Detailed seismic attenuation structure beneath Hokkaido, northeastern Japan: Arc-arc collision process, arc magmatism, and seismotectonics

摘要

In this study, we imaged a detailed seismic attenuation structure (frequency-independent Q~(-1)) beneath Hokkaido, Japan, by merging waveform data from a dense permanent seismic network with those from a very dense temporary network. Corner frequency of each event used for t~* estimation was determined by the S coda wave spectral ratio method. The seismic attenuation (Q_p~(-1)) structure is clearly imaged at depths down to about 120 km. For the fore-arc side of Hokkaido, high-Q_p zones are imaged at depths of 10 to 80 km in the crust and mantle wedge above the Pacific slab. Low-Q_p zones are clearly imaged in the mantle wedge beneath the back-arc areas of eastern and southern Hokkaido. These low-Q_p zones, extending from deeper regions, extend to the Moho beneath volcanoes, the locations of which are consistent with those of seismic low-velocity regions. These results suggest that the mantle wedge upwelling flow occurs beneath Hokkaido, except in the area where there is a gap in the volcano chain. In contrast, an inhomogeneous seismic attenuation structure is clearly imaged beneath the Hokkaido corner. A broad low-Q_p zone is located at depths of 0–60km to the west of the Hidaka main thrust. The location almost corresponds to that of the seismic low-velocity zone in the collision zone. The fault planes of the 1970M6.7 and 1982M7.1 earthquakes are located at the edges of this broad low-Q_p zone. Observations in this study indicate that our findings contribute to understanding the detailed arc-arc collision process, magmatism, and seismotectonics beneath Hokkaido.