UC San Diego

In this dissertation I investigate the physics of tsunami generation and seismic/acoustic radiations of two megathrust earthquakes in the Japan Trench. Elastic dislocation theory has been widely used for modeling shallow subduction earthquakes and tsunami generation, but several important observations in the 2011 MW 9.1 Tohoku-Oki earthquake contradict the theory, including diminishing shallow slip observed in the northern Japan Trench from differential bathymetry observations before and after the earthquake where the largest tsunami (up to 40 m) was generated and deficiency in high-frequency radiation associated with large tsunamigenesis. The 1896 Sanriku earthquake was a devastating tsunami earthquake with similar characteristics in the northern Japan Trench. Here I test an inelastic wedge deformation hypothesis in explaining these anomalous but important observations. Chapter 1 serves as an introduction to this dissertation. In Chapter 2, inelastic wedge deformation in the thick sediment of northern Japan Trench is shown as a mechanism to produce impulsive tsunami that can explain the extreme runup of the 1896 Sanriku tsunami. Chapters 3 and 4 present models of the 2011 Tohoku-Oki earthquake and 1896 Sanriku earthquake by fully coupling dynamic rupture, ocean acoustic waves, and tsunami with a finite-element code. Inelastic wedge deformation is shown in both earthquakes to cause depletion in high-frequency radiation but efficient tsunami generation with diminishing shallow slip, consistent with the observations. The inelastic deformation hypothesis may be applicable to accretionary and other sediment-rich margins. These results have important implications for tsunami hazard assessment and reduction around the world.