joint inversion (CAPJoint method)of the earthquake focal mechanism. Two groups of nodal planes are obtained after 1000 times Bootstrap test. The strike, dip and rake of the best solution are calculated to be 302°, 76° and 84° (the nodal plane Ⅰ)and 138°, 27° and 104° (the nodal plane Ⅱ), respectively. This event was captured by interferometric synthetic aperture radar (InSAR)measurements from the Sentinel-1A radar satellite, which provide the opportunity to determine the fault plane, as well as the co-seismic slip distribution, and assess the seismic hazards. The overall trend of the deformation field revealed by InSAR is consistent with the GPS displacement field released by the Gan Wei-Jun's team. Geodesy (InSAR and GPS)observation of the earthquake deformation field shows the northeastern side of the epicenter uplifting and the southwestern side sinking. According to geodetic measurements and the thrust characteristics of fault deformation field, we speculate that the nodal plane Ⅰ is the true rupture plane. Secondly, based on the focal mechanism, we use InSAR data as the constraint to invert for the fine slip distribution on the fault plane. Our best model suggests that the seismogenic fault is a NW-SE striking thrust fault with a high angle. Combined with the slip distribution and aftershocks, we suggest that the earthquake is a high-angle thrust event, which is caused by the NE-dipping thrust beneath the Namche Barwa syntaxis subducted deeply beneath the Lhasa terrane.

USING UNMANNED AERIAL VEHICLE PHOTOGRAMMETRY TECHNOLOGY TO OBTAIN QUANTITATIVE PARAMETERS OF ACTIVE TECTONICS

AI Ming, BI Hai-yun, ZHENG Wen-jun, YIN Jin-hui, YUAN Dao-yang, REN Zhi-kun, CHEN Gan, LIU Jin-rui

2018, 40(6):  1276-1293.  DOI: 10.3969/j.issn.0253-4967.2018.06.006

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With the development of photogrammetry technology and the popularity of unmanned aerial vehicles (UAVs)technology in recent years, using UAV photogrammetry technology to rapidly acquire high precision and high resolution topographic and geomorphic data on the fault zone has gradually become an important technical means. This paper first summarizes the basic principle and workflow of a new digital photogrammetry technology, SfM (Structure from Motion), which is simple, efficient and low cost. Using this technology, we conducted aerial image acquisition and data processing for a typical fault landform on the northern of Caka Basin in Qinghai. The digital elevation model (DEM)with 6.1cm/pix resolution is generated and the density of point cloud is as high as 273 points/m². The coverage area is 0.463km². Further, the terrain and slope data parallel to the fault direction are extracted by topographic analysis method, and combined with the contour map and the slope diagram generated by the DEM, a fine interpretation and quantitative study of complex multilevel geomorphic surfaces is carried out. Finally, based on the results of sophisticated interpretation of geomorphology, we got the vertical displacements of the T₁ terrace to the T₃ terrace as (1.01±0.06)m, (1.37±0.13)m and (3.10±0.11)m, and the minimum vertical displacements of the T₄ terrace and the T₅ terrace as (3.77±0.14)m and (5.46±0.26)m, respectively, through the topographic profile data extracted by DEM. Such vertical displacement parameters are difficult to obtain directly by traditional remote sensing images, which shows the great application prospect of UAV photogrammetry technology in the quantitative study of active tectonics.

