Scattering of teleseismic P-waves by the Japan Trench: A significant effect of reverberation in the seawater column

We detected a scattered wave train in data from the high-sensitivity seismograph network in Japan (Hi-net) following the arrival of the near-vertically incident P-wave generated by the 2009 earthquake (Mw 7.8) off the South Island of New Zealand. The scattered wave train represented predominantly vertical ground motion at a period of 20 to 50 s and with an apparent velocity of 3.5 km/s; it propagated cylindrically westward through the Kanto area of central Japan. Array analysis showed that the scattered wave train developed beneath the Pacific Ocean near the Boso triple junction, southeast of the Kanto area. A 3D finite-difference simulation of seismic wave propagation using a high-resolution model incorporating subsurface structure, topography, and bathymetry revealed that the strong scattered waves that were generated along the Japan Trench and propagated normal to the trench axis represented multiple reverberations of seismic waves between the seafloor and the Pacific plate boundary. In addition, strong reverberation of acoustic waves in the seawater column above the Boso triple junction causes elongated scattered waves, which reasonably explains our observations.