HR: 0800h
AN: S51B-0500    [Abstracts]
TI: Quasi-3D Waveform Inversion for Velocity Structures and Source Process Analyses Using its Results
AU: * Hikima, K
EM: hikima@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Koketsu, K
EM: koketsu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AB: In this study, we propose an efficient waveform inversion method for 2-D velocity structures and 3-D velocity structures are constructed by interpolating the results of the 2-D inversions. We apply these methods to a source process study of the 2003 Miyagi-ken Hokubu earthquake. We will first construct a velocity model, then determine the source processes of this earthquake sequence using the Green's function calculated with the resultant 3-D velocity model. We formulate the inversion procedure in a 2-D cross section. In a 2-D problem, an earthquake is forced to be a line source. Therefore, we introduce approximate transformation from a line source to a point source (Vidale and Helmberger, 1987). We use the 2-D velocity-stress staggered-grid finite difference scheme, so that the source representation is somewhat different from the original 'source box method' and we apply additional corrections to calculated waveforms. The boundary shapes of layers are expressed by connected nodes and we invert observed waveforms for layer thicknesses at the nodes. We perform 2-D velocity inversions along cross sections which involve a medium-size earthquake and observation points. We assemble the results for many stations and interpolated them to construct the 3-D velocity model. Finally, we calculate waveforms from the target earthquake by the 3-D finite difference method with this velocity model to confirm the validity of the model. We next perform waveform inversions for source processes of the 2003 Miyagi-ken Hokubu earthquake sequence using the resultant 3-D velocity model. We divide the fault plane into northern and southern subplanes, so that the southern subplane includes the hypocenter of the mainshock and the largest foreshock. The strike directions of the northern and southern subplanes were N-S and NE-SW, respectively. The Green's functions for these source inversions are calculated using the reciprocal theorem. We determine the slip models using the 3- D structure and compare them with the models determined using the 1-D structures (Hikima and Koketsu, 2004). The synthesized waveforms in the 3-D structure better explain the observed waveforms than those in the 1-D structures. While the large slip area (asperity) of the mainshock is recovered at the shallow southern part of the northern subplane in the 1-D inversion result, the asperity of the 3-D inversion result is located on the deep central part of the northern subplane. Most of the aftershocks occurred around the asperity of the 3-D result. The asperity of the 3-D result is consistent with that of a previous study from geodetic data. In addition, the 3-D inversion result is in good agreement with the distribution of estimated strong motions in the source area. To examine the reason why the different slip distributions were recovered using the 1-D and 3-D structures, we performed synthetic comparisons. The variations of the Green's functions due to changes in the subfault depths were different between the 1-D and 3-D models, and this difference results in the difference of the two inversion results. Because the waveform difference is larger mostly in a later part, an inversion of body wave parts alone did not produce such difference, which we confirmed by a synthetic calculation. However, in some situation, we cannot extract clear body waves not contaminated by later phases. Therefore, a source process inversion should be performed using accurate Green's functions which include later phases based on well-calibrated 3-D velocity models.
DE: 7209 Earthquake dynamics (1242)
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: 2007 Fall Meeting