Masayuki Takemura

Revision of attenuation formula considering the effect of fault size to evaluate strong motion spectra in near field

Abstract We propose a new distance measure, Equivalent Hypocentral Distance (EHD), which is the distance from a virtual point source that provides the same energy to the site as does a finite-size fault. As EHD includes the effects of fault size, fault geometry, inhomogeneous slip distribution and station location, we can use a far-source attenuation formula to estimate near-source spectra with EHD. We investigated the 1979 Imperial Valley and the 1984 Morgan Hill earthquakes and we found that the attenuation of Fourier spectra in the range from far to near source can be interpreted by the attenuation of body waves predicted by the point-source theory with EHD. We also confirmed that EHD can be applied to the estimation of the maximum accelerations. For near-source data, it is worth emphasizing that EHD does not require any new regression coefficients except those used in far-source attenuation formula. Therefore, if we use EHD, we can estimate the near-source spectrum by using regression coefficients obtained from the far-source data.

Inversion analysis of the 2004 off the Kii peninsula earthquakes using seismic intensity data

An inversion analysis has been developed to evaluate short-period radiation zones on an earthquake fault plane using seismic intensity data. It is robust and widely applicable even for historical earthquakes without ground motion, tsunami and geodetic data observed by instruments. It was applied to the 2004 off the Kii peninsula earthquakes that occurred in the Philippine Sea plate along the Nankai Trough. The short-period seismic wave radiation zone of each event during the 2004 off the Kii peninsula earthquakes from the inversion analysis was included in epicenter area of the corresponding aftershocks and slipped fault zone derived from the waveform inversion analysis as well. It may be also found that there were slip areas where little short period energy radiated. It also applied to the 1899/3/7 earthquake whose felt area was similar to that of the 2004 off the Kii peninsula earthquakes. However, it was indicated that the short-period radiation zones of the 1899 earthquake was located near the coast of the Kii peninsula and departed from that of the 2004 earthquake. The 1899 event might be an intermediate-depth earthquake in the Philippine Sea plate or a relative small interplate event at the west margin of the Tonankai earthquake source region.

Seismic Risk Analysis Using Both Earthquake Data and Active Fault Data

A method of seismic risk analysis is described which can consider local characteristics of seismicity by using both earthquake data and active fault data. To compare the results of the analysis with observed data, a seismic intensity is defined by extending the scale of intensity of the Japan Meteorological Agency, and an attenuation equation for the intensity is derived with the earthquake magnitude and the source-to-site distance as parameters. Local characteristics of seismicity are described by the distribution pattern and slip rates of active faults around a specified site. Seismic risks for three sites in Japan are evaluated and the analysis shows that the results are in good agreement with observed data and that the effect of active faults appears clearly in the range of high intensity.