FOCAL MECHANISM OF THE AUGUST 8, M7.0 SICHUAN JIUZHAIGOU AND AUGUST 9, M6.6 XINJIANG JINGHE EARTHQUAKES OF 2017

GUO Zhi, CHEN Li-chun, LI Tong, GAO Xing

2018, 40(6):  1294-1304.  DOI: 10.3969/j.issn.0253-4967.2018.06.007

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The W-phase is a long period phase arriving between the P and S wave phases of a seismic source, theoretically representing the total near-and far-field long-period wave-field. Recent study suggests that the reliable source properties of earthquake with magnitude greater than ~M_W4.5 can be rapidly inverted by using the W-phase waveform data. With the advantage of W-phase, most of major earthquake research institutes in the world have adopted the W-phase based inversion method to routinely assess focal mechanism of earthquake, such as the USGS and GFZ. In this study, the focal mechanism of the August 8, 2017 M7.0 Sichuan Jiuzhaigou and August 9, 2017 M6.6 Xinjiang Jinghe earthquakes were investigated by W-phase moment tensor inversion technique using global seismic event waveform recordings provided by Incorporated Research Institutions for Seismology, Data Management Center. To get reliable focal mechanism, we strictly select raw waveform data and carry out inversion in stages. At first, we discard waveform without correct instrument information. Then we carry out an initial inversion using selected waveform data to get primary results. Using the preliminary results as input, we carry out grid-search based inversion to find the final optimal source parameters. The inverted results indicate that the August 8, M7.0 Sichuan Jiuzhaigou shock resulted from rupturing on a NW-trending normal fault with majority of strike-slip movement. The parameters of two nodal planes are strike 152.7°, dip 61.4°, rake -4.8° and strike 245.0°, dip 85.8°, rake -151.3° respectively, and focal depth is 14.0km. The August 9, Xinjiang Jinghe M6.6 shock resulted from rupturing on a south-dipping thrust fault with left-lateral strike-slip. The parameters of two nodal planes are strike 100.6°, dip 27.5°, rake 114.1° and strike 259.3°, dip 65.1°, rake 78.0°, and the focal depth is 16.0km. The direction of two nodal planes is consistent with regional seismotectonic background.

ANALYSIS OF THE CONFIDENCE INTERVAL OF THE COMPOSITE FAULT PLANE SOLUTION OBTAINED FROM THE GRID SEARCH METHOD: AN EXAMPLE OF SOUTHERN JIANGXI PROVINCE

WANG Gan-jiao, HU Xing-ping, GAO Wei, CUI Xiao-feng

2018, 40(6):  1305-1317.  DOI: 10.3969/j.issn.0253-4967.2018.06.008

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In order to quantitatively analyze the reliability of the composite fault plane solution of small earthquakes, the Bootstrap sampling technique is introduced into the grid search method, and the ideas and methods for calculating the confidence interval from the grid search method are proposed initially. There are two sample sets that can represent the characteristics of the composite fault plane solution. One sample set is the optional solutions obtained by the grid search method and the other is obtained by the Bootstrap sampling technique. Then, we calculate the confidence intervals of the two sample sets (P, B and T axis). The research results of tectonic stress field in southern Jiangxi Province are relatively few. In view of such situation, we use the focal mechanism solutions of small earthquakes to calculate and analyze the composite fault plane solution and the confidence interval. This study shows that the confidence interval of the principal stress axis can be obtained well by both of the sample methods. The reliability of the results and the confidence range of the principal stress axis can be better represented by the confidence intervals. The middle principal stress in southern Jiangxi Province is nearly vertical, and the maximum and minimum principal stresses are nearly horizontal. The direction of maximum principal stress is NWW-SEE and that of the minimum principal stress is NNE-SSW. And, the area is in a strike-slip stress regime. The results are consistent with previous studies, and the stress directions obtained by previous researchers are within the confidence interval calculated by this paper.

