ON THE STUDY OF THE CRUST AND UPPER MANTLE STRUCTURE BENEATH THE SEISMIC TELEMETRY NETWORK IN SOUTH SICHUAN AND NORTH YUNNAN PROVINCES
ZHU PEI-DING LI YOU-MING ZHANG LI-MING SHU PEI-YI LIANG SHANG-HONG(Institute of Geophysics, Academia Sinica)
Sichuan and Yunnan Provinces in southwest China are in the "South-North" earthquake belt, a transition region from the Qing-Zang Plateau in the western part to the central and south China flatlands in the eastern part. This is a very active region, with a lot of disorganized faults and widespread seismicity. It is of great scientific significance to probing the deep structure of this region, improving the accuracy of earthquake location in Sichuan and Yunnan Provinces, determining the source mechanism of regional earthquakes, investigating the intraplate earthquake background, and expounding some problems in the study of Geodynamics.We have reported a method, for the study of crust-mantle structure beneath the seismic recording stations, and provided feasible test and examples of different application [1, 2, 3]. Main points of our work may be summed up as follows: (1) We have successfully developed the modified delta matrix of separational form, and the algorithm of effective combination of basic propagation terms of the wave field. It results in a simpler and economical computation process for the synthetic seismograms of teleseisinic P wave. (2) It makes full use of all dynamic informations recorded on seismograms. The optimum layered crust-mantle model chosen by this method depends on the degree of similarity between the synthetic and observed P waveforms. (3) Data of this method can be obtained very easily from existing seismic records, and therefore the expense involved is quite small.The observed data used in this paper are deep and teleseismic P waveforms recorded by twenty-one short period seismographs of the region in south Sichuan and north Yunnan Provinces. With observed P waveforms in comparison, we have obtained the synthetic P waveforms quite well by means of gradually adjusting the crust-mantle model. Our results reveal that the crust-mantle structure beneath this region is of rather complicated character, with multilayers. It consists of some thin layers with high or low velocity. Most of the crust thickness beneath these recording stations reach or exceed 54 km. In addition, we have studied the attenuation feature of the medium of this region using synthetic seismograms. The layered QP values are within the normal range of Eurasian continent. It indicates that the attenuation for seismic wave through this region is not very strong.