ALBERT

Description: Detailed 3-D P - and S -wave velocity ( V p and V s ) structures and P -wave anisotropy of the crust and upper mantle under the southern Kuril arc from the Kuril–Japan trench to the eastern margin of Japan Sea are investigated using a large number of high-quality arrival-time data from local earthquakes. The suboceanic earthquakes used in the tomographic inversion are relocated precisely using sP depth phase data, which are collected from three-component seismograms recoded by the dense Japanese seismic network. Our results show that three prominent high-velocity (high- V ) zones separated by low-velocity (low- V ) anomalies exist in the megathrust zone under the forearc region. These high- V zones coincide with areas with large coseismic slips of great megathrust earthquakes as well as the areas with large slip deficit on the plate interface, whereas the low- V anomalies are generally consistent with the afterslip distribution of the 2003 Tokachi-oki megathrust earthquake ( M w 8.0). We think that these high- V zones probably represent asperities (strongly coupled areas) in the megathrust zone, and the low- V anomalies around the asperities may contain fluids, which play an important role in the nucleation of the megathrust earthquakes, in addition to the stress accumulation. Our results also show an obvious boundary between the Northeast Japan arc and the Kuril arc, especially at the Hidaka collision zone. This boundary extends southwards to the source areas of the 1952 and 2003 Tokachi-oki earthquakes ( M w 8.1 and 8.0), which may have contributed to forming the asperity for the two megathrust events.