The Subduction Structure Beneath the New Britain Island Arc and the Adjacent Region from Double-Difference Tomography

We applied double-difference tomography to relocate seismic events and determine the lithospheric velocity structure beneath the New Britain Island arc and the South Bismarck Sea Basin,based on the local P wave arrival time dataset collected by the International Seismological Centre.Results of the seismic relocation and velocity inversion show that the subduction of Solomon Sea Plate along the New Britain Trench is spatially different above 150 km,and the subduction angle of the slab on the west side is higher than that on the east side.The relocated earthquakes also show that there are double seismic zones at the depths of about 30–90km beneath the New Britain Island Arc.The velocity structure shows that the dehydration of the subducting slab caused the low-velocity anomalies in mantle wedge above the slab,which are associated with the magmatic activities around the New Guinea-New Britain Island arc.Moreover,it shows that there is another low-velocity anomaly zone beneath the Bismarck mid-oceanic ridge with spatial variation.Beneath the west of the Bismarck mid-oceanic ridge,the low-velocity anomaly is weakly connected to the subducted Solomon Sea slab.Conversely,the low-velocity anomaly beneath the Manus Sea Basin is highly intertwined to the subducting slab and its mantle wedge,indicating that the subduction of the Solomon Sea Plate might be a key deep dynamic factor that drives the spreading of the Manus Sea Basin and the separation of the Bismarck Plate.

Double-difference tomography of P- and S-wave velocity structure beneath the western part of Java, Indonesia

West Java in the western part of the Sunda Arc has a relatively high seismicity due to subduction activity and faults.In this study,double-difference tomography was used to obtain the 3D velocity tomograms of P and S waves beneath the western part of Java.To infer the geometry of the structure beneath the study area,precise earthquake hypo・center determination was first performed before tomographic imaging.For this,earthquake waveform data were extracted from the regional Meteorological,Climatological,Geophysical Agency(BMKG)network of Indonesia from South Sumatra to Central Java.The P and S arrival times for about 1,000 events in the period April 2009 to July 2016 were selected,the key features being events of magnitude>3,azimuthal gap<210°and number of phases>8.A nonlinear method using the oct-tree sampling algorithm from the NonLinLoc program was employed to determine the earthquake hypocenters.The hypocenter locations were then relocated using double-difference tomography(tomoDD).A significant reduction of travel-time(root mean square basis)and a better clustering of earthquakes were achieved which correlated well with the geological structure in West Java.Double-difference tomography was found to give a clear velocity structure,especially beneath the volcanic arc area,i.e.,under Mt Anak Krakatau,Mt Salak and the mountains complex in the southern part of West Java.Low velocity anomalies for the P and S waves as well as the vp/vs ratio below the volcanoes indicated possible partial melting of the upper mantle which ascended from the subducted slab beneath the volcanic arc.

Application of the Double-difference Relocation Method Combined with Waveform Cross-correlation on the Three Gorges Reservoir Seismicity

In this paper,we use the double difference location method based on waveform crosscorrelation algorithm for precise positioning of the Three Gorges Reservoir( TGR)earthquakes and analysis of seismic activity. First,we use the bi-spectrum cross-correlation method to analyze the seismic waveform data of TGR encrypted networks from March,2009 to December,2010,and evaluate the quality of waveform cross-correlation analysis.Combined with the waveform cross-correlation of data obtained, we use the double difference method to relocate the earthquake position. The results show that location precision using bi-spectrum verified waveform cross-correlation data is higher than that by using other types of data,and the mean 2 sig-error in EW,NS and UD are 3.2 m,3.9 m and 6.2 m,respectively. For the relocation of the Three Gorges Reservoir earthquakes,the results show that the micro-earthquakes along the Shenlongxi river in the Badong reservoir area obviously show the characteristics of three linear zones with nearly east-west direction,which is in accordance with the small faults and carbonate strata line of the neotectonic period,revealing the reservoir water main along the underground rivers or caves permeated and induced seismic activity. The stronger earthquakes may have resulted from small earthquakes through the active layers.

