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.

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.

Study on Integrated Recurrence Behaviors of Strong Earthquakes Along Entire Active Fault Zones in the Sichuan-Yunnan Region, China

Based on historical earthquake data, we use statistical methods to study integrated recurrence behaviors of strong earthquakes along 7 selected active fault zones in the Sichuan-Yunnan region. The results show that recurrences of strong earthquakes in the 7 fault zones display near-random, random and clustering behaviors. The recurrence processes are never quasi-periodic, and are neither strength-time nor time-strength dependent. The more independent segments for strong earthquake rupturing a fault zone has, the more complicated the corresponding recurrence process is. And relatively active periods and quiescent periods for earthquake activity occur alternatively. Within the active periods, the distribution of recurrence time intervals between earthquakes has relatively large discretion, and can be modelled well by a Weibull distribution. The time distribution of the quiescent periods has relatively small discretion, and can be approximately described by some distributions as the normal. Both the durations of the active periods and the numbers of strong earthquakes within the active periods vary obviously cycle by cycle, leading to the relatively active periods having never repeated quasi-periodically. Therefore, the probabilistic assessment for middle- and long-term seismic hazard for entireties of active fault zones based on data of historical strong earthquakes on the fault zones still faces difficulty.

The Statistical Analysis of Migration of Strong Earthquakes─Taking the North China Region as an Example

The migration of strong earthquakes is an important research topic because the migration phenomena reflect partly the seismic mechanism and involve the prediction of tendency of seismic activity. Research on migration of strong earthquakes has mostly focused on finding the phenomena. Some attempts on getting regularity were comparatively subjective. This paper suggests that there should be indices of migration in earthquake dataset and the indexes should have statistical meaning if there is regularity in the migration of strong earthquakes. In this study, three derivative attributes of migration, i.e., migration orientation, migration distance and migration time interval, were statistically analyzed. Results in the North China region show that the migration of strong earthquakes has statistical meaning. There is a dominant migration orientation (W by S to E by N), a dominant distance (≤100km and on the confines of 300～700km), and a dominant time interval (≤1a and on the confines of 3～4a). The results also show that the migration will differ slightly with different magnitude range or earthquake activity phase.

Did the MS 7.0 Lushan earthquake dynamically trigger earthquakes in the Datong volcanic region(Shanxi Province)?

Immediately following the Ms7.0 Lushan earthquake on April 20, 2013, using high-pass and low- pass filtering on the digital seismic stations in the Shanxi Province, located about 870-1,452 km from the earthquake epicenter, we detected some earthquakes at a time corre- sponding to the first arrival of surface waves in high-pass filtering waveform. The earthquakes were especially noticed at stations in Youyu （YUY）, Shanzizao （SZZ）, Shanghuangzhuang （SHZ）, and Zhenchuan （ZCH）, which are located in a volcanic region in the Shanxi Province,but they were not listed in the Shanxi seismic observation report. These earthquakes occurred 4-50 rain after the passage of the maximum amplitude Rayleigh wave, and the periods of the surface waves were mainly between 15 and 20 s following. The Coulomb stresses caused by the Ray- leigh waves that acted on the four stations was about 0.001 MPa, which is a little lower than the threshold value of dynamic triggering, therefore, we may conclude that the Datong volcanic region is more sensitive to the Coulomb stress change. To verify, if the similar phenomena are widespread, we used the same filtering to observecontrastively continuous waveform data before, and 5 h after, the Ms7.0 Lushan earthquake and Ms9.0 Tohoku earthquake in 2011. The results show that the similar phenomena occur before the earthquakes, but the seis- micity rates after the earthquakes are remarkably increased. Since these weak earthquakes are quite small, it is hard to get clear phase arrival time from three or more stations to locate them. In addition, the travel time differences between P waves and S waves （S-P） are all less than 4 s, that means the events should occur in 34 km around the stations in the volcanic region. The stress of initial dynamic triggering of the Ms9.0 Tohoku earthquake was about 0.09 MPa, which is much higher than the threshold value of dynamic triggering stress. The earthquakes after the Ms9.0 Tohoku earthquake are related to dynamic triggering stress, but the events before the earthquake cannot be linked to seismic events, but may be related to the back- ground seismicity or from other kinds of local sources, such as anthropogenic sources （i.e., explosions）. Using two teleseismic filtering, the small background earthquakes in the Datong volcanic region occur frequently, thus we postulate that previous catalog does not apply bandpass filter to pick out the weak earthquakes, and some of the observed weak events were not triggered by changes in the dynamic stress field.

