経験的グリーン関数による高周波地震動の予測

Abstract

The empirical Green's function techniques are examined to synthesize high frequencymotions for large earthquakes. The target earthquake is the 1980 Izu-Hanto-Toho-Oki (Eastoff Izu Peninsula) earthquake with MJMA 6.7. The record of MJMA 4.9 foreshock observedat each station is used as the empirical Green's function. First, the mainshock ground motionis synthesized by Irikura's method (1986), in which the summation of small earthquakerecords is made to match the seismic moment at low frequencies and to obey the w 2-modelat high frequencies. It was already reported that the mainshock and the foreshock havew 2 source spectral structure. The peak accelarations and velocities and the spectral levelsof the synthetic ground motions give a good fit to those of the observed ones, although theindividual phases do not always correspond to each other. Second, Imagawa and Mikumo'smethod (1982) is applied, using small earthquake records as the empirical Green's functionby convolving a correction function for the differences in the slip velocity function betweenthe mainshock and the small earthquake. The slip velocity is assumed to have time dependence t 1/2 with the finite duration (rise time). The resultant synthetic ground motionsare overestimated, compared to the observed ones. This is because the slip velocity functionassumed here may not be fit and/or the uniform slip velocity over the mainshock fault maybe too simplified.