Apparent stress as an indicator of stress meta-instability:The 2021 M_(S)6.4 Yangbi earthquake in Yunnan,China

Investigating spatiotemporal changes in crustal stress associated with major earthquakes has implications for understanding seismogenic processes.However,in individual earthquake cases,the characteristics of the stress after it reaches its maximum value are rarely discussed.In this study,we use the 2021 M_S6.4 Yangbi earthquake in Yunnan,China and events of magnitudes M_L≥3.0 occurred in the surrounding area in the previous 11 years to investigate the spatiotemporal evolution of apparent stress.The results indicate that apparent stress began to increase in January 2015 and reached a maximum in January 2020.Apparent stress then remained at a high level until October 2020,after which it declined considerable.We suggest that the stress was in the accumulation stage from January 2015 to January 2020,and entered the meta-instability stage after October 2020.During the meta-instability stage,the zone of decreasing stress expanded continuously and the apparent stress increased around the Yangbi earthquake source region.These features are generally consistent with the results of laboratory rock stress experiments.We propose that apparent stress can be a good indicator for determining whether the stress at a specific location has entered the meta-instability stage and may become the epicenter of an impending strong earthquake.

Source mechanism of small-moderate earthquakes and tectonic stress field in Yunnan Province

In the paper, source mechanisms of 33 small-moderate earthquakes occurred in Yunnan are determined by modeling of regional waveforms from Yunnan digital seismic network. The result shows that most earthquakes occurred within or near the Chuandian rhombic block have strike-slip mechanism. The orientations of maximum compressive stresses obtained from source mechanism are changed from NNW-SSN to NS in the areas from north to south of the block, and tensile stresses are mainly in ENE-WSW or NE-SE. In the eastern Tibetan Plateau, the orientations of maximum compressive stress radiate toward outside from the plateau, and the tensile stress orientations mostly parallel to arc structures. Near 28N the orientations of both maximum compressive stress and tensile stress changed greatly, and the boundary seems to correspond to the southwestern extended line of Longmenshan fault. Outside of the Chuandian rhombic block, the orientations of P and T axes are some different from those within the block. The comparison shows that the source mechanism of small-moderate events presented in the paper is consistence with that of moderate-strong earthquakes determined by Harvard University, which means the source mechanism of small-moderate events can be used to study the tectonic stress field in this region.

Regional stress field in Yunnan revealed by the focal mechanisms of moderate and small earthquakes

We determined focal mechanism solutions of 627 earthquakes of magnitude M ≥ 3.0 in Yunnan from January 2008 to May 2018 by using broadband waveforms recorded by 287 permanent and temporary regional stations. The results clearly revealed predominantly strike-slip faulting characteristics for earthquakes in Yunnan, with focal depths concentrated in the top 10 km of the crust. The earthquake mechanisms obtained were combined with the global centroid moment tensor solutions of 80 additional earthquakes from 1976 to 2016 to invert for the regional variations of stress field orientation by using a damped regional-scale stress inversion scheme.Results of the stress field inversion confirmed that the Yunnan region is under a strike–slip stress regime, with both maximum and minimum stress axes being nearly horizontal. The maximum compressional axes are primarily oriented in a northwest-southeast direction, and they experience a clockwise rotation from north to south, whereas the maximum extensional axes are oriented largely northeast-southwest. The maximum compressional axes are in line with the global positioning system–inferred horizontal velocity field and the southeastward escape of the Sichuan–Yunnan Rhombic Block, whereas the maximum extensional axes are consistent with anisotropy derived from SKS splitting. Against the strike–slip background, normal faulting stress regimes can be seen in the Tengchong volcanic area as well as in other areas with complex crisscrossing fault zones.

Deep medium environment of strong earthquakes occurrence in Yunnan region

The deep structure background of earth medium for strong earthquakes ccurrence in Yunnan area is discussed inthis paper, by using the results on the study of the velocity structure, elect fieal conductivity stricture, geothermalstructure in the crust and upper mantle in Yunnan area. The results show that the occurrence of strong earthquakes in Yunnan region is obviously related to the deep medium and tectonic environment such as the existenceof the high velocity zone in the upper crust, the low velocity zone or high electrical conductivity layer in themiddle crust, local uplift in the upper mantle, high geothermal activity and deep and large fault, etc. The large earthquakes could not take place at anywhere, they often occur at some regions which have a certainbackground in the deep medium structure. The activity of the earthquakes with magnitude of 5 or less is quite random,the occurrence of them have not the obvious background of the deep medium strUcture.

