汶川MS8.0级地震成因三“层次”分析——基于深部电性结构

2008年5月12日汶川M_S8级地震的发生不是局部地区孤立的构造事件，研究汶川地震的孕震机制，应该把局部分析和区域分析相结合，关注地壳上地幔直至地幔过渡带的深部结构.基于近年来在东北、华北和汶川地震附近地区进行的深部结构电磁探测结果，结合地震学等其他资料，从太平洋板块的俯冲、印度板块的碰撞和松潘甘孜地块的推挤三个“层次”探讨分析汶川特大地震的成因.太平洋板块向亚洲大陆的俯冲作用，导致中国大陆东部地幔过渡带深度较普遍地存在着停滞的板片，它对汶川地震的影响不可忽视.印度板块与青藏高原的碰撞，使组成高原的各地块发生向北和向东的运动，各地块向东的运动作用于南北地震带中南段，影响到该区域的地震活动.松潘甘孜地块向四川地块的推挤，使松潘甘孜地块运动方向和龙门山断裂带形成“丁”字形结构，龙门山断裂带显示为较陡直的电性边界，加剧了汶川地震前的应力积累，可能是汶川地震发生的最直接的诱因.

Generation mechanism of Wenchuan strong earthquake of MS8.0 inferred from EM measurements in three levers

Wenchuan earthquakes occurred on May 12, 2008 is not an independent tectonic event confined in a local regions. It is reasonable to study the deep structure of the crust, upper mantle and mantle transition zone combining between the local and regional observation data for understanding the preparation and development of Wenchuan earthquakes. The analysis on Wenchuan earthquake generation in this paper is on basis of electromagnetic measurement data obtained recent years with considering the other data e.g., seismic data, which are relative to three level of tectonic movements, Pacific plate subduction and penetration, collision of India plate with Tibet plateau and pressure of Songpan-Garzê block upon Sichuan block. The stagnant slabs formed by Pacific plate subduction exist generally in the mantle transition zones under eastern China continent of North-South seismic belts, which can not be ignored when the generation of Wenchuan earthquakes are investigated. Both northward and eastward movements of different blocks in the Tibet caused by collision of India plate appear. Their eastward movements act on the eastern margin of Tibet i.e., mid- and south- sections of North-South seismic belt and affect the shocks occurred there including Wenchuan earthquakes. The south-east-east direction along which Songpan-Garzê block moves is approximately normal to Longmenshan faults and formed a T-geometry structure by both of them. The Longmenshan faults are represented by a large steep electric boundary in the resistivity profile. This kind of pattern of deep structure is prone to accumulate the stress before Wenchuan earthquakes and is a direct factor for generation of the earthquakes.