3-D Finite-Difference Simulation of Fault Zone Trapped Waves -Application to the Fault Zone Structure of the Mozumi-Sukenobu Fault, Central Japan-

Fault zone trapped waves are thought to be a useful tool to reveal fine structure of the fault zone down to the seismogenic depth. 3D numerical simulation is necessary to determine the velocity and Q structures and geometry of the low velocity zone (LVZ) with relatively complex fault structure. In the present study, a program was coded for performing the 3D simulation of trapped waves. Then we computed the synthetics for fitting the real seismograms recorded by a linear seismometer array at the depth of 300m across the Mozumi-Sukenobu fault with explosion sources. We solved the equation of motion and stress-strain relation numerically in velocity-stress scheme by using the staggered-grid finite-difference method with a second-order approximation for the time derivative and fourth-order approximation for the spatial one. We used the free surface boundary condition for the earth_fs surface and absorbing boundary condition for the other model boundaries. The observations (Ito et al. 2001) were performed in the following two cases of the source location: (i) in the center of the LVZ and (ii) 100m outside from the boundary of the LVZ. The distances of sources and linear array were 2km and 4km for the both cases. The following features are noted from the observational results of the source distance 2km in the case (i): (a) Distinctive phases with a large amplitude appear about 0.2s after the first arrival especially for the fault zone-parallel component recorded in the LVZ. (b) Following the previous phase, wave trains with relative large amplitude can be clearly seen only for the seismograms in the LVZ. On the other hand, phase (a) and wave trains (b) are not clearly seen in the case (ii) where the source is outside the LVZ. To compare the observational results with the synthetics, we adopted a 200m-wide uniform LVZ with 40% velocity reduction comparing the P-waves of 4.5km/s and the S-waves of 2.7 km/s for the surrounding rock. For the case (i) in the fault zone-parallel component of synthetic seismograms in the LVZ, we could identify 3 major phases such as the fault zone head waves at the first arrival, direct P-wave arrival with large amplitude following the head waves, and then wave trains of the trapped waves with relative large amplitude. For the case (ii) the amplitudes of the direct P-wave and trapped waves become small compared with that for the case (i). These characteristics are sufficiently consistent with the observational results (a) and (b). This program is, thus, expected to be able to determine details of 3D complex fault zone structure.