汶川M_S 8.0地震:龙门山断裂两侧地壳各向异性的初步研究

WENCHUAN EARTHQUAKE M_S 8.0:PRELIMINARY STUDY OF CRUSTAL ANISOTROPY ON BOTH SIDES OF THE LONGMENSHAN FAULTS

利用中国地震局地质研究所地震动力学国家重点实验室在川西地区布设的大规模密集流动宽频带地震台阵记录的远震P波波形数据,分别采用波形互相关和加权叠加方法研究了地壳各向异性。作为初步结果,得到了龙门山断裂两侧4个宽频带流动台站的接收函数莫霍面Ps转换震相快波方向和分裂时间延迟。结果表明:1)波形互相关方法总体上优于方位加权叠加方法,它不但可以给出快波方位,而且可以给出快慢波的时间延迟;方位加权叠加方法测量结果存在不确定性,其原因在于难以确定介质各向异性对称轴的方位;同时采用上述2种不同方法研究地壳各向异性参数有助于判断测量结果的可靠性;2)四川盆地内快波偏振方位基本一致,表明四川盆地地壳整体性较好,横向非均匀变形较弱;3)以汶川地震主震的震中区为界,松潘-甘孜地块北侧快波偏振方位与龙门山断裂近于平行,表明在四川盆地坚硬地壳的阻挡作用下,龙门山断裂附近松潘-甘孜地块北侧可能存在中下地壳软弱物质沿断层向NE方向的扩张变形,而其南侧处于正向挤压的状态。该结果有助于解释汶川MS8.0地震的单侧破裂过程及其余震发育的特征。

From the teleseismic P-waveform data recorded at the dense 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 using waveform correlation method and weighted stacking method. As a preliminary result,we measured the fast polarization azimuth and time delay of the Ps converted wave in the receiver functions at 4 stations on both sides of Longmen Shan Faults. Our results show： 1 ） The waveform correlation method is better than the weighted stacking method and it turns out not only the fast polarization azimuth ,but also the time differences between the fast-and slow-wave; The results obtained by using the weighted stacking method are something undetermined due to that the symmetric axis of the crustal anisotropy medium is unclear previously ; Application of both methods will be in favor of judging the reliability of the results. 2） The fast polarization azimuths are consistent each other at the stations in Sichuan Basin, suggesting the crust beneath Sichuan Basin has well integrality and a weak lateral deformation. 3）Taking the epicenter of the Wenchuan earthquake as a boundary, the fast polarization azimuth is parallel with the Longmen Shan Faults on the north side of the Sonpan-Ganzi block,and perpendicular to the faults on the south side. This suggests that under the obstruction of the Sichuan Basin,the soft lower crust beneath the north side of the Sonpan-Ganzi block has a NE direction extended deformation along the Longmen Shan Faults,and the crust on its south side is in the status of compressive deformation perpendicular to the faults. Our results can be used for interpreting the single-side rupture of the Wenchuan earthquake and the aftershock evolution.