Study on the Moment Magnitude of Small and Moderate Earthquakes Located in the Inner Mongolia Region

Based on digital seismic waveform data from Inner Mongolia Digital Seismic Network,the source spectrum parameters of 182 small and moderate earthquakes from January,2009to September,2016 are derived,and the seismic momentmoment magnitude MW of the earthquakes are calculated.Theand the relationship between stress drop and magnitude are obtained using the linear regression method.It is clear that incorporating the moment magnitude into the seismic quick report catalog and the official earthquake catalog can enrich earthquake observation report content,thus providing better service for earthquake emergency and earthquake scientific research.

Study on Relation Between Dynamic Pattern of Regional Vertical Strain Rate and Several Strong Earthquakes such as Lijiang(M_s7.0)and Menyuan(M_s6.4)Earthquakes

Making use of the method of obtaining regional vertical strain rate from regional preciseleveling data and gaining dynamic pattern combining with deformation data on spanningfaults, the regional vertical strain dynamic evolution characteristics of several moderatelystrong earthquakes such as Lijiang (M_s 7.0) and Menyuan (M_s 6.4) earthquakes occurredin crustal deformation monitoring areas located in the western Yunnan and Qilianshan-Hexiregion. Based on the above-mentioned facts, by studying the time-space nonhomogeneity andstrain energy accumulation status, some criteria for judging the medium. and short-termstrong seismic risk regions according to the regional vertical strain rate dynamic informationare proposed.

Study on the Relationship Between Recent Crustal Deformation and the Temporal-Spatial Distribution of Strong Earthquakes (M_s≥6.0) in the Sichuan-Yunnan Region

Analysis of deformation data measured across the faults, regional vertical deformation data and GPS measurements in the Sichuan-Yunnan region made since the 1980s permitted us to conclude that the crustal deformation in the region during this period of time was relatively weak and caused the occurrence of earthquakes (M S≥6.0), which were not distributed along the major boundary active faults in the region after the 1981 Dawu M S 6.9 earthquake and that the seismic activity is characterized by quasi-clockwise migration. Thus, it follows that earthquake prediction research should be focused on the central part of the Sichuan-Yunnan region in the coming years. Finally, a concept of temporal division of the region into active blocks is suggested and the preliminary result of the division is given in the paper.

Relocation of the M_S≥2.0 Earthquakes in the Northern Tianshan Region, Xinjiang, Using the Double-Difference Earthquake Relocation Algorithm

We applied the double-difference earthquake rdocation algorithm to 1348 earthquakes with Ms ≥2.0 that occurred in the northern Tianshan region, Xinjiang, from April 1988 to June 2003, using a total of 28701 P- and S-wave arrival times recorded by 32 seismic stations in Xinjiang. Aiming to obtain most of these Ms ≥ 2.0 earthquakes relocations, and considering the requirements of the DD method and the condition of data, we added the travel time data of another 437 earthquakes with 1.5 ≤ Ms 〈 2.0. Finally, we obtained the relocation results for 1253 earthquakes with Ms ≥2.0, which account for 93 % of all the 1348 earthquakes with Ms ≥ 2.0 and includes all the Ms ≥ 3.0 earthquakes. The reason for not relocating the 95 earthquakes with 2.0 ≤ Ms 〈 3.0 is analyzed in the paper. After relocation, the RMS residual decreased from 0.83s to 0.14s, the average error is 0.993 km in E-W direction, 1.10 km in N- S direction, and 1.33 km in vertical direction. The hypocenter depths are more convergent than before and distributed from 5 km to 35 kin, with 94% being from 5km to 35 kin, 68.2% from 10 km to 25 kin. The average hypocenter depth is 19 kin.

