January 11, 2011
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
On December 22, there was a M7.4 earthquake under the ocean area near Chichijima,and caused tsunamis observed on part of the Tohoku region and the Pacific coast from the southern Kanto region through the Okinawa region, with a tsunami height of 0.5m at Yaene, Hachijojima.
* On December 2, there was a M4.6 earthquake at a depth of approximately 5km in the central Ishikari region. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred within the crust.
* On December 6, there was a M5.8 earthquake off-shore east of Aomori prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. It is thought that this event occurred at the boundary between the Pacific and the continental plates.
* On December 22, there was a M7.4 earthquake under the ocean area near Chichijima. The focal mechanism showed a normal fault with a tension axis in a NE-SW direction. This event occurred within the Pacific plate. It caused tsunamis observed on part of the Tohoku region and the Pacific coast from the southern Kanto region through the Okinawa region, with a tsunami height of 0.5m at Yaene, Hachijojima. According to the GPS data, crustal movements associated with this earthquake were observed with a displacement over 1cm to the WSW at the Chichijima and Hahajima stations.
* There have been no anomalous changes observed in the GPS data for the Tokai region.
There was no remarkable activity.
There was no remarkable activity.
* On January 3, there was a M4.7 earthquake at a depth of approximately 10km, off-shore of the Kaetsu region, Niigata prefecture. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction.
January 11, 2011
Earthquake Research Committee
The following are the numbers of earthquakes by magnitude that occurred in Japan and the surrounding area during December 2010.
(reference)
The following events for the period from December 2009 to the end of November 2010 have been listed as "Major Seismic Activity" in monthly "Evaluation of Seismic Activity".
| – Off-shore East of the Izu Peninsula | Seismic activity with M5.0 on December 17, M5.1 on December 18, 2009 |
|
| – Under the Ocean Area near Ishigakijima | M6.5 on February 7, 2010 | |
| – Under the Ocean Area near the Okinawa Main Island | M7.2 on February 27, 2010 | |
| – Along the Central Coast of Chile | Mw8.8 on February 27, 2010 | |
| – Off-shore of Fukushima Prefecture | M6.7 on March 14, 2010 (Depth of approximately 40km) |
|
| – Off-shore of Fukushima Prefecture | M6.2 on June 13, 2010 (Depth of approximately 40km) |
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| – Northeastern Chiba Prefecture | M5.0 on July 23, 2010 (Depth of approximately 35km) |
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| |
– Joetsu Region, Niigata Prefecture | M4.7 on October 3, 2010 (Depth of approximately 20km) |
| – Under the Ocean near Miyakojima | M6.4 on October 4, 2010 |
"On December 2, there was a M4.6 earthquake at a depth of approximately 5km in the central Ishikari region. (The rest is omitted.)":
After this event, there were 6 earthquakes with seismic intensity of 1 or over.
There is no supplementary information for the Tohoku region.
"On December 22, there was a M7.4 earthquake under the ocean area near Chichijima. (The rest is omitted.)":
After this event, there was brisk activity of earthquakes such as M6.5
on December 23. The activity has gradually been decaying.
"There have been no anomalous changes observed in the GPS data for the Tokai region.":
(This evaluation is in agreement with the views (reference below) presented at the procedural meeting on December 20 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster.)
(Reference) Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, December 20, 2010)
"So far, there have been no changes of activity which are likely to be associated with the Tokai earthquake.
1. Status of seismicity
General seismicity was slightly strong since mid-2005 within the crust under mid-western Shizuoka prefecture, but it has tended to resume a normal level from around this spring.
There continue to be fewer events within the Philippine Sea plate under the region around Hamanako.
In other regions, seismic activity is generally at a normal level.
In addition, deep low-frequency earthquakes were observed near the plate boundary under Aichi to southern Nagano prefectures from November 11 through November 30. In this vicinity, a sequence of deep low-frequency earthquakes was observed in late April through early May this year.
2. Status of crustal movements
There have been no remarkable changes observed over the entire region.
According to the GPS data and leveling results, the trend of long-term subsidence at Omaezaki has been continuing as before.
In addition, crustal movements were observed on the strainmeters in the neighboring regions from around November 13 through around November 22, which are thought to have been caused by short-term slow slip around the plate boundary under Aichi prefecture, and were concurrent with the deep low-frequency seismicity mentioned above. Crustal movements related to short-term slow slip had not been observed since April this year."
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
There is no supplementary information for the Kyushu-Okinawa region.
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: