| 9 May 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in April 2001
On April 3rd, there was an M5.1 earthquake at a depth of about 35 km in central Shizuoka Prefecture, with a maximum seismic intensity of 5 upper and attendant damage. to supplementary reports
*On April 14th, there was an M5.4 earthquake at a depth of about 50 km in the sea off Kushiro. This earthquake occurred at the edge of the land plate.
*On April 27th, there was an M5.9 earthquake at a depth of about 80 km in the sea southeast of the Nemuro Peninsula. The event occurred within the Pacific Plate, and its focal mechanism was a normal fault type with a NNW-SSE tension axis. 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. This event occurred within the Pacific Plate. to supplementary reports
*On April 10th, there was an M4.5 earthquake at a depth of about 100 km in southern Chiba Prefecture. This event occurred within the Pacific Plate.
*On April 16th, there was an M4.0 earthquake at a depth of about 15 km at the border of Fukui and Shiga Prefectures (in the Reinan region of Fukui Prefecture).
*On April 20th, there was an M4.0 earthquake at a depth of about 60 km in eastern Saitama Prefecture. This event occurred within the Philippine Sea Plate.
*The seismic activity in the area from Miyakejima to Niijima-Kodushima is declining, but continues, and the results of GPS observation of the area indicate that crustal movement has not yet stopped completely.
*On April 3rd, there was an M5.1 earthquake in central Shizuoka Prefecture at a depth of about 35 km. The maximum seismic intensity was 5 upper and there was attendant damage (see the evaluation of the 86th Earthquake Survey Committee). Aftershock activity subsequent to this event has been abating over time.
*There was a period of increased microearthquake activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which is thought to have been triggered by the April 3rd earthquake, but that activity is now declining. Seismic activity in Suruga Bay and the vicinity, however, continues to be at a slightly low level. In addition, the results of GPS observation of the Tokai region show no marked changes from trends seen to date.
*On April 15th, there was a shallow M6.4 earthquake in the sea east of Torishima. The focal mechanism was a normal fault type and the event occurred in conjunction with the subduction of the Philippine Sea Plate. to supplementary reports
*Aftershock activity subsequent to the 2001 Geiyo Earthquake that occurred on March 24th continues to decline over time.
*Aftershock activity subsequent to the 2000 Western Tottori Earthquake that occurred on October 6th 2000 has declined considerably. to supplementary reports
*On April 25th, there was an M5.6 earthquake at a depth of about 40 km in Hyuga Nada. The focal mechanism had a tension axis in approximately an east-west orientation, and the event occurred within the subducting Philippine Sea Plate. to supplementary reports
*On May 8th, there was an M4.5 earthquake at a depth of about 50 km in the South of Tokachi Administrative District.
| May 9, 2001 |
| Earthquake Research Committee |
There were46 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 51 in March and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, 10 events were M5.0 or greater (as compared to six in March).
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 approximately 16 per year). There have been four such events in the period from January to April 2001.
There was the following major seismic activity in the period from April 2000 to the end of March 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, M6.1*) (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 M7.3*) 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 | 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) | ||
| *) see reference 3 | ||
- On April 30, there was an M5.2 earthquake in the sea east of Hokkaido
"On April 3rd, there was an M5.4 earthquake at a depth of about 60 km in the sea east of Aomori Prefecture. This event occurred within the Pacific Plate.":
Aftershock activity subsequent to the event is declining and there have been no marked signs of an increase in activity. In the sea zone extending northeast-southeast from this earthquake, there have been the following M7 class earthquakes in the past: an M7.6 event in 1931, an M7.9 event in 1968, an M7.6*) event in 1994, and an M7.2*) event in 1995. There have been no M5.0 or greater earthquakes within a 25 km radius of the epicenter of this event since 1926.
*) see reference 3
- On April 12th, there was an M5.0 event in the sea off Fukushima Prefecture. On February 25th, there was an M5.8 earthquake 50 km further out to sea.
"There was a period of increased microearthquake activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which is thought to have been triggered by the April 3rd earthquake, but that activity is now declining. Seismic activity in Suruga Bay and the vicinity, however, continues to be at a slightly low level. In addition, the results of GPS observation 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 and 10 such events in the two thirty-day periods that followed. In the most recent period (March 14th to April 12th 2001), there were 10 such events, including the event on April 3rd. The microearthquakes triggered by the April 3rd event were less than M1.5.
