Seismic Activity Off-shore of Miyagi Prefecture

* On August 16, at 11:46 (JST) there was a M7.2 earthquake at a depth of approximately 40km off-shore of Miyagi prefecture (maximum seismic intensity 6 Lower). A small tsunami was observed on the Pacific coast in the Tohoku region (see the 146th Evaluation of the Earthquake Research Committee " Earthquake Off-shore of Miyagi Prefecture on August 16, 2005"). The seismic activity was a main shock - aftershock sequence. The aftershock activity has been declining. The largest aftershock, as of 16:00 on September 14, was a M4.7 earthquake that occurred at 04:28 on September 12. The aftershocks locations were mostly distributed dipping downwards to the west along the subduction of the Pacific plate, within an area off-shore of the Oshika Peninsula approximately 40km from east to west and 30km from north to south. The main shock was located at the southeastern edge. This event occurred at the boundary between the Pacific and the continental plates.

* From the analyses of the focal process using seismic observations, it is determined that the region where the major slip of the main shock occurred was located near the rupture initiation point.

* According to the GPS data, crustal movements in southeast to east directions, associated with the main shock were observed centered on Miyagi prefecture, with a displacement of approximately 6cm to the east at the Oshika observation station (Ishinomaki city, Miyagi prefecture). Also, there was some subsidence observed in the region near the Oshika Peninsula, with the largest subsidence of approximately 5cm at the Oshika observation station. The estimated fault model determined from these GPS data is approximately consistent with the focal mechanism of the main shock and the aftershock distribution. There have been no significant postseismic movements observed so far.

* In the region approximately 80km ENE of the main shock, seismic activity started from August 18, and increased on August 24 and 31 with M6.3 earthquakes. Focal mechanisms of both events showed reverse faults with compression axes in a NW-SE direction. It is thought that these events occurred at the boundary between the Pacific and the continental plates. The activity has been gradually declining since September 4.

* It is thought that the recent event ruptured part of the focal region of the Off-shore Miyagi Prefecture Earthquake that the Earthquake Research Committee has expected in the near future. However, the magnitude of the recent earthquake was small, and the source area, which was estimated from the aftershock distribution and seismic waves, did not cover the entire expected source region. Therefore there is still a possibility for the occurrence of the expected Off-shore Miyagi Prefecture Earthquake.

• Earthquake Off-shore of Miyagi Prefecture on August 16
• Earthquake Off-shore of Miyagi Prefecture on August 16 (Comparison with Past Events)
• Earthquakes Off-shore of Miyagi Prefecture (near the Ocean Trench) on August 24 and 31
• Source Processes of the Earthquake Off-shore of Miyagi Prefecture on August 16, 2005
• Hypocenter Distribution of the Main Shock and Aftershocks, and Three-Dimensional P Wave Velocity Model
• Relation between the Earthquakes Off-shore of Miyagi Prefecture in 1933, 1936 and 1937 and the 1978 Off-shore Miyagi Prefecture Earthquake (preliminary)
• Horizontal and Vertical Crustal Movements of the Earthquake Off-shore of Miyagi Prefecture on August 16, 2005
• Changes in GPS Data by Component
• Fault Model Associated with the Earthquake Off-shore of Miyagi Prefecture on August 16, 2005 (Final analysis)