Bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the Kurile Trench

Resolution of earthquake source characteristics is limited by structural complexities in the source region. This study explores the effects of dipping near-source bathymetry and structure on long-period teleseismic body waveforms from shallow subduction zone earthquakes. Effects of nonhorizontal structure are observed in the P waveforms from shallow Kurile Trench events (M_s 5.8-7.2), which show prominent later arrivals not observed on SH waveforms. Synthetics calculated for a dipping water-crust interface provide a better fit to these arrivals than those calculated for a horizontal interface, suggesting that the P waveforms show systematic effects of seafloor dip. Large reverberations in the P waveforms of the October 20, 1963 and June 10, 1975 Kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture; however, SH waveforms suggest simple source time functions with durations of 25 and 43 s. Inversion of P and SH waveforms and amplitudes yields shallowly dipping thrust mechanisms, with no conclusive evidence for a previously proposed steepening of the fault dip during rupture. The anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments. -from Author