Analysis on Double-difference Earthquake Location and the Seismicity Pattern of the Yangjiang Earthquake Sequences

The locations of about 400 earthquakes in Yangjiang, Guangdong Province are determined using the double, difference earthquake location algorithm （DDA）. The seismicity pattern becomes concentrated from discrete grids. The rupture characteristics of the Yangjiang earthquake sequence show a conjugated distribution in NW and NE directions. The major distribution trends NE and dips NE with an angle of 30^o and a length of 30km,and the minor distribution trends NW and dips SE with an angle of 30^o and a length of 20km. The focal depth is 5km - 15km. The distribution of the Enping earthquake sequence,which is not far from Yangjiang,is NW-trending. The relationship between hypocenter distribution and geological structure is discussed.

Accurate relocation of earthquakes in central-western China using the double-difference earthquake location algorithm

The double-difference earthquake relocation algorithm (DD algorithm) has been applied to the accurate relocation of 10057 earthquakes in the central-western China (21°-36°N, 98°-112E°) during the period of 1992-1999. In total, 79706 readings for P waves and 72169 readings for S waves were used in the relocation, and the source parameters of 6496 events were obtained. The relocation results revealed a more complete picture of the hypocentral distribution in the central-western China. In several seismic belts the relocated epicenters present a more defined lineation feature, reflecting the close correlation between the seismicity and the active tectonic structures. The relocated focal depths confirmed that most earthquakes (91 percent of the 6496 relocated events) in the central-western China were located at shallower depths not deeper than 20 km. The distribution of focal depths indicates that the seismogenic layer in the central-western China is located in the upper-mid crust with its thickness no deeper than 20 km.

The MW5.5 earthquake on August 6,2023,in Pingyuan,Shandong,China:A rupture on a buried fault

On August 6,2023,a magnitude MW5.5 earthquake struck Pingyuan County,Dezhou City,Shandong Province,China.This event was significant as no large earthquakes had been recorded in the region for over a century,and no active fault had been previously identified.This study collects 1309 P-wave arrival times and 866 S-wave arrival times from 74 seismic stations less than 200 km to the epicenter to constrain the spatial distribution of the mainshock and its 125 early aftershocks by the double difference earthquake relocation method,and selects 864 P-waveforms from 288 stations located within 800 km of the epicenter to constrain the focal mechanism solution of the mainshock through centroid moment tensor inversion.The relocation and the inversion indicate,the Pingyuan MW5.5 earthquake was caused by a rupture on a buried fault,likely an extensive segment of the Gaotang fault.This buried fault exhibited a dip of approximately 75°to the northwest,with a strike of 222°,similar to the Gaotang fault.The rupture initiated at the depth of 18.6 km and propagated upward and northeastward.However,the ground surface was not broken.The total duration of the rupture was~6.0 s,releasing the scalar moment of 2.5895×1017 N·m,equivalent to MW5.54.The moment rate reached the maximum only 1.4 seconds after the rupture initiation,and the 90%scalar moment was released in the first 4.6 s.In the first 1.4 seconds of the rupture process,the rupture velocity was estimated to be 2.6 km/s,slower than the local S-wave velocity.As the rupture neared its end,the rupture velocity decreased significantly.This study provides valuable insights into the seismic characteristics of the Pingyuan MW5.5 earthquake,shedding light on the previously unidentified buried fault responsible for the seismic activity in the region.Understanding the behavior of such faults is crucial for assessing seismic hazards and enhancing earthquake preparedness in the future.

Decadal Forecasts of Large Earthquakes along the Northern San Andreas Fault System, California: Increased Activity on Regional Creeping Faults Prior to Major and Great Events

The three largest earthquakes in northern California since 1849 were preceded by increased decadal activity for moderate-size shocks along surrounding nearby faults. Increased seismicity, double-difference precise locations of earthquakes since 1968, geodetic data and fault offsets for the 1906 great shock are used to re-examine the timing and locations of possible future large earthquakes. The physical mechanisms of regional faults like the Calaveras, Hayward and Sargent, which exhibit creep, differ from those of the northern San Andreas, which is currently locked and is not creeping. Much decadal forerunning activity occurred on creeping faults. Moderate-size earthquakes along those faults became more frequent as stresses in the region increased in the latter part of the cycle of stress restoration for major and great earthquakes along the San Andreas. They may be useful for decadal forecasts. Yearly to decadal forecasts, however, are based on only a few major to great events. Activity along closer faults like that in the two years prior to the 1989 Loma Prieta shock needs to be examined for possible yearly forerunning changes to large plate boundary earthquakes. Geodetic observations are needed to focus on identifying creeping faults close to the San Andreas. The distribution of moderate-size earthquakes increased significantly since 1990 along the Hayward fault but not adjacent to the San Andreas fault to the south of San Francisco compared to what took place in the decades prior to the three major historic earthquakes in the region. It is now clear from a re-examination of the 1989 mainshock that the increased level of moderate-size shocks in the one to two preceding decades occurred on nearby East Bay faults. Double-difference locations of small earthquakes provide structural information about faults in the region, especially their depths. The northern San Andreas fault is divided into several strongly coupled segments based on differences in seismicity.