Seismogenic Tectonics and Dynamics of the 2011 Ms5.9 Yingjiang Earthquake in Yunnan, China

In the southern South-North Seismic Zone,China,seismic activity in the Yingjiang area of western Yunnan increased from December 2010,and eventually a destructive earthquake of Ms5.9 occurred near Yingjiang town on 10 March 2011.The focal mechanism and hypocenter location of the mainshock suggest that the Dayingjiang Fault was the site of the mainshock rupture.However,most of foreshocks and all aftershocks recorded by a portable seismic array located close to the mainshock occurred along the N-S-striking Sudian Fault,indicating that this fault had an important influence on these shocks.Coulomb stress calculations show that three strong （magnitude ≥5.0） earthquakes that occurred in the study region in 2008 increased the coulomb stress along the plane parallel to the Dayingjiang Fault.This supports the Dayingjiang Fault,and not the Sudian Fault,as the seismogenic fault of the 2011 Ms5.9 Yingjiang earthquake.The strong earthquakes in 2008 also increased the Coulomb stress at depths of ≤5 km along the entire Sudian Fault,and by doing so increased the shallow seismic activity along the fault.This explains why the foreshocks and aftershocks of the 2011 Yingjiang earthquake were located mostly on the Sudian Fault where it cuts the shallow crust.The earthquakes at the intersection of the Sudian and Dayingjiang faults are distributed mainly along a belt that dips to the southeast at ～40°,suggesting that the Dayingjiang Fault in the mainshock area also dips to the southeast at ～40°.

A preliminary report of the Yangbi, Yunnan, M_(S)6.4 earthquake of May 21, 2021

According to the reports of China Earthquake Networks Center,an Ms6.4 earthquake occurred in Yangbi City,Dali Prefecture,Yun-nan Province,on May 21,2021;the epicenter was located at 25.67°N and 99.87°E with a focal depth of 8 km.Within 5 km from the epicenter the average elevation is 2268 m.

Gravity variations before the Menyuan Ms6.4 earthquake

In order to study the relationship between gravity variation and Menyuan Ms6.4 earth- quake, gravity variation characteristics in mid-eastern of Qilian Mountain were analyzed based on the 2012-2015 relative gravity datasets. The results indicated that the gravity changes in mid-eastern of Qilian Mountain increased gradually, while gravity changes around Menyuan remarkably. Besides, great positive-negative gravity changing gradients appeared along the Lengiongling Fault which was located at the north of Menyuan, and the 2016 Menyuan Ms6.4 earthquake occurred near the junction of positive and negative gravity changes.

Subarea characteristics of earthquake types in Yunnan

Studies on the earthquake sequences and the source mechanisms of the strong earthquakes show that Yurman has more obvious subarea characteristics of earthquake type. Strike-slip seismic fault and mainshock-aftershock earthquake sequences are dominant in whole Yunnan area. Considering the ratio of non strike-slip faults and non mainshock-aftershock, Yurman area can be divided into four subareas with different characteristics, which are strike-slip mainshock-aftershock in central Yunnan （A1）, incline-slip swarm in northwestern Yunnan （A2）, strike-slip double shocks in western Yunnan （B1） and quasi-strike-slip mainshock-aftershock in southwestern Yunnan （BE）, respectively.

Establishment of the ELF Network in Yunnan and Electromagnetic Precursory Monitoring Results of the Yangbi MS5.1 Earthquake on March 27,2017

This paper summarizes the layout of the Yunnan seismic ELF electromagnetic observation network,site selection,ELF electromagnetic instrument system,data processing and other construction. The principle and method of using the ELF electromagnetic wave technique to monitor and predict earthquakes are expounded. The long term monitoring of ELF electromagnetic fields is carried out in the Yunnan earthquake prone area,and at the same time,the changes in electrical parameters and spatial electromagnetic fields of the regional crustal medium structure are monitored. The functions such as automatic,quasi real time, remote monitoring, network monitoring, data processing specialization, data service,data sharing and industrialization of the ELF electromagnetic observation data have been realized. In order to capture the deep electromagnetic precursory information of the earthquakes,service for earthquake prediction research,which has broad application prospects and development potential. Through the research of the seismicity of Yunnan in the trial run period of the project,the preliminary results of the extreme low frequency electromagnetic observation of the Yunnan earthquake in recent years are given. The electromagnetic precursors and the electromagnetic effects of the Yangbi earthquake are recorded. In the 3-month period before the earthquake,the power spectrum of the electric and magnetic fields,the apparent resistivity and the impedance phase in the observed signals are all abnormal,and gradually increased with time. The maximum value is reached 20 days before the earthquake,and an earthquake occurs when the change is restored to normal.

