A Seismic Regional Tomography Study And Application On Upper-crust Structure Beneath The BaChu-JiaShi In China

Earthquake is bright lamp, which illuminate the interior of the Earth.Geophysicist can 'see' the internal structure of the earth by means of seismictomographic methods. There have been significant advances in the theoryand application of seismic tomographic methods in the last decades, as aresult of efforts of geophysicist and with the development of other sciences,e.g. math, and observation technique. At the present time, the seismictomography methods developed by Dapen Zhao (hereinafter is called zhao'sseismic tomography) is an effective, economical, accurate and practicalmethods among methods of numerous seismic tomography. T_s-T_p methodof seismic tomography was proposed, in purpose to get a high-resolutiontomographic imagings, which takes advantage of not only a normal seismicdata set but also seismic data set with clock error, and the T_s-T_p methodwas realized by modifying the zhao's seismic tomography, after studying thezhao's seismic tomography in earnest. Moreover, a process was designed toverify the correctness of T_s-T_p.On Feb. 24, 2003, a strong earthquake (Ms6.8) occurred at the border ofBachu and Jiashi in Xinjiang autonomous region, western China. There arelarge numbers of aftershock. The location of Ms6.8 is near to a strong swarmin 1997. We collected the phase of Ms6.8 earthquake and aftershocks inBachu-Jiashi in 2003, and sift out initial 1-D velocity model, after readingrelated papers and trying a mass of models. At last we select a reliable size ofresolution and network grid from large number of test result. T_s-T_p seismictomography was applied to Bachu-Jiashi area and upper-crust velocity imagein this area was obtained. Results as following: A) P-Velocity at 2km depth isstrongly horizontal heterogeneity. This phenomenon maybe are related to theuppermost illuvial horizon about 4km thickness. B) P-velocity between 5kmand 14km depth suggests that there exist two high and conjugated velocityzone, one trending NE and the other NW. The NW high velocity zone issimilar in the shape to the distribution of aftershocks. C) According to the relation of P velocity imaging and the faults of studied area, it is consideredthat the hidden Meigaiti-Xiasuhong fault pass throng the crystallinebasement and is a fault for Jiashi strong earthquake in 2003. D) TheS-velocity slice along 39.5°N suggest that there exit slow-velocity layerbetween 5km and 10km depth. E) The high Vp/Vs region at 8km and 14kmmaybe caused by a softer medium than medium in same layer or invadingfluid.