THE DEPTH OF MOHO INTERFACE AND CRUSTAL THICKNESS IN SICHUAN-YUNNAN REGION, CHINA

XU Zhi-ping, WANG Fu-yun, JIANG Lei, XU Shun-qiang, TANG Lin

2018, 40(6):  1318-1331.  DOI: 10.3969/j.issn.0253-4967.2018.06.009

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By using moving average method to separate Bouguer gravity anomaly field in Sichuan-Yunnan region, we got the low-frequency Bouguer gravity anomaly field which reflects the undulating of Moho interface. The initial model is obtained after seismic model transformation and elevation correction. Then, we used Parker method to invert the low-frequency Bouguer gravity anomaly field to obtain the depth of Moho interface and crustal thickness in the area. The results show that the Qinghai-Tibet block in the northwest of the study area deepens and thickens from the edge to the interior, with the depth of Moho interface and the crust thickness of about 52~62km and 54~66km, respectively. The depth of Moho interface in Sichuan Basin is about 38~42km. In Sichuan-Yunnan block, the depth of Moho interface is about 42~62km from southeast to northwest. Beneath the West Yunnan block, west of the Red River fault zone, the Moho depth is about 34~52km from south to north. The Longmen Mountains and Red River fault zone are the gradient zone of the Moho depth change. Along the Red River fault zone, the depth difference of Moho interface is increasing gradually from north to south. No obvious uplift is found on the Moho interface of Panzhihua rift valley. The depth of Moho interface distribution in Sichuan and Yunnan is obviously restricted by the collision between the Indian plate and the Eurasian plate and the lateral subduction of the Indo-China peninsula. The mean square error of the depth of Moho interface is less than 1.7km between the result of divisional density interface inversion and artificial seismic exploration. At the same time, we compared the integral with divisional inversion result. It shows that:in areas where there is obvious difference between the crust velocity and density structure in different tectonic blocks, the use of high resolution seismic exploration data as the constraints to the divisional density interface inversion can effectively improve the reliability of inversion results.

DISCUSSION ON THE SEISMOGENIC STRUCTURE OF ZHAN-JIANG BAY AREA FROM THE VIEW OF DEEP FAULT SYSTEM INTERPRETED BASED ON THE GRAVITY DATA

BI Li-si, REN Zhen-huan, YE Xiu-wei, WU Ye-biao, LIU Tian-you, QIAO Ji-hua

2018, 40(6):  1332-1348.  DOI: 10.3969/j.issn.0253-4967.2018.06.010

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The neotectonics in Zhanjiang Bay area is almost the inferred faults and there are not any active faults seen on the ground surface. So it is difficult for research on the seismogenic structure. This paper analyzes and interpretes the gravity data that can reflect the feature of deep faults and then discusses the seismogenic structure of Zhanjiang Bay area in combination with its geology and earthquake activity. There is a huge NEE-trending high gravity gradient belt lying in the coastal region among Guangdong, Guangxi, and Hainan, and Zhanjiang Bay is located in this gravity gradient belt. We analyzed and interpreted more than eighty images obtained with many different methods one by one, then, got the result that Zhanjiang Bay area is embraced by two giant fault belts trending in the NEE and NW direction respectively, and its interior is crossed over by the NE-trending fault belt. These three fault belts are well shown in the gravity images, especially the NEE-trending fault belt and NW one. The gravity isolines and gradient belts or the thick black stripes of the NEE-and NW-trending fault belts are displayed apparently. Also, these gravity structures are good in continuity, extend vastly and cut deeply. What is more, the NEE-trending fault belt plays a leading and region-controlling part. It shows good continuity, and cuts off the NW-and NE-trending faults frequently and intensively. The NW-trending fault belt also is good in continuity and cuts the NEE-and NE-trending faults relatively frequently and strongly, but it is restricted by the NEE-trending one. Last, the continuity of the NE-trending fault is worse and the strength cutting off NE-and NW-trending faults is significantly weak, just in some segments and in the shallow positions. According to the characteristics above and combined with the analyses of geological structure and earthquake activity, the conclusion can be drawn that the NEE-trending fault is the controlling structure and the main seismogenic structure in Zhanjiang Bay area, and the NW-trending fault is the second one. They conjugate and act together. Therefore, Zhanjiang Bay has the tectonic condition for generating M_S=6.5 earthquakes.

