Variations of shear-wave splitting parameters in the source region of the 2023 Türkiye doublet earthquakes

In this study,the shear-wave splitting parameters of local seismic events from the source regions of the 2023 Türkiye MW7.7 and MW7.6 doublet earthquakes(event 1 and event 2,respectively)were measured from June 1,2022,to April 25,2023,and their spatiotemporal characteristics were analyzed.The results revealed clear spatial and temporal differences.Spatially,the dominant fast-wave polarization direction at each station shows a strong correlation with the direction of the maximum horizontal principal compressive stress,as characterized by focal mechanism solutions of seismic events(MW≥3.5)near the station.The dominant fast-wave polarization direction and the regional stress field also showed a strong correlation with the intermovement of the Arabian Plate,African Plate,and Anatolian Block.Along the Nurdagi-Pazarcik fault zone,the seismic fault of event 1,stations closer to the middle of the fault where the mainshock occurred exhibited notably greater delay times than stations located towards the ends of the fault and far from the mainshock.In addition,the stations located to the east of the Nurdagi-Pazarcik fault and to the north of the Sürgüfault also exhibited large delay times.The spatial distribution of shear-wave splitting parameters obtained from each station indicates that the upper-crust anisotropy in the source area is mainly controlled by the regional stress field,which is closely related to the state of the block motion.During the seismogenic process of the MW7.7 earthquake,more stress accumulated in the middle of the Nurdagi-Pazarcik fault than at either end of the fault.Under the influence of the MW7.7 and MW7.6 events,the stress that accumulated during the seismogenic process of the earthquake doublet may have migrated towards some areas outside the aftershock intensive area after the earthquakes,and the crustal stress and its adjustment range near the outer stations increased significantly.With the exception of two stations with few effective events,all stations showed a consistent change in shear-wave splitting parameters over time.In particular,each station showed a decreasing trend in delay times after the doublet earthquakes,reflecting the obvious intensification of crustal stress adjustment in the seismogenic zone after the doublet earthquakes.With the occurrence of the earthquake doublet and a large number of aftershocks,the stress accumulated during the seismogenic process of the doublet earthquakes is gradually released,and then the adjustment range of crustal stress is also gradually reduced.

Shear-wave elastography to predict hepatocellular carcinoma after hepatitis C virus eradication:A systematic review and meta-analysis

BACKGROUND Direct-acting antiviral agents(DAAs)are highly effective treatment for chronic hepatitis C(CHC)with a significant rate of sustained virologic response(SVR).The achievement of SVR is crucial to prevent additional liver damage and slow down fibrosis progression.The assessment of fibrosis degree can be performed with transient elastography,magnetic resonance elastography or shear-wave elastography(SWE).Liver elastography could function as a predictor for hepato-cellular carcinoma(HCC)in CHC patients treated with DAAs.AIM To explore the predictive value of SWE for HCC development after complete clearance of hepatitis C virus(HCV).METHODS A comprehensive literature search of clinical studies was performed to identify the ability of SWE to predict HCC occurrence after HCV clearance.In accordance with the study protocol,a qualitative and quantitative analysis of the evidence was planned.RESULTS At baseline and after 12 wk of follow-up,a trend was shown towards greater liver stiffness(LS)in those who go on to develop HCC compared to those who do not[baseline LS standardized mean difference(SMD):1.15,95%confidence interval(95%CI):020-2.50;LS SMD after 12 wk:0.83,95%CI:0.33-1.98].The absence of a statistically significant difference between the mean LS in those who developed HCC or not may be related to the inability to correct for confounding factors and the absence of raw source data.There was a statist-ically significant LS SMD at 24 wk of follow-up between patients who developed HCC vs not(0.64;95%CI:0.04-1.24).CONCLUSION SWE could be a promising tool for prediction of HCC occurrence in patients treated with DAAs.Further studies with larger cohorts and standardized timing of elastographic evaluation are needed to confirm these data.

Algebraic processing technique for extracting frequency-dependent shear-wave splitting parameters in an anisotropic medium

Based on the dual source cumulative rotation technique in the time-domain proposed by Zeng and MacBeth（1993）,a new algebraic processing technique for extracting shear-wave splitting parameters from multi-component VSP data in frequency-dependent medium has been developed.By using this dual source cumulative rotation technique in the frequency-domain（DCTF）,anisotropic parameters,including polarization direction of the shear-waves and timedelay between the fast and slow shear-waves,can be estimated for each frequency component in the frequency domain.It avoids the possible error which comes from using a narrow-band filter in the current commonly used method.By using synthetic seismograms,the feasibility and validity of the technique was tested and a comparison with the currently used method was also given.The results demonstrate that the shear-wave splitting parameters frequency dependence can be extracted directly from four-component seismic data using the DCTF.In the presence of larger scale fractures,substantial frequency dependence would be found in the seismic frequency range,which implies that dispersion would occur at seismic frequencies.Our study shows that shear-wave anisotropy decreases as frequency increases.