P-wave velocity structure beneath reservoirs and surrounding areas in the lower Jinsha River

The lower reaches of the Jinsha River are rich in hydropower resources because of the high mountains,deep valleys,and swift currents in this area.This region also features complex tectonic structures and frequent earthquakes.After the impoundment of the reservoirs,seismic activity increased significantly.Therefore,it is necessary to study the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds,thus providing seismological support for subsequent earthquake prevention and disaster reduction work in reservoir areas.In this study,we selected the data of 7.670 seismic events recorded by the seismic networks in Sichuan.Yunnan,and Chongqing and the temporary seismic arrays deployed nearby.We then applied the double-difference tomography method to this data,to obtain the P-wave velocity structure and earthquake locations in the lower reaches of the Jinsha River and surrounds.The results showed that the Jinsha River basin has a complex lateral P-wave velocity structure.Seismic events are mainly distributed in the transition zones between high-and low-velocity anomalies,and seismic events are particularly intense in the Xiluodu and Baihetan reservoir areas.Vertical cross-sections through the Xiangjiaba and Xiluodu reservoir areas revealed an apparent high-velocity anomaly at approximately 6 km depth:this high-velocity anomaly plays a role in stress accumulation,with few earthquakes distributed inside the high-velocity body.After the impoundment of the Baihetan reservoir,the number of earthquakes in the reservoir area increased significantly.The seismic events in the reservoir area north of 27°N were related to the enhanced activity of nearby faults after impoundment:the earthquakes in the reservoir area south of 27°N were probably induced by additional loads(or regional stress changes),and the multiple microseismic events may have been caused by rock rupture near the main faults under high pore pressure.

Relocation of the 10 March 2011 Yingjiang,China,earthquake sequence and its tectonic implications

An earthquake with Ms5.8 occurred on 10 March 2011 in Yingjiang county, western Yunnan, China. This earthquake caused 25 deaths and over 250 injuries. In order to better understand the seismotectonics in the region, we collected the arrival time data from the Yunnan seismic observational bulletins during 1 January to 25 March 2011, and precisely hand-picked the arrival times from high-quality seismograms that were recorded by the temporary seismic stations deployed by our Institute of Crustal Dynamics, China Earthquake Administration. Using these arrival times, we relocated all the earthquakes including the Yingjiang mainshock and its aftershocks using the double-difference relocation algorithm. Our results show that the relocated earthquakes dominantly occurred along the ENE direction and formed an upside-down bow-shaped structure in depth. It is also observed that after the Yingjiang mainshock, some aftershocks extended toward the SSE over about 10 km. These results may indicate that the Yingjiang mainshock ruptured a conjugate fault system consisting of the ENE trending Da Yingjiang fault and a SSE trending blind fault. Such structural features could contribute to severely seismic hazards during the moderate-size Yingjiang earthquake.

High-precision relocation of the aftershock sequence of the January 8,2022,M_(S)6.9 Menyuan earthquake

The 2022 Menyuan M_(S)6.9 earthquake,which occurred on January 8,is the most destructive earthquake to occur near the Lenglongling(LLL)fault since the 2016 Menyuan M_(S)6.4 earthquake.We relocated the mainshock and aftershocks with phase arrival time observations for three days after the mainshock from the Qinghai Seismic Network using the double-difference method.The total length and width of the aftershock sequence are approximately 32 km and 5 km,respectively,and the aftershocks are mainly concentrated at a depth of 7-12 km.The relocated sequence can be divided into 18 km west and 13 km east segments with a boundary approximately 5 km east of the mainshock,where aftershocks are sparse.The east and west fault structures revealed by aftershock locations differ significantly.The west fault strikes EW and inclines to the south at a 71°-90°angle,whereas the east fault strikes 133°and has a smaller dip angle.Elastic strain accumulates at conjunctions of faults with different slip rates where it is prone to large earthquakes.Based on surface traces of faults,the distribution of relocated earthquake sequence and surface ruptures,the mainshock was determined to have occurred at the conjunction of the Tuolaishan(TLS)fault and LLL fault,and the west and east segments of the aftershock sequence were on the TLS fault and LLL fault,respectively.Aftershocks migrate in the early and late stages of the earthquake sequence.In the first 1.5 h after the mainshock,aftershocks expand westward from the mainshock.In the late stage,seismicity on the northeast side of the east fault is higher than that in other regions.The migration rate of the west segment of the aftershock sequence is approximately 4.5 km/decade and the afterslip may exist in the source region.

