Tectonic Stress Analysis of Future Large Earthquake Zones along the Bayan Har Block Boundary,Tibet Plateau

The Bayan Har block is mainly bounded by the east Kunlun fault zone to the north, Garze-Yushu -Xianshuihe fault zone to the south and Longmenshan fault zone to the east （Fig. 1）. In the past 20 years, large earthquakes have occurred frequently along this block＇s boundaries, which has received much attention among geoscientists. Whether large earthquakes will happen （and where） along this block＇s boundary faults in the future are two key problems that need to be addressed. This study calculates the accumulated tectonic stress and superposition of the coulomb stress caused by fault slip of 16 large earthquakes since 1904, and evaluates the possible locations of future earthquakes that may occur around this block.

Internal structures and high-velocity frictional properties of Longmenshan fault zone at Shenxigou activated during the 2008 Wenchuan earthquake

This paper reports internal structures of a wide fault zone at Shenxigou,Dujiangyan,Sichuan province,China,and high-velocity frictional properties of the fault gouge collected near the coseismic slip zone during the 2008 Wenchuan earthquake.Vertical offset and horizontal displacement at the trench site were 2.8 m（NW side up）and 4.8 m（right-lateral）,respectively.The fault zone formed in Triassic sandstone,siltstone,and shale about 500 m away from the Yingxiu-Beichuan fault,a major fault in the Longmenshan fault system.A trench survey across the coseismic fault,and observations of outcrops and drill cores down to a depth of 57 m revealed that the fault zone consists of fault gouge and fault breccia of about0.5 and 250-300 m in widths,respectively,and that the fault strikes N62°E and dips 68° to NW.Quaternary conglomerates were recovered beneath the fault in the drilling,so that the fault moved at least 55 m along the coseismic slip zone,experiencing about 18 events of similar sizes.The fault core is composed of grayish gouge（GG） and blackish gouge（BG） with very complex slip-zone structures.BG contains low-crystalline graphite of about 30 %.High-velocity friction experiments were conducted at normal stresses of 0.6-2.1 MPa and slip rates of 0.1-2.1 m/s.Both GG and BG exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient lpto steadystate friction coefficient lssover a slip-weakening distance Dc.Deformation of GG and BG is characterized by overlapped slip-zone structures and development of sharp slickenside surfaces,respectively.Comparison of our data with those reported for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault zones are quite uniform and the high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.

Some thoughts on seismotectonics of major earthquake occurrence zones in China

A major earthquake occurrence zone means a place where M ≥6 events have occurred since the Holocene and similar shocks may happen again in the future. The dynamic context of the major earthquake occurrence zones in China is primarily associated with the NNE-directed push of the India plate, next with the westward subduction of the Pacific plate. The Chinese mainland is a grand mosaic structure of many crust blocks bounded by faults and sutures. When it is suffered from boundary stresses, deformation takes place along these faults or sutures while the block interiors remain relatively stable or intact. Since the Quaternary, for example, left slip on the Xianshuihe-Xiaojiang fault zone in southwestern China has produced a number of fault-depression basins in extensional areas during periods Q1 and Q2. In the Q3, the change of stress orientation and enhancement of tectonic movement made faults of varied trends link each other, and continued to be active till present day, producing active fanlt zones in this region. Usually major earthquakes occur at some special locations on these active fault zones. During these events, in the epicenter areas experience intensive deformation character- ized by large-amplitude rise and fall of neighboring sections, generation of horst-graben systems and dammed rivers. The studies on palaeoearthquakes suggest that major shocks of close magnitudes often repeated for several times at a same place. By comparison of the Chi-Chi, Taiwan event in 1999 and Yuza, Yunnan event in 1955, including contours of accelerations and intensities, destruction of buildings, and in contrast to the Xigeda formation in southwestern China, a sandwich model is established to account for the mechanism of deformation caused by major earthquakes. This model consists of three layers, i.e. the two walls of a fault and the ruptured zone intercalated between them. This ruptured zone is just the loci where stress is built up and released, and serves as a channel for seismic waves.