Shear-wave splitting in the crust:Regional compressive stress from polarizations of fast shear-waves

When propagating through anisotropic rocks in the crust, shear-waves split into faster and slower components with almost orthogonal polarizations. For nearly vertical propagation the polarization of fast shear- wave （PFS） is parallel to both the strike of the cracks and the direction of maximum horizontal stress, therefore it is possible to use PFS to study stress in the crust. This study discusses several examples in which PFS is applied to deduce the compressive stress in North China, Longmenshan fault zone of east edge of Tibetan plateau and Yunnan zone of southeast edge of Tibetan plateau, also discusses temporal variations of PFS orientations of 1999 Xiuyan earthquake sequences of northeastern China. The results are consistent to those of other independent traditional stress measurements. There is a bridge between crustal PFS and the crustal principal compressive stress although there are many unclear disturbance sources. This study suggests the PFS results could be used to deduce regional and in situ principal compressive stress in the crust only if there are enough seismic stations and enough data. At least, PFS is a useful choice in the zone where there are a large number of dense seismic stations.

Shear-wave splitting beforeand after the 1999 Xiuyan earthquake in Liaoning, China

Using seismic waveform data recorded at station YK (Yingkou) of Liaoning Telemetry Digital Seismic Network, this paper studied the characteristics of shear-wave splitting before and after the Xiuyan MS5.9 (ML5.3) earthquake in November 29, 1999 with SAM method. The results show that the predominant polarizations of fast shear-waves at YK is in direction of ENE-WSW, consistent with the direction of regional principal compressive stress and also consistent with the direction of the regional tectonic stress field in North China; time-delays increasing before Xiuyan earthquake may shows accumulation of stress before earthquake. The predominant polarizations of fast shear-waves at YK are also related to the spatial distribution of small earthquakes and correlate with the fault strike. The histogram of monthly average polarizations of fast shear-waves shows that polarizations of fast shear-waves also seems to change from two months before the earthquake, but it still needs more data for verification.

Variational characteristics of shear-wave splitting on the 2001 Shidian earthquakes in Yunnan, China

In 2001 three earthquakes occurred in Shidian in Yunnan Province, which were the MS=5.2 on April 10, the MS=5.9 on April 12 and the MS=5.3 on June 8. Based on the data from the station Baoshan of Yunnan Telemetry Digital Seismograph Network, the variational characteristics of shear-wave splitting on these series of strong earthquakes has been studied by using the systematic analysis method (SAM) of shear-wave splitting. The result shows the time delays of shear-wave splitting basically increase with earthquake activity intensifying. However the time delays abruptly decrease immediately before strong aftershocks. It accords with the stress relaxation before earthquakes, which was found recently in study on shear-wave splitting. The result suggests it is significant for reducing the harm degree of earthquakes to develop the stress-forecasting on earthquake in strong active tectonic zones and economic developed regions or big cities under the danger of strong earthquakes.

Variational characteristics of shear-wave splitting on the 2001 Shidian earthquakes in Yunnan, China

In 2001 three earthquakes occurred in Shidian in Yunnan Province, which were the MS=5.2 on April 10, the MS=5.9 on April 12 and the MS=5.3 on June 8. Based on the data from the station Baoshan of Yunnan Telemetry Digital Seismograph Network, the variational characteristics of shear-wave splitting on these series of strong earthquakes has been studied by using the systematic analysis method (SAM) of shear-wave splitting. The result shows the time delays of shear-wave splitting basically increase with earthquake activity intensifying. However the time delays abruptly decrease immediately before strong aftershocks. It accords with the stress relaxation before earthquakes, which was found recently in study on shear-wave splitting. The result suggests it is significant for reducing the harm degree of earthquakes to develop the stress-forecasting on earthquake in strong active tectonic zones and economic developed regions or big cities under the danger of strong earthquakes.

Radiologic-pathologic correlation of three-dimensional shear-wave elastographic findings in assessing the liver ablation volume after radiofrequency ablation

AIM: To evaluate the usefulness of three-dimensional (3D) shear-wave elastography (SWE) in assessing the liver ablation volume after radiofrequency (RF) ablation.

