Decoupling of Pacific subduction zone guided waves beneath central Japan: Evidence for thin slab

Abstract

AbstractThe fine‐scale seismic structure of the northeast Japan subduction zone is studied based on waveform analyses of moderate‐sized (M4.5–6), deep‐focus earthquakes (h >350 km) and the finite difference method (FDM) simulation of high‐frequency (up to 8 Hz) wave propagation. Strong regional S wave attenuation anomalies for specific source‐receiver paths connecting the cluster of events occurring in central part of the Sea of Japan recorded at fore arc stations in northern and central Japanese Islands (Honshu) are used to model the deeper structure of the subducting Pacific Plate, where recent teleseismic tomography has shown evidence for a possible slab tear westward beneath the Sea of Japan. The character of the observed anomalous S wave attenuation and the following high‐frequency coda can be captured with the two‐dimensional (2‐D) FDM simulation of seismic waves in heterogeneous plate model, incorporating the thinning of the plate at depth, which is also compared with other possible causes of dramatic attenuation of high‐frequency S wave due to low‐Q or much weaker heterogeneities in the slab. The results of simulation clearly demonstrate that the dramatic loss of high‐frequency S wavefield from the plate into the surrounding mantle occurred due to the variation in the plate geometry (i.e., thinning of the plate) at depth near the source rather than due to variation in physical properties, such as due to the lowered‐Q and weaker heterogeneities in the plate. The presence of such a thin zone defocuses the high‐frequency slab‐guided S wave energy from the subducting plate into the surrounding mantle and acts as a geometric antiwaveguide. Based on the sequence of simulation results obtained, we propose thinning of Pacific Plate at depth subducting beneath northeastern Japan, localized to central part of Honshu, in agreement with the observations.