云南遮放—宾川和孟连—马龙宽角地震剖面的层析成像研究

TOMOGRAPHY RESEARCH OF THE ZHEFANG—BINCHUAN AND MENGLIAN—MALONG WIDE—ANGLE SEISMIC PROFILES IN YUNNAN PROVINCE

依据有限差分反演和射线反演的方法 ,利用走时、振幅比和重力布格异常数据对云南地区遮放—宾川和孟连—马龙宽角地震剖面的地壳上地幔结构进行层析成像研究 .遮放—宾川测线地壳厚度为 35～ 4 6km ,孟连—马龙测线地壳厚度 33～ 4 4 .5km .局部位置上地幔顶部P波速度值偏低 ,速度值变化范围大 ,反映出云南地区是典型的构造活动区的特点 .地壳上地幔结构存在一定程度的深浅构造一致性 ,意味着浅部物质活动存在深部背景 .遮放—宾川剖面速度结构显示地表怒江断裂东侧存在贯穿地壳的大规模低速异常 ,可能与深部物质上涌活动有关 .遮放—宾川线和孟连—马龙线速度结构显示作为一级构造单元分界线的红河断裂是超壳断裂 ,怒江断裂深及上地幔 .而昌宁—双江断裂显示为低角度铲式断层 ,意味着该断裂切割并不深 .云南地区强震的发生往往与延伸到上地幔的深大断裂有关 ,且一些浅源地震经常位于中上地壳深大断裂与其他断裂的交汇部位、高速块体与低速块体接触带上速度等值线弯曲的位置 .推测这样的位置有利于能量和区域应力的积累 .

Based on the finite difference inversion and ray inversion method, the crust and upper mantle's structure on the Zhefang—Binchuan and Menglian—Malong wide-angle seismic profiles, both of which are located in Yunnan province,are tomographyed using the geophysical data of travel-time, amplitude ratio and Bouger anomaly. Thus some new recognitions about the geodynamics and the seismotectonic environment are derived. The crustal thickness of the Zhefang—Binchuan profile is 35～46km,and that of the Menglian—Malong profile is 33～44.5km. The geometry of the Moho interface is like that given by Hu Hongxiang. The P wave velocity on the top of the mantle of some places is relatively low and the fluctuating range of the velocity is very large, which may indicates that the Yunnan region is a typical tectonic active area. Moreover, there exits a prominent consistency between the shallow and deep structures, which implys that the activity of shallow material always has a deep background. The velocity structure of the Zhefang—Binchuan profile indicates that there exists a large low velocity anomaly across the crust to the east of the Nujiang fault exposed on the ground,and this maybe related with the deep material upwelling. The characteristic of the large faults which are the borders between some first order tectonic units can also be derived by both of the velocity structure profiles: the Honghe fault is a super-fault that cut the lithosphere,the Nujiang fault cuts deeply in the crust and even stretches downward into the top of the upper mantle, yet the Changning—Shuangjiang fault has a listric shape with a small dip angle, which may suggest that its incision depth is not very deep. The large earthquakes in Yunnan province always have a relationship with the large and deep faults that stretch into the upper mantle. Some earthquakes with shallow focal depths are generally located at converge positions of faults within the upper and middle crust or the positions where the velocity contours on the belt between the high and low velocity anomaly blocks are crooked. It is inferred estimated that these locations are prone to the accumulation of the energy and the regional stress.