Quantitative Seismicity Analysis for the Risk of Historical Large Earthquake Rupture Zone:Application to the Mid-North Segment of the North-South Seismic Belt

Although seismic gap theory plays an important role in the med-and long-term earthquake prediction,the potential risk of the non-seismic gap in historical earthquake rupture areas will need to be simultaneously taken into account in the study of med-and long-term earthquake prediction,due to the temporally clustering or non-linear behavior of large earthquake recurrence.In order to explore technical methods which can be based on observational data,and identify historical earthquake rupture zones( including the seismic gap in historical and prehistoric earthquake rupture zones),we select eight historical large earthquake rupture zones with different elapsed times on the mid-north segment of the North-South Seismic Belt to make quantitative analysis on the characteristics of modern seismicity of these zones and preliminarily explore the seismicity method for determining the urgency degree of potential earthquake hazards.The results mainly show that the pvalue,which reflects the attenuation of earthquake sequence,and the a-value,which reflects the seismicity rate,are strongly related to the elapsed time of the latest earthquake in the rupture zone.However,the corresponding relationships in some rupture areas are not clear perhaps due to the complex fault structure and faulting behavior.The b-value,which represents the state of tectonic stress accumulation,does not easily reflect the elapsed time information of different evolution stages.The b-value temporal scanning shows a steady evolution over time in most of the rupture zones,but in the rupture zone of the Wudu M8.0 earthquake of 1879,the b-value shows significant fluctuations with a decreasing trend for 20 years.By comparative analysis,we conclude that the rupture zones of the 1933 M7.5 Maoxian earthquake and the 1976 M7.2 Songpan-Pingwu earthquake are still in the decaying period of earthquake sequences,and thus do not have the background for recurrence of M7.0 earthquakes.The low b-value Maqu segment,which is located at the north margin of the rupture zone of the 842A.D.M7.0 Diebu earthquake,is more dangerous than the Diebu segment.The continuous decline of the b-value in the 1879 M8.0 Wudu earthquake rupture zone may also indicate a new round of seismogenic process.

A Statistical Analysis on Recent Tidal Triggering of the Earthquake in the Tianshan Seismic Zone

A statistical analysis is done to study the spatio-temporai features of earthquake activity in the Tianshan seismic belt triggered by tide, based on Schuster＇s test. The data we choose is the ML ≥2. 0 earthquakes from January 1, 2010 to August 31, 2012 in eastern Tianshan, and the calculation is on tidal body stress. The results show that the p-value based on the time window smoothing of Schuster＇s test corresponds better with the strong earthquakes in the Tianshan seismic belt, especially for a long time before the November 1, 2011 Nilka Ms6. 0 earthquake, when the p-value of the Schuster＇s test was always lower than the threshold of 0. 05 for tidal trigger of earthquake, but after the Niika Ms6. 0 earthquake, that value was quickly restored to a high level, which reflects a close relationship between the Nilka Ms6. 0 earthquake and the Earth tide. According to the p-value based on the spatial window smoothing of Schuster＇s test, the Nilka Ms6. 0 earthquake was at or near the tidal triggering area. Thus we can see from the spatio-temporal results that the Nilka Ms6. 0 earthquake was obviously triggered by Earth fide.

Analysis of Dynamic Characteristics of Crustal Horizontal Deformation in the South Segment of Longmenshan Fault Zone Revealed by GPS Data after the Wenchuan Earthquake

In order to obtain deformation parameters in the south segment of Longmenshan fault zone,Euler datum transformation and the least square collocation for data interpolation and smoothing are used to process GPS displacement time series data in the south segment of Longmenshan fault zone,and the rigid and elastic-plastic block motion model is used to calculate the strain parameters in each subarea. Conjoint analysis of displacement,velocity of each station and strain parameters of each subarea reveals that the influence of the Wenchuan earthquake on the south segment of Longmenshan fault zone increases from southeast to northwest,causing a highest deformation rate 6 times the background value and heightening the influence of the hidden faults on the difference of the earth surface along its two sides,which leads to the seismic risk of the southern segment increasing from north to south. The comparison of seismic risk among subareas based on the tectonic and seismicity background indicates that the most dangerous area is on the southeast of Longmenshan faults,and the background strain accumulation and the promoting effect of the Wenchuan earthquake advanced the occurrence of Lushan earthquake and the sinistral strike-slip on the rupture plane. The Wenchuan earthquake also caused a slight two-year long continuous strain release in the south segment of Xianshuihe fault,but the influence is far less than the effect of the compressive strain caused by the Sichuan-Yunnan block.

