Seismic Attenuation in Relation to the Tectonic Force in the Vicinity of Kyoto

Abstract

This paper discusses seismic wave attenuation in the seismically active region in the vicinity of Kyoto based on the regional and azimuthal distributions of the ratios of short- to long-period amplitude recorded by two types of seismographic instruments with different frequency responses. One of the most interesting results shows that seismic wave attenuation differs with propagation directions. Attenuation is small for seismic waves propagating in the direction of EW which coincides with the direction of the global tectonic force acting in the region concerned. A possible cause to diminish attenuation for waves propagating in the EW direction might be a high strain accumulation caused by the tectonic compressional force. The results also show that attenuation of seismic waves in the northern part of the region is low if compared with that in the southern part. Low attenuation in the northern part can be interpreted as caused by the in situ stress added to the global one. The regional distribution of the ratios of P- to S-maximum amplitude, the data concerning earth quake magnitudes and the regional difference of the aftershock activity seem to support the above interpretation.