Source Characteristics of Interplate Earthquakes in Northeast Japan Inferred From the Analysis of Broadband Strong-Motion Records

Observed broadband strong-motion records can be simulated assuming a relatively simple distinctive source model, which consists of large slip-velocity area within the total fault area with no explicit heterogeneity inside it, called ``strong motion generation area (SMGA)'', using the empirical Green's function (EGF) method. Scaling relationship of SMGAs is examined for crustal earthquakes (Miyake et al., 2003) and intraslab earthquakes (Asano et al., 2003). In order to investigate the broadband source characteristics of subduction zone, interplate earthquakes, we estimated the size and the location of SMGAs of Mw6.0--7.0 earthquakes which occurred along the Japan (the 2002 Miyagi-oki earthquake) and the Kuril (eight earthquakes which occurred in the off-shore of Hokkaido from 2003 to 2005) trenches. Model parameters, such as the length, the width, the rise time, and the rupture starting subfault of the SMGA are estimated by fitting S-waves simulated using the EGF method by Irikura (1986) to the observed ones. We used the residual value of velocity waveforms and acceleration envelopes between observation and simulation so as to evaluate broadband strong motions. For the 2002 Miyagi-oki earthquake, we estimated the best model by trial and error considering the residual value. For eight Hokkaido events, we employed the genetic algorithm to derive the source model which gave the minimum residual values. The estimated SMGA sizes are smaller than those predicted from the empirical relation for crustal earthquakes with the same seismic moment, which indicates the stress drop of the SMGA of these interplate earthquakes are large. Actually, the stress drop of SMGAs of analyzed earthquakes calculated using the circular crack formula is larger than that of crustal earthquakes. For several earthquakes, slip distribution has been deduced by strong-motion or teleseismic waveform inversion analyses and the SMGA is found to be located in the large slip area. It was not obvious that the stress drop depends on the focal depth, which Asano et al. (2003) found for the shallow intraslab earthquakes whose depth ranges from 30 to 100 km. Four events in the Kushiro-oki region tend to have larger stress drop than the other four events in the Tokachi-oki region. This shows that broadband strong-motion radiation by interplate earthquakes analyzed in this study seems to have regional characteristics in spite of their spatial proximity.