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Spatial and Temporal Stress Variations before and after the 2008 Wenchuan M_(w)7.9 Earthquake and its Implications:A Study based on Borehole Stress Data

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Acta Geologica Sinica(English Edition) 2023年第1期97卷 226-242页

作者： MENG Wen LIN Weiren chen qunce LI Yonghua Institute of Geophysics China Earthquake AdministrationBeijing 100081China Key Laboratory of Earthquake Source Physics China Earthquake AdministrationBeijing 100081China Key Laboratory of Active Tectonics and Geological Safety Ministry of Natural ResourcesInstitute of GeomechanicsChinese Academy of Geological SciencesBeijing 100081China Graduate School of Engineering Kyoto UniversityKyotoJapan

In situ stress measurement data was analyzed to estimate the temporal and spatial stress variations at shallow depths in the Longmenshan fault zone(LMSF),prior to and following the 2008 Wenchuan earthquake(WCEQ).Analysis of the stress field related to fault strength and behavior is useful for understanding geodynamic processes and conducting hazard *** shallow stress changes after the WCEQ show clear along-strike *** of stress orientation rotations have a negative correlation with the horizontal principal stress ratios and the WCEQ apparently reduced the magnitude difference between horizontal principal *** stress magnitudes and orientation distribution relative to the fault strike into account,we propose an intermediate-strength of LMSF,with a friction coefficient generally constrained between 0.35 and *** addition,high-pressure fluids in the fault zone reduce the effective normal stress and to a certain degree weaken the fault *** accumulated stress over a certain period following release of the WCEQ indicates the start of another earthquake *** changing crustal stress field makes the LMSF stable or slipping optimally during geodynamic *** segmentation feature of the shallow crustal stress field in the LMSF may imply a different tectonic loading and seismic release processes along the *** southwestern section to the epicenter of the WCEQ favors the occurrence of future earthquakes,as highμm in a state of critical failure was present in this area,which indicates that the Wenchuan and Lushan earthquakes did not release the accumulated stress to a sufficient extent there.

关键词： stress variation in situ stress measurement fault strength hazard assessment Longmenshan fault zone

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Piezomagnetic In-situ Stress Monitoring and its Application in the Longmenshan Fault Zone

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Acta Geologica Sinica(English Edition) 2014年第5期88卷 1592-1602页

作者： ZHANG Chongyuan WU Manlu chen qunce LIAO Chunting Key Laboratory of Neotectonic Movement and Geohazard of Ministry of Land and Resources Institute of Geomechanics Chinese Academy of Geological Sciences

The relative change of in-situ stress is an inevitable outcome of differential movement among the crust plates. Conversely, changes of in-situ stress can also lead to deformation and instability of crustal rock mass, trigger activity of faults, and induce earthquakes. Hence, monitoring real-time change of in-situ stress is of great significance. Piezomagnetic in-situ stress monitoring has good and longtime applications in large engineering constructions and geoscience study fields in China. In this paper, the new piezomagnetic in-situ stress monitoring system is introduced and it not only has overall improvements in measuring cell＇s structure and property, stressing and orienting way, but also enhances integration and intelligence of control and data transmission system, in general, which greatly promotes installing efficiency of measuring probe and quality of monitoring data. This paper also discusses the responses of new piezomagnetic system in large earthquake events of in-situ stress monitoring station at Qiaoqi of Baoxing and Wenxian of Gansu. The monitoring data reflect adjustments and changes of tectonic stress field at the southwestern segment of and the northern area near the Longmenshan fault zone, which shows that the new system has a good performance and application prospect in the geoscience field. Data of the Qiaoqi stress-monitoring station manifest that the Lushan Earthquake did not release stress of the southwestern segment of the Longmenshan fault zone adequately and there still probably exists seismic risk in this region in the future. Combined with absolute in-situ stress measurement, carrying out long-term in-situ stress monitoring in typical tectonic position of important regions is of great importance for researchers to assess and study regional crust stability.

