川滇及邻区Pn波速度结构和各向异性研究

川滇地区是中国大陆构造变形及地震活动最强烈的区域之一.本文利用从国际地震中心(ISC)、中国地震台网以及云南、四川、重庆三个省级地震台网的地震观测报告中严格挑选出的53673个Pn波到时数据,用Pn波层析成像方法反演了川滇地区的Pn波速度结构和各向异性.结果表明该地区上地幔顶部速度结构的横向变化与现代构造运动有明显的关联:盆地地区上地幔顶部速度明显高于其周边区域,四川盆地尤为突出,其上地幔顶部P波速度是整个研究区域的高值;上地幔顶部P波速度低值区与研究区现代火山活动区域(如腾冲)及高热泉活动区域(如康定)有明显的相关性.强震大都发生在Pn波速低值区或Pn波速低值边界区.Pn波速度低速带与狭义的南北地震带相吻合,反映出上地幔顶部的流变性对构造应力的传递起着重要作用.Pn波各向异性表明:位于川青块体与四川盆地交界的龙门山地区,快波方向与该区域绝对运动方向不一致,而是基本沿龙门山断裂走向,这表明该地区的各向异性主要受龙门山断裂对Moho面切割变形的影响.而在其他区域的Pn波各向异性方向大体与当地块体的绝对运动方向一致,说明其各向异性主要是由板块运动产生的软流圈变形引起.

Study on Pn-wave velocity structure and anisotropy in the Sichuan-Yunnan and its adjacent areas

Sichuan-Yunnan is adjacent to the eastern border of Tibet and is one of the most active tectonic blocks in China. We obtained 53673 Pn travel time picks from the earthquake bulletins of International Seismological Center, China national seismic network and Sichuan, Yunnan, Chongqing provincial seismic networks to invert for Pn velocity variation and anisotropy and station delays in Sichuan-Yunnan and its adjacent areas. The main results are as follows: (1) The Pn velocities show a mosaic of very fast and very slow anomalies, mirroring the heterogeneous geology at the surface. The Pn velocities are high beneath the major basins like Sichuan basin and low in areas of active volcanoes like Tengchong or hot spring areas like Kangding. Most of strong earthquakes occurred in or around the low Pn velocity areas. The famous China South-North seismically active belt is overlap with a low Pn velocity belt, which implies that the visco-plasticity of the top-mantle material might play an important role in transferring the tectonic stress to the upper crust. (2) The Pn anisotropy is consistent with the regional lithosphere movement direction in the interest region except Longmenshan area. The fast Pn direction in Longmenshan area is parallel to the strike of Longmenshan fault, which implies the whole crust of Longmenshan area may be highly deformed and the Longmenshan fault has extended to Moho.