Distribution characteristics of historical earthquake classes in Jiangsu Province and South Huanghai Sea region

According to the analysis on the characteristics of historic earthquakes in Jiangsu Province and South Huanghai Sea region, the historical earthquakes in the studied area are divided into two kinds of comparatively safe class and comparatively dangerous class. Then the statistical result of earthquake class, the characteristics of geo-graphical distribution and geological structures are studied. The study shows: a) In Jiangsu Province and South Huanghai Sea region, the majority of historical strong earthquakes belong to comparatively safe class, only 13.8% belong to comparatively dangerous class; b) Most historical earthquakes belong to comparatively safe class in the land area of Jiangsu, eastern sea area of Yangtze River mouth and northern depression of South Huanghai Sea region. However, along the coast of middle Jiangsu Province and in the sea area of South Huanghai Sea, the distribution of historical earthquake classes is complex and the earthquake series of comparatively dan-gerous class and comparatively safe class are equivalent in number; c) In the studied area, the statistical results of historical earthquake classes and the characteristics of spatial distribution accord very well with the real case of present-day earthquake series. It shows that the seismic activity in the region has the characteristic of succession, and the result from this study can be used as a reference for early postseismic judgment in the earthquake emer-gency work in Jiangsu Province.

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.

Shear wave splitting analysis of local earthquakes from dense arrays in Shimian,Sichuan

The Shimian area of Sichuan sits at the junction of the Bayan Har block.Sichuan-Yunnan rhombic block,and Yangtze block,where several faults intersect.This region features intense tectonic activity and frequent earthquakes.In this study,we used local seismic waveform data recorded using dense arrays deployed in the Shimian area to obtain the shear wave splitting parameters at 55 seismic stations and thereby determine the crustal anisotropic characteristics of the region.We then analyzed the crustal stress pattern and tectonic setting and explored their relationship in the study area.Although some stations returned a polarization direction of NNW-SSE.a dominant polarization direction of NW-SE was obtained for the fast shear wave at most seismic stations in the study area.The polarization directions of the fast shear wave were highly consistent throughout the study-area.This orientation was in accordance with the direction of the regional principal compressive stress and parallel to the trend of the Xianshuihe and Daliangshan faults.The distribution of crustal anisotropy in this area was affected by the regional tectonic stress field and the fault structures.The mean delay time between fast and slow shear waves was 3.83 ms/km.slightly greater than the values obtained in other regions of Sichuan.This indicates that the crustal media in our study area had a high anisotropic strength and also reveals the influence of tectonic complexity resulting from the intersection of multiple faults on the strength of seismic anisotropy.

Crustal Motion Characteristics in the Eastern Margin of the Tibetan Plateau and Adjacent Regions after the Wenchuan Earthquake

The Wenchuan earthquake has altered the crustal motion characteristics in the eastern margin of the Tibetan Plateau and adjacent regions.Using discontinuous GPS survey data for 2008–2012, the velocity field for the Eurasia reference framework has been obtained, and the general trend of contemporary crustal motion after the occurrence of the Wenchuan earthquake has been studied.In addition, using the velocity field, the block movement velocity has been estimated by least-squares fitting.Furthermore, the properties and displacement rates of main faults have been obtained from the differences in velocity vectors of the blocks on both sides of the faults.The results reveal that there are no obvious changes in the general characteristics of crustal motion in this area after the Wenchuan earthquake.The earthquake mainly changed the rate of the movement of the Chuan-Qing block and caused variation in the movement direction of the South China block.The effect of the earthquake on faults is mainly reflected in variations in fault displacement velocity; there is no fundamental change in the properties of fault activity.The displacement rates of the Xianshuihe fault decreased by 3–4 mm/a, the Longmenshan fault increased by 9–10 mm/a, and the northern segment of the Anninghe fault increased by approximately 9 mm/a.Furthermore, the displacement rates of the Minjiang, Xueshan, Huya, Longquanshan, and Xinjin faults increased by 2–3 mm/a.This implies that the effects of the Wenchuan earthquake on crustal movement can mainly be observed in the Chuan-Qing, South China, and N-Chuan-Dian blocks and their internal faults, as well as the Xianshuihe and Longmenshan faults and the northern section of the Anninghe fault.The reason for this is that the Wenchuan earthquake disturbed the kinematic and dynamic balance in the region.