关键词： in-situ stress monitoring new piezomagnetic in-situ stress monitoring system theLongmenshan fault zone regional stress field dynamic changes

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Crustal Stress State and Seismic Hazard along Southwest Segment of the Longmenshan Thrust Belt after Wenchuan Earthquake

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Journal of Earth Science 2014年第4期25卷 676-688页

作者： Xianghui Qin chengxuan Tan qunce chen Manlu Wu chengjun Feng Institute of Geomechanics Chinese Academy of Geological Sciences Key Laboratory of Neotectonic Movement & Geohazard Ministry of Land and Resources

The crustal stress and seismic hazard estimation along the southwest segment of the Longmenshan thrust belt after the Wenchuan Earthquake was conducted by hydraulic fracturing for in-situ stress measurements in four boreholes at the Ridi, Wasigou, Dahegou, and Baoxing sites in 2003, 2008, and 2010. The data reveals relatively high crustal stresses in the Kangding region （Ridi, Wasigou, and Dahegou sites） before and after the Wenchuan Earthquake, while the stresses were relatively low in the short time after the earthquake. The crustal stress in the southwest of the Longmenshan thrust belt, especially in the Kangding region, may not have been totally released during the earthquake, and has since increased. Furthermore, the Coulomb failure criterion and Byerlee＇s law are adopted to analyzed in-situ stress data and its implications for fault activity along the southwest segment. The magnitudes of in-situ stresses are still close to or exceed the expected lower bound for fault activity, revealing that the studied region is likely to be active in the future. From the conclusions drawn from our and other methods, the southwest segment of the Longmenshan thrust belt, especially the Baoxing region, may present a future seismic hazard.

关键词： Wenchuan Earthquake Longmenshan thrust belt in-situ stress measurement crustal stress state Coulomb failure criterion seismic hazard.

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Tectonic Stress State Changes Before and After the Wenchuan M_s 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

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Acta Geologica Sinica(English Edition) 2015年第1期89卷 77-89页

作者： MENG Wen chen qunce WU Manlu FENG chengjun QIN Xianghui Key Laboratory of Neotectonic Movement and Geohazard Ministry of Land and Resources Institute of Geomechanics Chinese Academy of Geological Science

Crustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan M. 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.

关键词： Wenchuan Ms 8.0 earthquake stress state in situ stress measurement finite elementmethod Longmenshan active fault zone

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In Situ Stress Measurement Method and its Application in Unconventional Oil and Gas Exploration and Development

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Acta Geologica Sinica(English Edition) 2015年第2期89卷 685-686页

作者： SUN Dongsheng chen qunce WANG Zhongxiu ZHAO Weihua LI Awei Key Laboratory of Neotectonic Movement and Geohazard Ministry of Land and Resources Institute of Geomechanics Chinese Academy of Geological Sciences

Unconventional oil and gas resources have become more and more important for meeting our increasing energy needs, tbr conventional petroleum resources are becoming depleted. During the exploration and development of unconventional oil and gas resources, the magnitude and direction of in situ stresses are key parameters for sweet spot forecast, stimulation parameters selection, horizontal well arrangement and borehole stability analysis （Olsen et al., 2009; John, et al., 2011）. In situ stresses have been previously focused on, but unfortunately they are not well understood when stimulation and completion decisions are made.

关键词： MPa ASR

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The Stress State of the Beichuan-Jiangyou Segment of the Longmenshan Fault before and after the Wenchuan M_S 8.0 Earthquake

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Journal of Earth Science 2014年第5期25卷 861-870页

作者： chengjun Feng qunce chen chengxuan Tan Xianghui Qin Peng Zhang Wen Meng Institute of Geomechanics Chinese Academy of Geological Sciences Key Laboratory of Neotectonic Movement & Geohazard Ministry of Land and Resources

