Variation of shearwave splitting in earthquake clusters with very similar waveforms

Among the records of aftershock observation of the 1991 Datong, China ML=5.8 earthquake, very similar waveforms from clusters of small earthquakes were found. Cross-correlation of the waveforms of each pair in the cluster confirmed the similarities. Re-sample technology is used to improve the sampling rate, which is helpful to distinguish the small variation of shear wave splitting. The variation of shear wave splitting could be found directly from seismograms of each pair in a cluster.

Stepwise joint inversion of surface wave dispersion,Rayleigh wave ZH ratio,and receiver function data for 1D crustal shear wave velocity structure

Accurate determination of seismic velocity of the crust is important for understanding regional tectonics and crustal evolution of the Earth. We propose a stepwise joint linearized inversion method using surface wave dispersion, Rayleigh wave ZH ratio （i.e., ellipticity）, and receiver function data to better resolve 1D crustal shear wave velocity （Vs） structure. Surface wave dispersion and Rayleigh wave ZH ratio data are more sensitive to absolute variations of shear wave speed at depths, but their sensi- tivity kernels to shear wave speeds are different and complimentary. However, receiver function data are more sensitive to sharp velocity contrast （e.g., due to the existence of crustal interfaces） and Vp/Vs ratios. The stepwise inversion method takes advantages of the complementary sensitivities of each dataset to better constrain the Vs model in the crust. We firstly invert surface wave dispersion and ZH ratio data to obtain a 1D smooth absolute vs model and then incorporate receiver function data in the joint inver- sion to obtain a finer Vs model with better constraints on interface structures. Through synthetic tests, Monte Carlo error analyses, and application to real data, we demonstrate that the proposed joint inversion method can resolve robust crustal Vs structures and with little initial model dependency.

Preliminary seismic anisotropy in the upper crust of the south segment of Xiaojiang faults and its tectonic implications

The Xiaojiang faults,striking north-tosouth(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array,the Honghe-Xiaojiang temporary Seismic Array(HX Array),across first-order block boundaries in the southern SYB.By using the waveform data of small earthquakes recorded by stations in the HX Array across Xiaojiang faults from 2017 to 2019,and by permanent seismic stations of the China National Earthquake Networks from 2012 to 2019,this paper adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain preliminary results for seismic anisotropy in the upper crust.The study area can be divided into two subzones according to the spatial distribution of the directions of polarization of the fast shear-wave(PFS)at the stations:the northern zone(zone A,where the HX Array is located)and the southern zone(zone B,to the south of the HX Array).The results show that the directions of the PFS at stations in zone A were highly consistent,dominant in the NE direction,correlated with the in-situ principal compressive stress,and were seemingly unaffected by the Xiaojiang faults.The directions of the PFS as recorded at stations in zone B were more complicated,and were dominant in the NS direction parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise in the southeast margin of the TP,the NS direction of the PFS in the middle and lower crust,and its EW direction in the upper mantle,this paper concludes that azimuthal anisotropy in the upper crust differed from that in the lower crust in the south segment of Xiaojiang faults,at least beneath the observation area,and azimuthal anisotropy in the crust was different from that in the upper mantle.The results support the pattern of deformation of ductile flow in the lower crust,and the decoupling between the upper and lower crusts as well as that between the crust and the mantle in the study area.The crustal directions of the PFS appeared to be independent of the Xiaojiang faults,suggesting that the influence of the South China block on the SYB passed through the Xiaojiang faults to the Yimen region.The results of this study indicate that anisotropic studies based on data on the dense temporary seismic array can yield clearer tectonic information,and reveal the complex spatial distribution of stress and deformation in the upper crust of the south segment of Xiaojiang faults.

Site response of heterogeneous natural deposits to harmonic excitation applied to more than 100 case histories

Variation of shear-wave propagation velocity （SWV） with depth was studied by analyzing more than one hundred actual SWV profiles. Linear, power, and hyperbolic variation schemes were investigated to find the most representative form for naturally occurred alluvial deposits. It was found that hyperbolic （asymptotic） variation dominates the majority of cases and it can be reliably implemented in analytical or analytical-numerical procedures. Site response analyses for a one-layer heterogeneous stratum were conducted to find an equivalent homogeneous alternative which simplifies the analysis procedure but does not compromise the accuracy of the resonance and amplification responses. Harmonic average, arithmetic average and mid-value equivalents are chosen from the literature for investigation. Furthermore, full and partial depth averaging schemes were evaluated and compared in order to verify the validity of current practices which rely upon averaging shallow depths, viz., the first 30 m of the strata. Engineering bedrock concept was discussed and the results were compared.

