Step-over of strike-slip faults and overpressure fluid favor occurrence of foreshocks:Insights from the 1975 Haicheng fore-main-aftershock sequence,China

This study analyzed and summarized in detail the spatial and temporal distributions of earthquakes,tidal responses,focal mechanisms,and stress field characteristics for the M 7.3 Haicheng earthquake sequence in February 1975.The foreshocks are related to the main fault and the conjugate faults surrounding the extension step-over in the middle.The initiation timing of the foreshock clusters and the original time of the mainshock were clearly modulated by the Earth's tidal force and coincided with the peak of dilational volumetric tidal strain.As a plausible and testable hypothesis,we proposed a fluid-driven foreshock model,by which all observed seismicity features can be more reasonably interpreted with respect to the results of existing models.Together with some other known examples,the widely existing step-over along strike-slip faults and associated conjugate faults,especially for extensional ones in the presence of deep fluids,favor the occurrence of short-term foreshocks.Although clustered seismicity with characteristics similar to those of the studied case is not a sufficient and necessary condition for large earthquakes to occur under similar tectonic conditions,it undoubtedly has a warning significance for the criticality of the main fault.Subsequent testing would require quantification of true/false positives/negatives.

Possible link between long-term and short-term water injections and earthquakes in salt mine and shale gas site in Changning,south Sichuan Basin, China

Late at night on 17 June 2019,a magnitude 6.0 earthquake struck Shuanghe Town and its surrounding area in Changning County,Sichuan,China,becoming the largest earthquake recorded within the southern Sichuan Basin.A series of earthquakes with magnitudes up to 5.6 occurred during a short period after the mainshock,and we thus refer to these earthquakes as the Changning M6 earthquake sequence(or swarm).The mainshock was located very close to a salt mine,into which for^3 decades fresh water had been extensively injected through several wells at a depth of 2.7–3 km.It was also near(within^15 km)the epicenter of the 18 December 2018 M5.7 Xingwen earthquake,which is thought to have been induced by shale gas hydraulic fracturing(HF),prompting questions about the possible involvement of industrial activities in the M6 sequence.Following previous studies,this paper focuses on the relationship between injection and seismicity in the Shuanghe salt field and its adjacent Shangluo shale gas block.Except for a period of serious water loss after the start of cross-well injection in 2005–2006,the frequency of earthquakes shows a slightly increasing tendency.Overall,there is a good correlation between the event rate in the Shuanghe area and the loss of injected water.More than 400 M≥3 earthquakes,including 40 M≥4 and 5 M≥5 events,had been observed by the end of August 2019.Meanwhile,in the Shangluo area,seismicity has increased during drilling and HF operations(mostly in vertical wells)since about 2009,and dramatically since the end of 2014,coincident with the start of systematic HF in the area.The event rate shows a progressively increasing background with some fluctuations,paralleling the increase in HF operations.More than 700 M≥3 earthquakes,including 10 M≥4 and 3 M≥5 in spatially and temporally clustered seismic events,are correlated closely with active fracturing platforms.Well-resolved centroid moment tensor results for M≥4 earthquakes were shown to occur at very shallow depths around shale formations with active HF,in agreement with some of the clusters,which occurred within the coverage area of temporary or new permanent monitoring stations and thus have been precisely located.After the Xingwen M5.7 earthquake,seismic activity in the salt well area increased significantly.The Xingwen earthquake may have created a unidirectional rupture to the NNW,with an end point close to the NW-trending fault of the Shuanghe earthquake.Thus,a fault in the Changning anticline might have terminated the fault rupture of the Xingwen earthquake,possibly giving the Xingwen earthquake a role in promoting the Changning M6 event.

Laboratory acoustic emission study for earthquake generation process

Since the similarity in size distribution of earthquakes and acoustic emissions （AE） was found in the 1960s, many laboratory studies have been motivated by the need to provide tools for the prediction of mining failures and natural earthquakes. This paper aims, on the one hand, to draw an outline of laboratory AE studies in the last 50 years, which have addressed seismological problems. Topics include the power laws in which the similarity between AEs and earthquakes is involved and progress that has been made in AE technology and laboratory AE study. On the other hand, this study will highlight some key issues intensively discussed, especially in the last three decades, such as aspects related to the pre-failure damage evolution, fault nucleation and growth in brittle rocks and discuss factors governing these processes.

