Preliminary Study Of Anisotropic Receiver Function Of The Crust Beneath Western Sichuan

On Monday 12 May 2008, an devastating earthquake of magnitude 8.0 struck northwestern Sichuan province of China, which shook the whole world up and was totally unexpected by anybody. Researches on the Wenchuan earthquake has become a hotspot both domestically and overseas and a series of studies came out in a short time. Zhang et al.(2008) and Xu et al.(2008) discussed the geological background of the Wenchuan earthquake. Chen et al.(2008) and Wang et al.(2008) gave the faulting process of the earthquake. Zhu et al.(2008) relocated the earthquake sequence. Liu et al.(2009) presented the high resolution deep-seated tectonic environment of this event. However, the genesis of the Wenchuan earthquake is still an issue worth intensive study.At present, the seismic anisotropy study of the earth has already become a major technique in understanding the movement and deformation of the earth medium(Savage & Silver, 1993). McNamara & Owens(1993) studied the crustal anisotropy utilizing the Ps converted phase on the Moho of the receiver functions. Due to the importance of the crustal anisotropy in understanding the movement and deformation of the crust, the study of anisotropic receiver function has already attracted the world' s attention.From the teleseismic P-waveform data recorded at the dens mega seismic array deployed in the western Sichuan area by the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, we investigate the crustal anisotropy beneath the stations in terms of the waveform correlation method and the weighted stacking method , respectively. As a preliminary result, we measured the fast polarization azimuth and time delay of the Ps converted phases in the receiver functions at eight stations located in the Chuandian block, the Songpan-Ganzi block and the Sichuan basin.Our results manifest:1) The fast polarization azimuths are consistent with each other at the stations in the Sichuan basin, suggesting the crust beneath the Sichuan basin has well integrality and its lateral deformation is much weaker than that of the Songpan-Ganzi block and Chuandian block; The fast direction of the Ps converted phases at the Mono discontinuity beneath the Sichuan basin is nearly perpendicular to the Longmen Shan faults, suggesting that the crustal material of the Tibetan plateau is moving eastward and the Sichuan basin is in a compressive status;2) Except the area nearby the Longmen Shan faults, the fast direction of Ps converted phases at the Mono discontinuity beneath the Songpan-Ganzi block is nearly along the west-east direction and do not have obviously lateral variations, suggesting the azimuthal anisotropy of the crust is mainly controlled by the rheological property and status of the soft materials inside the crust;3) The fast direction of Ps converted phases at the Moho discontinuity beneath the Chuandian block is very different from that in the Songpan-Ganzi block and has obviously lateral variations, which are mainly controlled by the boundary faults and the faults inside the block;4) In most of the cases, the measurements of the anisotropic parameters by using the waveform correlation method and the weighted stacking method are consistent with each other; However, the waveform correlation method is better than the weighted stacking method; The results obtained by using the weighted stacking method are somehow undetermined due to that the symmetry axis of the crustal anisotropy medium is unclear previously; Joint application of both the two methods will be in favor of judging the reliability of the results.