| 11 April 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in March 2001
On March 24th, there was an M6.4 earthquake (named the 2001 Geiyo Earthquake) at a depth of about 50 km in Aki Nada, which registered a maximum seismic intensity of 6 lower. There was damage attendant to this event, including two deaths. On March 26th, there was an M5.0 event in approximately the same location, which registered a maximum seismic intensity of 5 upper. to supplementary reports
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
*On March 31st, there was an M4.9 earthquake at a depth of about 10 km at the border of Tochigi and Gunma Prefectures. The focal mechanism was a strike-slip type with a northwest-southeast pressure axis. Aftershock activity is declining, but included an M3.9 event on April 1st.
*The seismic activity in the area from Miyakejima to Niijima-Kodushima is declining, but continues. Results of GPS observation of the area also indicate that crustal movement is diminishing, but has not yet completely ceased.
*Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which had been at a low level since August of 1999, has showed signs of a return to normal activity since October 2000. Since that time, there have been some fluctuations in the activity, but it appears to be returning to its regular average level. Crustal seismic activity in Suruga Bay and the surrounding area also continues at a slightly low level. However, the results of GPS observation of the Tokai Region show no marked changes in trends seen to date. to supplementary reports
*On March 24th, the 2001 Geiyo Earthquake occurred, registering a maximum seismic intensity of 6 lower. Then on March 26th, the largest aftershock to date, an M5.0 event with a maximum seismic intensity of 5 upper, occurred in approximately the same location as the epicenter of the main shock (see reports of the 84th and 85th earthquake evaluation committee). Aftershock activity has continued to diminish since that time.
*The seismic activity that has been continuing in northern Hyogo Prefecture (near the border with Tottori Prefecture) since December 2000 is declining, although there was some aftershock activity, including an M3.1 earthquake.
*Aftershock and triggered seismic activity subsequent to the 2000 Western Tottori Earthquake (M7.3, tentative) that occurred on October 6th 2000 has declined. to supplementary reports
There was no marked activity. to supplementary reports
*On April 3rd, there was an M5.4 earthquake at a depth of about 60 km in the sea east of Aomori Prefecture.
*On April 3rd, there was an M5.1 earthquake at a depth of about 35 km in central Shizuoka Prefecture, which registered a maximum seismic intensity of 5 upper and had attendant damage. The area remained active for about one hour, but activity decreased thereafter and has followed the pattern of main shock-aftershock subsequent to the initial M5.1 event.
The focal mechanism had an east-west tension axis, and is believed to have occurred within the subducting Philippine Sea Plate. No marked changes were seen in GPS measurements of the area prior to or following this event.
*On April 10th, there was an M4.5 earthquake at a depth of about 100 km in southern Chiba Prefecture. to supplementary reports
| April 11, 2001 |
| Earthquake Research Committee |
There were 51 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 45 in February and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, six events were M5.0 or greater (as compared to five in February).
The average number of M6.0 or greater earthquakes in the 1998-2000 period was 16 per year (the 30-year average up to 2000 was also 16 per year). There have been three such events in the period from January to March 2001.
There was the following major seismic activity in the period from March 2000 to the end of February 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(The 2000 Western Tottori Earthquake in 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) | |
| -Near Niijima-Kodushima | On February 13th 2001, an M3.9 earthquake (at a depth of about 10 km) | |
-On March 23rd, there was an M5.8 earthquake in the sea east of Hokkaido. This event occurred in the northeastern tip of the aftershock region of the 1994 eastern Offshore Hokkaido Earthquake. The focal mechanism was a reverse fault type with a northwest-southeast pressure axis.
-On March 25th, there was an M5.2 earthquake in the sea off Sanriku.
-On February 25th, there was an M5.8 earthquake in the sea off Fukushima Prefecture, accompanied by aftershock activity, which included a maximal M4.0 aftershock on March 18th, but is gradually declining.
"Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which had been at a low level since August of 1999, has showed signs of a return to normal activity since October 2000. Since that time, there have been some fluctuations in the activity, but it appears to be returning to its regular average level. Crustal seismic activity in Suruga Bay and the surrounding area also continues at a slightly low level. However, the results of GPS observation of the Tokai Region show no marked changes in trends seen to date":
Although there were three times 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 and 10 such events in the two thirty-day periods since that time.
(This coincides with the views (see reference below) expressed at the procedural meeting, on January 29th, of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(reference) Recent earthquakes and crustal movement in the Tokai region and surrounding area (March 29, 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 its 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.
Moreover, no marked changes in crustal movement in the Tokai region have been observed."
"*On March 24th, the 2001 Geiyo Earthquake occurred, registering a maximum seismic intensity of 6 lower.":
Aftershocks have occurred in an area extending a little more than 20 km southward from initial fracture point of the main shock. The strongest aftershock to date was an M5.0 event on March 26th, which occurred in the northern part of the aftershock zone. Currently, the southern part of the aftershock zone is more active than the northern part.
-On February 8th, there was seismic activity including a maximal M5.8 event in the sea near Miyakojima, but frequency has been decreasing since the beginning of March.
"*On April 3rd, there was an M5.1 earthquake at a depth of about 35 km in central Shizuoka Prefecture, which registered a maximum seismic intensity of 5 upper and had attendant damage.":
A review of M6.0 or greater earthquakes since 1900 shows an M6.1 event in 1917, an M6.4 event in 1935 and an M6.1 event in 1965, all of which occurred within 30 km of the epicenter of this event. There have been no M6.0 or greater events in the area since 1965.
"The focal mechanism had an east-west tension axis, and is believed to have occurred within the subducting Philippine Sea Plate.":
The fracture was average for a normal type fault.
(Reference) The Meteorological Agency released the following report on "Information on earthquakes and crustal movement in the Tokai Region (Explanatory Information)" on April 4th at 2:00.
"On April 3rd at 11:57 p.m., there was an M5.1 earthquake at a depth of 33 km (tentative data) in central Shizuoka Prefecture. This event was focused in the Tokai region, registering a seismic intensity of 5 upper in the city of Shizuoka, 5 lower in the city of Shimada and the towns of Okabe-cho and Kawane-cho, and seismic intensities of 1 or greater throughout the Kanto and Kinki regions.
There have been three aftershocks registering seismic intensities of 1 or greater as of the present time of 2:00 a.m., and the aftershocks are declining in a normal fashion. The focal mechanism of the earthquake is believed to be a normal fault type occurring within the subducting Philippine Sea Plate. On October 5th 1996, there was an M4.3 earthquake near Kawane-cho, which had a similar focal mechanism. Step changes were recorded in the crustal movement at the time of the earthquake, but have not been observed since that time.
For the above reasons, it is not believed that this event was linked to the Tokai earthquake."
In addition, there was the following seismic activity:
-On April 3rd, there was an M4.4 earthquake at a depth of about 70 km within the Philippine Sea Plate in Suo Nada.
| 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. |