Source Characteristics of Interplate Earthquakes Estimated from Strong Motion Modeling
- Analysis of Earthquakes in the Off-shore of Tokachi and Kushiro Regions -
Wataru Suzuki
Observed broadband strong motion records can be simulated assuming a relatively simple source model, which consists of extended regions with large slip velocity within the total fault area, strong motion generation area (SMGA), using the empirical Green's function method. In order to examine the microscopic characteristics of interplate earthquakes, we estimated the SMGA of seven earthquakes in the off-shore Tokachi and Kushiro regions. We estimated the size, rise time of the SMGA, and the rupture propagation direction by fitting the simulated S-wave waveforms to the observation. For this nonlinear search, we used the genetic algorithm whose performance function is the sum of the residual of the acceleration envelopes (0.2 - 10 Hz) and the velocity waveforms (0.2 - 1 Hz) in order to evaluate broadband strong motion.
The sizes of the derived SMGA are smaller than that predicted from the empirical relation for crustal earthquakes with the same seismic moment. This shows that the earthquakes we analyzed have larger stress drop on the SMGA than crustal earthquakes. Dependency of the stress drop on the SMGA on the focal depth, which was pointed out in the analysis of intraslab earthquakes, is not obvious probably because the depth of these earthquakes is not so much different from each other. Earthquakes in the off-shore Kushiro region tend to have the larger stress drop on the SMGA than those in the off-shore Tokachi region, which may indicate regional characteristics. The SMGA of several earthquakes is located in the large slip area deduced from waveform inversion of strong motion or teleseismic data. The size is relatively smaller than the stretch of large slip area. Therefore, it is possible that the strong motion at higher frequency range (> about 0.2 Hz) is mainly radiated from the smaller area where stress drop is concentrated. The rise time is smaller than that derived for crustal earthquakes. It is necessary to analyze interplate earthquakes in other regions in order to confirm whether these characteristics are common among interplate earthquakes.