Fault plane parameters of Tancheng M81/2 earthquake on the basis of present-day seismological data

The great Tancheng earthquake of M81/2 occurred in 1668 was the largest seismic event ever recorded in history in eastern China. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We relocated those earthquakes with the double-difference method and found focal mechanism solutions using gird test method. The inversion results are as follows： the strike is 21.6°, the dip angle is 89.5°, the slip angle is 170°, the fault length is about 160 km, the lower-boundary depth is about 32 km and the buried depth of upper boundary is about 4 km. This shows that the seismic fault is a NNE-trending upright right-lateral strike-slip fault and has cut through the crust. Moreover, the surface seismic fault, intensity distribution of the earthquake, earthquake-depth distribution and seismic-wave velocity profile in the focal area all verified our study result.

Velocity calibration for microseismic event location using surface data

Because surface-based monitoring of hydraulic fracturing is not restricted by borehole geometry or the difficulties in maintaining subsurface equipment, it is becoming an increasingly common part of microseismic monitoring. The ability to determine an accurate velocity model for the monitored area directly affects the accuracy of microseismic event locations. However, velocity model calibration for location with surface instruments is difficult for several reasons： well log measurements are often inaccurate or incomplete, yielding intractable models; ori- gin times of perforation shots are not always accurate; and the non-uniqueness of velocity models obtained by inver- sion becomes especially problematic when only perforation shots are used. In this paper, we propose a new approach to overcome these limitations. We establish an initial velocity model from well logging data, and then use the root mean square （RMS） error of double-difference arrival times as a proxy measure for the misfit between the well log velocity model and the true velocity structure of the medium. Double-difference RMS errors are reduced by using a very fast simulated annealing for model perturbance, and a sample set of double-difference RMS errors is then selec- ted to determine an empirical threshold. This threshold value is set near the minimum RMS of the selected samples, and an appropriate number of travel times within the threshold range are chosen. The corresponding velocity models are then used to relocate the perforation-shot. We use the velocity model with the smallest relative location errors as the basis for microseismic location. Numerical analysis with exact input velocity models shows that although large differences exist between the calculated and true velocity models, perforation shots can still be located to their actual positions with the proposed technique; the location inaccuracy of the perforation is 〈2 m. Further tests on field data demonstrate the validity of this technique.

The 20 April 2013 Lushan, Sichuan, mainshock, and its aftershock sequence: tectonic implications

Using the double-difference relocation algo- rithm, we relocated the 20 April 2013 Lushan, Sichuan, earthquake （Ms 7.0）, and its 4,567 aftershocks recorded during the period between 20 April and May 3, 2013. Our results showed that most aftershocks are relocated between 10 and 20 km depths, but some large aftershocks were relocated around 30 krn depth and small events extended upward near the surface. Vertical cross sections illustrate a shovel-shaped fault plane with a variable dip angle from the southwest to northeast along the fault. Furthermore, the dip angle of the fault plane is smaller around the mainshock than that in the surrounding areas along the fault. These results suggest that it may be easy to generate the strong earthquake in the place having a small dip angle of the fault, which is somewhat similar to the genesis of the 2008 Wenchuan earthquake. The Lushan mainshock is underlain by the seismically anomalous layers with low-Vp, low-Vs, and high-Poisson＇s ratio anomalies, possibly suggesting that the fluid-filled fractured rock matrices might signifi- cantly reduce the effective normal stress on the fault plane to bring the brittle failure. The seismic gap between Lushan and Wenchuan aftershocks is suspected to be vulnerable to future seismic risks at greater depths, if any.