Microseismic Concentration Zones before and after the February 12,2014 M_S 7.3 Yutian Earthquake and the Possible Indication of an Earthquake Risk Zone

Since 2001, there have occurred in succession the 2001 Kunlun Mountains M S8. 1earthquake,the 2008 Wenchuan M S8. 0 earthquake,the 2010 Yushu M S7. 1 earthquake and the 2012 Lushan M S7. 0 earthquake in the periphery of the Bayan Har block. By comparison of the characteristics of seismic strain release variations before and after the Kunlun Mountains M S8. 1 earthquake in the same time length in the geodynamical related regions,we found that the seismic strain release was obviously enhanced after the earthquake in the Longmenshan area,Batang area,and the NS-trending valleys at the west of the Hot Spring Basin. The Wenchuan earthquake occurred in the first area,and the Yushu earthquake is related to the second area. After the earthquake rupture occurred on the East Kunlun fault zone on the northern boundary of the Bayan Har Block,crustal materials on the south side of the fault zone migrated to the southeast,leading to a concentration of tectonic deformation in the Longmenshan thrust belt, e ventually rupturing on the Longmenshan thrust belt. This earthquake case illustrates that seismicity enhancement zones are possibly prone to long-term destructive earthquakes. After the M S7. 3 earthquake in Yutian,Xinjiang on February 12,2014,earthquake frequency and seismic strain release markedly increased in the junction area between the eastern Qilian Mountain tectonic belt and the Altun Tagh fault zone,where more attention should be paid to the long-term seismic risk.

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.

Recent advances in imaging crustal fault zones: a review

Crustal faults usually have a fault core and surrounding regions of brittle damage, forming a low-velocity zone （LVZ） in the immediate vicinity of the main slip interface. The LVZ may amplify ground motion, influence rupture propagation, and hold important information of earthquake physics. A number of geophysical and geodetic methods have been developed to derive high-resolution structure of the LVZ. Here, I review a few recent approaches, including ambient noise cross-correlation on dense across-fault arrays and GPS recordings of fault-zone trapped waves. Despite the past efforts, many questions concerning the LVZ structure remain unclear, such as the depth extent of the LVZ. High-quality data from larger and denser arrays and new seismic imaging technique using larger portion of recorded waveforms, which are currently under active development, may be able to better resolve the LVZ structure. In addition, effects of the alongstrike segmentation and gradational velocity changes across the boundaries between the LVZ and the host rock on rupture propagation should be investigated by conducting comprehensive numerical experiments. Furthermore, high-quality active sources such as recently developed large-volume airgun arrays provide a powerful tool to continuously monitor temporal changes of fault-zone properties, and thus can advance our understanding of fault zone evolution.

Pre-seismic gravity anomalies before Linkou Ms6.4 earthquake by continuous gravity observation of Crustal Movement Observation Network of China

A Ms6.4 earthquake occurred at Linkou country, Heilongjiang Province （44.8°N, 129.9°E） on January 2, 2016 at a depth of 580 km. Pre-seismic graviW anomalies obtained at a 1 Hz sampling rate from Crustal Movement Observation Network of China （CMONOC） are analyzed after the earthquake. The results show that： （1） different from previous studies, both pre-seismic amplitude perturbation and co-seismic amplitude perturbation are not critical inversely proportional to epicentral distance; （2） unlike shallow earthquake, the pre-seismic and co-seismic amplitude perturbation of gravity illustrate syn- chronous spatial variation characters with decrease of epicentral distance for Linkou earthquake. This may because Linkou earthquake is a deep earthquake and occurred in Pacific Plate subduction zone; （3） compared to basement and semi-basement, cave can provide a better observation environment for gPhone gravimeter to detect pre-seismic gravity anomalies.