Seismic anisotropy of the crust in Yunnan,China:Polarizations of fast shear-waves

Using seismic data recorded by Yunnan Telemetry Seismic Network from January 1, 2000 to December 31, 2003, the dominant polarization directions of fast shear-waves are obtained at 10 digital seismic stations by SAM technique, a systematic analysis method on shear-wave splitting, in this study. The results show that dominant directions of polarizations of fast shear-waves at most stations are mainly at nearly N-S or NNW direction in Yunnan. The dominant polarization directions of fast shear-waves at stations located on the active faults are consistent with the strike of active faults, directions of regional principal compressive strains measured from GPS data, and basically consistent with regional principal compressive stress. Only a few of stations.show complicated polarization pattern of fast shear-waves, or are not consistent with the strike of active faults and the directions of principal GPS compressive strains, which are always located at junction of several faults. The result reflects complicated fault distribution and stress field. The dominant polarization direction of fast shear-wave indicates the direction of the in-situ maximum principal compressive stress is controlled by multiple tectonic aspects such as the regional stress field and faults.

Assessment of Fibrosis during the Development of Fatty Liver in Rabbits using Real-time Shear-wave Elastography

Nonalcoholic and alcoholic rabbit models of fatty liver were established by feeding on high-fat diet and alcohol, respectively, and fatty liver stiffness at different pathological stages was as- sessed with real-time shear-wave elastography （SWE）, so as to investigate the fibrosis process during the development of fatty liver. The fatty liver stiffness of rabbit in nonalcoholic and alcoholic groups was higher than that in the control group, and that in alcohol group was higher than that in the nonalco- holic group （P〈0.01）. The elasticity modulus of liver in fatty liver rabbits of nonalcoholic and alcoholic groups showed a positive correlation with progression of liver fibrosis （P〈0.01）. Real-time SWE, as a noninvasive diagnostic method, can objectively reflect the liver stiffness change and progression of liver fibrosis during the development of fatty liver.

Preliminary analysis of the shear-wave splitting observations from the Qiaojia seismic array

A total of 351 shear-wave splitting results at 25 stations were obtained from the seismic data recorded in period of January,2013 to December,2016,by a broadband seismic array deployed in the northern segment of Xiaojiang Fault Zone(n-XJFZ).Meanwhile,the stress field of the n-XJFZ was determined by inverting 140 focal mechanism solutions of the small earthquakes within the study area which were recorded in the same period.This determination confirmed a compressive stress in NW-SE orientation and an extensional stress in the NE-SW orientation,with little difference from those released by previous studies.The shear-wave splitting results show a spatial complexity in polarization orientation,different from one site to another.How-ever,the polarization orientations integrated for the subareas suggest that the fault trends seemingly played important roles.All the subareas bear two dominant orientations,N10°E and N90°E,both of which are different from the azimuths of the principal compressive stress,due to the fault distribution.The time delay averaged over the entire region is 4.56 ms/km,close to that of the upper boundary of the generally accepted interval worldwide but larger than those in most of the investigated regions in the Chinese mainland,which probably implies an alignment of more micro-cracks in the n-XJFZ.Interestingly,the 2014 Ms6.5 Ludian earthquake was found to have caused a variation in the time delays of the slow shear waves within the study area though its epicenter was outside.This earthquake resulted in an evident drop of the time delays remaining for 4 months,however,lifted a bit the time-delay level with respect to that prior to the earthquake.

Systematic Analysis Method of Shear-Wave Splitting:SAM Software System

In order to make a more effective use of the data from regional digital seismograph networks and to promote the study on shear wave splitting and its application to earthquake stress-forecasting, SAM software system, i.e., the software on systematic analysis method of shear wave splitting has been developed. This paper introduces the design aims, system structure, function and characteristics about the SAM software system and shows some graphical interfaces of data input and result output. Lastly, it discusses preliminarily the study of shear wave splitting and its application to earthquake forecasting.

The Character of Shear-wave Splitting in Marble in the Critical State of Rupture

This paper mainly observed and analyzed the character of shear-wave splitting in rock specimens while they were in the critical state of rupture. The rock specimens for study are made of Laizhou marble from Shandong, China. A series of records were obtained from two rock specimens when they were in the critical state of rupture. The result shows that, in the critical state just before rock rupture, there may be the phenomenon of rise and fall in the time delay of shear-wave splitting, even though the load was kept constant. That is to say, the time delay of shear-wave splitting may have a falling process before rock rupture.

