Abstract

Variable-slip rupture models for large earthquakes, obtained by the source inversion of long-period (>4 sec) seismic waves, are taken into account in a semi-empirical method for simulating broadband (< about 10 sec) strong ground motions. The high-frequency (>0.25 Hz) source spectrum of the (p, q)th subfault is inferred by the θ−2 mode with two circular corner frequencies. The first is ωDpq = Vpq/Dpq, due to the temporal integration of the slip-velocity time function, where Vpq is the maximum slip velocity and Dpq is the final slip. The other is ωSpq = 2βpqpq, due to the spacial integration of the slip-velocity time function on the subfault, where βpq is the S-wave velocity of the medium and λpq is the equivalent radius of the subfault. Here, Vpq, Dpq, βpq, and λpq are specified by the long-period source-inversion results.

First, we describe this new method by applying it to the variable-slip rupture model for the 1985 Michoacan, Mexico, earthquake of MS 8.1 obtained by Mendoza and Hartzell (1989). The simulated accelerations and velocities at CAL (Caleta de Campos) and VIL (La Villita), both located above the ruptured zone, are in good agreement with the observed ones. Next, the method is applied to the variable-slip rupture model for the 1923 Kanto, Japan, earthquake of MS 8.2 obtained by Wald and Somerville (1995). This earthquake is one of the most important earthquakes for the mitigation of earthquake disaster in the Tokyo metropolitan area; unfortunately, strong-motion records for this earthquake were off-scaled in the region of strong shaking and significant damage. The pseudo-velocity response spectrum of the simulated acceleration of TOK (Tokyo JMA) averages 60 cm/sec in the period range of 0.5 to 10 sec and is consistent with that of the Kanto earthquake record observed at HNG (Hongo, Tokyo), whose off-scaled parts were restored as well as possible by Yokota et al. (1989). The instrumental JMA seismic intensities (JMA, 1996) of the simulated accelerations at TOK and YOK (Yokohama JMA) are consistent with the observed JMA seismic intensity 6 (JMA, 1983). The instrumental JMA seismic intensity of the simulated accelerations at KNS (soil site in Odawara) is also consistent with the JMA seismic intensity 7, estimated from the ratio of collapsed houses (Mononobe, 1925). The simulated broadband (0.1 to 10 sec) motions will be useful in the mitigation of earthquake disaster in the Tokyo metropolitan area.

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