Relation between the evolution of seismic apparent strain field and the region of strong earthquake occurrence

In this paper, according to the data on the middle and strong earthquakes in China, we have preliminary studied the relation between the characteristic of space-time evolution of the seismic apparent strain field and the regions of 31 macroseism events since 1955. The result shows that, there is a rather well correlation between the anomaly region of seismic apparent strain and the zone of macroseism event occurrence within the time range of one to about five years. The R value of the application of the abnormal region of seismic apparent strain to predicting the area of strong earthquake occurrence is 0.458, and the empirical possibility of forecasting the region of macroseism occurrence is 0.625, and so the forecasting effect is comparatively well. Finally, the main results obtained above are discussed preliminarily.

Seismicity acceleration model and its application to several earthquake regions in China

With the theory of subcritical crack growth, we can deduce the fundamental equation of regional seismicity acceleration model. Applying this model to intraplate earthquake regions, we select three earthquake subplates: North China Subplate, Chuan Dian Block and Xinjiang Subplate, and divide the three subplates into seven researched regions by the difference of seismicity and tectonic conditions. With the modified equation given by Sornette and Sammis (1995), we analysis the seismicity of each region. To those strong earthquakes already occurred in these region, the model can give close fitting of magnitude and occurrence time, and the result in this article indicates that the seismicity acceleration model can also be used for describing the seismicity of intraplate. In the article, we give the magnitude and occurrence time of possible strong earthquakes in Shanxi, Ordos, Bole Tuokexun, Ayinke Wuqia earthquake regions. In the same subplate or block, the earthquake periods for each earthquake region are similar in time interval. The constant α in model can be used to describe the intensity of regional seismicity, and for the Chinese Mainland, α is 0.4 generally. To the seismicity in Taiwan and other regions with complex tectonic conditions, the model does not fit well at present.

In search of potential earthquake source regions in the Chinese mainland in the light of ambient shear stress field

Earth media are incomplete media.There exist many cracks in it. The achievements of fracture mechanics showthat the strength of the incomplete materials will be much lower than that of the complete materials. We consider that earthquake occurrence is the result of unstable propagation of a crack in crust media in proper conditionand the earthquake rupture is the phenomenon of a failure by fast fracture under applied low shear stress. It hasalready been explained by fracture mechanics.The occurrence of failure by fast fracture is necessarily associated with the presence of high level concentration of local stress and strain. The elastic/plastic stress analysis in cracked pieces by Dugdale indicates that thestate of stress at the tip of a crack takes a very important role to crack propagation. A plastic zone has necessarilyformed in the tip of a crack due to stress concentration. Therefore, the dislocations st the tip of a crack are naturally a plastic displacement, rather than elastic one. The plastic displacement, where τ0 is appliedshear stress which is equivalent to initial or tectonic shear stress when the quake occurs, a is the half length of acrack, It is the rigidity,τy is the yield stresses in shear. The main seismic dislocations take place exactly at theends of the crack where the plastic zone had been formed. SO, a critical assumption is adopted, i. e. we assumethe dislocation D(1,,t) as formula (5) in text. The maximum earthquake dislocation, whereL is the fault length. If p is taken the value in the upper crust, μ=33 GPa; and τy is taken the average valuegiven from laboratories,τy= 30 MPa. Thus, according to observation values of Dmax and L, using the formula,one can estimate the initial shear stresses for large earthquakes. Computations show that the initial shear stressesfor large earthquakes all over the world are about 5-20 MPa which have some differences between regions.We further research the characteristics of source spectra and have derived the dependent relation of bodywave magnitude mb on the shear stress τ0 and seismic moment M, as formula (11)in text. Thus, the formulaprovides a POssibility of computation of large amount of tectonic shear stress values from seismic data. We consider that the tectonic shear stress field is a main factor which controls the earthquake occurrence. The regions withhigh tectonic shear stress values are considered to be prone to occur great earthquakes (Ms>6) and called earthquake hazard regions. Based on this criterion, τ0 values for all earthquakes with mb≥3. 8 all over China since1987 have been computed, and the great earthquake hazard regions with magnitude ranges have been zoned inthe Chinese mainland.During April 1992 -January 31, 1994, there were 9 Ms≥6 earthquakes which occurred in the Chinesemainland, 8 earthquakes of the 9 had fallen into the regions delineated by us prior to the earthquake occurrence,with only one failure. This new approach as a method for medium--term prediction of strong earthquakes hasbeen proved by practice to be an efficient one.It has good physical bases and bright prospect and worth furtherresearch. Received February 7,1994 1 Accepted February 10, 1995.Contribution No. 95A0061, Institute of Geophysics,SSB, China.

