Seismic Activity Off-shore East of the Izu Peninsula in April 2006

* Since April 17, seismic activity started off-shore east of the Izu Peninsula. The strong activity continued with some decreases and increases for a week, and has subsequently been subsiding gradually. The events extend from an area 1 km east of Kawanazaki for approximately 4km in an E-W direction, also another area extends 8 km to the south in an approximately N-S direction from the eastern edge of the previously mentioned area. These events are mostly distributed at depths of approximately 5km or deeper. The largest earthquake so far was a M5.8 event (maximum seismic intensity 4) that occurred at 02:50 (JST) on April 21 near the eastern edge of the E-W trending active area. After this event, seismicity started in the N-S trending area. In addition, there were relatively large earthquakes in neighboring areas slightly apart from the active regions, such as a M4.5 earthquake (maximum seismic intensity 5 Lower) off-shore of Ajiro, Atami city on April 30, and a M5.1 event (maximum seismic intensity 4) approximately 10km to the east of the main active regions on May 2. The focal mechanisms of the largest event and other many events 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 centered on the east coast of the Izu Peninsula, with a displacement of approximately 4cm to the southeast at the Ito Yawatano observation station. The estimated source location of the crustal movements corresponds to the active region off-shore east of Kawanazaki. Also crustal movements were observed on tiltmeters and strainmeters in the neighboring area, which is thought to indicate that the crust at the source appears to be dilating. In addition, the observed changes corresponding to the recent seismicity are currently declining.

* Earthquake swarm activity and related crustal movements have been repeatedly observed since 1978 off-shore east of the Izu Peninsula. The recent activity is the first strong seismicity since the activity from April through June in 1998. Characteristically, the largest event of recent seismicity is similar to the maximum size events in 1997 and 1998 (all were M5.9 earthquakes), however current activity is continuing with a smaller total number of events. Also, while the seismicity was gradually declining, some relatively large earthquakes occurred in the areas slightly separated from the active region.

* Judging from the combination of the seismic activity, the condition of the crustal movements, and the previous cases of seismicity in that region, it is thought that this activity will gradually end.

• Seismic Activity Off-shore East of the Izu Peninsula
• Seismic Activity Off-shore East of the Izu Peninsula (Comparisons of the recent activity)
• Seismic Activity Off-shore East of the Izu Peninsula (Recalculation of hypocenters using the Double Difference (DD) method)
• Earthquake Off-shore East of the Izu Peninsula on April 30
• Earthquake Off-shore East of the Izu Peninsula on May 2
• Seismic Activity Off-shore East of the Izu Peninsula (Table of Felt Earthquake Occurrences by Maximum Seismic Intensity)
  (00:00 on April 17, 2006 - 24:00 on May 9, 2006 )
• Seismic Activity Off-shore East of the Izu Peninsula (Recent activity with M6 class earthquakes)
• Changes on Strainmeters and Tiltmeters Associated with Seismic Activity Off-shore East of the Izu Peninsula
• Relation of Seismic Activity Off-shore East of the Izu Peninsula and Strain (First Maximum Value of Compressional Change in 24 hours (using ε2))
• Crustal Movements and Changes in GPS Data by Component on the Eastern Izu Peninsula
• Izu Peninsula Area and Model Calculation
• Past Crustal Movements and Changes in GPS Data by Component in the Eastern Izu Peninsula (in 1997, 1998)