Attenuation Property of Seismic Waves and Source Characteristics of Small Earthquakes

Abstract

In order to discuss the source characteristics of the local small earthquakes occurring in and around the Kinki district, the attenuation property of body wave is estimated, and the spectral densities are examined through the analysis of the coda parts of the seismograms observed at the Sumiyama Seismic Station in the southern parts of Kyoto. As for the attenuation of the body waves in the frequency range of 1～20 Hz in the crust, Q increases with frequency quite similarly to that of the coda parts. Q_s is approximated by Q_s=q√<f>, in which q=110 for the S travel time, t_s=5～50sec. It was found that the source spectral density of the far-field shear wave can be explained fairly well by the ω-square model in the range of the magnitude, M=2～5. When M is smaller than about 3, the corner frequency of the spectrum takes nearly constant value regardless of the seismic moment. When M=3～5, the frequency becomes smaller with increase of the moment. Consequently, according to Brune's model of the dislocation, the stress drop increases from a few bars to several tens of bars with increase of the moment in the low level of the magnitude, M<3. While the stress drop is ranging from 10 to 100 bars in relation to the source location and depth in the range of M=3～5. In the Kinki district, the seismically active regions, where small and micro earthquakes occur frequently or in a swarm, are characterized by the low stress drop compared with the other non-active regions.