Estimation of residual shear strength ratios of liquefied soil deposits from shear wave velocity

For sites susceptible to liquefaction induced lateral spreading during a probable earthquake, geotechnical engineers often need to know the undrained residual shear strength of the liquefied soil deposit to estimate lateral spreading displacements, and the forces acting on the piles from the liquefied soils in order to perform post liquefaction stability analyses. The most commonly used methods to estimate the undrained residual shear strength （Su~） of liquefied sand deposits are based on the correlations determined from liquefaction induced flow failures with SPT and CPT data. In this study, 44 lateral spread case histories are analyzed and a new relationship based on only lateral spread case histories is recommended, which estimates the residual shear strength ratio of the liquefiable soil layer from normalized shear wave velocity. The new proposed method is also utilized to estimate the residual lateral displacement of an example bridge problem in an area susceptible to lateral spreading in order to provide insight into how the proposed relationship can be used in geotechnical engineering practice.

Application of passive source surface-wave method in site engineering seismic survey

Site engineering seismic survey provides basic data for seismic effect analysis. As an important parameter of soil, shear-wave velocity is usually obtained through wave velocity testing in borehole. In this paper, the passive source surface-wave method is introduced into the site engineering seismic survey and practically applied in an engineering site of Shijingshan District. By recording the ubiquitous weak vibration on the earth surface, extract the dispersion curve from the surface-wave components using the SPAC method and obtain the shear-wave velocity structure from inversion. Over the depth of 42 m under- ground, it totally consists of five layers with interface depth of 3.31, 4.50, 7.23, 17.41, and 42.00 m; and shear-wave velocity of 144.0, 198.3, 339.4, 744.2, and 903.7 m/s, respectively. The inversion result is used to evaluate site classification, determine the maximum shear modulus of soil, provide basis for further seismic hazard analysis and site assessment or site zoning, etc. The result shows that the passive source surface-wave method is feasible in the site engineering seismic survey and can replace boreholes,shorten survey period, and reduce engineering cost to some extent.

Application research of seismic method in assessment of active fault

eismic method is one of the main tools of assessing fault activity. There are Pwave reflection and shearwave splitting methods when active fault is investigated using seismic methods. The Pwave reflection method is according to the fact that whether bedrock fault extends up to Quaternary strata Q3 or not, while shearwave splitting method is according to anisotropy of Quaternary strata Q3 determined by the shearwave splitting in assessing activity of bedrock fault. The Pwave reflection method can be used under the conditions of large scale bedrock fault up to Quaternary strata, whereas the shear wave splitting method can be used.

A Strong Stability-Preserving Predictor-Corrector Method for the Simulation of Elastic Wave Propagation in Anisotropic Media

In this paper,we propose a strong stability-preserving predictor-corrector(SSPC)method based on an implicit Runge-Kutta method to solve the acoustic-and elastic-wave equations.We first transform the wave equations into a system of ordinary differential equations(ODEs)and apply the local extrapolation method to discretize the spatial high-order derivatives,resulting in a system of semi-discrete ODEs.Then we use the SSPC method based on an implicit Runge-Kutta method to solve the semi-discrete ODEs and introduce a weighting parameter into the SSPC method.On top of such a structure,we develop a robust numerical algorithm to effectively suppress the numerical dispersion,which is usually caused by the discretization of wave equations when coarse grids are used or geological models have large velocity contrasts between adjacent layers.Meanwhile,we investigate the performance of the SSPC method including numerical errors and convergence rate,numerical dispersion,and stability criteria with different choices of the weighting parameter to solve 1-D and 2-D acoustic-and elastic-wave equations.When the SSPC is applied to seismic simulations,the computational efficiency is also investigated by comparing the SSPC,the fourth-order Lax-Wendroff correction(LWC)method,and the staggered-grid(SG)finite differencemethod.Comparisons of synthetic waveforms computed by the SSPC and analytic solutions for acoustic and elastic models are given to illustrate the accuracy and the validity of the SSPCmethod.Furthermore,several numerical experiments are conducted for the geological models including a 2-D homogeneous transversely isotropic(TI)medium,a two-layer elastic model,and the 2-D SEG/EAGE salt model.The results show that the SSPC can be used as a practical tool for large-scale seismic simulation because of its effectiveness in suppressing numerical dispersion even in the situations such as coarse grids,strong interfaces,or high frequencies.