| 10 January 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in December 2000
There was no marked activity. to supplementary reports
On December 22, there was a moderately deep M6.5 earthquake at a depth of about 140 km near Etorofu-to. The focal mechanism had a tension axis in the direction of the subduction of the Pacific Plate. to supplementary reports
There was no marked activity. to supplementary reports
*On December 5th, there was an M5.3 earthquake at a depth of about 40 km in the sea off Ibaraki Prefecture. This earthquake occurred in conjunction with the subduction of the Pacific Plate.
*The seismic activity that began at the end of June in the area from Miyakejima to Niijima-Kodushima is subsiding, but continues and on December 6th there was an M3.8 earthquake in northern Kodushima. Changes in crustal movement have also nearly ceased, but it cannot yet be said that they have stopped entirely.
*Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture has been at lower than average levels since August 1999, but there have been recent signs of a return in activity. However, the results of GPS observation of the Tokai Region show no changes in the trends seen to date. to supplementary reports
Aftershock activity subsequent to the 2000 Western Tottori Earthquake (M7.3; tentative) that occurred on October 6th continue to decline gradually, but since the beginning of December, there was an M4.2 aftershock on the 7th and two consecutive M4.1 aftershocks on the 19th in the northern part of the aftershock zone. The activity that was triggered by the M7.3 (tentative) event in the areas 25 km WSW and 30 km ENE of the aftershock zone is also gradually declining. to supplementary reports
There was no marked activity. to supplementary reports
*On January 2nd 2001, there was an M4.4 earthquake at a depth of about 10 km in the Chuetsu region of Niigata Prefecture, with a maximum seismic intensity of 5 lower. The focal mechanism was a reverse fault type with a NW-SE pressure axis. Also, on January 4th, there was an M5.1 earthquake with a maximum seismic intensity of 5 lower at a depth of about 10 km in the same region, approximately 40 km SSE of the epicenter of the event that occurred on the 2nd. The focal mechanism was a reverse fault type with an E-W pressure axis. Seismic activity has subsided in both areas.
The results of GPS observation of this area over the past three years show east-west contraction in a WNW-ESE direction, and the focal mechanisms of the two events conform to the characteristics of the region. The results of GPS observation of the area show no marked crustal deformation related to these events.
As the earthquakes that occurred on the 2nd and the 4th were Class M4 and M5, and were separated by approximately 40 km, it is not believed that they are directly connected. However, an examination of seismic activity in the Chuetsu region of Niigata Prefecture over the past ten years shows that in December 1990 there was activity including a maximal M5.4 event with attendant damage in an area 5 km SW of the epicenter of the event that occurred on January 2nd.
*On January 6th, there was an M4.6 earthquake at a depth of about 50 km in the eastern part of Mino in Gifu Prefecture.
*On January 9th, there was an M4.6 earthquake at a depth of about 50 km in the Iyo-nada.
| January 10, 2001 |
| Earthquake Research Committee |
In Japan and the surrounding area, there were 43 events of M4.0 or greater (compared with 44 such events in November, and a monthly average of 40 in the 30-year period up to the end of 1999). Within that number, there were four events of M5.0 or greater (five in November).
Additionally, although the average annual occurrence of M6.0 or greater events in the 1997-1999 period was 11 (or eight if events in the Taiwan region are not included), in 2000 there were a total of 23 by the end of December (or 18 if events in the area from Miyakejima to Niijima-Kodushima are not included). None of the above statistics includes the event that occurred on August 5th in central Sakhalin.
There was the following major seismic activity from December 1999 to the end of November 2000:
| -Off the eastern coast of Hokkaido | 28 January, 2000, an M6.8 event (at a depth of about 60 km) |
| -Iburi administrative district (in the vicinity of Mt. Usuzan) | |
| 30 March, 2000, an M4.3 event (at a depth of less than 10 km) and 1 April, an M4.6 event (at a depth of less than 10 km), seismic activity associated with the volcanic activity | |
| -Northeastern Chiba Prefecture | 3 June 2000, M6.0 (at a depth of about 50km) |
| -Off the west coast of Ishikawa Prefecture | 7 June 2000, M6.1 (at a depth of less than20 km) |
| -The Kumamoto region of Kumamoto Prefecture | 8 June 2000, M4.8 (at a depth of about 10km) |
| -from the area near Miyakejima to the area near Niijima-Kozushima | |
| In the period from the end of June to September, there was continued seismic activity in association with magma activity in the area, and on July 1st there was an M6.4 event (at a depth of about 10 km), and another M6.4 event (at a depth of about 10 km) on July 30th. | |
| -The sea off Ibaraki Prefecture | July 21st 2000, M6.0 (at a depth of 50 km) |
| -Western Tottori Prefecture | On October 6th, there was an M7.3 (tentative) event (at a depth of about 10 km) |
| -Near Iriomotejima | On November 14th 2000, there was an M4.4 earthquake (at a depth of about 10 km) |
"On December 22, there was a moderately deep M6.5 earthquake at a depth of about 140 km near Etorofu-to. The focal mechanism had a tension axis in the direction of the subduction of the Pacific Plate.":
This earthquake occurred on the lower plane of a double seismic zone within the subducting Pacific Plate. In December 1978, there was an M7.7 earthquake in an area near the epicenter of this event.
There are no supplements in the Tohoku Region.
"Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture has been at lower than average levels since August 1999, but there have been recent signs of a return in activity.":
Although activity was at higher than the average of six M1.5 or greater events per 30-day period at some points since August 1999, the overall activity level in the subducting Philippine Sea Plate in central Shizuoka Prefecture was below average. However, in recent consecutive 30-day periods there were nine and ten such events, which exceeds the average.
In addition, there was the following seismic activity:
-On December 6th, there was an M5.1 event in the sea near Torishima.
There was microearthquake activity including M3.1 and M3.0 events in northern Hyogo Prefecture (near the border with Tottori Prefecture).
-The seismic activity that began in June 2000 in the Kumamoto region of Kumamoto Prefecture is subsiding.
-The seismic activity that began in mid-November 2000 near Iriomotejima has continued at a reduced level since December.
In addition, there was the following seismic activity:
-On December 13th, there was an M5.1 event near Taiwan.
| Reference Material 1 Standards to select earthquakes which are to be described in "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) Events of M6.0 or greater. Or, M4.0 or greater (M5.0 or greater at sea) and having a maximum intensity of 3 or greater. Reference Material 2 Standards to describe additional explanation in "Supplementary repot of the evaluation." 1 Reference items relating to seismic activity described in "Evaluation of Seismic Activity". 2 Activity related to seismic activity (of the past 12 months or so) described in "Major Seismic Activity in Evaluation of Seismic Activity." 3 Activity that have been evaluated but not described in "Evaluation of Seismic Activity" because of being within the bounds of normal activity or because the activity is not prominent. |