Fault-zone trapped waves at Muyu in Wenchuan earthquake region

Trapped waves in the Qingchuan fault zone were observed at Muyu near the northeastern end of the fractured zone of the Wenchuan Ms8. 0 earthquake. The results indicate a fault-zone width of about 200 m and a great difference in physical property of the crust on different sides of the fault. The inferred location of crustal changes is consistent with land-form boundary on the surface

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.

A Framework of Massively Parallel Analysis of Regional Earthquake Activities

Recent rapid progress in cyberinfrastructure in geosciences is providing seismologists an enormous boost for addressing multi-physical phenomena of regional seismic activities. The inherent nature of their multi-scale properties, from temporal to spatial spaces, makes it inevitably to be solved using large-scale computations and distributed parallel data processing schemes. Under such circumstance, using the advanced numerical algorithms and unstructured mesh generation technologies become the obstacles for modern seismologists. The main objective of this paper is to present a framework, which includes a parallel finite element simulation and distributed data infrastructure, to address the novel algorithms, state-of-the-art modeling and their implementation in regional seismicgenic systems. We also discuss and implement this framework to analyze the strong earthquake evolution processes in the Sichuan-Yunnan region. This study is the key to long-term seismic risk by estimates, providing a platform for predictive large-scale numerical simulation modeling of regional earthquake activities.

Moment tensor inversion of small to moderate earthquakes in the Capital Region in 2004

The moment tensor solutions of 51 small to moderate earthquakes occurred in the Capital Region in the year of 2004 are obtained by inverting the broadband waveform data. Accordingly, other source parameters, such as scalar seismic moments, moment magnitudes, double-couple （DC） components and compensated-linear-vector-dipole （CLVD） components, are determined as well as fault parameters and stress-axis parameters. The inverted results are evaluated by groups of numerical tests.

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.

A Preliminary Study on Rainfall Pattern before and after the January 26, 2001 Bhuj Earthquake (<i>M_w</i>7.7) over Kachchh Region of Western Peninsular, India

Under the influence of great debate on relation between earthquake and rainfall, some scientists have carried out detailed study and now commonly accepted that heavy rainfall can trigger earthquake at the faults or fractures depending upon the local geology. Here, an attempt is made to check relation between earthquake and rainfall with different scientific approaches. We have attempted to critically examine the relation between the Bhuj earthquake-aftershocks sequence and the rainfall pattern over the region as large earthquake (Mw 7.7) has occurred on January 26, 2001 in Kachchh region of western peninsular shield of India and the aftershocks are being reported till the date. We have analyzed rainfall data for 20 years, i.e. 10 years before and 10 years after the main shock of January 26, 2001, recorded by three meteorological observatories in the Kachchh region. We have studied annual total rainfall for two decades, annual rainfall departures from the climate normals, number of rainy days and number of heavy rainfall days during the period for all the three meteorological observatories of Kachchh region. We have found significant increase in all the measured rainfall parameters i.e. annual total rainfall, number of rainy days and number of heavy rainfall days over the Kachchh region during last decade i.e., from 2001 to 2010 after the main shock. Numbers of negative departures have been decreased during the last decade compared to previous decade. Thus rainfall pattern over Kachchh region is being changed. This increase in rainfall activity over Kachchh region may have been influenced by large earthquake and continuing aftershock activities over the region.