Temporal variation of gravity field prior to the Ludian Ms6.5 and Kangding Ms6.3 earthquakes

Using mobile gravity data from the central area of Sichuan and Yunnan Provinces, the relationship between gravity variation and earthquakes was studied based on the Ludian Ms6.5 earthquake that occurred on August 3rd, 2014, and the Kangding Ms6.3 earthquake that occurred on November 22 nd, 2014; the mechanism of gravity variation was also explored. The results are as follows：（1） Prior to both earthquakes, gravity variation exhibited similar characteristics as those observed before both the Tangshan and Wenchuan earthquakes, in which typical precursor anomalies were positive gravity variation near the epicenter and the occurrence of a high-gravity-gradient zone across the epicenter prior to the earthquake.（2） A relatively accurate prediction of the occurrence locations of the two earthquakes was made by the Gravity Network Center of China（GNCC） based on these precursor anomalies. In the gravity study report on the 2014 earthquake trends submitted at the end of 2013, the Daofu-Shimian section at the junction of the Xianshuihe and Longmenshan fault zones was noted as an earthquake-risk region with a predicted magnitude of 6.5, which covered the epicenter of the Kangding Ms6.3 earthquake. In another report on earthquake trends in southwestern China submitted in mid-2014, the Lianfeng, Zhaotong fault zone was also classified as an earthquake-risk region with a magnitude of 6.0, and the central area of this region basically overlapped with the epicenter of the Ludian Ms6.5 earthquake.（3） The gravity variation characteristics are reasonably consistent with crustal movements, and deep material migration is likely the primary cause of gravity variation.

The relationships between earthquakes and positions of the sun and moon（Ⅱ）——Sometemporalcharacteristicsoftheaftershocksequencesofstrongearthquakes

This paper deals with the distributive characteristics of the occurrence time of earthquakes with respect to the aftershock sequences of strong earthquakes. The distribution of lunar and solar local hour angles at the time of commencement of moderate and strong aftershocks indicates that the time of commencement of moderate and strong aftershocks is modulated by the positions of the sun and moon and then the earthquake restrained time zones exists also. In this paper the differences of earthquake restrained time zones between the preshock sequences and the aftershock sequences are compared, and the possible mechanism is analyzed preliminarily. And the possible maximum scope of accuracy in predicting the occurrence time of an earthquake is determined as well.

Goce derived geoid changes before the Pisagua 2014 earthquake

The analysis of space-time surface deformation during earthquakes reveals the variable state of stress that occurs at deep crustal levels, and this information can be used to better understand the seismic cycle. Understanding the possible mechanisms that produce earthquake precursors is a key issue for earthquake prediction. In the last years, modern geodesy can map the degree of seismic coupling during the interseismic period, as well as the coseismic and postseismic slip for great earthquakes along subduction zones. Earthquakes usually occur due to mass transfer and consequent gravity variations, where these changes have been monitored for intraplate earthquakes by means of terrestrial gravity measurements. When stresses and correspondent rupture areas are large, affecting hundreds of thousands of square kilometres（as occurs in some segments along plate interface zones）, satellite gravimetry data become relevant. This is due to the higher spatial resolution of this type of data when compared to terrestrial data, and also due to their homogeneous precision and availability across the whole Earth.Satellite gravity missions as GOCE can map the Earth gravity field with unprecedented precision and resolution. We mapped geoid changes from two GOCE satellite models obtained by the direct approach,which combines data from other gravity missions as GRACE and LAGEOS regarding their best characteristics. The results show that the geoid height diminished from a year to five months before the main seismic event in the region where maximum slip occurred after the Pisagua Mw = 8.2 great megathrust earthquake. This diminution is interpreted as accelerated inland-directed interseismic mass transfer before the earthquake, coinciding with the intermediate degree of seismic coupling reported in the region. We highlight the advantage of satellite data for modelling surficial deformation related to preseismic displacements. This deformation, combined to geodetical and seismological data, could be useful for delimiting and monitoring areas of higher seismic hazard potential.

Discussion on Deep Tectonic Background of Moderately Strong Earthquake in Anhui Province and Its Neighboring Areas Using Results of Seismic Tomography

