Effects of near source bathymetry on teleseismic P waveforms

Teleseismic P waveforms from submarine earthquakes show water reverberations that are often poorly modeled by synthetic seismograms. This study suggests that considerable misfit results from the non‐horizontal nature of the seafloor in the epicentral region and examines this effect for a planar dipping water‐crust interface. Geometric ray theory synthetics calculated for a double‐couple source show that P waveforms are extremely sensitive to seafloor orientation. Because of the large impedance contrast across the boundary, seafloor dips as small as 0.5° significantly modify the ray paths, affecting the take‐off angle at the source, reflection and transmission coefficients at the interface, geometric spreading, and travel time. To test the applicability of such modeling, P waveforms from the March 29, 1976 Cocos Ridge intraplate earthquake were studied. The waveforms, many of which show prominent water multiples poorly modeled with flat lying bathymetry, were inverted for the best‐fitting planar ocean bottom orientation using a grid search technique. Synthetics computed for the best fitting seafloor orientation match the data much better than synthetics for a flat lying structure, and reproduce the observed azimuthal dependence of water multiple amplitudes. The derived ocean bottom dip angle and direction are in agreement with bathymetric charts, suggesting P waveforms provide constraints on the seafloor orientation in the epicentral region. Green's functions incorporating the effects of bathymetry should provide better resolution of the source parameters of submarine earthquakes.