Validation of the deep velocity structure model of the north-west region of the Kanto basin using long-period S-waves from moderate earthquakes.

Abstract

There are several 3-D velocity structure models for the urban Kanto basin in Japan. However, the velocity models are different because they have been constructed using separate data sets such as geophysical, geological, and earthquake data. It is, therefore, important for the reliable prediction of long-period ground motions to validate the existing models based on the observed earthquake records. Yamanaka and Yamada (2006) reconstructed the 3-D velocity structure model of the Kanto basin based on the Rayleigh wave phase velocities at periods from 0.5 to 5 s deduced from the microtremor array observations at more than 300 sites in the area. One of the advantages of using the deep sedimentary models from microtremor survey method for the simulation of long-period ground motions is that the models, by this method, are based on the principle of propagation of long-period surface waves in the sedimentary plains, where the surface waves are dominant for longer durations during large earthquakes. In this paper, we compare the observed S-waveforms with the synthetic ones using a 1-D simulation of long-period S-waves (2 to 10s) to validate the Yamanaka and Yamada model. We select the north-west region of the Kanto-basin and use strong-motion data from several nearby intermediate depth moderate earthquakes (depth 60-100km, Mw5.2-5.9). Based on the S-waveform comparisons, we find that, despite some notable discrepancies at some sites, the model reproduces satisfactorily the observed S-waveforms at most sites thus validating the model for long-period (2-10s) ground motion simulations.