FOCAL MECHANISMS, DISPLACEMENT RATE AND MODE OF MOTION OF THE SICHUAN-YUNNAN BLOCK
CHENG Wan-zheng 1) DIAO Gui-ling 2) LAU¨G Yi-pei 1) ZHANG Yong-Jiu 1) LI Gui-fang 1) CHEN Tian-chang 1) 1) Seismological Bureau of Sichuan Province, Chengdu 610041, China 2) Seismological Bureau of Hebei Province, Shijiazhuang 050021, China
In this paper, Sichuan-Yunnan and its adjacent areas are divided into Yajiang, Central Yunnan, Sichuan-Qinghai, Central Sichuan, and Myitkyina-West Yunnan blocks. The mode and rates of motion of these blocks and their boundaries are studied separately. The predominant direction of the principal compressive stress axes within the blocks is analyzed in term of focal mechanism solutions of 442 moderate strong earthquakes. The models of focal faulting are determined through the rake angles λ obtained from source parameters of 771 events with magnitudes about 3 or more, and they are evidenced by focal faulting or slip modes derived from the distributions of N axis plunges of P-wave first motion solutions for moderate-strong earthquakes. A comparison is made between the moment tensor rates for moderate-strong earthquake and the calculated annual average slip rates of each seismotectonic zones within the blocks. Based on regular resurveying of across fault short level lines and baselines during the period of 1980-2001 along the border of the Sichuan-Qinghai, Yajiang and Central Yunnan blocks, the annual average rates of horizontal and vertical deformation for each site are analyzed. The motion rates of the Sichuan-Yunnan Block are inhomogeneous. The Sichuan-Qinghai Block moves toward SEE direction, and the predominant direction of the principal compressive stress axis falls in the range of N100°～130°E. The motion rate of this block is smaller than that of the Yajiang block, implying a delayed motion relative to that of the later. As a result, the Xianshuihe Fault between these two blocks displays a strong left lateral displacement. In addition to earthquakes of strike-slip type, the earthquakes of reverse dip-slip type are also predominant in the Sichuan-Qinghai Block. The predominant direction of the principal compressive stress axes in the Yajiang Block is within the range of N160°～170°E, i.e. SSE. The motion rate of this block is larger than that of the Sichuan-Qinghai Block, and the percentage of strike-slip type earthquakes in this block is larger then that in the other blocks. The motion direction of the Central Yunnan Block is nearly the same as that of the Yajiang Block, leaning slightly to the east. The predominant direction of the principal compressive stress axes in this block is in the range of N150°～160°E, and there are various types of earthquake failure including strike-slip, normal dip-slip, and reverse dip-slip. The direction of the principal compressive stress axes in the Central Sichuan Block falls in the range of N110°～140°E, approximately the same as the motion direction of the South China Block. The percentage of strike-slip type events in this block is relatively low, and there is a portion of events belonging to normal dip-slip or oblique dip-slip types. The motion rates at the northeast boundary between the Yajiang and Central Yunnan Blocks are greater than those at the west and south boundaries. The SSE-directed motion of the blocks is associated with right lateral component. Similarly, as the motion rates of the eastern boundary of the Sichuan-Qinghai Block is greater than that of the Longmenshan belt to the southeast of the block, the SSE-directed motion of the Sichuan-Qinghai Block is also associated with right lateral component. Two predominant directions of the principal compressive stress axes in the Myitkyina-West Yunnan Block indicate the NE-directed squeezing and the SE-directed escaping of the block, while the motion rate of the block is larger than that of the Yajiang and Central Yunnan Blocks. This might be attributed to the dynamic mechanism of the formation of collision and squeezing belts due to the direct action of the Asam Wedge while Indian Plate moving northward.
【Fund】： 国家重点基础研究发展规划项目 (G95 130 5 0 5 )资助。
【CateGory Index】： P315.3
【CateGory Index】： P315.3