SLC Analysis on Earthquake Activity in the Regions of Sichuan and Yunnan of China

Using the method of Single-link Cluster(SLC),analyzing the pattern of time sliding of SLCparameters,the earthquake activity in the western Sichuan-Yunnan of China is studiedcombining with the regional earthquake catalogue since 1970 and the tectonic background.Comparing with the high level of earthquake activity in 1970’s and the low in 1980’s,theearthquake activity is in general at the middle level in 1990’s.This paper suggests that SLCmethod,which considers the temporal and spatial relationship among earthquake events,could be adopted to analyze the trend of regional earthquake activity,it is very useful forstudying seismic activity.

Coseismic fault model of the 2017 M_(W)6.5 Jiuzhaigou earthquake and implications for the regional fault slip pattern

On August 8,2017,an M_(W)6.5 earthquake occurred in Jiuzhaigou County,Sichuan Province,China,on the eastern margin of the Qinghai-Tibet Plateau.This study investigates the coseismic deformation field and fault model with ascending and descending Sentinel-1 synthetic aperture radar(SAR)images,aftershock distribution,and elastic half-space dislocation model.The regional fault slip pattern is then quantita-tively examined using the boundary element method.The results show that the ascending and descending interferometric synthetic aperture radar(InSAR)coseismic deformation fields display an overall NNW-SSE trend,with more significant deformation on the southwest side of the fault.The coseismic fault geometry is divided into NW and SE sub-faults with strikes of 162.1°and 149.3°,respectively.The coseismic fault slip is dominated by a left-lateral strike-slip movement with an average rake of-2.31°,mainly occurring at a depth of 0-13.04 km with a shape of an approximately inverted triangle.The fault slip features two peak slip zones,with a maximum of 1.39 m.The total seismic moment is 6.34×10^(18) N·m(M_(W)6.47).The boundary element calculation quantitatively indicates that the regional fault slip pattern may be mainly attributable to the changing strike and dip.The strike changes from NNWeSSE to nearly NS direction,and the dip gradually decreases from the Jiuzhaigou earthquake fault in the north to the Huya fault in the south.With these characteristics,the Huya and the Jiuzhaigou earthquake faults form the eastern boundary of the Minshan uplift zone and accommodate the accumulated deformation.

Regional seismicity triggered by the HyogoKen Nanbu, Japan, M=7.2 earthquake on January 17, 1995

The time and spatial feature of the regional seismicity triggered by the Hyogo-Ken Nanbu, Japan, M=7.2 earthquake on January 17, 1995, was studied. The concerned region is about several hundred kilometers in length and breadth surrounding the epicenter (33°-37°N, 133°-138°E). It is divided into 16 subregions. The seismicity of these subregions from January of 1976 to June of 1996 has been analyzed. It is showed that, 1) there were significant seismicity changes in 10 subregions triggered by the Hyogo-Ken Nanbu, Japan, M=7.2 earthquake on January 17, 1995. These changes passed a Z statistic test exceeding 0.95 confidence level and the greatest epicenter distance of these subregions was 280 km; 2) seismicity changes were triggered within 1-5 days in three subregions near the main shock while in other subregions the seismicity changes were triggered within several ten days after the main shock; 3) the greatest triggered event is 5.4, which is about the same size as the greatest aftershock; 4) the regional stress change resulted from the main shock may be the triggered mechanism of the regional seismicity.

