Itsuo Furuya

Delamination structure imaged in the source area of the 1982 Urakawa-oki earthquake

[1] The Kuril arc collides with the northeast Japan arc in the southern part of Hokkaido, Japan. 3-D tomographic inversion of data from a dense network of sensitive ocean-bottom seismographs and land stations has allowed imaging of previously unseen details of the arc-arc collision structure. A low velocity body dips gently southwestward, at depths of 35 to 45 km, from east of the Hidaka Mountains to the source area of the 1982 Urakawa-oki destructive earthquake (Ms 6.8). The low velocity body is the lower half of the lower crust of the Kuril arc, which must have been delaminated by the collision. We believe that the continuing collision of the delaminated lower crust with the northeast Japan arc resulted as an episode of aseismic slow slip prior to the 1982 Urakawa-oki earthquake as well being the reason for the high seismic activity in this region.

Gravity Observation by Means of a Superconducting Gravimeter at Matsushiro, Japan

Gravity observation by means of the superconducting gravimeter #11 has been started at Matsushiro Seismological Observatory, Japan Meteorological Agency. This gravimeter is the one that used to be located at Kakioka, Japan. The new gravimeter site is inside a tunnel where the environmental conditions are very stable. The gravimeter is bottom mounted to eliminate the disturbances from local ground motions. Absolute gravity measurements were carried out to calibrate the scale factor of the superconducting gravimeter. Preliminary analysis of the gravity data shows that the noise level of the gravimeter at seismic bands has been improved with its move to Matsushiro.