RELATIONSHIPS BETWEEN TSUNAMI SIZE AND EARTHQUAKE MAGNITUDE IMPROVED BY FAULT PARAMETERS

Abstract

Megathrust earthquakes are the main source of tsunamis. The rupture at the plate interface deforms the seafloor, displacing seawater over a large region. The earthquake magnitude is not the only factor that affects the tsunami amplitude. A tsunami earthquake, which produces a much larger tsunami than what can be inferred from the seismic energy release, exemplifies this phenomenon. This thesis examines relationships between tsunami size and key geophysical attributes such as fault depth, fault dip, fault size, rigidity, and water depth, besides moment magnitude. The parametric study involves four sets of simplified megathrust-ocean models with an elastic planar-fault solution to define the earth surface deformation and a non-hydrostatic model to describe the resulting tsunami. The first set of models contains a flat seafloor to provide a baseline for comparison. The second set includes a flat seafloor abutting a 2° slope, and by varying the fault depth, fault dip, and water depth, explores the contributions from wave shoaling and wave energy anisotropy to peak tsunami amplitude. The third set utilizes the same topography to demonstrate effects of reduced rigidity or fault size for the same seismic moment. The fourth set examines the combined effects of the geophysical parameters as well as their trade-off. The results highlight the importance of depth-dependent fault rigidity and size in describing the two orders of magnitude variability in observed peak tsunami amplitude for given moment magnitude.