Three-dimensional P-wave Velocity Structure Modelling of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Crustal Architecture and Metallogenic Implications

In this study,we compiled and analyzed 69310 P-wave travel-time data from 6639 earthquake events.These events(M≥2.0)occurred from 1980 s to June 2019 and were recorded at 319 seismic stations(Chinese Earthquake Networks Center)in the study area.We adopted the double-difference seismic tomographic method(tomo DD)to invert the 3-D P-wave velocity structure and constrain the crust-upper mantle architecture of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt(MLYB).A 1-D initial model extracted from wide-angle seismic profiles was used in the seismic tomography,which greatly reduced the inversion residual.Our results indicate that reliable velocity structure of th e uppermost mantle can be obtained when Pn is involved in the tomography.Our results show that:(1)the pattern of the uppermost mantle velocity structure corresponds well with the geological partitioning:a nearly E-W-trending low-velocity zone is present beneath the Dabie Orogen,in contrast to the mainly NE-trending low-velocity anomalies beneath the Jiangnan Orogen.They suggest the presence of thickened lower crust beneath the orogens in the study area.In contrast,the Yangtze and Cathaysia blocks are characterized by relatively high-velocity anomalies;(2)both the ultra-high-pressure(UHP)metamorphic rocks in the Dabie Orogen and the low-pressure metamorphic rocks in the Zhangbaling dome are characterized by high-velocity anomalies.The upper crust in the Dabie Orogen is characterized by a low-velocity belt,sandwiched between two high velocity zones in a horizontal direction,with discontinuous low-velocity layers in the middle crust.The keel of the Dabie Orogen is mainly preserved beneath its northern section.We infer that the lower crustal delamination may have mainly occurred in the southern Dabie Orogen,which caused the mantle upwelling responsible for the formation of the granitic magmas emplaced in the middle crust as the low-velocity layers observed there.Continuous deep-level compression likely squeezed the granitic magma upward to intrude the upper crustal UHP metamorphic rocks,forming the'sandwich'velocity structure there;(3)high-velocity updoming is widespread in the crust-mantle transition zone beneath the MLYB.From the Anqing-Guichi ore field northeastward to the Luzong,Tongling,Ningwu and Ningzhen orefields,high-velocity anomalies in the crust-mantle transition zone increase rapidly in size and are widely distributed.The updoming also exists in the crust-mantle transition zone beneath the Jiurui and Edongnan orefields,but the high-velocity anomalies are mainly stellate distributed.The updoming high-velocity zone beneath the MLYB generally extends from the crust-mantle transition zone to the middle crust,different from the velocity structure in the upper crust.The upper crust beneath the Early Cretaceous extension-related Luzong and Ningwu volcanic basins is characterized by high velocity zones,in contrast to the low velocity anomalies beneath the Late Jurassic to Early Cretaceous compression-related Tongling ore field.The MLYB may have undergone a compressive-to-extensional transition during the Yanshanian(Jurassic-Cretaceous)period,during which extensive magmatism occurred.The near mantle-crustal boundary updoming was likely caused by asthenospheric underplating at the base of the lower crust.The magmas may have ascended through major crustal faults,undergoing AFC(assimilation and fractional crystallization)processes,became emplaced in the fault-bounded basins or Paleozoic sequences,eventually forming the many Cu-Fe polymetallic deposits there.

Seismic activities before and after the impoundment of the Xiangjiaba and Xiluodu reservoirs in the lower Jinsha River

