综合震源机制解法反演湖南地区构造应力场的初步结果

基于435个地震的1580条P波数据，通过综合震源机制解方法，根据地震与网格点之间的距离不一样来确定P波符号的权重，反演得到了湖南地区0.2°×0.2°的二维构造应力场.相较以往利用震源机制求解应力场的方法，该方法不仅能直接采用P波初动资料反演应力场而略去计算震源机制的中间步骤，而且能够尽可能多地使用本地区小震P波数据对应力场方向进行限定.初步研究结果表明：湖南地区综合震源机制解总体分布特点为逆断型和不确定型；P轴倾角近水平，方位具有一定的分区性，总体上呈NWW-SEE向；T轴倾角偏高，方位在局部地区变化较大，统一性相对较差；湖南地区整体上受NWW向的挤压应力，对该区的活动断裂具有一定的控制，这或许也是该区地震多发生于郴州、娄邵、湘西州所构成的NW向空间条带的主要原因之一；湖南地区主要受菲律宾海板块NWW向的推挤作用，其次才受太平洋板块向西俯冲的影响，西面虽受印度洋与欧亚板块的水平碰撞，但由于碰撞主要引起青藏高原向东扩张，同时又受到云贵高原的阻隔，其影响至本区已大为减弱.

Preliminary results of inversion of tectonic stress field in Hunan region by the composite focal mechanism method

Applying the composite focal mechanism method, we use 1580 P-wave polarity data of 435 earthquakes to invert the tectonic stress field in Hunan region. The study area is divided into two-dimensional grids of 0.2°×0.2°, and different weights of P-wave first motion polarity are given according to the distances between earthquakes and grid points to obtain the stress field at each grid point. Comparing with the traditional method of determining stress field by focal mechanisms, this method can not only directly invert the stress field using P-wave polarity data without the intermediate step of calculating focal mechanisms, but also make full use of a large number of P-wave data of small earthquakes in the region to constrain the direction of stress field. Preliminary results of the study show that, the main types of composite focal mechanisms are thrust faults and uncertainty types in Hunan region. The dip angle of the P axis is nearly horizontal, and the P axis azimuth is NWW-SEE overall, with divisions in different regions. In comparison, the dip angle of the T axis is relatively high, and the T axis azimuth varies greatly in some areas with a relatively poor uniformity. It shows extrusion stress in NWW direction overall in Hunan region, which controls the active faults in this region to some extent and may be one of the main reasons for earthquakes in this area mostly occurring in the NW-trending spatial strip composed of Chenzhou, Loudi-Shaoyang, and Xiangxi Autonomous Prefectures. The Hunan area is primarily pushed by the Philippine Sea plate in the NWW direction, and secondarily impacted by the westward subduction of the Pacific plate. Although the west is subjected to horizontal collisions between the Indian Ocean and the Eurasian plates, the influence is very weak in the Hunan region because of the collisions mainly cause the eastward expansion of the Qinghai-Tibet Plateau and the obstruction of the Yunnan-Guizhou Plateau.