GROUND MOTION CHARACTERISTICS OF GAOYOU-BAOYING M4.9 EARTHQUAKE

WANG Heng-zhi, YU Yan-xiang, HE Bin, PENG Xiao-bo, ZHAN Xiao-yan, ZHU Sheng-chu

2018, 40(6):  1349-1361.  DOI: 10.3969/j.issn.0253-4967.2018.06.011

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An M4.9 earthquake occurred at the junction of Gaoyou and Baoying on July 20, 2012. In this paper, 43 sets of strong motion records of the main shock are analyzed. With these data, we analyzed the characteristics of the peak ground motion value, attenuation relation, duration and acceleration response spectrum. We draw the peak acceleration contour map of the region near the epicenter. The contour line is smooth and the trend of long axis is northwest-southeast. Distribution of peak acceleration of the observed records is basically consistent with the real intensity distribution. Compared with the predicted result based on the seismic attenuation relation proposed by Yu Yanxiang and Wang Suyunon for eastern China and the Fifth-generation ground motion zonation map, the horizontal PGA and PGV of Gaoyou-Baoying earthquake are higher than the predicted results that are based on the model of Fifth-generation ground motion zonation map, while the PGV is similar with the predicted results which are based on Yu Yanxiang and Wang Suyun's model. We regressively analyzed the spatial-temporal change curves of the two types of relative ground motion durations. Compared with the predicted results proposed by Bommer et al. (2009) based on the NGA strong motion records, the durations of all the three components of this earthquake are higher. 10 typical recordings' acceleration response spectra with 5% damping are calculated, their peak periods are around 0.1~0.3s. The acceleration response spectrum of the station 32BYT, which has the largest amplitude, is considerably larger than the Chinese code design spectra, while it becomes notably smaller when the period is larger than 0.4s. Compared with the horizontal bedrock acceleration response spectrum predicted by the attenuation relationship for the eastern part of China, the observed response spectrum shape is similar with the predicted ones, while almost all the observed response spectrum values (except station 32YCT)are smaller than the predicted bedrock acceleration response spectrum. These phenomena suggest that this earthquake has a weak impact on the seismic fortification standards in this area. Using H/V single-station spectral ratio method, amplitude and site amplification effect of the two typical stations are calculated, and the results show the H/V values are obviously larger than that of ground microtremor. This suggests that the site of the station has obvious amplification effect on ground motion.

STUDY ON DYNAMIC CHANGE OF GRAVITY FIELD IN XIANSHUIHE FAULT ZONE AND THEIR RELATION TO THE OCCURRENCE OF M_S ≥ 5.0 EARTHQUAKES

LI Tie-ming, XU Yun-ma, YANG Yong-lin, FENG Sheng-tao

2018, 40(6):  1362-1379.  DOI: 10.3969/j.issn.0253-4967.2018.06.012

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Xianshuihe Fault, a main strong earthquake activity belt in southwest China, begins from Ganzi in the northwest, passes through Luhuo, Daofu, and Kangding, and then extents along the Dadu River valley. The fault is divided into two parts at Shimian, one part turns to south and converses to Anninghe Fault extending further to south, the other part, continuing to extend to southeast, cutting through Xiaoxiangling and then changing to Daliangshan Faults in the north of the Yuexi Basin, has the length of about 400km. Since 1700AD, there have happened 22 earthquakes larger than magnitude 6.0 and 8 earthquakes larger than magnitude 7.0. In this paper, we systematically collated and computed the gravity repetition measurement data along the Xianshuihe fault zone since 1988, and by referring to the anomaly index of gravity field of the predecessor achievements, analyzed the spatial-temporal variation of the regional gravity field and the relation to the occurrence of ≥ M_S5.0 earthquakes. The mechanism of the regional gravity changes is further studied, and also the implication of strong earthquake risk because of the dynamic variation of gravity field in the near future is discussed.The results show that:1)The mobile gravity observation has the ability to detect crustal activity and M_S ≥ 5.0 earthquake events. 2)There is definite correspondence between interannual gravitational field change and the 8 earthquakes among the 13 M_S ≥ 5.0 earthquakes occurring in the surveying area since 1988, which can be determined according to the change of interannual gravitational field. Three M ≥ 6.0 earthquakes occurred 3~4 years after the abnormal image was developed, 4 earthquakes that occurred in the region of no data available were not determined. 3)A significant feature of the spatial-temporal variation of the regional gravity is a north-south run-through image before 2004, and characterized by the alternatively positive or negative variation in different year, the earthquakes of M_S ≥ 5.0 occurring in this period were not distributed along the fault. Gravity variation magnitude indicates that there were two similar crustal material movement waves before 2004, corresponding to the course of earthquake space-time distribution from strong to weak in the study area. After 2010, the variation image shows that the local positive and negative zones are concurrent within a year, different from the image before 2004, and earthquakes of M_S ≥ 5.0 basically occurred on the fault. It is believed that the variation of gravity field since 1988 and the seismic distribution fit with the geodynamic mode of strong and weak stages of the northeast motion of Indian plate. According to the conclusion we can try to optimize gravity anomaly index. After the Kangding earthquake in 2014, the north segment of Moxi Fault was still subject to negative high value changes till 2017 and then the gravity variation was further developed to a four quadrant distribution image. Based on the analysis of this paper and the previous variation trend of gravity field, we believe that the north segment of Moxi Fault has the background of medium-long term, strong or large earthquake risk.

