[S03-1] Imaging of heterogeneities in the Earth with seismic scattered waves and ambient noise I

[S03-1-05] Modeling waveform anomaly across central Japan with scattered seismic waves as inferred from high-frequency simulations

^○Simanchal Padhy^{1, 2}, Takashi Furumura¹ (1.Earthquake Research Institute, The University of Tokyo, 2.CSIR-National Geophysical Research Institute, Hyderabad)

We model significant waveform attenuation anomaly observed in central Japan for intermediate to deep-focus (>190 km) earthquakes in subduting Pacific (PAC) plate. Analyses of Hi-net waveform data from the moderate sized (M 4-6) PAC plate events show differences in amplitude and duration of the high-frequency coda of P- and S-waves and their frequency content between the events occurring on northern and southern ends of the PAC plate. The northern events experience abnormally high attenuation of high-frequency (>1 Hz) seismic waves giving rise to spindle-shaped seismograms with strong converted phases and extended coda with very slow decay at the eastern stations. We investigate the cause of such waveform anomaly using the finite difference method (FDM) simulation of wave propagation through the subduction zone. Results of simulation carried out for a suite of plate models with key subduction zone elements show that the waveform anomaly is primarily explained by strong attenuation and defocusing of seismic waves by the localized highly heterogeneous low-wave speed anomaly, representing partial melting of the upper mantle. The data are secondarily explained by partial melting of the Philippine Sea (PHS) plate, mainly the hydrated basaltic oceanic crust, causing strong scattering of high-frequency seismic waves. The findings of this study have wide geodynamic implications such as the magma genesis and mechanism of intermediate earthquakes in young (<25 Ma) and warm subduction zones of central Japan.