A laboratory acoustic emission experiment and numerical simulation of rock fracture driven by a high-pressure fluid source

In order to improve our understanding of rock fracture and fault instability driven by high-pressure fluid sources, the authors carried out rock fracture tests using granite under a confining pressure of 80 MPa with fluid injection in the laboratory. Furthermore, we tested a number of numerical models using the FLAC;modeling software to find the best model to represent the experimental results. The high-speed multichannel acoustic emission（AE） waveform recording system used in this study made it possible to examine the total fracture process through detailed monitoring of AE hypocenters and seismic velocity.The experimental results show that injecting high-pressure oil into the rock sample can induce AE activity at very low stress levels and can dramatically reduce the strength of the rock. The results of the numerical simulations show that major experimental results, including the strength, the temporal and spatial patterns of the AE events, and the role of the fluid can be represented fairly well by a model involving（1） randomly distributed defect elements to model pre-existing cracks,（2） random modification of rock properties to represent inhomogeneity introduced by different mineral grains, and（3）macroscopic inhomogeneity. Our study, which incorporates laboratory experiments and numerical simulations, indicates that such an approach is helpful in finding a better model not only for simulating experimental results but also for upscaling purposes.

An effective method for laboratory acoustic emission detection and location using template matching

In this paper, a template matching and location method, which has been rapidly adopted in microseismic research in recent years, is applied to laboratory acoustic emission(AE) monitoring. First, we used traditional methods to detect P-wave first motions and locate AE hypocenters in three dimensions. In addition, we selected events located with sufficient accuracy(normally corresponding AE events of relatively larger energy, showing clear P-wave first motion and a higher signal-to-noise ratio in most channels) as template events. Then, the template events were used to scan and match other poorly located events in triggered event records or weak events in continuous records. Through crosscorrelation of the multi-channel waveforms between the template and the event to be detected, the weak signal was detected and located using a grid-searching algorithm(with the grid centered at the template hypocenter). In order to examine the performance of the approach, we calibrated the proposed method using experimental data of different rocks and different types of experiments. The results show that the proposed method can significantly improve the detection capability and location accuracy, and can be applied to various laboratory and in situ experiments, which use multi-channel AE monitoring with waveforms recorded in either triggering or continuous mode.

Influence of bedding structure on stress-induced elastic wave anisotropy in tight sandstones

To understand the evolution of stress-induced elastic wave anisotropy,three triaxial experiments were performed on sandstone specimens with bedding orientations parallel,perpendicular,and oblique to the maximum principal stress.P-wave velocities along 64 different directions on each specimen were monitored frequently to understand the anisotropy change at various stress levels by fitting Thomsen’s anisotropy equation.The results show that the elastic wave anisotropy is very sensitive to mechanical loading.Under hydrostatic loading,the magnitude of anisotropy is reduced in all three specimens.However,under deviatoric stress loading,the evolution of anisotropic characteristics(magnitude and orientation of the symmetry axis)is bedding orientation dependent.Anisotropy reversal occurs in specimens with bedding normal/oblique to the maximum principal stress.P-wave anisotropyε0 is linearly related to volumetric strain Sv and dilatancy,indicating that stress-induced redistribution of microcracks has a significant effect on P-wave velocity anisotropy.The closure of initial cracks and pores aligned in the bedding direction contributes to the decrease of the anisotropy.However,opening of new cracks,aligned in the maximum principal direction,accounts for the increase of the anisotropy.The experimental results provide some insights into the microstructural behavior under loading and provide an experimental basis for seismic data interpretation and parameter selection in engineering applications.

Fluid-driven seismicity in relatively stable continental regions:Insights from the February 3^(rd),2020 M_(S)5.1 Qingbaijiang isolated earthquake

On February 3rd,2020,an isolated MS5.1 earthquake occurred in the northern section of the Longquanshan fault zone.This study aims at defining the geometry of seismogenic structures of this earthquake.In detail,centroid moment tensor inversion results show that the earthquake is characterized by a focal depth of 3.8 km with no corresponding surface faults.The strike/dip/rake angles for the two nodal planes are 205°/54°/96°and 15°/36°/82°,respectively.With the analyses of coseismic deformation of the surface obtained from InSAR measurements,together with the information of relocated hypocenters for a small number of aftershocks,it is concluded that a northwest-dipping nodal plane corresponds well to the source fault.The fault is suggested to have a length of about 2.8 km and a depth range of 2-5 km,and the centroid of the earthquake is located at 104.48°E and 30.71°N.Furthermore,multiple pieces of evidence indicate that this earthquake is partly driven by the overpressure effect associated with the adjacent natural gas packets,which is similar to several other moderate natural earthquakes in Sichuan Basin.

