青藏高原东南缘三维S波速度和径向各向异性及泸定M_(S)6.8和芦山M_(S)7.0地震孕震环境探讨

S-wave velocity and radial anisotropy structure in Southeastern Tibetan Plateau and the deep seismogenic environment of Luding M_S6.8 earthquake and Lushan M_S7.0 earthquake

利用青藏高原东南缘布设的684个流动地震台和150个固定地震台的连续背景噪声波形数据,提取了周期5~50 s的Rayleigh波和Love波相速度频散,并反演获得了该区域下方0~70 km三维S波速度和径向各向异性结构.结果表明:(1)受川滇地块南部高速体的阻挡,东南缘中下地壳内存在两条空间分布独立的低速带.西侧(L1)从川滇地块北部向南延伸至滇西南地块,其中下地壳平均V_(S)小于3.4 km·s^(-1)并表现为正径向各向异性结构,反映了高原中下地壳可能存在部分熔融和韧性变形;东侧低速带(L2)沿着小江断层南北分布,受到红河断层的阻挡,弱物质不太可能进入滇西南地块,该区域下地壳和上地幔顶部均显示较强的正各向异性,推测其更可能是沿小江断层相互驱动的块体在横向挤压过程中引起的地壳部分熔融而导致.(2)芦山M_(S)7.0地震位于龙门山逆冲断层南段,泸定M_(S)6.8地震位于鲜水河走滑断层东南段,两者都发生在地壳浅层高低速异常过渡带上,但其深部孕震环境有所不同.芦山震区西北部中下地壳低速体为负各向异性,推测韧性物质沿着龙门山下方陡倾断层向上运移,上地壳积累应力并通过薄弱区域释放导致了芦山地震的发生;泸定震区为正各向异性的中下地壳韧性变形促进了川滇地块SE向的水平运动,受刚性的四川盆地阻挡,加剧了上地壳发震断层的滑动变形和应力积累,脆性上地壳突然破裂导致了泸定地震的发生.

Based on the continuous waveform data of 684 temporary seismic stations and 150 permanent seismic stations on the southeastern Tibetan Plateau,we obtain 5~50 s period Rayleigh wave and Love wave phase velocity dispersions by using the ambient noise tomography method,then the dispersions are inverted to determine the 3-D S-wave velocity and radial anisotropy structure of depth 0~70 km in the study area.The results demonstrate:(1)Blocked by the high-velocity zone in the south Chuan-Dian block,there are two independent low-velocity belts with spatial distribution in the mid-lower crust of the southeastern Tibetan Plateau.The west belt(L1)extends from the north Chuan-Dian block to the southwest Dian block.The average S-wave velocity in the mid-lower crust is less than 3.4 km·s~(-1)and shows the positive radial anisotropic,which may reveal the partial melting and ductile deformation;The east belt(L2)is distributed along the Xiaojiang fault zone from south to north.Blocked by the Red River fault,weak materials are unlikely to enter the southwest Dian block.The lower crust and uppermost mantle in this area show strong positive radial anisotropy,which is more likely to be caused by partial melting of the crust during the process of lateral compression between blocks and mutual drive along the fault zone.(2)Lushan M_S7.0 earthquake is located in the southern segment of the Longmenshan thrust fault,and Luding M_S6.8 earthquake is located in the southeastern segment of the Xianshuihe strike-slip fault.Both occurred in the transition zone of high and low velocity anomalies in the shallow crust,but their deep seismogenic environments are different.The low velocity zone of the mid-lower crust in the northwest of the Lushan earthquake area is negative radial anisotropy,it is speculated that the ductile material is transferred upward along the steep fault below Longmenshan,the accumulation of stress in the upper crust and release through the weak area caused the Lushan earthquake;The Luding earthquake area shows that the positive radial anisotropy of the ductile deformation in the mid-lower crust has promoted the SE horizontal movement of the Chuan-Dian block.Blocked by the rigid Sichuan basin,the sliding deformation and stress accumulation of the upper crustal seismogenic fault have further intensified,and the sudden rupture of the brittle upper crust caused the Luding earthquake.