THE ANALYSIS OF VERTICAL MOTION CHARACTERISTICS IN YUNNAN AREA BASED ON PRECISE LEVELING

SU Guang-li, CHANG Liu, XU Ming-yuan

2018, 40(6):  1380-1389.  DOI: 10.3969/j.issn.0253-4967.2018.06.013

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In this study, vertical deformation of different regions of Yunnan area in 1993-2013, 2001-2006, 2011-2017 is obtained using observational data of precise leveling. The results show that:1) In the whole, Yunnan area exhibits uplifting in the east of Yunnan and subsiding in the south of Yunnan, which is well consistent with the current horizontal velocity field obtained by GPS. In the east of Yunnan, southeastward horizontal velocity at the east boundary of Sichuan-Yunnan block is significantly decreased, which indicates extrusion deformation. This result is in accordance with the result that there is uplift in the east of Yunnan with precise leveling data. GPS velocity field rotates clockwise at Eastern Himalayan Syntaxis, therefore east-west extension is formed in central and southern Yunnan, which coincides with crustal subsidence observed by precise leveling. 2)The vertical movement in the northwest of Yunnan mainly exhibits the succession movement of basin subsidence and mountain uplift, in which, in the rift zone, Chenghai Basin, Qina Basin, Binchuan Basin and Midu Basin distributed along Chenghai Fault are all in the sinking state and the sinking velocity of Binchuan Basin located in the end of the sinistral strike-slip Chenghai Fault is the maximum. The sinking velocity of Dali Basin distributed along Honghe Fault is approximately 0.5mm/a and the sinking velocity of Midu Basin is approximately 1mm/a under the comprehensive action of right-lateral Honghe Fault and left-lateral Chenghai Fault. On the northwest boundary of the fault zone, the vertical movement of the basins (Lijiang Basin, Jiangchuan Basin)under the control of the nearby Lijiang-Jianchuan Fault is not obvious and the nearby mountain area exhibits uplift. 3)In the Honghe Fault, the southern region still possesses strong activity. Seeing from the leveling profile and vertical deformation field, the Honghe Fault still possesses the significance of block boundary fault and strong activity. GPS velocity field reveals that the southeast movement velocity of the Sichuan-Yunnan rhombic block is rapidly decreased near Xiaojiang Fault and the earth's crust is shortened and deformed. In the vertical deformation field, the uplift is formed near Xiaojiang Fault and there is obvious vertical deformation gradient. 4)Notably, deformation contour in the junction of Qujiang Fault and Xiaojiang Fault is characterized by four quadrant distribution, which indicates the possibility of earthquake.