Is clustered seismicity an indicator of regional stress?Insights from earthquake sequences in Yongning-Luguhu faulted basin,Southwest China

Using hypocenter relocation,moment tensor inversion,stress field inversion,and fault slip tendency analysis,this study systematically investigated three M5.5-5.8 earthquake sequences that occurred after 2000 in the Yongning-Luguhu faulted basin in the middle of the Lijiang-Xiaojinhe fault zone within the Sichuan-Yunnan block,Southwest China.Our results show that since the 2008 Wenchuan earthquake,the tectonic stress pattern in this area may have changed and that b-values estimated for the earthquake sequences show evidence of an increasing trend in stress in the study area.Seismicity in the small-scale faulted basin adjacent to the large-scale fault zone is a possible indicator of regional stress.We also note that the aftershocks of the M5.7 earthquake sequence in 2012 and the M5.5 earthquake sequence in 2022 show relatively clear fluid diffusion-triggering characteristics.Overpressure of deep fluids is still the main factor driving seismic activity in the region,and we propose that the background tectonic stresses have not yet reached critical levels.

用ASTER三维影像观测引发2003年6.6级伊朗巴姆地震的活动断裂

卫星影像展示了伊朗东南部地区活动断层的地貌和几何学特征。断层活动引发的巴姆(Bam)地震造成超过40000人死亡,发震时间为2003年12月26日1点56分52秒(UTC),震中位于巴姆和Baravat附近的城镇,震级为6.6级。利用ASTER的可见光和近红外(VNIR)数据生成震前、震后三维(3D)鸟瞰影像,可以观测到断层活动引起的地表变化。卫星影像的解译和分析显示,Bam—Baravat地区发育一条长约65km的右旋走滑断层。断层破碎带在两个地区之间边界的一段地表出露,走向北北西方向。震源机制解及在野外观察到的地貌证据显示,本次地震的地震破裂可能是扭压力引发的。在过去的2000年里,巴姆地区还没有一次地震的破坏像2003年这次一样,结合卫星遥感技术和野外实地考察,这场灾难发生的可能性足以被发现。

Causative fault and seismogenic mechanism of the 2010 Suining M5.0 earthquake from joint modeling of seismic and InSAR data

Although the Sichuan basin is a stable block with low historical seismicity,the Suining M5.0 earthquake on January31,2010 occurred near the center of the basin,causing casualty and substantial damage.Previous studies have shown that the earthquake is very shallow and may occur in the sedimentary cover rocks,but its causative fault has not been identified.Based on local broadband seismic waveform data as well as a pair of ALOS PALSAR ascending orbit data,we explore the seismogenic mechanism via further constraining the source depth and the ruptured fault.The earthquake caused ground uplift in the southeast of the epicenter area,with a maximum line of sight displacement of about 13.6 cm,much larger than the displacement caused by a M5 earthquake at a typical depth of 10 km,which indicates that the earthquake is very shallow.Through joint inversion of seismic waveform and InSAR data,we obtain the moment magnitude of Suining earthquake as MW4.5,with the strike,dip,and rake of its fault plane as 17°,66° and 90°,respectively,and the centroid depth less than 1 km,supporting that the earthquake occurred at the shallow part of a high angle thrust fault dipping to the southeast.It is further confirmed that the earthquake may be triggered by the diffusion of high-pressure fluid migrating from the underside gas reservoir.

Ms8.0汶川地震产生的应力变化空间分布及其对地震活动性的影响

采用已公布的5个震源破裂模型,计算了Ms8.0汶川地震产生的库仑破裂应力变化,分析了应力变化与后续地震活动空间分布的相关性.以震源模型引起的不确定性分析为基础,研究了汶川地震在周边断裂上引起的库仑应力变化,并重点探讨了应力变化引起的地震活动性随时间的相对变化趋势.结果表明,以Ji模型和王卫民模型计算出的应力变化空间分布较好地与后续地震活动的空间分布相对应.最大的应力减小区主要分布于震源断层的东西两侧,最大的应力增加区分布于震源断层面以及断层面南北两个端部的邻近区域,此外还有一个北西-南东向的显著增加区位于主震断层面南段附近的汶川-映秀一带,该区域已发生了较多的余震活动,且地震活动呈现继续扩散的趋势,是未来地震危险性较高的地区.汶川地震引起了周边多数断裂不同程度的地震活动性增强,活动性增强最显著的是彭县-灌县断裂和青川断裂,未来100年内都维持在震前的2倍以上,地震活动恢复至背景水平所需的时间基本都达到800～900年;西秦岭南缘断裂、龙泉山断裂、鲜水河断裂、玉龙希断裂、安宁河断裂、岷江断裂和阿坝断裂的地震活动性受汶川地震的影响较小,震后地震活动率与震前相比不足2倍,并且扰动恢复时间较短.因抚边河断裂和虎牙断裂处于应力减小区域,未来地震活动性可能会减弱.受到汶川地震产生的应力变化影响,青川断裂和彭县-灌县断裂的未来发震危险性较高.