Temporal evolution of the focal mechanism consistency of the 2021 Yangbi M_(S) 6.4 earthquake sequence in Yunnan

Using the Cut And Paste(CAP)method,we invert the focal mechanism of 38 moderate earthquakes(M_(S)≥3.0)recorded by Yunnan seismic network and analyze the corresponding focal mechanism consistency based on the minimum spatial rotation angle.Our results indicate that the M_(S)6.4 mainshock is induced by a lateral strike slip fault(with a rake angle of~-165°)and a little normal-faulting component event along a nearly vertical plane(dipping angle~79° and strike~138°).Combining our results with high resolution catalog,we argue that the seismogenic fault of this earthquake sequence is a secondary fault western to the major Weixi-Qiaohou-Weishan fault.The focal mechanism evolution can be divided into three periods.During the first period,the foreshock sequence,the focal mechanism consistency is the highest(KA<36°);during the second period which is shortly after the mainshock,the focal mechanism shows strong variation with KA ranging from 8° to 110°;during the third period,the seismicity becomes weak and the focal mechanism of the earthquakes becomes more consistent than the second period(18°<KA<73°).We suggest that the KA,to some extent,represents the coherence between local tectonic stress regime and the stress state of each individual earthquake.Furthermore,high focal mechanism consistency and high linearity of seismic distribution may serve as indicators for the identification of foreshock sequence.

Gravity changes and crustal deformations before the Menyuan,Qinghai Ms6.4 earthquake of 2016

In this study, the relative gravity data(2012 e2015), GPS data-derived horizontal deformation(2011 e2014) and the background vertical deformation from the leveling measurements(1970 e2011) in the northeastern margin of Tibetan Plateau were processed to systematically analysis the mechanism of temporalespatial patterns and the relationship with Menyuan Ms6.4 earthquake. It can be summarized in the following: 1) The regional gravity changes, the GPS and the vertical deformational showed an intense spatial relationship: the gravity increased along with the direction of horizontal movement, and decreased with the crustal uplift and vice versa, which reflected the inherited characteristics of geotectonic activities. 2) The crustal deformations were closely related to the active faults. The contour lines of gravity changes and vertical deformation were generally along with the Qilian-Haiyuan fault(strike is NWW), and the crustal horizontal deformation showed left-lateral strike slip motion near the Qilian-Haiyuan fault. 3) Menyuan Ms6.4 earthquake occurred in the high negative gravity variation area and a high gradient formed in regions, positive and negative variation of gravity amount to 110 m Gal.Specifically, a borderline of positive and negative gravity located in the south of epicenter along the north edge of Qilianshan fault and Lenglongling fault, as well as the vertical and/or horizontal deformation is intensely. The extrusion deformation, surface compression rate and gravity changes were obvious near the epicenter of 2016 Menyuan Earthquake.

Performance of the Yangbi M_(S)6.4 earthquake disaster in different geomorphic units in Yunnan

An M_(S)6.4 earthquake occurred in Yangbi,Yunnan province,on May 21,2021.According to related investigations,the macro-epicenter of the earthquake is 6 km northwest of Yangbi County,and the seismogenic structure is the NW-trending Weixi-Qiaohou fault.The earthquake area is located in the hinterland of the Hengduan Mountains in the northwest of Yunnan Province,a region dominated by high and medium-high mountains,with deep canyons and tectonic basins in between.Various geomorphic features are derived from drastic topographic changes and huge geological differences in the earthquake area.There are a variety of buildings in the earthquake-affected zone,including civil and brick-wood structures ones with weak seismic performance,as well as brick-concrete and frame ones with better seismic performance.This paper summarizes and analyzes different characteristics of the earthquake in different geomorphic units through field investigations of different buildings and geological disasters in the affected area.The results show that under the same earthquake intensity,the damage to most buildings(located in slope areas or rooted in weak strata)is amplified by the earthquake.The earthquake has exerted an obvious propagation effect along the direction of the seismogenic structure.Moreover,local ground fissures will aggravate the damage to the buildings even without surface dislocation.Thus,we suggest that attention should be paid to the ground fissures caused by the slope effect.The fissure areas may also be the disaster spot of collapses and landslides in case of a high-magnitude earthquake.