SIMULATION STUDY OF ROAD-CUT EFFECTS ON SLOPE STABILITY

CHEN Xiao-li, WANG Ming-ming, ZHANG Ling

2018, 40(6):  1390-1401.  DOI: 10.3969/j.issn.0253-4967.2018.06.014

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Landslides and rock falls along the highway are common geological hazards in Southwest China. As an influencing factor on potential landslides behavior, roads or distance to roads have been successfully used in landslide susceptibility assessments in mountainous area. However, the relationship between the road-cut and the slope stability is not clear. Therefore, we performed two-dimensional slope stability calculation using the general limit equilibrium (GLE)method incorporated in the software SLOPE/W of GeoStudio for stability analysis of slopes. Our studies show that the man-made roads influence on the slope stability mainly exists in two ways:One is to create a new steep slope, which will result in rock falls and shallow landslides along the roads; the other is to influence the stability of the original slope, which will result in comparatively huge landslides. For the latter, our simulation study reveals that the road location, namely at which part of a natural slope to construct a road is important for the slope stability. For a natural slope with a potential slip surface, if a road is constructed at or near the slope toe where the potential slip surface surpasses, it will greatly degrade the slope's factor of safety (Fs) and make the slope unstable; however, if a rode-cut is near the top of the slope, it will increase the slope's Fs and make the slope more stable. The safety location is different for different slope angle, steeper slope needs a higher location for a safety road-cut in comparison with gentle slopes. Moreover, the slope stability decreases when loading a seismic force and it varies with the slope angle. Firstly, the Fs decreases when the slope angle increasing, and when the slope angle reaches 45°, the Fs then becomes greater with the slope angle increasing.

CORRELATION BETWEEN GAS GEOCHEMICAL EMISSION AND FAULT ACTIVITY OF THE YUXIAN-GUANGLING AND KOUQUAN FAULTS

ZHAO Jian-ming, LI Ying, CHEN Zhi, LIU Zhao-fei, ZHAO Rong-qi, RONG Wei-jian

2018, 40(6):  1402-1416.  DOI: 10.3969/j.issn.0253-4967.2018.06.015

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Soil gas emission is closely related to tectonic and seismic activity and has been widely used to track active faults and monitor seismicity in the upper crust. Because active fault plays an important role as the channel of the earth's deep gas upward migration due to its high permeability and porosity, the geochemical characteristics of soil gas in fault zone is a good indicator of tectonic fracture and activity. In order to study the soil gas geochemical emission intensities and its correlation to fault activity, fluxes of Rn, Hg and CO₂ in soil gas and the ground resistivity were surveyed across the Yuxian-Guangling Fault and Kouquan Fault which are both Quaternary active faults in the border area of Shanxi Province, Hebei Province and Inner Mongolia Autonomous Region. In 2017, soil gas fluxes were measured in 2 profiles consisting of 10 and 9 wells of depth of 3.0m across the fault scarps in Yuxian-Guangling Fault and Kouquan Fault, respectively. Resistivity tomography sections were attained by ground resistivity survey with electrode spacing of 5.0m along the profiles of soil gas measurement. The gas geochemical data show that there exist two abnormal flux peaks across the Yuxian-Guangling Fault and one in the Kouquan Fault. The high density resistivity measurement shows that fault breccia and fractured rocks zones are developed under the measured faults, where higher values of soil gas flux are also observed. Fractures with high gas permeability in the strata favor the transfer and migration upward of soil gases, which results in the anomalies of gas flux value. In addition, the anomalies of gas flux values are spatially identical with the occurrence of the fault scarps. The soil gas degassing rate of Yuxian-Guangling Fault is higher than that of Kouquan Fault. The research results of high density electrical prospecting and previous tectonic activity show that low-resistance bodies are more developed and the fault activity is stronger with higher slip rate, which leads to the more intense emission of soil gas in Yuxian-Guangling Fault. The conclusions can be made that soil gas geochemical characteristics and degassing rate in fault zone is closely correlated to the tectonic activity and fracture degree. Combination of geochemical and geophysical methods is an efficient way for the monitoring and study of fault activity to estimate the possible earthquake hazards.

