Small‐scale heterogeneities in the oceanic lithosphere inferred from guided waves

Abstract

We analyze seismic waveforms from deep‐focus earthquakes occurring in the subducting slab beneath Japan, recorded by broadband ocean bottom seismometers (BBOBSs) installed on the northwestern Pacific Ocean seafloor. The data reveal waveforms with a low‐frequency direct P onset, followed by large‐amplitude, high‐frequency, long‐duration Po and So waves. From the analysis of the BBOBS records and a numerical finite‐difference method simulation of seismic wave propagation, we elucidate the generation and propagation processes of such guided waves. We demonstrate that the low‐frequency direct P and S waves propagate in the asthenosphere and that the following high‐frequency, long‐duration Po and So waves are developed by multiple forward scattering of P and S waves. The scattering occurs due to laterally elongated heterogeneities in both the subducting and horizontal parts of the oceanic lithosphere, with the apparent velocities (V p = 8.1 km/s, V s = 4.6 km/s) being close to the velocities of oceanic lithosphere.