Study on Fine Crustal Structure of the Sanhe-Pinggu Earthquake (M8.0) Region by Deep Seismic Reflection Profiling

Two near-vertical deep seismic reflection profiles (140km-long, 24-fold) were completed in the 1679 Sanhe-Pinggu earthquake (M8.0) region. The profiles ran through the Xiadian fault and the Ershilichangshan fault. The profiling result shows that the crust in this region is divided into the upper crust, the lower crust and the crust-mantle transitional zone by two powerful laminated reflectors: one at the two-way travel-time of about 7.0s (21km), the other at about 11.0～12.5s (33～37km). Crustal structure varies significantly in vertical direction. The shallow part is characterized by obvious stratification, multilayers and complexity. The upper crust on the whole features reflection “transparency”, while the lower crust features distinct reflectivity. Crustal structure also varies a lot in the lateral direction. The main fracture in this region is the deep fault under the Xiadian fault. This deep fault is steeply inclined (nearly vertical), and is supposed to be the causative fault of the Sanhe-Pinggu M8.0 earthquake. The two profiles respectively reveal the existence of local strong reflectivity in the lower crust and the lower part of the upper crust, which is assumed to be a dike or rock mass formed by the upwelling and cooling down of materials from the upper mantle. Magmatic activity in this part brought about differences in regional stress distribution, which then gave rise to the formation of the deep fault. That is supposed to be the deep structural setting for the Sanhe-Pinggu M8.0 earthquake.

Research on the Dynamic Change of Regional Stress Fields before the M_S8.0 Wenchuan Earthquake

Using the digital telemetric seismic waveform data of Chengdu and Kunming, this article studies the focal mechanism solutions and the apparent stress values of a large number of small earthquakes, and then analyzes the dynamic variation of regional stress fields and the spatio- temporal distribution of apparent stress values. The annual variation values of the azimuth of average principal stress field before the May 12, 2008 Ms8.0 Wenchuan earthquake in the Sichuan-Yunnan region were 58° from 2003 to 2004, 85° from 2003 to 2005,61° from 2006 to 2007 and 90° from 2006 to April 2008 respectively. In recent years, deflection or disturbances occurred in the azimuth of the average principal stress field in the Sichuan-Yunnan region. Analysis shows that this may be related to the change of stress field states of crustal blocks before and after the December 26, 2004 Ms9.0 Sumatra earthquake and the 2008 Ms8.0 Wenchuan earthquake. The ratio of thrust-type earthquakes in the Sichnan-Qinghai block was on the higher side in the period from 2006 to 2007, and the source faulting type of the regional moderate and small earthquakes had changed before the Ms8.0 Wenchnan earthquake. The change of state of the stress field is consistent with the changes in block displacement fields revealed by GPS data and the crustal shortening velocity vertical to the Longmenshan fault zone. Based on the radiation energy calculated from all bands of the seismic waveform, the value of apparent stress σapp is obtained. The fluctuation shape of the fitting trend of the apparent stress is related to the intensity of regional seismicity. It reveals that the micro- dynamic fluctuation process of the regional stress value is similar to the azimuth transition of the regional principal compressive stress field, which can be used to probe for pregnant physical processes. Areas with a higher value of apparent stress σapp are possible areas of potential seismic risk. It can be seen from the spatial distribution of the medium and shortterm apparent stress σapp before the Ms8.0 Wenchuan earthquake, the Longmenshan fault zone is in a low stress distribution area, and the relatively high apparent stress is in the peripheral area. These images may show medium and short-term locking phenomena near the seismogenic tectonics of the Ms8.0 Wenchuan earthquake. For example, changes with time of the focal parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces, continual increase of thrust-type earthquakes in the Sichuan-Qinghai block and the appearance of spatial distribution areas of high apparent σapp stress. The work on this aspect was continued after the Ms8.0 Wenchuan earthquake, and the results seem to be shown a clearer relationship between these phenomena and future great earthquakes.

Damage and Seismic Intensity in the Meizoseismal Region of the M_S8.0 Wenchuan Earthquake,Sichuan,China

This paper presents the damage in the meizoseismal region of the M_S8.0 Wenchuan earthquake,Sichuan,China,and the seismic intensities determined according to "the Chinese Seismic Intensity Scale",and discusses briefly the types of earthquake-generating faults and some features of seismic damage.