The Two-dimensional Time Coordinate System and Time Prediction Research of M≥6.7 Strong Earthquakes in the Sichuan-Yunnan Region

Since the 20 thcentury,the time intervals of M ≥6.7 strong earthquakes in the SichuanYunnan region show obvious regularity.Using the years of the strong events,a twodimensional time coordinate system is generated,based on which,the time prediction model is constructed for strong earthquakes in the Sichuan-Yunnan region.Prediction analysis shows that there is risk of generating four earthquakes with M ≥ 6.7 in the Sichuan-Yunnan region in the future 16 years,and there are strong signals for M ≥6.7earthquakes for periods 2012-2021 and 2025-2029.The strong earthquakes may occur around 2014-2015,2019 and 2027.

The Quantitative Identification of Regional Mine and Natural Earthquakes and Its Application

Based on the Morlet wavelet transform and digital data from the Fushun and Beizhen seismic stations, Liaoning Province, we put forward a new method in the paper, called time-frequency energy attenuation factor （α-value）. The characteristics of the α-value and its variation with magnitude of natural and mine earthquakes are studied, and the statistic relations between the α-value and specific earthquake magnitude are obtained. From the results, some conclusions can be drawn as follows： （1） in general, the α-values of mine and natural earthquakes of the same intensity have obvious difference and the ranges of their variation do not overlap each other; （2） the α-value decreases with the increase of earthquake magnitude, and the α-value of mine earthquake decreases faster than that of natural earthquake; （3） based on the earthquake magnitude and on the relations between the α-value and earthquake magnitude, we can distinguish the mine earthquakes from the natural ones; （4） the difference in focal mechanism of mine and natural earthquakes would be the main cause for obvious difference of the α-value; （5） the α-value variation is relatively steady for mine and natural earthquakes that occur in the same region, but it has obvious regional difference. The above results are of inspirational meaning for the study of abnormal change of the α-value before strong earthquakes.

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.

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.

Strong paleoearthquakes along the Talas-Fergana Fault,Kyrgyzstan

The Talas-Fergana Fault, the largest strike-slip structure in Central Asia, forms an obliquely orien- ted boundary between the northeastern and southwestern parts of the Tianshan mountain belt. The fault under- went active right-lateral strike-slip during the Paleozoic, with right-lateral movements being rejuvenated in the Late Cenozoic. Tectonic movements along the intracontinental strike-slip faults contribute to absorb part of the regional crustal shortening linked to the India-Eurasia collision; knowledge of strike-slip motions along the Ta- las-Fergana Fault are necessary for a complete assessment of the active deformation of the Tianshan orogen. To improve our understanding of the intracontinental deformation of the Tianshan mountain belt and the occurrence of strong earthquakes along the whole length of the Talas-Fergana Fault, we identify features of relief arising during strong paleoearthquakes along the Talas-Fergana Fault, fault segmentation, the length of seismogenic ruptures, and the energy and age of ancient catastrophes. We show that during neotectonic time the fault devel- oped as a dextral strike-slip fault, with possible dextral displacements spreading to secondary fault planes north of the main fault trace. We determine rates of Holocene and Late Pleistocene dextral movements, and our radi- ocarbon dating indicates tens of strong earthquakes occurring along the fault zone during and interval of 15800 years. The reoccurrence of strong earthquakes along the Talas-Fergana Fault zone during the second half of the Holocene is about 300 years. The next strong earthquake along the fault will most probably occur along its southeastern chain during the next several decades. Seismotectonic deformation parameters indicate that M〉7 earthquakes with oscillation intensity I〉IX have occurred.

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