川西龙门山及邻区地壳上地幔远震P波层析成像

Teleseismic P-wave tomography of the crust and upper mantle in Longmenshan area, west Sichuan

本文利用川西地震台阵记录到的远震P波走时数据和非线性层析成像算法,获得龙门山地区400 km深度范围内的三维P波速度结构.为了适应川西地区复杂的地质结构,本文的层析成像方法采用了快速行进三维走时计算算法和Tarantola非线性反演算法.我们的结果揭示了川滇地块、松潘—甘孜地块和四川盆地三个不同地块构造差异及该区深部动力学特征.本文的研究表明:1)研究区地壳上地幔P波速度结构具有较为明显的分区特征,松潘—甘孜地块和川滇地块岩石圈速度较低,四川盆地岩石圈速度较高,四川盆地的岩石圈厚度从南250 km向北逐渐减薄至100 km.松潘—甘孜地块上地幔存在地幔上涌的特征.2)川滇地块和四川盆地仅是垂直接触关系,而在龙门山地区四川盆地前缘存在减薄的现象,并伴随松潘—甘孜地块上地幔低速物质有侵入四川盆地岩石圈下方的特征,这显示了四川盆地与松潘—甘孜地块和川滇地块的动力学关系的差异.3)以映秀为界,龙门山断裂带被从松潘—甘孜侵人的低速异常分为南北两段:龙门山南段和龙门山北段,汶川大地震及其余震序列均分布在龙门山断裂带的北段.在青藏高原向东挤压和地幔上涌的双重作用下造成松潘—甘孜地块隆升,由于汶川处于龙门山北段的最南端,应力容易在此集中.这些因素可能是汶川Ms8.0地震的基本动力学背景.本文的结果不支持四川盆地的俯冲及层间流动的动力学模型.

The 3D P-wave velocity structure of the crust and upper mantle within the depth range of 400 km was obtained by using teleseismic traveltimes data recorded by West Sichuan Seismic Array distributed in Longmenshan region and non-linear tomography inversion technique. For adapting to the complicated structure, the fast marching traveltime calculation method and Tarantolats inversion method was used. Our results show the tectonic differences of the crustal and upper-mantle structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. Our results show that： 1） the crustal structure of the study area correlates with the surface geological features. The Sichuan basin is imaged as a high-velocity feature, while the Songpan- Garze and Chuandian block as low-velocity feature. The lithosphere thickness of Sichuan basin has lateral variations from 250 km in south part to 100 km in north part. There exist upwelling features under Songpan-Garz6 block. 2） Sichuan basin vertically contacts with Chuandian block, but the thickness of Sichuan basin front in Longmenshan area decreases from east to west with the feature of Songpan-Garze block incursion into Sichuan basin at upper mantle. This feature shows the difference in dynamic mechanism between Sichuan basin with Chuandian block and Sichuan with Songpan-Garze block. 3） The Longmen Shan faults belt is divided into two parts at Yingxiu. the south part and north part. The great Wenchuan earthquake and aftershocks distribute in the north part of Longmen Shan faults. The eastward extrusion of Tibetan plateau and upper mantle upwelling caused the uplifting of Songpan-Garze block. Because Yingxiu is located in the south edge of north part of Longmenshan faults belt, it＇s possible to accumulate high stress. This should be the deep dynamic background of the Wenchuan earthquake. Based on our results, the velocity structure does not support the subduction of the Sichuan basin downward and channel flow.