| 13 June 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in May 2001
There was no marked activity. to supplementary reports
*On May 8th, there was an M4.6 earthquake at a depth of about 55 km in the Tokachi Administrative District. The focal mechanism was a reverse fault type with a northwest-southeast pressure axis. This earthquake occurred near the juncture of the subducting Pacific Plate and the Continental Plate.
*On May 25th, there was a shallow M6.6 earthquake in the sea near Etorofu. It is believed that this earthquake occurred in conjunction with the subduction of the Pacific Plate. to supplementary reports
There was no marked activity. to supplementary reports
*On May 24th, there was an M4.2 earthquake at a depth of about 75 km in southern Ibaraki Prefecture. It is believed that this earthquake occurred in conjunction with the subduction of the Pacific Plate.
*On May 25th, there was an M4.8 earthquake at a depth of about 50 km in northeastern Chiba Prefecture. The focal mechanism was a reverse fault type with an east-west pressure axis. This earthquake occurred near the juncture of the subducting Philippine Sea Plate and the Pacific Plate. An M6.1 earthquake occurred in this area on June 3rd, 2000.
*On May 31st, there was an M4.5 earthquake at a depth of about 55 km in southwestern Ibaraki Prefecture (southern Ibaraki Prefecture). This event occurred within the subducting Philippine Sea Plate.
*In the area from Miyakejima to Niijima-Kozushima, there were seven earthquakes of M3.0 or greater, including an M4.2 earthquake at a depth of about 10 km in the sea east of Kozushima on May 1st. In the period since October 2000, activity in this area from May to the beginning of June has been slightly increased (see supplement). But the results of GPS observation of the area show that, while crustal movement in the area has not yet stopped completely, the trend of changes has become regular and no new changes have been observed in association with the recent activity.
*In central Shizuoka Prefecture, while there was a brief period of increased activity in the area of the aftershock zone subsequent to the earthquake on April 3rd, the activity subsided thereafter and returned to about the normal level by the end of the month;. On May 31st, there was an M4.1 earthquake within the aftershock zone of the April 3rd event, with an accompanying increase in seismic activity. (See reference)
However, crustal seismic activity in Suruga Bay and the surrounding area continues to be at a slightly low level. In addition, the results of GPS observation and leveling surveys of the Tokai region show no marked changes from trends seen to date. to supplementary reports
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
*On June 3rd, there was an M4.6 earthquake at a depth of about 10 km in the sea near Niijima-Kozushima (northern tip of Kozushima), with a transient increase in seismic activity focused in the northern tip of Kozushima.
*The aftershock activity subsequent to the M5.1 earthquake that occurred in central Shizuoka Prefecture on April 3rd diminished over time, and had subsided by the end of the month. But there was an increase in activity in the period from May 31st to June 3rd, with an M4.1 event on May 31st and then an M4.8 event on June 1st. The activity that began on May 31st occurred within the subducting Philippine Sea Plate, near the southeastern border of the active zone of the April 3rd event. For the most part, the focal mechanisms had east-west tension axes, similar to that of the April 3rd earthquake.
The activity that began on May 31st, when considered as occurring within the aftershock region of the main shock on April 3rd, is believed to be triggered seismic activity. The aftershock activity to this event has tended to be considerably greater than the average seen in Japan and the surrounding area in the past.
A review of the data for this region shows that there has been no similar occurrence of multiple consecutive M4 class events in the same location within a short time frame (3 to 4 days) in the past.
No extraordinary crustal movement has been in the surrounding area to present.
| June 13, 2001 |
| Earthquake Research Committee |
There were 56 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 46 in April and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, 2 events were M5.0 or greater (as compared to 10 in April).
The average number of M6.0 or greater earthquakes in the period from 1998 to 2000 was 16 per year (the 30-year average up to 2000 was also approximately 16 per year). There have been five such events in the period from January to May 2001.