Three-dimensional velocity structure around the focal area of the 2021 MS6.4 Yangbi earthquake

On May 21,2021,an MS6.4 earthquake occurred in Yangbi,Yunnan province,China,which exhibited typical foreshock-mainshock-aftershock characteristics.To better understand the velocity structure of the focal area and adjacent fault zones,Pg/Sg travel times at 12 seismic stations for the local earthquakes with ML≥1.5 from 2009-2019 and the Yangbi sequence in May of 2021 were used to invert the three-dimensional(3D)structures for both vP and v_(P)/v_(S).The obtained structure extends deeply to 15 km for area(25°N-26.5°N,99.5°E-101°E)at a horizontal resolution of 10×10 km,and the accuracy of the v_(P) velocity was verified using airgun signals excited by the Binchuan Airgun Transmitting Seismic Station(BATSS).The resulting v_(P) and v^(P)/v_(S) images correlate with existing fault zones and the Yangbi sequence,including:(1)The shallow velocity structure at 0 km agrees with local topography,where the Binchuan basin exhibits low-v_(P) and high-v_(P)/v_(S) values.From 3-15 km,v_(P) and v_(P)/v_(S) show variations,and the boundaries are consistent with the main faults(e.g.,the Weixi-Qiaohou-Weishan,Honghe,and Chenghai faults).(2)The largest foreshock(M_(S)5.6),main-shock(MS6.4),and largest aftershock(M_(S)5.2)occurred near the boundaries where both vP and v_(P)/v_(S) have clear contrasts.(3)Small earthquakes are also concentrated in the transition zone between high-and low-vP and v_(P)/v_(S) anomalies,and are biased toward low-v_(P)/v_(S) zones.(4)Boundaries in v_(P) and v_(P)/v_(S) are observed at 20 km west of the Weixi-Qiaohou-Weishan fault,indicating that there may exist one hidden fault.

Representative value of cross-fault in the northeastern margin of the Qinghai-Tibet block and case analysis of the 2016 Menyuan Ms6.4 earthquake

The equation for determining cross-fault representative value is calculated based on hanging wall and foot wall reference level surfaces. The cross-fault data reliability are analyzed base on the stability of reference datum and observation points, thereby facili- tating plotting of the representative value curves after removing interference. The spatial and temporal characteristics of fault deformation abnormalities before the 2016 Menyuan Ms6.4 earthquake, as well as the fault-movement characteristics reflected by representa- tive value, are summarized. The results show that many site trends had changed 1-3 years before the Menyuan Ms6.4 earthquake in the Qilian Fault, reflecting certain background abnormalities. The short-term abnormalities centrally had appeared in the 6 months to 1 year period before the earthquake near and in the neighborhood of the source region, demonstrating a significantly increased number of short-term abnormalities. Many sites near and in the neighborhood of the source region had strengthened inverse activities or had changed from positive to inverse activities in the most recent 2-3 years, which reflect stress-field enhancements or adjustment features.

Complex Seismic Focus Structure and Earthquake-Triggered Landslide Distribution:Analysis of the 2014 Ludian M_w6.1 Earthquake in Yunnan

Objective The 2014 Ludian Mw6.1 earthquake in Yunnan occurred in a mountainous area with complex tectonics and topography, which caused serious damage as well as co-seismic landslides of an unusual large scale. Because the suspected seismogenic faults on the surface, distribution of aftershocks and focal mechanism solutions are not consistent, it remains difficult to determine what is the real causal fault or seismogenic structure for this event. Actually, it may imply the complicity of the seismic source at depth. In addition, the distribution of the co- seismic landslides also exhibits some diffusion that is different from general eases, likely associated with the seismic focus structure.

Study of the Seismicity of Strong Earthquakes in the Yunnan Area

Yunnan is located in the east margin of the collision zone between the India Plate and the Eurasian Plate on the Chinese Continent, where crustal movement is violent and moderatestrong earthquakes are frequent. In addition, the area features marked active block movement. Therefore, Yunnan is a perfect place for research on strong earthquake activity. Through the study on the temporal and spatial distribution of the M ≥ 6.7 earthquakes and the related earthquake dynamics in Yunnan in the last century, we conclude that the four seismically active periods, which are characterized by alternative activity in the east and the west part of Yunnan, possibly result from a combination of active and quiescent periods in each of the east and west part. And for every 100 years, there may be a period in which strong earthquakes occur in the east and west parts simultaneously. In addition, the seismicity of strong earthquakes in Yunnan corresponds well to that in the peripheral region. The seismicity of the great earthquakes in the Andaman-Myanmar Tectonic Arc belt indicates, to some extent, the beginning of a seismically active period in Yunnan. The seismicity of strong earthquakes in east Yunnan is closely related to that in Sichuan. Strong earthquakes in Sichuan often occur later than those in Yunnan. Furthermore, in the east part of Ynnnan, the three procedures including continuous occurrence of moderate-strong earthquake, quiescent period, and the occurrence of the first strong earthquake may be the style of the beginning of the earthquake active period. The above cognition is helpful to the study of earthquake prediction, seismogenic mechanism, and the dynamics of the plate margin in Yunnan.

Influence of 2008 Wenchuan earthquake on earthquake occurrence trend of active faults in Sichuan-Yunnan region

The Wenchuan earthquake coseismic deformation field is inferred from the coseismic dislocation data based on a 3-D geometric model of the active faults in Sichuan-Yunnan region. Then the potential dislocation displacement is inverted from the deformation field in the 3-D geometric model. While the faults＇ slip velocities are inverted from GPS and leveling data, which can be used as the long-term slip vector. After the potential dislocation displacements are projected to long-term slip direction, we have got the influence of Wenchuan earthquake on active faults in Sichuan-Yunnan region. The results show that the northwestern segment of Longmenshan fault, the southern segments of Xianshuihe fault, Anninghe fault, Zemuhe fault, northern and southern segments of Daliangshan fault, Mabian fault got earthquake risks advanced of 305, 19, 12, 9.1 and 18, 51 years respectively in the eastern part of Sichuan and Yunnan. The Lijiang-Xiaojinhe fault, Nujiang fault, Longling-Lancang fault, Nantinghe fault and Zhongdian fault also got earthquake risks advanced in the western part of Sichuan-Yunnan region. Whereas the northwestern segment of Xianshuihe fault and Xiaojiang fault got earthquake risks reduced after the Wenchuan earthquake.

Strong Earthquake Activity and Its Relation to Regional Neotectonic Movement in Sichuan-Yunnan Region

Based on analyzing space inhomogeneous image of strong earthquake activity, the image of source rupture and the mechanical property of the source fault in Sichuan-Yunnan region, the relations among the strong earthquake activity, active fault, modern movement status of active blocks and structural background of the deep media have been discussed, and the characteristics of strong earthquake activity and possible mechanism have been also discussed.

The housing loss assessment of rural villages caused by earthquake disaster in Yunnan Province

Based on the assessment report of destructive housings caused by more than 20 earthquakes occurred in Yunnan Province in 1990~2004, the vulnerability models of 4 types housings of rural residents in Yunnan Province are setup. The scenario earthquake disaster loss model is used to simulate the housing loss if the historical earthquakes that occurred since A.D. 886 in Yunnan Province reoccur in 2002. The analyses show the simulation deviation of the usual earthquakes is less than 30% and the method is of high practicality. Meanwhile, the simulation result of 398 historical earthquakes in Yunnan Province shows that the annual economic loss caused by the earthquakes is about RMB 410 million Yuan that accounts for 0.18% of GDP of Yunnan Province for the year. Because the per capita living area and the price of the housing increases year by year, if the historical destructive earthquake reoc- curs today, the loss of Yunnan Province will be greater than in those years.

Dynamic variation of ambient shear stress field before and after three strong earthquakes in Yunnan

A method calculating relative shear stress values in seismic source areas using data of seismic wave is used in thispaper which is introduced by PEI-SHAN CHEN, based on a rupture model studying seismic rupture process fromtheory of fracture mechanics. The shear stress values τ0 in mid-small earthquakes occurred in 1986~June of 1997in Yunnan and its adjacent areas are used to trace the variation process of ambient shear stress field before andafter three strong earthquakes in Yunnan and China-Myanmar border areas. The results show that there exists abackground of high values in ambient shear stress field. In the development process of a strong earthquake, theambient shear stress field in and nearby its potential source area increases obviously, for a long time, experiences aprocess of multiple down - up - down - up. The pattern of dynamic variation of ambient shear stress field hasreflected the multi-source fields of seismogenesis in and nearby Yunnan. There exist multi-increased areas of ambient shear slress fields synchronously in southwest, northwest. west and east of Yunnan, and three strong eathquakes of magnitude about 7 occurred in the most obviously increased areas of ambient shear stress fields.