Tsunami earthquakes: slow thrust-faulting events in the accretionary wedge

The November 20, 1960, Peru, October 20, 1963, Kurile and June 10, 1975, Kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude. Long-period surface wave analysis indicates double-couple (faulting) mechanisms for all three events rather than single-force mechanisms indicative of submarine landslides. The earthquakes have shallow depths (< 15 km) and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism. Body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events, with dip angles of 6°-8°. Most of the "anomalous' tsunami excitation results from underestimation of earthquake size by M_S due to the long source duration; the tsunami heights are not significantly anomalous relative to seismic moment. The slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism. -from Authors