TELESEISMIC SYNTHETICS OBTAINED FROM 3-D CALCULATIONS IN 2-D MEDIA

At subduction zones, the laterally heterogeneous medium such as the steeply dipping sea bottom and thick sedimentary layers causes large effects on the teleseismic body waveforms from the shallow events occurred in this region. We develop a new method to incorporate these effects from near-source structure in calculating the teleseismic body waveforms. To incorporate the structural effects we simulate 3-D elastic-wave propagation in a two-dimensionally heterogeneous structure. Although the simulated wavefield is 3-D our method does not require huge computer storage. This is because we assume a 2-D structure to reduce the dimension of the equations of motion by spatial Fourier transform. Because of the strong laterally heterogeneous features such as steeply dipping sea bottom and thick sedimentary layers, and because of the 2-D geometry of the oceanic trench, our method is particularly applicable to the shallow subduction-zone earthquakes. In our formulation, we assume a 2-D model structure in which the physical properties are constants in the direction parallel to the trench axis. Then we apply the spatial Fourier transform to the equations of motion in that direction. The resultant wavenumber-domain equations can be solved as independent sets of 2-D equations on a computer with a moderate-sized storage. We solve these wavenumber-domain equations by the staggered-grid finite-difference method. Finally the inverse Fourier transform provides the 3-D displacement field. The reciprocal principle is used to obtain the teleseismic displacement. First, the response of the near-source structure to an input plane P wave is calculated by the 3-D method introduced here. Then, based on the reciprocal principle, convolution of the response strain at the source position with the moment tenser is calculated to get far-field P-wave displacement. As an example of the application of the present method, we calculated the waveforms from a shallow thrust event that occurred at the Kurile trench (1964 October 16, Ms = 6.5). The synthetics calculated by the new method reproduce the large-amplitude later arrivals in the observed seismograms. Furthermore, a reasonable source position is obtained from the waveform analysis.