(This coincides with the views (see reference below) expressed at the procedural meeting, on April 27th, of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(reference) Recent earthquakes and crustal movement in the Tokai region and surrounding area (April 27, 2001; Department of Earthquakes and Volcanoes, Meteorological Agency)
"In the Tokai region, on April 3rd there was an M5.1 earthquake at a depth of 33 km in central Shizuoka, which registered shocks of maximum seismic intensity 5 upper in Shizuoka Prefecture. This event was the first earthquake of M5 or greater in Shizuoka Prefecture since April 21st 1965 (M6.1). The April 3rd earthquake occurred in the upper section of the subducting slab, and was followed by aftershocks of up to M4.0. Aftershock activity is subsiding normally, and it is not believed that this event is linked to the Tokai earthquakes.
It is acknowledged however that activity within the slab in the aftershock region of this event did experience a transient rise in seismic activity. Activity within the slab has been decreasing since then, but it will be necessary to monitor the activity' for changes for some time. No crustal changes were observed related to the earthquake under discussion in Suruga Bay and the western coast, and it appears that activity continues at a moderately low level.
Volumetric strainmeter measurements of the epicenter of this event recorded strain step, with aftereffects recorded in a single monitoring station, but the changes are believed to be localized to the area of the station and are not thought to be linked to any unusual crustal movement."
- Near Mt Fuji at a depth of 15 km, there has been an increase in low frequency earthquakes since around April 30th (maximum magnitude M2.3)
"Aftershock activity subsequent to the 2000 Western Tottori Earthquake that occurred on October 6th 2000 has declined considerably.":
In the eastern and western zones that experienced seismic activity triggered by the Western Tottori Earthquake, each zone experienced one event of M2.0 or greater in the period.
-On April 3rd, there was an M4.4 earthquake at a depth of about 70 km within the Philippine Sea Plate in Suo Nada.
There are no supplementary notes for the month of April, but there was the following activity outside the period under discussion in May:
- On May 7th, in western Oita Prefecture, there was seismic activity of maximum M3.2.
3 Regarding Magnitudes
On April 23rd 2001, in order to maintain the integrity of its magnitude reporting, the Meteorological Agency reviewed its formulas for determining magnitudes. It was resolved to introduce new formulas for determining magnitude, which will correct some of the results of the changes to the observation network in 1994. For the following 17 earthquakes that occurred since 1994, it is believed that the magnitude determining formula has been reliably recreated, and the Meteorological Agency will correct its magnitude data to reflect the new figures. The Agency also plans to revise magnitude figures based on the new formula for other earthquakes that occurred after 1994.
As the Earthquake Survey Committee intends to continue to use the Meteorological Agency magnitude data in its evaluations in the future, we hereby correct the magnitudes for the following events, in accordance with the revisions described above.
| Date and time of occurrence | Magnitude reported to date | Revised magnitude | |
| 1994/10/04 22:22 EasternOffshore Hokkaido Earthquake | 8.1 | 8.2 | |
| 1994/12/28 21:19 Sanriku Haruka Earthquake | 7.5 | 7.6 | |
| 1995/01/07 07:37 Offshore Iwate Prefecture | 7.1 | 7.2 | |
| 1995/01/17 05:46 Southern Hyogo Prefecture Earthquake | 7.2 | 7.3 | |
| 1996/02/17 00:22 Eastern Offshore Fukushima | 6.5 | 6.8 | |
| 1996/09/11 11:37 Near Choshi | 6.2 | 6.4 | |
| 1996/10/19 23:44 Hyuga Nada | 6.6 | 6.9 | |
| 1996/12/03 07:17 Hyuga Nada | 6.6 | 6.7 | |
| 1997/06/25 18:50 Northern Yamaguchi Prefecture | 6.3 | 6.6 | |
| 1999/01/24 09:37 Sea Near Tanegashima | 6.2 | 6.6 | |
| 2000/01/28 23:21 Southeast Offshore Nemuro Peninsula | 6.8 | 7.0 | |
| 2000/06/03 17:54 Near Choshi (Northeast Chiba Prefecture) | 6.0 | 6.1 | |
| 2000/07/15 10:30 Sea Near Niijima-Kodushima | 6.3 | 6.3(unchanged) | |
| 2000/07/21 03:39 Offshore Ibaraki Prefecture | 6.0 | 6.4 | |
| 2000/07/30 21:25 Sea Near Miyakejima | 6.4 | 6.5 | |
| 2000/10/06 13:30 Western Tottori Prefecture Earthquake | 7.3(tentative) | 7.3 | |
| 2001/03/24 15:27 Geiyo Earthquake | 6.4 | 6.7 |
| 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. |