A seismological evidence for the northwestward movement of Africa with respect to Iberia from shear-wave splitting

Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting. Waveform data generated by 95 teleseismic events recorded at 17 broadband stations deployed in the western Mediterranean region are used in the present study. Although the station coverage is not uniform in the Iberian Peninsula and north- west Africa, significant variations in the fast polarization directions and delay times are observed at stations located at different tectonic domains. Fast polarization directions are oriented predominantly NW-SE at most stations which are close to the plate boundary and in central Iberia; being consistent with the absolute plate motion in the region. In the northern part of the Iberian Peninsula, fast velocity direc- tions are oriented nearly E--W; coincident with previous results. Few stations located slightly north of the plate boundary and to the southeast of lberia show E--W to NE-SW fast velocity directions, which may be related to the Alpine Orogeny and the extension direction in lberia. Delay times vary significantly between 0.2 and 1.9 s for individual measurements, reflecting a highly anisotropic structure beneath the recording stations. The relative motion between Africa and lberia represents the main reason for the observed NW-SE orientations of the fast velocity directions. However, different causes of anisotropy have also to be considered to explain the wide range of the splitting pattern observed in the western Mediterranean region. Many geophysical observations such as the low Pn velocity, lower lithospheric Q values, higher heat flow and the presence of high conductive features support the mantle flow in the western Mediterranean, which may contribute and even modify the splitting pattern beneath the studied region.

Middle and upper crust shear-wave velocity structure of the Chinese mainland

In order to give a more reliable shallow crust model for the Chinese mainland, the present study collected many short-period surface wave data which are better sensitive to shallow earth structures. Different from traditional two-step surface wave tomography, we developed a new linearized surface wave dispersion inversion method to directly get a 3D S-wave velocity model in the second step instead of inverting for 1D S-velocity profile cell by cell. We convert all the regionalized dispersions into linear constraints for a 3D S-velocity model. Checkerboard tests show that this method can give reasonable results. The distribution of the middle- and upper-crust shear-wave velocity of the Chinese mainland in our model is strongly heterogeneous and related to different geotectonic terrains. Low-velocity anomalies delineated very well most of the major sedimentary basins of China. And the variation of velocities at different depths gives an indication of basement depth of the basins. The western Tethyan tectonic domain （on the west of the 95°E longitude） is characterized by low velocity, while the eastern Tethyan domain does not show obvious low velocity. Since petroleum resources often distribute in sedimentary basins where low-velocity anomaly appears, the low velocity anomalies in the western Tethyan domain may indicate a better petroleum prospect than in its eastern counterpart. Besides, low velocity anomaly in the western Tethyan domain and around the Xing＇an orogenic belt may be partly caused by high crustal temperature. The weak low-velocity belt along -105°E longitude corresponds to the N-S strong seismic belt of central China.

Shear-wave velocity structure of the crust and uppermost mantle in the Shanxi rift zone

The Shanxi rift zone is one of the largest and active Cenozoic grabens in the world, studying the velocity structure of the crust and upper mantle in this region may help us to understand the mechanisms of rift processes and the seismogenic environment of active seismicity in continental rifts. In this work, using the broadband seismic data of Shanxi, Hebei, Henan, Shaanxi provinces, and the Inner Mongolia Autonomous Region from February 2009 to November 2011, we have picked out 350 high-quality phase velocity dispersion curves of fundamental mode Rayleigh waves at periods from 8 to 75 s, and Rayleigh wave phase velocity maps have been constructed from 8 to 75 s period with horizontal resolution ranging from 40 to 50 km by two-station surface-wave tomography. Then, using a genetic algorithm, a 3D shear-wave speed model of the crust and uppermost mantle have been derived from these maps with a spatial resolution of 0.4° × 0.4°. Four characteristics can be outlined from the results： （1） Except in the Datong volcanic zone, in the depth range of 11-30 km, the location of a transition zone between the highand low-velocity regions is in agreement with the seismicity pattern in the study region, and the earthquakes are mostly concentrated near this transition zone; （2） In the depth range of 31-40 km, shear-wave velocities are higher to the south of the Taiyuan Basin and lower to the north, which is similar to the distribution pattern of Moho depth variations in the Shanxi region; （3） The shear-wave velocity pattern of higher velocities to the south of 38×N and lower velocities to the north is found to be consistent with that from the upper crustal levels to depth of 70 km. At the deeper depths, the spatial scale of the low-velocity anomalies zone in the north is gradually shrinking with depth increasing, the low-velocity anomalies are gradually disappearing beneath the Datong volcanic zone at the depth of 151-200 km. We proposed that the root of the Datong volcano may reach to a depth around 150 km; （4） Along the N-S vertical profile at 112.8°E, the 38°N latitude is the boundary between high and low velocities, arguing the tectonic difference between the Shanxi rift zone and its flanks, in the rift zone the seismic velocity is dominated by low-velocity anomalies while in the flanks it is high.

