Seismic Zoning and Seismic Ground Motion in the Southern Parts of Kyoto, Southwest Japan

Abstract

The vibrational characteristics and the process of vibration during earthquakes were examined in regard to the source properties, seismic wave attenuations and amplification effects of soil grounds. In and around the Kinki district, Southwest Japan, the regional variations of stress drop and attenuation, depending on the geological structure and the regional seismic activity, have been obtained from coda analysis for small earthquakes of M<5. The effects of source properties were investigated for larger events with magnitude of up to 6.8 by the time domain analysis and the response spectral analysis. It was found that the high frequency vibrational characteristics reflect the regionality of stress drop derived from small events. As a result, large acceleration even by moderate earthquake may be predicted in the northernmost region of the Kinki district, where high stress drop and small attenuation have been suggested. The long period seismic effects due to the soil deposits in the Kyoto basin have been studied by observation of microseism caused mainly by sea waves. The process of vibration during the 1984 West Nagano Prefecture Earthquake was examined in relation to the ground characteristic frequency inferred from microseism. The amplification by soil deposits was explained with the amplification and elongation of duration for each wave mode and the resultant interference of various modes in the characteristic frequency. It was suggested that the amplification factor for microseism corresponds to the maximum value of that for seismic surface waves. The instrumentation and the method of seismometry at the Sumiyama Seismic Observatory of Kyoto University will be described briefly. In the Appendix, the regional variations of attenuation for seismic coda, Q_c^-1, observed in and around the Kinki district are mentioned.