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Reliability of Envelope Inversion for the High-Frequency Radiation Source Process Using Strong Motion Data: Example of the 1995 Hyogoken Nanbu Earthquake

Instituto de Ingeniería, UNAM
Ciudad Universitaria, Apdo. 70-472
Coyoacán 04510, México, D. F., Mexico
joagg{at}pumas.iingen.unam.mx
(J.A.)

Disaster Prevention Research Institute
Kyoto University
Uji 611, Japan
irikura{at}egmdpri01.dpri.kyoto-u.ac.jp
(K.I.)

This article presents an application of a procedure to invert the high-frequency radiation process at the source during the 1995 Hyogo-ken Nanbu earthquake using the envelopes of acceleration waveforms from 16 stations. The inversion uses genetic algorithms that compare observed ground motions with synthetic ones calculated using empirical Green's functions. Before the inversion, the reliability of the solutions for models with different grid sizes is checked. It is found that the resolution of the high-frequency radiation is strongly dependent on the number of reliable data and, for this case, it is shown that a coarse grid model with 60 parameters provides reliable results. With use of this model, the inversion of the high-frequency radiation distribution for the 1995 Hyogo-ken Nanbu earthquake was performed, and results showed four distinct zones of high-frequency radiation. The first zone is located in subfault 1 near a step-over from subfault 1 to the Nojima fault and the starting point of the rupture. The second zone is located in subfault 2 near the lower limit of the slip inverted from low-frequency data by many authors. The third and fourth zones are located in subfault 3, one of them coinciding with the bifurcation of the fault plane into two fault planes (Sekiguchi et al., 2000).

It is concluded that the procedure applied in this article successfully inverts the high-frequency radiation distribution for the 1995 Hyogo-ken Nanbu earthquake. Resolution is restricted to the coarse grid model with 60 parameters because of the limited number of stations, according to the resolution test. The results of the resolution test done in this work can not be generalized; however, they show that it is very important to check the resolution before doing this kind of inversion.