| 14 March 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in February 2001
On February 13th, there was an M3.9 earthquake at a depth of about 10 km in the sea near Niijima-Kozushima, registering a maximum seismic intensity of 5 lower. to supplementary reports
On February 26th, there was a deep M6.2 earthquake at a depth of about 420 km in the southern part of the Sea of Okhotsk. This event occurred within the subducting Pacific Plate. to supplementary reports
On February 25th, there was an M5.8 earthquake at a depth of about 20 km in the sea off Fukushima Prefecture, and aftershocks including a maximal M5.4 event occurred in about the same area on the 26th. The focal mechanisms for these events were reverse fault type, with a WNW-ESE pressure axis. Aftershock activity is gradually declining. to supplementary reports
*On February 2nd, there was an M4.2 earthquake at a depth of about 20 km near the border between Kanagawa and Yamanashi Prefectures.
*On February 13th, there was an M3.9 earthquake at a depth of about 10 km in the sea near Niijima-Kozushima (about 5 km west of Shikinejima), which registered a maximum seismic intensity of 5 lower. This earthquake was part of the seismic activity in the area from Miyakejima to Niijima-Kozushima, which is declining, but continues. No marked changes in that seismic activity were observed subsequent to this event. Also, there have been no changes seen in the results of GPS observation of the area, and the activity has not entirely stopped.
*On February 23rd, there was an M4.9 earthquake at a depth of about 40 km in western Shizuoka Prefecture (near Lake Hamanako). The focal mechanism of this event had a tension axis in the same direction as other earthquakes occurring in this vicinity in the past.
*Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which had been at lower-than-average levels since August 1999, but began to show indications of a resumption of activity in October 2000. Since that time and through February as well, activity continues to rise and fall, although it appears to be returning to a regular activity level. Crustal seismic activity in Suruga Bay and the surrounding area is also continuing at a somewhat low level. However, the results of GPS observation and leveling surveys of the Tokaido region show no marked changes in the trends observed to present. to supplementary reports
*The seismic activity that has continued since December 2000 in northern Hyogo Prefecture is moving in a main shock-aftershock pattern, and although there was aftershock activity including two M3.9 events, the activity is gradually declining.
*On February 8th, there was an M4.2 earthquake at a depth of about 15 km in southern Shimane Prefecture.
*On February 11th, there was an M4.3 earthquake at a depth of about 15 km in eastern Tokushima Prefecture. This event was an aftershock to the 2000 Western Tottori Earthquake (M7.3, tentative), which occurred on October 6th, 2000. The aftershock activity to the M7.3 (tentative) event has declined even more since January, as has activity triggered by that event. to supplementary reports
*On February 8th, there was an M5.8 earthquake in the sea near Miyakojima (about 50 km south of Miyakojima). There was some moderate seismic activity in the area prior to and following this event. Seismic activity continues in the area, although the frequency of occurrence has declined. to supplementary reports
| March 14, 2001 |
| Earthquake Research Committee |
There were 45 M4 or greater earthquakes in Japan and the surrounding area (compared with 44 in January, and a monthly average of 46 in the 30 years up to the end of 2000). Of that number, 5 were M5 or greater (compared with 5 in January).
The average annual occurrence of M6 or greater events in the years from 1998-2000 was 16 (the average over the past 30 years was also approximately 16). In February 2001, there was one such event, in the southern part of the Sea of Okhotsk.
There was the following major seismic activity in the period from February 2000 to the end of January 2001:
| -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) | |
| -Chuetsu region of Niigata Prefecture | 1/2/2001 M4.4 (at a depth of about 10 km) | |
| -Chuetsu region of Niigata Prefecture(about 40 km SSE of the 1/2/2001 event) | ||
| 1/4/2001 M5.1 (at a depth of about 10 km) | ||
| -Northern Hyogo Prefecture | 1/12/2001 M5.4 (at a depth of about 10 km) | |
There are no supplements in the Hokkaido Region.
There are no supplements in the Tohoku Region.
"On February 2nd, there was an M4.2 earthquake at a depth of about 20 km near the border between Kanagawa and Yamanashi Prefectures.":
The focal mechanism had a NNW-SSE pressure axis. This area is near the border between the subducting Philippine Sea and Continental Plates. Aftershock activity had ended by the first half of February.
"On February 23rd, there was an M4.9 earthquake at a depth of about 40 km inn western Shizuoka Prefecture (near Lake Hamanako). The focal mechanism of this event had a tension axis in the same direction as other earthquakes occurring in this vicinity in the past.":
This event occurred within the Philippine Sea Plate.
"Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which had been at lower-than-average levels since August 1999, but began to show indications of a resumption of activity in October 2000. Since that time and through February as well, activity continues to rise and fall, although it appears to be returning to a regular activity level. Crustal seismic activity in Suruga Bay and the surrounding area is also continuing at a somewhat low level.":
Although there were three times since August 1999 where the occurrence 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. However, beginning on October 15th, there were three successive 30-day periods in which the occurrence was 9, 10 and 7 such events. There were 5 such events in the 30 days from January 13th to February 11th.
(This coincides with the views (see reference below) expressed at the procedural meeting, on January 26th, of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(reference) Recent earthquakes and crustal movement in the Tokai region and surrounding area (January 26, 2001; Department of Earthquakes and Volcanoes, Meteorological Agency)
"A decrease in activity had been observed in the subducting slab in the Tokai region since the year before last, but it appears that the activity, with some fluctuations, is returning to a regular level. It is also thought that the crustal seismic activity in Suruga Bay and the western coastal area is continuing at a somewhat low level.
On February 23rd, there was an M4.9 earthquake near lake Hamanako, and it is believed that this event occurred within the subducting Philippine Sea Plate. The focal mechanism was of a type frequently seen in the area. Furthermore, there were few aftershocks to the event and no noteworthy changes in crustal movement have been observed in the Tokaido region."
There was the following seismic activity:
*In the low-frequency seismic activity near Mt. Fuji at a depth of about 15 km, which suddenly increased in October 2000, although activity in December was comparatively lower than that in October, the activity level remained somewhat higher than usual. This was also the case in January and February.
"On February 8th, there was an M4.2 earthquake at a depth of about 15 km in southern Tokushima Prefecture.":
The focal mechanism had a WNW-ESE pressure axis.
"On February 8th, there was an M5.8 earthquake in the sea near Miyakojima (about 50 km south of Miyakojima). There was some moderate seismic activity in the area prior to and following this event. Seismic activity continues in the area, although the frequency of occurrence has declined.":
From the 7th to the 9th, there was seismic activity including three M4.5 or greater events (with a total of about 60 events). There was also an M4.7 event in the latter half of February.
In addition, there was the following seismic activity:
*On February 9th, there was an M3.3 earthquake near Beppu-wan.
*Near Yonagunijima (about 50 km northeast of Yonagunijima), on February 21st there was an M4.9 earthquake. On February 14th, there was also an M4.4 earthquake in the sea about 70 km further northeast of the 2/21 event.
| 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. |