Research Of Focal Parameter Of Medium And Small Earthquakes In Yunnan And Sichuan

The record effect of small and moderate earthquake has a great improvement since the area that covered by digital seismic network in our country increased. On this condition, first of all, 47 waveform records occurred during the time from 2000 to 2004 were collected from Chengdu and Kunming telemetered seismic network. The magnitudes of them are between 4.0 and 4.5. Then the waveform data were transformed to displacement spectra in frequency field. After correction of instrument and noise on displacement spectra, the coefficient of non-elastic attenuation would be determined by using Atkinson method. Based on that, the source spectral parameter of three typical earthquakes, including plateau level Q 0 and corner frequency f 0, occurred in Ludian county , Yunnan province in 2003 and 2004 with magnitude of Ms≥5.0, was calculated. Concrete method was using Atkinson method on displacement spectra of three earthquakes to get rid of the effect of path and site, and searching optimal Q 0 and f 0 by using genetic algorithm to minimize cumulate error between observed source spectra and theoretic source spectra. Then the optimal Q 0and f 0 were the source spectra parameter that we needed. We could not only calculate the source parameters that earthquake moment, stress drop, rupture scale and so on, but analyze the process of rupture of three Ludian earthquakes. Suppose the extending direction of the unilateral rupture is an only one, and the source-time function of a rupture fault is a step function. Based on the Seismic Doppler Effect, the rupture direction and velocity could be estimate by using least square to fit the corner frequency changing along with the station direction. After calculation we obtained the main rupture directions of three earthquakes which were NNW direction and propagated from SE to NW.Additionally, in this paper, 925 seismic waveform records were selected from M≥2.5 earthquakes(1994~2005), which distribute in the four sub-blocks, they were Sichan-Qinghai, Yajiang, Central Sichuan and Central Yunnan sub-blocks. The focal mechanism solutions of small earthquakes were obtained by inversion of waveform data by using the algorithm of generalized transmission coefficient approach to fit the maximum amplitude ratio of direct waves. According to these solutions, we analyzed the characteristics of stress field of four sub-blocks and calculated mean stress moment tensor of each sub-block. The difference between the force axis direction of focal mechanism solution and the direction of the mean stress tensor was define as consistency parameterθ, and then the variation ofθversus time was analyzed, the variation information of dynamic stress field was given as well.