| 8 November 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in Japan October 2000
On October 2nd, there was an M5.7 earthquake at a depth of about 10 km in the vicinity of Toshimamura on Akusekijima (near Amami Oshima), which registered a maximum seismic intensity of 5 upper on Akusekijima. Also, on October 6th, there was an M7.3 event (tentative data) at a depth of about 10 km in western Tottori Prefecture, which registered a maximum seismic intensity of 6 upper in western Tottori Prefecture. Then, on October 31st, there was an M5.5 event at a depth of about 40 km in central Mie Prefecture, which registered a maximum seismic intensity of 5 lower. There was damage attendant to all of the above events. Each of the events occurred in patterns characteristic of the regions of occurrence, and it is not believed that there was a direct connection between any of the events. to supplementary reports
On October 7th, there was an M4.4 event at a depth of about 60 km in the eastern Hidaka administrative district. It is believed that this event occurred near the juncture of the Continental and subducting Pacific Plates. to supplementary reports
On October 3rd, there was an M5.9 event in the sea off Sanriku. The focal mechanism was a reverse fault type with a northwest-southeast pressure axis. This event occurred within the aftershock region of the 1994 Earthquake far off the Coast of Sanriku. to supplementary reports
*The seismic activity that began on June 26th in the area from Miyakejima to Niijima-Kozushima decreased to even lower levels than September's, but on October 3rd there was an M4.4 event about 10 km west of Miyakejima, and on October 31st, there was an M4.7 event about 10 km west of Niijima. And, as was the case in September, crustal movement has nearly stopped.
*On October 11th, there was a moderately deep M4.1 event at a depth of about 90 km in eastern Kanagawa Prefecture. This earthquake occurred in the same location as an M4.5 event that occurred on September 29th, and is believed to have been an aftershock to that event.
*On October 12th, two earthquakes of M4.1 and M4.2 occurred within a two-minute period in the Kaetsu region of Niigata Prefecture. These two events occurred within a relatively inactive area; events of M4.0 or greater occur about once every ten to twenty years in the area within about 20 km from the epicenter.
*On October 18th, there was an M4.5 event at a depth of about 10 km in northern Tochigi Prefecture. There was continuing microearthquake activity in the area of the epicenter following the October 18th event, and an M4.0 event occurred on October 19th. The microearthquake activity continued thereafter, but had decreased by the end of October.
*On October 27th, there was a deep M6.3 event at a depth of about 410 km in the sea near Chichijima. It is believed that this event occurred within the Pacific Plate.
*On October 31st, there was an M5.5 event at a depth of about 40 km in central Mie Prefecture, which registered a maximum seismic intensity of 5 lower. The focal mechanism was a reverse fault type with a north-south pressure axis and it is believed that the event occurred within the Philippine Sea Plate subducting northwest from the Pacific side. There was some continued seismic activity following this event around the epicenter, but the activity level gradually decreased and almost no events were recorded after two days. It is believed that the activity took the form of aftershocks to the original event, and there were about 20 aftershocks in the two-day period following the event.
*Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture has continued at lower than average levels since August of last year. However, the results of GPS observation of the area from Kakegawa to Omaezaki in the Tokai region show no changes from established trends. to supplementary reports
On October 6th, there was an M7.3 event (tentative data) at a depth of about 10 km in western Tottori Prefecture, which registered a maximum seismic intensity of 6 upper in western Tottori, and was followed by continued aftershock activity. By the end of October, the aftershock activity had declined to an average incidence of 2-3 M3.0 or greater events per day (see separate document). to supplementary reports
On October 2nd, there was an M5.7 event at a depth of about 10 km in the sea southwest of Toshimamura, Akusekijima, which registered a maximum seismic intensity of 5 upper on Akusekijima. The focal mechanism was a normal fault type with a northwest-southeast tension axis. Approximately 15 minutes prior to the event, an M5.2 event occurred in the same sea zone. Following this event, there was continued seismic activity in the sea southwest of Akusekijima, with more than 200 events recorded in the first half of October, and it is believed that all of these events were aftershocks to the M5.7 event. Since mid-October, there were two events of around M4.0, but aftershock activity has declined. to supplementary reports
| November 8, 2000 |
| Earthquake Research Committee |
There were 90 events of M4.0 or greater in Japan and the surrounding area (or 60 such events, not including the aftershock activity in western Tottori Prefecture). In September, about 40 such events were recorded (the average monthly incidence for the past thirty years has also been around 40). Of this total, there were 12 events of M5.0 or greater (compared with five such events in September). In addition, although the average yearly incidence of M6.0 or greater earthquakes in the 1997-1999 period was 11 (or eight, not including events in the Taiwan region), there have already been 21 such events as of the end of October this year (or 16, not including events in the area from Miyakejima to Niijima-Kozushima). Please note that none of the above figures includes that earthquake that occurred in central Sakhalin on August 5th.
There has been the following major seismic activity in the period from October of last year to October of this year:
| -Central Inland Sea | 30 October, 1999, an M4.5 event (at a depth of about 15 km) |
| -Kumamoto region of Kumamoto Prefecture (at a depth of about 10 km), off the coast of Fukui Prefecture (at a depth of about 15km) | |
| and western Aichi Prefecture (at a depth of about 50 km), in November 1999, all M4.0 or greater earthquakes | |
| -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) |
| -In the area from Miyakejima to Niijima-Kozushima | There was seismic activity associated with magma activity, including events of M6.4 (at a depth of 10 km) on July 1st, and M6.4 (at a depth of 10 km) on July 30th. |
| -The sea off Ibaraki Prefecture | July 21st 2000, M6.0 (at a depth of 50 km) |
| -from the area near Miyakejima to the area near Niijima-Kozushima | |
| In August 2000, there were five M5.0 or greater events (on August 18th, maximum M6.0). | |
| -from the area near Miyakejima to the area near Niijima-Kozushima | |
| September 11th 2000, in the sea west of Toshima, an M5.3 event | |
There are no supplements in the Hokkaido Region.
There are no supplements in the Tohoku Region.
"...there was an M5.5 event at a depth of about 40 km in central Mie Prefecture.... it is believed that the event occurred within the Philippine Sea Plate subducting northwest from the Pacific side....It is believed that the activity took the form of aftershocks to the original event, and there were about 20 aftershocks in the two-day period following the event.":
Compared to shallow earthquakes occurring in inland areas, events occurring within a subducting plate frequently show less continued aftershock activity. This was the case for this event.
In central Mie Prefecture, about 20 km from the epicenter of this event, there was seismic activity which began in January, 1999 and had largely subsided by June, 2000. The regions of occurrence and focal mechanisms for that activity and the current event were different.
"Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture has continued at lower than average levels since August of last year.":
The average monthly incidence for M1.5 and greater events within the Philippine Sea Plate was six in central Shizuoka, but since August of 1999 the frequency of occurrence has been lower than average.
"On October 6th, there was an M7.3 event (tentative data) at a depth of about 10 km in western Tottori Prefecture, which registered a maximum seismic intensity of 6 upper in western Tottori, and was followed by continued aftershock activity."
The moment magnitude (Mw) for this event, reflecting the area of the fault and the slip volume was estimated at 6.6 (Disaster Prevention Research Center, National Geographical Institute) or 6.8 (Meteorological Agency). The M7.2 Southern Hyogo Prefecture Earthquake in 1995 was estimated to have had an Mw of 6.9 (Tokyo University Earthquake Research Center).
There are no supplements in the Kyushu-Okinawa Region.
Corrections:
In the Seismic activity in September 2000 report issued by this committee on October 11th, there was an error in the "Major seismic activity" supplementary section.
Error: (4 to 6 lines from the top of the section) "... as of the end of August this year."
Correct: (4 to 6 lines from the top of the section) "... as of the end of September this year."
| 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. |