9:00 AM - 9:15 AM
[SSS26-01] P- and S-wave velocity structure in southern Hokkaido deduced from ocean-bottom seismographic and land observations
Keywords:ocean-bottom seismograph (OBS), seismicity, tomography, Hidaka Collision Zone, Hokkaido, 1982 Urakawa-Oki earthquake
From the tomographic images, distinct low-velocity anomalies are detected at the western side of the Hidaka Main Thrust (HMT). They are considered to be the crust of the northeast Japan arc. In the eastern side of it, we find high-velocity anomalies, which are considered to be the crust of the Kuril arc obducted towards the west. The low-velocity anomalies appear to reach a depth of the upper boundary of the subducting Pacific plate. These results are similar to Murai et al. (2003). Murai et al. (2003) considered the low-velocity anomalies deeper than 30 km as the delaminated Kuril arc lower crust because the velocity inversion occurs at around 30 km depth. However we cannot image the delamination structure clearly although the velocity inversion is also detected. From the vertical cross section along the subducting direction of the Pacific plate, the depth of the upper boundary of the high-velocity anomalies increases towards the northwest, which represents the upper boundary of the subducting Pacific plate. The low-velocity anomalies beneath the western side of the HMT disappear in the offshore area southeast of Cape Erimo. Moreover many microearthquakes occurred in the low-velocity anomalies whereas the seismic activity was low outside them. These results suggest arc-arc collision has little influence on the offshore area as an extension of the Hidaka Mountains. S-wave velocity structure shows similar features to P-wave velocity structure.
We thank the crew of the R/V Kofu Maru of the former Hakodate Marine Observatory for their cooperation during deployment of OBSs. We thank D. Zhao for permitting us to use his computer program for 3-D tomographic inversion.
Katsumata et al., 2002, Bull. Earthq. Res. Inst., 77, 199-223.
Murai et al., 2003, G.R.L., 30(9), 1490, doi:10.1029/2002GL016459.
Zhao et al., 1992, J.G.R., 97, 19909-19928.