Using the latest results of seismic tomography, we studied the deep tectonic settings of the moderate and strong earthquakes in Anhul Province and its neighboring areas （28° - 39°N, 112°- 124°E）. The results are as follows： （1） There exists a certain correlation between the location of moderate-strong earthquake, the geologic structure of the surface and the partitioning of active tectonic elements with the upper-crust velocity structure. Most earthquakes recording M ≥ 6.0 occur in high-velocity zones or in the transitional areas between high-velocity and low-velocity zones in the upper crust. Seismicity in the low-velocity zone has a lesser impact. Earthquakes occuring in the high-velocity zone are distributed mainly in the velocity variation area. The boundary belts and the interior of the North China plain fault block are the main active sites of moderate-strong earthquakes. Beneath the fault basins in the western and northern sides of the block, the upper crust is characterized by a wide discontinuous distribution in the low-velocity zone, and in the transition zone from the low- to high velocities, the moderate strong seismicity shows a zonal distribution where active faults are developed. The NW-extension Zhoukou-Hefei-Xuancheng low-velocity zone separates the highvelocity zones of Dabieshan Mountains and west Shandong-Anhul, and moderate-strong earthquakes on its northern side bordering the high-velocity zones are relatively frequent. This low-velocity zone is probably an important and deeply structured boundary between the North China and the South China tectonic provinces. （2） The frequent moderate-strong earthquake recorded in the past and the recent small earthquake activities in the Huoshan-Lu＇ an area are the result of a low-velocity zone in the middle crust beneath the central part of Dabieshan and the two sets of deep faults that cut through the crust. （3） In terms of deep structures, the distribution of moderate-strong earthquake in Anhui Province has an obvious regional feature. Based on historical earthquake recurrence intervals and analogue principles of deep seismogenic tectonics, the potential earthquake risk zones recording Ms5.0 in Anhui Province are determined.

A Discussion of Some Problems Concerning the Tosonhu Lake M_S7.5 Earthquake in 1937

The East Kuulun active fault zone, which lies in the valley of the Kuulun Mountains above an elevation of 4,000 meters, is an important active fault zone in the Northeast Qinghai-Xizang （Tibet） Plateau. The 1937, the Tosonhu lake Ms7. 5 earthquake occurred in the eastern segment of the East Kuulun active fault zone. Four field investigations were launched on this seism in 1963, 1971, 1980, and between 1986 and 1990. However, due to different extents of the investigations, four different conclusions have been gained. Concerning the length aspect of the surface rupture zone of this earthquake, the unanimous consensus is that its eastern end lies in the west side of the main Ridge of the A ＇nyemaqen Mountains, but opinions about the western end and the location of the macro-epicenter are different. Based on investigation and comprehensive study, a series of scientific problems like geometric and kinetic characteristics, the length of the rupture zone, the maximum sinistral horizontal displacement and the macroepicenter were re-evaluated. We believe that the total length of this earthquake＇s surface deformation zone is at least 240km; the western end of the zone is at the west of Wnsuwuwoguole; the maximum sinistral horizontal displacement is 8m to the west of Baerhalasha gully on the east side of Sanchakou; the maximum vertical displacement is 3.5m in the south of Sanchakou and the macro-epicenter is in Sanchakou.

Research on the Activity Characteristics of Small Earthquake Swarms in the Muli Area of Sichuan Province

In this article,we relocated the seismic source location of the earthquakes in the Muli area of Sichuan,inverted the focal mechanism of the larger earthquakes and analyzed the relationship between the water level of the Jinping reservoir and the frequency of the earthquake swarm. The results show that:( 1) The epicenters of the relocated small earthquake swarms are distributed in a seismic zone,and the earthquake focal depths were in the range of 0- 12 km.( 2) By analyzing the earthquake swarm spatial distribution,we found that the swarms were generated by one branch fault on the west of Xiaojinhe fault.( 3) The focal mechanism of the three earthquakes with magnitude greater than 4. 0 is significantly different,with the shallow source thrust events affected by vertical stress,and the strike-slip events are related to regional stress tectonic activity.

Evaluation of Uncertainty of Earthquake Parameters for the Purpose of Seismic Zoning of Iran

An efficient procedure is used for explicit description and evaluation of uncertainty of earthquake parameters in the uniform catalog of earthquakes in Iran and neighboring regions.An inadequate number of local and regional seismographic stations,poor station distribution,and Inadequacy of velocity models have resulted in conspicuous uncertainty in different parameters of recorded events.In a comprehensive seismic hazard analysis such uncertainties should be considered.Uncertainty of magnitude and location of events are evaluated for three different time periods,namely,historical,early instrumental,and modern instrumental time periods,for which existing seismological information differ widely in quantity,quality,and type.It is concluded that an uncertainty of 0.2-0.3 units of magnitude and 10-15 km in epicenter determinations should be considered in the most favorable conditions.None of the hypocenters of earthquakes in Iran can be considered as reliable,unless supported by other information such as