There was the following major seismic activity in the period from May 2000 to the end of April 2001:
| -Northeastern Chiba Prefecture | 3 June 2000, M6.1(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 M6.5 event (at a depth of about 10 km) on July 30th. | ||
| -The sea off Ibaraki Prefecture | July 21st 2000, M6.4 (at a depth of 50 km) | |
| -Western Tottori Prefecture(The 2000 Western Tottori Earthquake in western Tottori Prefecture) | ||
| On October 6th, there was an M7.3event (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) | |
| -Chuetsu region of Niigata Prefecture | 2 January 2001 M4.4 (at a depth of about 10 km) | |
| -Chuetsu region of Niigata Prefecture(about 40 km SSE of the 1/2/2001 event) | ||
| 4 January M5.1 (at a depth of about 10 km) | ||
| -Northern Hyogo Prefecture | 12 January 2001 M5.4 (at a depth of about 10 km) | |
| -Near Niijima-Kodushima | On February 13th 2001, an M3.9 earthquake (at a depth of about 10 km) | |
| -Akinada "2001 Geiyo earthquake" | ||
| March 24 2001, M6.7 (at a depth of about 50 km) | ||
| -Central Shizuoka Prefecture | ||
| April 3rd 2001 M5.1 (at a depth of about 35 km) | ||
"On May 25th, there was a shallow M6.6 earthquake in the sea near Etorofu. It is believed that this earthquake occurred in conjunction with the subduction of the Pacific Plate.":
Aftershock activity to this event declined and had subsided by the end of May. In this area, there was the Eastern Offshore Hokkaido Earthquake (M8.2) on October 4th 1994 and an earthquake (M7.2) in the sea near Etorofu on December 4th 1995.
There are no supplements in the Tohoku Region.
"On May 25th, there was an M4.8 earthquake at a depth of about 50 km in northeastern Chiba Prefecture. The focal mechanism was a reverse fault type with an east-west pressure axis. This earthquake occurred near the juncture of the subducting Philippine Sea Plate and the Pacific Plate. An M6.1 earthquake occurred in this area on June 3rd, 2000.":
Following the M6.1 that occurred on June 3rd 2000, the seismic activity in this region, including aftershock activity to that event, has been at a higher level compared to that seen in the past.
"In central Shizuoka Prefecture, while there was a brief period of increased activity in the area of the aftershock zone subsequent to the earthquake on April 3rd, the activity subsided thereafter and returned to about the normal level. On May 31st, there was an M4.1 earthquake within the aftershock zone of the April 3rd event, with an accompanying increase in seismic activity."
Although there were three months since August 1999 where the incidence of M1.5 or greater earthquakes exceeded the previous average of 6 such events per 30 days, at no time were there consecutive months of above-average activity, and activity for the period taken as a whole was below average. However, beginning on October 15th, there were three successive 30-day periods in which the occurrence was 9, 10 and 7 such events. And there were 5, 10 and 10 such events in the three thirty-day periods that followed. In the most recent period (April 13th to May 12th 2001), there were 9 such events.
(Supplement)
"The aftershock activity subsequent to the M5.1 earthquake that occurred in central Shizuoka Prefecture on April 3rd diminished over time, and had subsided by the end of the month ...
The activity that began on May 31st, when considered as occurring within the aftershock region of the main shock on April 3rd, is believed to be triggered seismic activity. The aftershock activity to this event has tended to be considerably greater than the average seen in Japan and the surrounding area in the past.":
The activity that began on May 31st can also be thought of as included within the main shock- aftershock activity of the April 3rd earthquake. In this scenario as well, it can be said that the aftershock activity to this event has tended to be considerably greater than the average seen in Japan and the surrounding area in the past.
- In the period from April 30th to the beginning of May there was an increase in the number of low frequency earthquakes at depths of around 15 km in the area of Mt Fuji.
In addition, although it occurred in June, outside the period under discussion, there was the following activity:
- In the period from June 5th to June 7th, there was seismic activity including a maximal M3.1 earthquake in northern Tochigi Prefecture (area of Nikko-Mt Shirane). On March 31st, there was an M4.9 earthquake about 4 km southeast of this location near the border of Tochigi and Gunma Prefectures.
There was the following seismic activity in the Kinki-Chugoku-Shikoku region:
- Aftershock activity subsequent to the 2000 Western Tottori Earthquake that occurred on October 6th has diminished.
- Aftershock activity subsequent to the 2001 Geiyo Earthquake that occurred on March 24th has diminished.
- In the period from May 29th to the beginning of June, there was seismic activity including a maximal M3.6 earthquake at a depth of about 15 km at the border of Wakayama and Nara Prefectures.
The following earthquakes occurred within the Philippine Sea Plate in the area of the Chugoku and Shikoku regions:
- On May 9th and 12th, there were earthquakes of M4.1 and M4.0 (respectively) both of which occurred at a depth of about 45 km, in the Nan'yo region of Ehime Prefecture.
- On May 16th, there was an M4.2 earthquake at a depth of about 45 km in southern Hiroshima Prefecture.
- On May 27th, there was an M4.0 earthquake at a depth of about 55 km in Aki Nada.
There was the following seismic activity in the Kyushu-Okinawa region:
- In the period from May 7th to May 8th, there was seismic activity including a maximal M3.2 earthquake in northwestern Oita Prefecture.
- On May 7th, there was an M4.8 earthquake at a depth of about 50 km in the sea near the main island of Okinawa.
| 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. |