青藏高原东南缘地壳速度结构及云南漾濞M_(S)6.4地震深部孕震环境

Crustal velocity structure of the southeastern Tibetan plateau and its geological implications for the Yunnan Yangbi M_(S)6.4 earthquake

青藏高原东南缘岩石圈变形强烈、地震活动频繁,对其深部结构进行研究有助于提高对其演化及强震发震机理等问题的认识.本研究利用青藏东南缘固定和流动观测台站记录的地震P波走时数据,采用体波走时层析成像方法得到研究区地壳、上地幔顶部三维P波速度结构.结果显示,峨眉山大火成岩省内带范围呈明显的高速异常,推测为二叠纪时期地幔柱活动残留在地壳内的基性和超基性幔源物质.川西北次级块体和大火成岩省内带东西两侧存在低速带,可能是壳内部分熔融存在和中下地壳流动的证据:高原物质向南运移时受大火成岩省壳内高速体阻挡后分为两支,东支沿安宁河断裂—则木河断裂—小江断裂延伸,西支沿红河断裂向上地壳运移并逐渐穿过红河断裂.云南漾濞M_(S)6.4地震序列全部位于西支低速通道之上,推测构造块体SE向运动和地壳流作用使得应力在上地壳震源区进一步集中,共同驱动断裂活动导致漾濞地震发生.

The southeast margin of the Tibetan plateau is characterized by complex geological structure,strong lithospheric deformation and seismic activity.In an effort to improve our understanding of the seismic features of the crust and how it associated with the tectonic evolution and seismic activity,we present a new three-dimensional seismic velocity model through local seismic tomographic inversion using seismic data recorded by dense temporary and permanent stations in the southeastern Tibetan plateau.Our model images obvious high velocity anomalies beneath Emeishan Large Igneous Province(ELIP),which is related to the mafic and ultra-mafic composition associated with Permian mantle plume activity.Obvious low-velocity zones are observed in the middle-lower crust within the Chuanxibei sub-block and the eastern and western sides of the inner zone of ELIP,which may be indicative of partial melting and the crustal flow.The high-speed body of ELIP may have acted as a barrier to the southward flow of weak Tibetan crust and divided it into two branches.The east branch extends along the Anninghe-Zemuhe-Xiaojiang fault.The west branch moves upward to upper crust and gradually passes through the Red river fault.The Yunnan Yangbi M_(S)6.4 earthquake sequence is all located above the low-speed channel of the west branch.It is speculated that the SE movement of the tectonic block and the crustal flow further concentrated the stress in the upper crust of focal area,which jointly drove the fault activity and caused the Yangbi earthquake.