Study Of Shear-wave Splitting In The Crust By Liaoning Regional Seismograph Network

Anisotropy in crust is mainly the result of stress-aligned fluid-saturated grain-boundary cracks, when traveling through this anisotropic media, shear-waves split into fast shear-waves and slow shear-waves. The predominant polarization direction of fast shear-waves is consistent with the direction of cracks, and also consistent with the direction of principal compressive stress; time delays correlate to the level of anisotropy. The fecture of anisotropy in crust is affected by geologic structure, lithofacies, distrubiting of faults and stress environment, and correlate to movement of crust. Also, anisotropy in crust is strongly regionalized. With shear-wave parameters, can study the feature of crust, the condition and change of crustal stress field, and can study earthquake forecast issues with the change of shear-wave parameters before and after the large earthquake. It is also a new attempt to study the natures of faults with the feature of anisotropy in crust.Liaoning locates in north part of north China, and it shows NE trending tectonical characters mainly nowadays. There are Xialiaohe Basin in the middle of Liaoning, and both sides of it are uplift. In Neozoic, the tectonic movement is some active, and it is mainly the new activity of the primary faults. It is very common to find NE striking faults in the whole province, largely and actively. Antitheticly, the NW striking faults are smaller and commonly wrench faults to cut the NE striking faults(Ma X Y, 1989).On November 29,1999, an earthquake M_L5.3(Ms5.9) happened in Xiuyan, Liaoning, and the largest aftershock M_L5.4(Ms5.2) happened on January 12,2000. Xiuyan earthquake is a cluster of foreshock-main shock-aftershock, the cluster lasted from November 1999 to October 2000(Sun & Jiao, 2001). This study analyzed the seismic data recorded by Liaoning Telemetry Digital Seismic Network from June 1999 to September 2006 with SAM technique.The analyzed results at 8 stations of Liaoning Seismic Netwoke (Liaoning Seismic Netwoke has 16 stations in all) show that predominant polarization direction of fast shear-waves at most stations strike to NEE(nearly E-W), consistent with the direction of regional principal compressive stress, and also consistent with the direction of the regional tectonic stress field in north part of North China. However, the predominant polarization direction of fast shear-waves at SJ station in the middle of Liaoning and KD station in southeast of Liaoning strike to nearly N-S and NW, different from other stations, they may be controlled and influenced by the complex local geological structure. Also, there is not enough available recorded data at the two stations(SJ is 4, and KD is 1), we need more data to analyze.We also find that the average polarizations of fast shear-waves at YK station have a change of 8°～12°before and after the main shock. 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 needs more data to prove. Predominant direction of the polarizations of fast shear-waves at YK station is also consistent with the spatial distribution of small earthquakes, correlates to the fault strike.Time delays increase before Xiuyan earthquake at YK station shows accumulation of stress before earthquake. But we have not observe the sudden decrease before impending earthquake due to lack of data one month before the earthquake. From the average time delays in different stages at YK station, we find that the average time delays is the largest before main shock, the least between main shock and aftershock, and after the end of cluster, it gets back to the level of before main shock.According to the study results and other data, such as GPS, geological and geophysical data, we studied the regional distribution of faults and feature of principal compressive stress field, and analysed the feature of special and temporal change of shear-wave parameters in Liaoning, it has some significance to study regional tectonicstress field and stress-forecasting et al.