In the study of the shallow earthquakes in the subduction zones, reverberations inside the water layer near the source are known to affect very significantly the far field body waveforms. The usual correction is to take a horizontally layered model as the near source structure, but it is not realistic for the trench geometry. Therefore finite-difference simulation combined with far-field ray-theory is employed in order to examine the effects of the realistic non-flat seafloor topography.

As a realistic trench model the Japan trench is selected. The response of the structure near the source is calculated by two-dimensional finite-difference simulation. The teleseismic P waveforms are calculated by using reciprocal theorem. Our results suggest that (1) around the trench bottom, effects of non-planar structure are remarkable and the realistic seafloor cannot be approximated by any model of the planar structure, however, (2) effects of the continental slope are well approximated by a model of planar dipping seafloor of approximate dip. Therefore, the near source structure must include the realistic seafloor topography, or the waveform analyses may lead to spurious subevents in a later stage of rupture process and mislocation of events. Our results also show that present knowledge about the seafloor topography is enough to incorporate the complicated geometry. ¿ American Geophysical Union 1989