Contiguous rupture areas of two Nankai Trough earthquakes revealed by high‐resolution tsunami waveform inversion

Abstract

We have developed a new method for inverting tsunami waveforms that reveals considerable detail in megathrust slip during subduction zone earthquakes. Previous methods have ensured compliance with the shallow‐water theory used to compute tsunami waveforms by using large subfaults that generate only long‐wavelength seafloor deformation. We show that a better approach is to use small subfaults coupled with a smoothing criterion that ensures compliance with the shallow‐water approximation. In an application of the method to historical earthquakes in the Nankai Trough, southwestern Japan, we find that the areas with slip > 1 m for the earthquakes of 1944 and 1946, which ruptured adjacent segments of the subduction zone, are separated by a sharp, non‐overlapping boundary. This establishes that interseismic accumulation of strain energy extends very close to the boundary between rupture zones, and strongly suggests that this boundary is associated with a physical barrier to rupture.