Upper mantle structure of marginal seas and subduction zones in northeastern Eurasia from Rayleigh wave tomography

The upper mantle structure of marginal seas (the Seas of Japan and Okhotsk) and subduction zones in northeastern Eurasia is investigated, using the three-stage multimode surface wave tomography incorporating finite-frequency effects. Broadband waveform data from 305 events with magnitude greater than 5.5 from 1990 to 2005 recorded at 25 stations of the IRIS network in northeastern Eurasia and Japan and at 8 stations of the broadband seismic network in Far-Eastern Russia from 2005 to 2008 are employed in our analysis. The dispersion curves of the fundamental mode and first two higher modes of Rayleigh waves are simultaneously inverted for the shear-wave velocity structure of the region. The off-great circle propagation due to strong heterogeneities in the region is also taken into account in the construction of intermediary phase velocity models for each mode as a function of frequency. The obtained 3D S-wave velocity model is well resolved down to 200 km depth. Checkerboard tests show the average horizontal resolution of 5° in the study region. The subducting Pacific plate is clearly imaged as a high velocity anomaly up to 6%. The mantle wedge above the Pacific plate is associated with low velocity anomalies. The absolute minimum S-wave velocity in the mantle wedge is 4 km/s in the Sea of Okhotsk in the depth range from 80 to 160 km, probably indicating the presence of partial melt. The anomalous spot with conspicuous low velocity in the southern end of the Sea of Okhotsk may indicate the existence of hot upwelling flow in the mantle. A high velocity anomaly subparallel to the present subduction zone is found in the northwestern Sea of Okhotsk in the depth range from 100 to 200 km. The position of this anomaly correlates well with the high velocity anomaly found in the P-wave tomography of Gorbatov et al. (2000), which may be interpreted as a relict of the Okhotsk plate subducted in the past. We also attempted a mapping of azimuthal anisotropy in this region. The fast phase velocity directions near the Pacific plate are observed subparallel to the Kuril and Japan Trenches at all the periods, indicating a strong effect of the subducting Pacific plate on the mantle flow, while the anisotropy appears to be weak in tectonically inactive marginal seas.