Growing seismicity in the Sichuan Basin and its association with industrial activities

In the Sichuan Basin,seismic activity has been low historically,but in the past few decades,a series of moderate to strong earthquakes have occurred.Especially since 2015,earthquake activity has seen an unprecedented continuous growth trend,and the magnitude of events is increasing.Following the M5.7 Xingwen earthquake on 18 Dec.2018,which was suggested to be induced by shale gas hydraulic fracturing,a swarm of earthquakes with a maximum magnitude up to M6.0 struck Changning and the surrounding counties.Questions arose about the possible involvement of industrial actions in these destructive events.In fact,underground fluid injection in salt mine fields has been occurring in the Sichuan Basin for more than70 years.Disposal of wastewater in natural gas fields has also continued for about 40 years.Since 2008,injection for shale gas development in the southern Sichuan Basin has increased rapidly.The possible link between the increasing seismicity and increasing injection activity is an important issue.Although surrounded by seismically active zones to the southwest and northwest,the Sichuan Basin is a rather stable region with a wide range of geological settings.First,we present a brief review of earthquakes of magnitude 5 or higher since 1600 to obtain the long-term event rate and explore the possible link between the rapidly increasing trend of seismic activity and industrial injection activities in recent decades.Second,based on a review of previous research results,combined with the latest data,we describe a comprehensive analysis of the characteristics and occurrence conditions of natural and injection-induced major seismic clusters in the Sichuan Basin since 1700.Finally,we list some conclusions and insights,which provide a better understanding of why damaging events occur so that they can either be avoided or mitigated,point out scientific questions that need urgent research,and propose a general framework based on geomechanics for assessment and management of earthquake-related risks.

Temporal and spatial distribution of microfractures in granites of different structures under triaxial compression and its significance in seismology

The temporal and spatial distribution of microfracturing activity in two kinds of granite under triaxial compression has been studied by using a new acoustic emission system. For Inada granite, there is no clear clustering of acoustic emission events in time and space, thus it is difficult to exactly deduce the time and position of the major fracturing. While for Mayet granite,acoustic emission events are clustered in time and space, so the time and position of the major fracturing can be exactly predicted according to microfracturing process. Such a difference may result from the difference in deformation mode caused by different rock structures.

Study on anisotropy of Longmaxi shale using hydraulic fracturing experiment

Hydraulic fracturing reservoir reconstruction technology is crucial in the development of shale gas exploitation techniques.Large quantities of high-pressure fluids injected into shale reservoirs significantly alter compressional(P)and shear(S)wave velocities,rock mechanical parameters,and anisotropic characteristics.In this study,differentiated hydraulic fracturing petrophysical experiments were carried out on Longmaxi Formation shale under pseudo-triaxial stress loading conditions.The effects of stress loading methods,and water-rock physical and chemical reactions on P-and S-wave velocities and rock mechanical parameters were compared.The experimental results showed that isotropic stress loading may increase the P-and Swave velocities and Young’s modulus of dry shale kldnsample.Furthermore,it may lead to a weakening of the corresponding anisotropy.In contrast,differential stress loading was able to improve the anisotropy of Young’s modulus and accelerate the decrease in the compressive strength of shale in the vertical bedding direction.The water-rock physical and chemical reactions prompted by hydraulic fracturing was found to"soften"shale samples and reduce Young’s modulus.The influence of this"soften"effect on the compressional and shear wave velocities of shale was negligible,whilst there was a significant decrease in the anisotropy characteristics of Thomsen parameters,Young’s modulus,and Poisson’s ratio.The negative linear relationship between the Poisson’s ratios of the shale samples was also observed to lose sensitivity to stress loading,as a result of the"soften"effect of fracturing fluid on shale.The results of this study provide a reliable reference point and data support for future research on the mechanical properties of Longmaxi shale rocks.