Simulation of Long-Period Ground Motions for the 1923 Kanto Earthquake

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The 1923 Kanto earthquake was recorded in Tokyo with a Ewing seismograph and an Imamura seismograph. The ground motions estimated by earlier investigators from these two seismograms differed significantly. This conflict can be reconciled if we assume that the solid friction of the Ewing seismograph was very high during shaking. Solid friction can reduce the resonance of the instrument, and long-period ground motions with large amplitudes can be recorded correctly. We conclude that the ground motion of the Kanto earthquake had a vary large long-period component with a velocity response spectrum of 120 cm/sec at a period of 13 sec. This spectral peak position are considerably longer than that for micro-tremors usually observed in Tokyo. By way of examples to investigate the difference in the spectral peak positions inferred from small earthquake data and from large earthquake model, we take the 1923 Kanto earthquake and a hypothetical earthquake beneath the Los Angeles basin.Several numerical simulations produce large long-period ground-motions whose periods are considerably longer than that for micro-tremors usually observed in Tokyo and Los Angeles. Size of subfault on the fault, however, does not affect the peak position of response spectra in long-period range, even if the characteristic rupture duration of the subfault is longer than the peak period of response spectra. Our result shows that the difference in the spectral peak positions between small earthquake data and a large earthquake model is caused by the excitation of long-period surface waves due to a source of several to ten kilometers deep and by the directivity of rupture propagation.