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THE VARIATION OF P-WAVE SPECTRAL CHARACTERISTICS OF THE SMALL EARTHQUAKES BEFORE AND AFTER THE DAOFU EARTHQUAKE

Liao zhihe, Li guifang (Seismological Bureau of Sichuan Province)  
The source parameters of 42 small earthquakes (ML = 2.5-3.6) recorded in the Daofu region from 1974 to 1983 have been estimated with th eseismic-source model of Brune (1970,1971). The results show that the stress drops of the small earthquakes had distinctly increased for more than two years before the Daofu earthquake (Ms = 6.9). These earthquakes of high stress drops mainly located on the northwestern of the main fractured zone of the Daofu earthquake, coinciding with the end (the position of stress concentration) of a blocked segment of the fractured zone determined by earthquake activity and ground deformation measure before the Daofu earthquake occurred. This suggested that high stress drops could be related to the state of tectonic stress.Effects of slip-sampling at regular intenals on P-wave spectral parameters have been studied. The statistical results show that the corner frequency fo is not affected by sampling positions, but the long-period level (Ωo) has been reduced by about 16% while the sampling position removes 2 second backward and the epicenter distance equals 120 km. The certain numeral of the corner frequency (fo) is 1.31 and the uncertain one of the seismic moment (Mo) 1.43 respectively from the spectral lines The uncertain factor of stress drop can be reduced from 2.4 to 1.6 by using mean method.
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