After the Wenchuan MS 8.0 Earthquake, which occurred on May 12, 2008, in Sichuan Province, China, we conducted a series of hydraulic fracturing stress measurements in three 200 m deep boreholes（ZK01, ZK02, and ZK03） drilled in Beichuan and Jiangyou regions near the northeastern segment of Longmenshan fault belt in 2009. These measurements revealed the near-surface stress field in the fault region one year after the Wenchuan MS 8.0 Earthquake. However, the lack of the stress measurements before the earthquake in the same region makes it difficult to understand variations of the in situ stress field（near-surface） by comparative analysis. In order to determine the unknown horizontal principal stresses before the earthquake in Beichuan and Jiangyou regions, the following research method was tentatively applied. Firstly, we calculate the static co-seismic stress field by linear elastic finite element numerical simulation with ANSYS, based on the co-seismic static displacement generated by the Wenchuan MS 8.0 Earthquake along the central Longmenshan fault plane in Beichuan and Jiangyou. Secondly, combining hydraulic fracturing measurements（after the earthquake） with the co-seismic stress（simulation）, the magnitudes and orientations of horizontal principal stresses before the earthquake were calculated. Finally, the variation of the in situ stress（near-surface） in Beichuan and Jinagyou, both before and after the Wenchuan MS 8.0 Earthquake, were obtained by comparative analysis. To do this the magnitude of SHmax was decreased on average by 13.01 and 6.54 MPa after the earthquake in ZK02 and ZK03, respectively and the magnitude of SHmin was decreased by 2.54 and 5.29 MPa in ZK02 and ZK03, respectively. Following the earthquake, the average direction of SHmax rotated anticlockwise by 42.5°.

关键词： Longmenshan fault belt hydraulic fracturing situ stress co-seismic stress numerical simulation.

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Reexamining the Seismological Implications of the Present-day Stress State of the Yingxiu-Beichuan Fault after the Wenchuan Earthquake

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Acta Geologica Sinica(English Edition) 2016年第2期90卷 567-577页

作者： QIN Xianghui chen qunce FENG chengjun DU Jianjun WU Manlu MENG Wen SUN Dongsheng Institute of Geomechanics Chinese Academy of Geological Sciences Key Laboratory of Neotectonic Movement and Geohazard Ministry of Land and Resources

The present-day stress state of the Yingxiu-Beichuan fault after the Wenchuan earthquake was re-estimated using measured in-situ stress data obtained after the Wenchuan earthquake. The results reveal that the gradient coefficients of principal stresses versus depth decrease from south to north along the Yingxiu-Beichuan fault, revealing that the stress level decreases from south to north. The consistency between the present-day stress levels and surface ruptures generated during the earthquake indicates that the accumulated tectonic stress beneath the Yingxiu-Beichuan fault before the Wenchuan earthquake was relieved in form of surface ruptures. This resulted in the stress remaining high in the southern section of the Yingxiu-Beichuan fault but relatively low in the northern section. Abnormal high pore pressure conditions and an extremely low frictional coefficient play important role in the interpretation of the stress field adjustment and seismic events observed after the Wenchuan earthquake along this fault, according to the estimation results using the Coulomb frictional-failure theory incorporating frictional coefficients ranging from 0.4 to 1.0. To accurately estimate the seismological hazard of the Yingxiu-Beichuan fault by analyzing fault instability using the Coulomb frictional-failure theory, much attention should be focused on the pore pressure conditions and the evolution state of the frictional coefficient under the present-day stress state.

关键词： stress state fault activity seismological hazard Coulomb frictional-failure theory Yingxiu-Beichuan fault

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A Preliminary Study on the Present Crustal Stress of Northern South China Sea

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Acta Geologica Sinica(English Edition) 2015年第1期89卷 320-321页

作者： chen qunce MENG Wen WU Manlu FAN Taoyuan ZHANG Chongyuan QIN Xianghui LI Guoqi Key Laboratory of Neotectonic Movement and Geohazard Ministry of Land and Resources Institute of Geomechanics Chinese Academy of Geological Science

The South China Sea （SCS） is the largest marginal sea in West Pacific formed by a combined effect of the Eurasian plate, Pacific plate and India-Australian plate, and is thus considered to be a unique natural laboratory to study oceanic dynamics and tectonic evolutions （Xie et al., 2006; Luan and Zhang, 2009）. Besides, the SCS contains enormous petroleum resources for its favorable conditions of oil-gas generation, accumulation and preservation. It is therefore imperative to intensively study the marine in-situ stress and its controlling factors of the SCS.