The lower Jinsha River basin is located at the junction of Sichuan and Yunnan provinces in Southwest China,a region with intense tectonic movements and frequent moderate to strong seismic activities.Cascade hydropower stations have been constructed along the lower Jinsha River since 2012.However,research on the effect of the impoundment of large-scale cascade reservoirs in a river basin on local seismic activities is currently lacking.Accurately identifying earthquake locations is essential for studying reservoir-induced earthquakes.Analyzing the spatiotemporal migration process of seismic activities based on complete and precise earthquake relocation is fundamental for determining the fluid diffusion coefficient,constructing fault models for reservoir areas,identifying earthquake types,exploring earthquake mechanisms,and evaluating seismic hazards.The seismicity pattern in the Xiangjiaba and Xiluodu reservoir areas,where seismic activities had been weak for a long time,has changed with the successive impoundment of the two reservoirs,showing microseismic events and seismic clusters.We investigated the spatiotemporal characteristics of seismic activities in the Xiangjiaba and Xiluodu reservoir areas using the waveform cross-correlation-based double-difference relocation technique and the b-value analysis method.We discovered that seismic events after the impoundment of these two reservoirs exhibited different characteristics in different regions.The seismic activities at the Xiluodu dam quickly responded to the rising water level,with the seismic intensity decaying rapidly afterward.These events were concentrated in the limestone strata along both sides of the Jinsha River,with a shallow focal depth,generally within 5 km,and a high b-value of approximately 1.2.Such features are close to those of karst-type earthquakes.Microseismic activities frequent occur on the eastern bank of the Yongshan reservoir section downstream of the Xiluodu dam,with two parallel NW-trending earthquake strips visible after precise earthquake relocation.The M_(S)5.2 earthquake near Wuji town on August 17,2014,had prominent foreshocks and aftershocks distributed in a clear NW-trending 20-km-long strip,perpendicular to the riverbank.These seismic events had a low b-value of approximately 0.7.The orientation of the node plane revealed by the strike-slip focal mechanism of the mainshock is consistent with that of the strip formed by the foreshock-mainshock-aftershock sequence,indicating the existence of a NW-striking concealed fault.Seismic activities near the Yanjin-Mabian fault upstream of the Xiangjiaba reservoir area since 2013 were concentrated in a NW-trending strip,with several near EW-trending seismic clusters on its western side,and with the largest event having a magnitude of M_(L)3.7.So far,the impoundment of the Xiangjiaba and Xiluodu reservoirs has not triggered seismic activities on the large Jinyang-Ebian and Yanjin-Mabian faults nearby.

Spatiotemporal evolution of the 2021 M_(S)6.4 Yangbi earthquake sequence

Based on the seismic phase reports of the Yangbi area from January 1 to June 25,2021,and the waveform data of M≥4 earthquakes,we obtained the relocation results and focal mechanism solutions of the M_(S)6.4 Yangbi earthquake sequence using the HypoDD and CAP methods.Based on our results,our main conclusions are as follows:(1)the M_(S)6.4 Yangbi earthquake sequence is a typical foreshock-mainshock-aftershock sequence.The fore-shocks of the first two stages have the obvious fronts of migration and their migration rate increased gradually.There was no apparent front of migration during the third stage,and the occurrence of the mainshock was related to stress triggering from a M5.3 foreshock.We tentatively speculate that the rupture pattern of the Yangbi earthquake sequence conforms to the cascading-rupture model;and(2)the main fault of the M_(S)6.4 Yangbi earthquake sequence is a NW-trending right-lateral strike-slip fault.As time progressed,a minor conjugate aftershock belt formed at the northwest end of this fault,and a dendritic branching structure emerged in the southern fault segment,showing a complex seismogenic fault structure.We suggested that the fault of the Yangbi earthquake sequence may be a young sub-fault of the Weixi-Weishan fault.

The Characteristic Analysis and Seismic Triggering Study of the M6.2 and M6.1 Dayao Earthquake Sequences in 2003

The high-resolution hypocenter locations of the mainshocks on July 21 （M6.2） and October 16, 2003 （M6.1） and their aftershock sequences are determined in Dayao, Yunnan by using a double-difference earthquake location algorithm. The results show that the epicenters of the two mainshocks are very close to each other and the distribution of the aftershock sequence appears to be very linear. The distribution of the earthquake sequence is very consistent with the focal mechanism, and both mainshocks are of nearly vertical right-lateral fault. Unlike most other double earthquakes in the Yunmm area, the aftershock distribution of the M6.2 and M6.1 Dayao earthquakes does not appear to be a conjugated distribution but to be in a line, and there are some stacks in the two earthquake sequences. It can be inferred that they are all controlled by the same fault. The distribution of aftershocks is asymmetrical with respect to the mainshock location and appears to be unilateral. The aftershocks of the M6.2 mainshock centralize in the northwest of M6.2 earthquake and the aftershocks of the M6.1 earthquake are in the southeast of the mainshock, moreover, the M6.1 earthquake appears to be another rupture on the southeastern extensiou of the same fault as the M6.2 earthquake. The results of Coulomb failure static stress changes △σf show that the earthquake on July 21 （M6.2） apparently triggered the earthquake on October 16 （M6.1）, the two mainshocks have stress triggering to their off-fault aftershocks to different extents, and the M6.5 earthquake that occurred in Yao＇an in 2000 also triggered the occurrence of the two Dayao earthquakes.

