| 11 July 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in June 2001
There was no marked activity. to supplementary reports
On June 21st, there was an M4.4 earthquake at a depth of approximately 35 km in the Hidaka Administrative District. The focal mechanism had a northeast-southwest pressure axis. to supplementary reports
There was no marked activity. to supplementary reports
*On June 25th, there was an M4.0 earthquake at a depth of approximately 30 km near the border of Tokyo and Kanagawa Prefecture (eastern Kanagawa Prefecture). The focal mechanism was a reverse fault type with an east-west pressure axis, and it is believed the earthquake occurred within the crust.
*In the region from Miyakejima to Niijima-Kozushima, on June 3rd, there was an M4.6 earthquake at a depth of approximately 10 km in the sea near Niijima-Kozushima, and the area around the northern part of Kozushima experienced a transient increase in activity. However, although the results of recent GPS observation of the area indicate that crustal movement has not yet entirely ceased, the trend of changes has become regular, and no new changes have been observed following this latest activity.
*The seismic activity in central Shizuoka Prefecture that experienced an increased level of activity from May 31st to early June became quiet again by the end of June. Moreover, no marked aberrances in crustal movement have been observed in the surrounding area to present.
While it is acknowledged that seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture experienced a transient increase associated with the May 31st earthquake, activity subsequently decreased and has now returned to about normal levels.
Crustal seismic activity in Suruga Bay and the surrounding area had been at a slightly low level, but has showed recent signs of a return to normal activity. 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 14th, there was a shallow M6.2 earthquake in the sea near Taiwan.
On June 29th, seismic activity including earthquakes of M3.0 and greater began in the sea in an area 30 to 60 south of Aogashima island in Tokyo metropolitan. This seismic activity experienced a transient increase in intensity from the night of July 4th to July 5th, during which time there were four earthquakes of M5.0 or greater. The activity has since subsided. The epicenters for all of the events in this activity were shallow, and the focal mechanisms for the M5.0 and greater events were all normal fault types with east-west tension axes. An examination of seismic activity in this area since October 1997 shows that activity including an event of M3.0 class has tended to occur at the rate of about once a year.
| July 11, 2001 |
| Earthquake Research Committee |
There were 64 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 56 in May and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, 9 events were M5.0 or greater (as compared to 2 in May).
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 six such events in the period from January to June 2001.
There was the following major seismic activity in the period from June 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 June 21st, there was an M4.4 earthquake at a depth of approximately 35 km in the Hidaka Administrative District.":
There were two earthquakes of around M4.0 prior to this earthquake (6/21).
There was the following additional seismic activity in the Hokkaido region:
Microearthquake activity including a maximal event of M2.6 (on June 21) has been continuing in the vicinity of Yoichidake since April.
There are no supplements in the Tohoku Region.
"....
While it is acknowledged that seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture experienced a transient increase associated with the May 31st earthquake, activity subsequently decreased and has now returned to about normal levels.
Crustal seismic activity in Suruga Bay and the surrounding area had been at a slightly low level, but has showed recent signs of a return to normal activity. In addition, the results of GPS observation and leveling surveys of the Tokai region show no marked changes from trends seen to date.":
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, 10 and 9 such events in the four thirty-day periods that followed. In the most recent period (May 13th to June 11th 2001), there were 9 such events.
(This coincides with the views (see reference below) expressed at the June 25th procedural meeting of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(reference) Recent earthquakes and crustal movement in the Tokai region and surrounding area (July 25, 2001; Department of Earthquakes and Volcanoes, Meteorological Agency)
"In the period from the 31st of last month to the beginning of this month, an increase in seismic activity within the slab in central Shizuoka Prefecture, beginning with an M4.8 earthquake on June 1st and including four earthquakes of M4.0 or greater, was observed. This activity occurred at the southern end of the aftershock region for the earthquake (M5.1) that occurred in central Shizuoka Prefecture on April 3rd, and is believed to be linked to the April 3rd event. This activity declined afterwards, and has now returned to its normal level.
Seismic activity in Suruga Bay and the western coast had been at a moderately low level, but we recognize signs of a resumption of activity.
Additionally, no noteworthy changes have been observed in crustal movement in the Tokai region."
There was the following additional seismic activity in the Kanto-Chubu region:
- From June 5th to 7th, there was seismic activity including a maximal M3.1 earthquake in northern Tochigi Prefecture (near Nikko Shirane-san), but the activity had generally subsided by mid-month. This area is about 4 km southeast of the site of the M4.9 earthquake that occurred on March 31st at the border of Tochigi and Gunma Prefectures.
- On June 10th, there was an M3.4 earthquake in western Nagano Prefecture. This area is within the aftershock region of the 1984 Western Nagano Earthquake.
- There was an M2.2 earthquake near Hakone in late June.
There was the following seismic activity in the Kinki-Chugoku-Shikoku region:
- Microearthquake activity at a depth of 10 to 15 km near the border of Wakayama and Nara Prefectures began on May 29th. Then, beginning in late June, new microearthquake activity began in an area 2 to 3 km to the north. The strongest event in this activity to date was an M3.6 earthquake that occurred on May 29th.
- Aftershocks to the 2000 Western Tottori Prefecture Earthquake that occurred on October 6th 2000 have subsided.
- Aftershocks to the 2001 Geiyo Earthquake that occurred on March 24th 2001 have subsided.
- The area of the M5.4 earthquake that occurred on January 12th in northern Hyogo Prefecture experienced a transient increase in microearthquake activity from June 27th to 29th.
- On June 13th, there was an M4.1 earthquake at a depth of about 40 km in Bungo Suido.
There was the following seismic activity in the Kyushu-Okinawa region:
- From June 7th to late June, there was seismic activity including a maximal M3.5 earthquake in southern Nagasaki Prefecture.
- On June 13th, there was an M4.5 earthquake on the main island of Okinawa.
- On June 13th, there was a moderately deep M5.5 earthquake between Yonagunijima and Taiwan.
- On June 14, there was an M5.2 earthquake in the sea near Yonagunijima.
- On June 17th, there was a moderately deep M5.0 earthquake at a depth of about 140 km in the sea near Ishigakijima. There was an M5.2 earthquake in this vicinity on February 25th 2000.
| 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. |