A further source of Tokyo earthquakes and Pacific Ocean tsunamis
Earthquake hazard assessments for the Tokyo Region are complicated by the trench-trench triple junction where the oceanic Philippine Sea Plate not only underthrusts a continental plate but is also being subducted by the Pacific Plate. Great thrust earthquakes and associated tsunamis are historically recognized hazards from the Continental/Philippine Sea (Sagami Trough) and Continental/Pacific (Japan Trench) plate boundaries but not from the Philippine Sea/Pacific (Izu-Bonin Trench) boundary alone. Here we implicate possible ruptures along these three plate boundaries in generating an unusually large tsunami from sandy deposits found along 50 km of coastline east of Tokyo. Dating to about 1,000 years ago, this inferred tsunami predates local written history by several hundred years. We find that the inland extent of its sand sheet is best explained, in computer simulations, by displacement on one of the three plate boundaries offshore of the Boso Peninsula, corresponding to the triple junction. The minimum magnitude scenario capable of generating the inland extent of inundation involves displacement along the Philippine Sea/Pacific boundary megathrust. This plate-boundary fault adds another potential source for earthquakes in the Tokyo Region and tsunamis in the Pacific Ocean.