An improved method of evaluating liquefaction potential with the velocity of shear-waves

According to the results of cyclic triaxial tests, a linear correlation is presented between liquefaction resistance and elastic shear modulus, which shows the relation of Gmax (kPa) with (σd/2)1/2(kPa)1/2. When applied to soils from different sites, the correlation can be normalized in reference to its minimum void ratio (emin). Accordingly, an improved method is established to evaluate the liquefaction potential with shear-wave velocity. The critical shear-wave velocity of liquefaction is in linear relation with 1/4 power of depth and the maximum acceleration during earthquakes, which can be used to explain the phenomenon that the possibility of liquefaction decreases with the increment of the depth. Compared with previous methods this method turns out simple and effective, which is also verified by the results of cyclic triaxial tests.

Shear-wave Splitting in the Crust beneath Shandong and Its Adjacent Area

Based on the seismic data of near-field source from the Shandong Digital Seismic Network,this study obtained the shear-wave splitting parameters from multiple stations. The results show that dominant directions of polarizations of fast shear-waves reflect the spatial distribution characteristics of tectonic stress in this area,CHD and LIS stations show 2 dominant directions,and reveals that the crustal seismic anisotropy in the Shandong area is constrained by the regional stress background,local failure and structure.

The Temporal Evolution of Shear-wave Splitting and Geoelectrical Anisotropy During Yongdeng Earthquake

The temporal evolution of shear-wave splitting and geoelectrical anisotropy of earth media during the preparation and occurrence of the Yongdeng MS5.8 earthquake is studied based on the digital seismic data and continuous magnetotelluric (MT) records. The results show that the direction of polarization of the fast S-wave gradually rotated from north by east to north by west before the Yongdeng earthquake and returned to north by east after the earthquake. Moreover, the time delay between the fast S wave and slow S wave increased to the largest until the earthquake occurred. On the other hand, the directions of geoelectrical principal axes also changed before earthquake, and exhibited identical characteristics in the frequency domain. The direction of the axis (pxy), which was originally perpendicular to the fault near station, varied from N17°to N15°before earthquake, and returned to north by east after the earthquake. Correspondingly, the change of pxy was most obvious in the period range of 160 s to 226 s, but the variation of pyx was less obvious. The focal mechanism solution of the earthquake showed that the direction of P-axis was N15°. Conclusions are: (1) The fast S-wave polarization, one of the geoelectrical principal axes and the P-axis were in agreement in orientation during earthquake preparation, which is the result of the alignment of EDA cracks; (2) The fact that the geoelectrical axes identically varied with frequency demonstrated that EDA is really widespread in the crust; (3) The variation of MT apparent resistivity also showed the existence of anisotropy and has its own features: The static anisotropy of resistivity is controlled by the tectonic conditions of the station, while the dynamic anisotropy is caused and affected by the seismic stress field; The resistivity along the axis parallel to the P-axis showed the most obvious variation, while the difference in variation with frequency shows that the variation of resistivity depends not only on direction but also on buried depth.

A Shear-wave Splitting Study of the Yingjiang M_S6.1 Earthquake Sequence Occurring on May 30,2014

Earthquakes of M_S5. 6 and M_S6.1 occurred in Yingjiang,Yunnan on May 24 and May 30,2014 respectively. In this paper,we use the waveform data recorded by mobile seismic stations( KAC) which were set up in the source area after the Yingjiang MS5. 6earthquake on May 24,2014 to study the shear-wave splitting characteristics of Yingjiang M_S6.1 earthquake sequence with the SAM method. The result shows that predominant polarization of fast shear-waves before the M_S6.1 earthquake is consistent with the direction of regional principal compressive stress,and predominant polarization of fast shear-waves before the M_S6.1 earthquake show better consistency and smaller dispersion compared to after the M_S6.1 earthquake,and there may be a deflection for the fast shearwaves predominant polarization between the M_S6.1 earthquake sequence and foreshock sequence. We found that the time-delays generally exhibited a lower level before the M_S6.1earthquake and a relatively higher value after the M_S6.1 earthquake.