April 11, 2007
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
The Noto Hanto Earthquake in 2007
*On March 25, at 09:42 (JST) there was a M6.9 earthquake at a depth of approximately 10km near the west coast of the Noto Peninsula. This event had maximum seismic intensity 6 Upper in Ishikawa prefecture. Also, small tsunamis were observed in the Hokuriku region, with heights of 22cm in Nagahashi, Suzu city, and 18cm in Kanazawa. The focal mechanism of the main shock showed a reverse fault with a strike-slip component and a compression axis in a WNW-ESE direction (see the 166th Evaluation of the Earthquake Research Committee " The Noto Hanto Earthquake in 2007")
* Seismic activity was a main shock - aftershock sequence. The aftershock activity has been declining. The aftershock locations are distributed along a 40 km trend in a NE-SW direction and dipping downwards to the southeast. The largest aftershocks as of 14:00 on April 11, were two M5.3 events that occurred at 18:11 on March 25 and at 07:16 on March 26. One of these events occurred at the northeastern edge of the aftershock region and the other occurred at the southwestern edge. Since the largest aftershock at the southwestern edge, the aftershock area has been slightly extending to the southwest. In addition, there have been no remarkable changes observed in seismicity in the neighboring area.
* According to the GPS data, crustal movements associated with the main shock were centered on the Noto Peninsula. There was a displacement of approximately 21cm to the southwest with an uplift of approximately 7cm at the Shikamachi Togi observation station (Hakui-gun, Ishikawa prefecture), and a displacement of approximately 12cm to the northwest with a subsidence of approximately 2cm at the Anamizu observation station (Housu-gun, Ishikawa prefecture). In addition, there have been no significant postseismic movements observed so far.
* According to a geologic field survey, there were verticals changes observed along the northwest coast of the Noto Peninsula, in the area from Monzen-cho, Wajima city to Shika-machi, there was a maximum uplift of approximately 40cm and a maximum subsidence of approximately 10cm.
* There is a reverse fault off the west coast of the Noto peninsula, which is about 20 km long, trending in a NE-SW direction with a southeast dip. It is highly possible that part of this fault was associated with the recent event.
* It is estimated that there is a less than a 10 percent chance of occurrence for M5.0 or greater aftershocks (with seismic intensity 5 Lower in some areas), within the 3 days from 17:00 on April 11. Also the frequency of occurrence for aftershocks with seismic intensity 1 or larger, is estimated to decrease to about once per day by late April.