关键词： In FMI A Preliminary Study on the Present Crustal Stress of Northern South China Sea SCS

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Preliminary Results of In-situ Stress Measurements along the Longmenshan Fault Zone after the Wenchuan M_s 8.0 Earthquake

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Acta Geologica Sinica(English Edition) 2009年第4期83卷 746-753页

作者： WU Manlu ZHANG Yueqiao LIAO Chunting chen qunce MA Yinsheng WU Jinsheng YAN Junfeng OU Mingyi Key Laboratory of Neotectonic Movement and Geohazard Ministry of Land and Resources Beijing 100081 Institute of Geomechanics Chinese Academy of Geological Sciences Beijing 100081 The Institute of Exploration Technology Chinese Academy of Geological Sciences Chengdu Sichuan 611734

Four months after the Wenchuan Ms 8 earthquake in western Sichuan, China, in situ stress measurements were carried out along the Longmenshan fault zone with the purpose of obtaining stress parameters for earthquake hazard assessment. In-situ stresses were measured in three new boreholes by using overcoring with the piezomagnetic stress gauges for shallow depths and hydraulic fracturing for lower depths. The maximum horizontal stress in shallow depths （-20 m） is about 4.3 MPa, oriented N19°E, in the epicenter area at Yingxiu Town, about 9.7 MPa, oriented N51°W, at Baoxing County in the southwestern Longmenshan range, and about 2.6 MPa, oriented N39°E, near Kangding in the southernmost zone of the Longmenshan range. Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth. A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan range, and is still moderate in the epicenter zone. These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.

关键词： Wenchuan Ms 8.0 earthquake in-situ stress measurement overcoring hydraulic fracturing Longmenshan fault belt

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Three-dimensional in situ stress determination by anelastic strain recovery and its application at the Wenchuan Earthquake Fault Scientific Drilling Hole-1(WFSD-1)

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Science China Earth Sciences 2014年第3期57卷 1212-1220页

作者： SUN DongSheng LIN WeiRen CUI JunWen WANG HongCai chen qunce MA YinSheng WANG LianJie Institute of Geomechanics Chinese Academy of Geological Sciences Key Laboratory of Neotectonic Movement and Geohazard Ministry of Land and Resources Kochi Institute for Core Sample Research Japan Agency for Marine-Earth Science and Technology (JAMSTEC) State Key Laboratory for Continental Tectonics and Dynamics Institute of Geology Chinese Academy of Geological Sciences

In situ stress state becomes more and more significant with in-depth research on geodynamics and energy ***,there has not been an economic and effective method developed to determine deep three-dimensional in situ *** Anelastic Strain Recovery(ASR)method is a newly developed technique that can determine three-dimensional in situ *** the 12 May 2008 Ms8.0 Wenchuan earthquake,the ASR method was used for the first time in China's Mainland to measure the in situ stresses in the WFSD scientific boreholes in Sichuan Province,*** this paper,the basic procedure of the ASR method is introduced in detail and the compliances of ASR for boring cores are *** results show that the maximum principal stress direction was NW64°at a measured depth(MD)of 1173 m(vertical depth 1151 m)in *** ratio of shear mode to the volume mode compliance of ASR was *** the three principal stresses at 1173 m MD in WFSD-1are 43,28 and 25 *** with stress measurement results determined using other in situ measurement methods along the Longmenshan fault zone,the directions of the maximum horizontal principal stress changes from E-W to NEE-SWW to NWW-SEE when moving from NE to SW along the Longmenshan fault *** change is in agreement with the stress regime of the Longmenshan fault zone of the Wenchuan Earthquake,which supports a stress regime consisting predominantly of thrusts in the southwest and strike-slip in the northeast.

关键词： Wenchuan earthquake anelastic strain recovery compliance of anelastic strain recovery three-dimensional in situ stress scientific borehole

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