January 12, 2010
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On December 17, seismic activity started off-shore east of the Izu Peninsula and continued at a strong level until December 20. Seismicity declined subsequently and has returned to the previous level. Those events were mainly located within an area 4km in an east- west direction, centered on Cape Shiofukizaki, and generally distributed at depths of 3 to 8km. The largest event in this seismicity was the M5.1 earthquake that occurred at 08:45 (JST) on December 18 (maximum seismic intensity 5 Lower). The largest event and many of other events occurred with focal mechanisms that showed strike-slip faults with compression axes in a NW-SE direction, which are commonly seen in this vicinity.
* According to the GPS data, crustal movements associated with the recent seismic activity were observed in the area centered on the eastern coast of the Izu Peninsula, with a displacement of approximately 4cm to the SSW at the Komuroyama station. The estimated location of the deformation source generally corresponds to the active region around Cape Shiofukizaki. Some changes were also observed on strainmeters, tiltmeters and groundwater levels in the neighboring area. Data from those observations are thought to mainly show the expansion of the crust at the deformation source. Those observational values that changed in accordance with the seismicity have mostly returned to normal levels.
* In the area off-shore east of the Izu Peninsula, earthquake swarms and associated crustal movements have repeatedly been observed since 1978. The recent seismicity was the first strong activity since April-May, 2006. This time, the seismicity occurred in nearly the same region as in May-June, 1993 and declined more quickly than previous activity.