Three-dimensional crustal velocity structure and activity characteristics of the Madoi Ms7.4 earthquake in 2021

In this paper,using natural earthquake P-wave arrival time data recorded by the seismic network in the surrounding area of Madoi,the three-dimensional fine P-wave crustal velocity structure at depths above 60 km in the epicenter of the Madoi Ms7.4 earthquake was inverted using the double-difference seismic tomography method.On the basis of the relocation of the source of the aftershock sequence,we summarized the strip-shaped distribution characteristics along the strike of the Jiangcuo fault,revealing the significant heterogeneity of the crustal velocity structure in the source area.Research has found that most of the Madoi Ms7.4 aftershocks were located in the weak area of the high-speed anomaly in the upper crust.The focal depth changed with the velocity structure,showing obvious fluctuation and segmentation characteristics.There was a good correspondence between the spatial distribution and the velocity structure.The high-velocity bodies of the upper crust in the hypocenter area provided a medium environment for earthquake rupture,the low-velocity bodies of the middle crust formed the deep material,and the migration channel and the undulating shape of the high-speed body in the lower crust corroborated the strong pushing action in the region.The results confirmed that under the continuous promotion of tectonic stress in the Madoi area,the high-speed body of the Jiangcuo fault blocked the migration of weak materials in the middle crust.When the stress accumulation exceeded the limit,the Madoi Ms7.4 earthquake occurred.Meanwhile,the nonuniform velocity structure near the fault plane determined the location of the main shock and the spatiotemporal distribution of the aftershock sequence.

Three-dimensional velocity structure and tectonic characteristics of earthquake area in Yibin

In this study,on the basis of absolute first-arrival times of 84756 P-and S-waves from 6085 earthquakes recorded at 56 fixed stations in Yibin and surrounding areas in China from January 2009 to January 2019,focal parameters and three-dimensional(3 D)body-wave high-resolution velocity structures at depths of 0–30 km were retrieved by double-difference tomography.Results show that there is a good correspondence between the spatial distribution of the relocated earthquakes and velocity structures,which were concentrated mainly in the high-velocity-anomaly region or edge of high-velocity region.Velocity structure of P-and S-waves in the Yibin area clearly shows lateral inhomogeneity.The distribution characteristics of the P-and S-waves near the surface are closely related to the geomorphology and geologic structure.The low-velocity anomaly appears at the depth of 15–25 km,which is affected by the lower crust current.The Junlian–Gongxian and Gongxian–Changning earthquake areas,which are the two most earthquake-prone areas in the Yibin region,clearly differ in earthquake distribution and tectonic characteristics.We analyzed the structural characteristics of the Junlian–Gongxian and Gongxian–Changning earthquake areas on the basis of the 3 D bodywave velocity structures in the Yibin region.We found that although most seismicity in the Yibin area is caused by fluid injection,the spatial position of seismicity is controlled by the velocity structures of the middle and upper crust and local geologic structure.Fine-scale 3 D velocity structures in the Yibin area provide important local reference information for further understanding the crustal medium,seismogenic structure,and seismicity.

Seismic analysis of the Xiluodu reservoir area and insights into the geometry of seismogenic faults

The Xiluodu(XLD)reservoir is the second largest reservoir in China and the largest in the Jinsha River basin.The occurrence of two M>5 earthquakes after reservoir impoundment has aroused great interest among seismologists and plant operators.We comprehensively analyzed the seismicity of the XLD reservoir area using precise earthquake relocation results and focal mechanism solutions and found that the seismicity of this area was weak before impoundment.Following impoundment,earthquake activity increased significantly.The occurrence of M≥3.5 earthquakes within five years of impoundment also appear to be closely related to rapid rises and falls in water level,though this correlation weakened after five years because earthquake activity was far from the reservoir area.Earthquakes in the XLD reservoir area are clustered;near the dam(Area A),small faults are intermittently distributed along the river,while Area B is composed of multiple NW-trending left-lateral strikeslip faults and a thrust fault and Area C is composed of a NW-trending left-lateral strike-slip main fault and a nearly EWtrending right-lateral strike-slip minor fault.The geometries of the deep and the shallow parts of the NW-trending fault differ.Under the action of the NW-trending background stress field,a series of NW-trending left-lateral strike-slip faults and NE-trending thrust faults in critical stress states were dislocated due to the stress caused by reservoir impoundment.The two largest earthquakes in the XLD reservoir area were tectonic earthquakes that were directly triggered by impoundment.