Excitation of Back‐Arc Tsunamis From Megathrust Ruptures: Theory and Application to the Sea of Japan

Abstract

AbstractLarge megathrust ruptures can create notable tsunamis in tectonic back‐arc basins as was documented during the 2011 Tohoku earthquake in the Sea of Japan. We present a physical analysis of the excitation of back‐arc tsunamis by extending the fore‐arc deformation field from the earthquake centroid into the back‐arc basin and identify fault dip as the main geometrical contributor to the propagation of these events. As such, our theoretical model along with a large number of numerical simulations reveal that the dominant period of back‐arc tsunamis is different from that of the fore‐arc waves and thus they are a new class of tsunamis. Through numerical simulations and analysis of data from the 2011 Tohoku event, we show that a combination of near‐ to intermediate‐field horizontal and vertical deformation as well as transient surface waves is necessary to reconstruct the back‐arc propagation. We find that while seismic surface waves can affect coastal tsunami amplitudes in the back‐arc, their effect comes second to that of the horizontal component of deformation as manifested via bathymetric gradient. We then simulate back‐arc tsunamis and the hazard in the Sea of Japan from several potential future earthquake scenarios in the Japan Trench and Nankai Trough. Our results show that the coseismic excitation of back‐arc tsunamis can result in considerable waves close to 1 m in the Sea of Japan, near the Niigata Prefecture from megathrust earthquakes.