丽江地震序列的震源机制、发震应力场和破裂特征

THE FOCAL MECHANISM, SEISMOGENIC STRESS FIELD AND RUPTURE CHARACTERISTICS OF THE LIJIANG EARTHQUAKE SEQUENCE

丽江７０级地震震区位于我国西南地区现代构造应力场空间分布的复杂地区，区域应力场主压应力优势方位为南南东。震区位于可能受到多种构造动力源作用的特定构造运动环境中。获得了主震和２２个ＭＬ≥４０级余震的震源机制Ｐ波初动解，分析表明，主震发震应力场为北３°东，与震区区域应力场主压应力优势方位有一个小角度的偏差。主震的发震应力不仅有水平应力的作用，同时还有显著的垂直应力的作用。在余震序列发展中震区呈现出应力场分布的复杂图象。地质资料表明，震区处在北西、北东和北北西———北北东向三条较大规模构造断裂所围限的三角形断块内。推测主震发生在北西向边界断裂与断块内近南北向断裂的交汇部位，并且自交汇点分别向此两断裂两侧破裂，形成震源的两次破裂过程。主震的破裂错动以正倾滑动为主，兼具左旋剪切错动分量。余震主要发生在三角形断块内。

The seismic area of the M 7.0 Lijiang earthquaeke is located in the present tectonic stress field region with complex space distribution in southwestern China and lies in specific tectonic environment acted probably by multiple tectonic dynamic sources with the SSE dominant direction of principal pressure stress in regional stress field. The focal mechanism solutions for the main shock and 22 aftershocks with M≥4.0 have been obtained from the data of P wave first motion. The analysis of these results shows that the seismogenic stress field of the main shock is N 3° E, which deviates from the dominant direction of principal pressure stress of the regional stress field in the seismic area in a small angle; the seismogenic stress of the main shock has not only horizontal stress component, but also obvious vertical stress one, and the complicated image of stress field distrbution was displayed in development of aftershock sequence in the seismic area. The geological data incicate that the seismic area is located in a triangular fault bolck surrounded and confined by three large tectonic faults in NW, NE and NNW-NNE. It is inferred that the main shock occurred the junction position of the boundary fault in NW and the fault in near SN in the block and ruptured along the two sides of the two faults from the convergence point, which formed two rupture processes of the mainshock source. The rupture dislocation of the mainshock takes normal dip-slip as dominant, with left-lateral shearing dislocation component. The aftershocks mostly occurred in the triangular fault block and displayed the rupture feature in shattering type of the activities of many small faults.