A SYSTEM OF CAPACITIVE HIGH-VOLTAGE PULSE RECHARGE/DISCHARGE AND ITS IMPLICATION ON EXPERIMENTALLY TRIGGERED EARTHQUAKE

ZHANG Zhi-he, GUO Yan-shuang, CHEN Shun-yun

2018, 40(6):  1417-1426.  DOI: 10.3969/j.issn.0253-4967.2018.06.016

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An external small disturbance may trigger seismic events when the fault is in a critical state. The problems related with earthquakes triggered by the dynamic stress such as blasting loads, impact loads, volcanic eruptions and strong earthquakes, have usually drawn wide concerns in earthquake science, and the corresponding research contents are quite extensive, including earthquake triggering mechanisms, triggering earthquake uncertainty, aftershock triggering, and so on. Among them, experimental research is an important way to understand the stress triggering conditions and physical mechanisms, such as the influence of load disturbance on fault friction traits, the influence of periodic disturbance of tidal stress on fault instability, etc., all of which can be gained through experimental investigations. Among them, "how to trigger" is a basic scientific problem to increase the understanding of earthquake prediction theory, thus receiving more attention. There are also some studies that focus on "what happened after the trigger", that is, the sliding instability generated by the triggering method, and then the evolution characteristics of the sliding instability process. The well-known experimental study of the super-shear rupture process is conducted by using the electric explosion method to trigger the fault instability, and the high-speed camera records the super-shear rupture during the fault instability. This means that when the trigger source is controllable, it is possible to generate different types of instability processes, and then to explore which earthquakes will be triggered at different time and space positions under different stress states by means of active triggering. The study of stability analysis and instability process has important scientific significance.A stable system of capacitive high-voltage pulse discharge and recharge is one of basic techniques for studying the triggered earthquakes in laboratory. Based on the wire electric explosion method, this paper develops a controllable trigger experiment system. By designing a new capacitive high-voltage pulse charge and discharge system, while considering the actual needs of monitoring and system timing, multiple functions are integrated into one system. Functionally, in addition to realizing the dynamic loading and unloading function of the wire electric explosion method, the discharge process can be monitored, and the triggering, synchronization and timing signal output is performed with other observation systems, thus realizing the whole process monitoring of the dynamic disturbance action. After testing, the following functions are achieved:1)The voltage and current of the high-voltage charging power supply system can be automatically adjusted, and the system can be shut off after charging; 2)Control modes include manual and remote controls. These two modes can control the recharge, release and pulse discharge of the high voltage capacitor; 3)The system can produce multi-channel synchronous output, which satisfy multiple systems working together. In particular, the remote sensing method greatly improves the experimental maneuverability and security; 4)The system has multiple sets of gas discharge tube to trigger discharge, with a wide range of discharge voltage of 500~5 000V; 5)Roche coil resistance integral current detection can meet the transient resistance, large current detection. Test results indicate that this system has good repeatability and stability with the same discharge energy and discharge energy regulator, which is conducive to carry out single channel trigger of high-pressure discharge experiment. In short, the new charging and discharging system can meet the requirements of experimental study of triggering earthquake. In addition, this system can be used to generate the stress disturbance under certain static and dynamic conditions, and then judge whether this kind of mechanical conditions in active fault systems is currently stable.In short, a controllable single-shot discharge system is developed by a capacitive high-voltage pulse discharge system, which provides a good technical basis for experimental research on triggering earthquakes. In addition, the new system also has application significance:1)multiple triggering output can simultaneously start multiple systems and improve the efficiency of observation. Fault instability is a characteristic of transient response, so, its observation requires high-speed acquisition equipment, which is difficult to control on observation; the trigger system is controllable, with active synchronization observation using physical parameters; 2)stress disturbance can be triggered under static and dynamic loads to detect the safety and stability of the fault system with active trigger.