| 14 June 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan May 2000
There was no marked activity. to supplementary reports
The seismic activity associated with the volcanic activity at Mt. Usuzan, which began at the end of March, decreased in April and continued to lessen in May. There was an Magnitude(M)3.0 earthquake on 14 May, but there have been no events of M3.0 or greater since then. to supplementary reports
There was no marked activity. to supplementary reports
*On 16 May, there was an M4.6 earthquake at a depth of about 50 km in the Kashima Nada sea. This earthquake occurred in conjunction with the subduction of the Pacific Plate. In this same area, an M4.9 earthquake occurred on 26 March, 1999, and an M5.1 event occurred on 25 April in the same year.
*Seismic activity in the subducting Philippine Sea Plate of central Shizuoka Prefecture has continued to be lower than average levels since August of last year. Results of GPS observation of the Tokai region indicate no changes in the present trend.
*There has been seismic activity in central Mie Prefecture, beginning in January of last year, which has continued with a slight increase in activity and northward expansion of the zone of activity since February of this year. to supplementary reports
There was seismic activity in three locations in the area between southern Kyoto Prefecture and northern Osaka Prefecture: the first was, including a maximum of M4.3 event, at a depth of about 15km and occurred on 16 May, near the border of Kyoto Prefecture and Osaka Prefecture. The second was an M3.7 event at a depth of about 15 km, which occurred on 20 May in northern Osaka Prefecture. The third was, including a maximum M3.8 event, at a depth of about 15 km, which occurred on 21 May near the border of Kyoto and Shiga Prefectures. All the three had abated considerably by the end of May. Seismicities in these area have increased since the 1995 southern Hyogo earthquake to supplementary reports
The seismic activity in central Oita Prefecture (near Beppu Bay), which began on 29 April, had diminished considerably by the second half of May. to supplementary reports
*On 2 June, in northern Wakayama Prefecture, at a depth of about 60 km, there was an M4.0 seismic event.
*On 3 June, in northeastern Chiba Prefecture, at a depth of about 50 km, there was an M6.0 seismic event, with maximal seismic intensity of 5 lower and some damage observed.
*On 5 June, in the Reinan region of Fukui Prefecture, at a depth of about 10 km, there was an M4.7 seismic event.
*On 6 June, off the coast of Amami Oshima Island, there was an M6.1 seismic event.
*On 7 June, off the coast of western Ishikawa Prefecture, at a depth of less than 30 km, there was an M6.1 seismic event, with a maximal seismic intensity of 5 lower and some damage observed.
*On 8 June, in the Kumamoto region of Kumamoto Prefecture, at a depth of about 10 km, there was an M4.8 seismic event with a maximal seismic intensity of 5 lower and some damage observed.
*On 10 June, off the coast of Tokaido, at a depth of about 530 km, there was an M6.5 seismic event.
*On 13 June, off the coast of Kushiro, at a depth of about 60 km, there was an M5.0 (tentative) seismic event. to supplementary reports
*At 5:54 p.m. on 3 June, there was an M6.0 seismic event at a depth of about 50 km in northeastern Chiba Prefecture, and some attendant damage. Seismic intensity of 5 lower was recorded near the epicenter.
*It is thought that this earthquake occurred near the border between the subducting Philippine Sea Plate and the Pacific Plate. The focal mechanism was a reverse fault type with an east-west pressure axis, similar to these of events that have occurred in this region in the past. The subsequent seismic activity is believed to be aftershock activity, and is gradually attenuating. The aftershock activity accompanying this event is slightly less than the average level for Japan and the surrounding region.
*The results of GPS observation did not show any marked changes before and after the event.
*An analysis of the occurrence of M6 class and greater earthquakes within a 40 km area of the epicenter of this earthquake since 1926 shows that such earthquakes occur every ten to twenty years, and tend to occur two or more times within a relatively short period. The most recent M6 class earthquake occurred on 1 June 1990.
*At 6:16 a.m. on 7 June, there was an M6.1 event at a depth of less than 30 km off the west coast of Ishikawa Prefecture, with some attendant damage. Seismic intensity of 5 lower was recorded in Komatsu City, about 100 km southeast of the epicenter.
*It is thought that this event occurred within the Eurasian Plate about 80 km northwest of the border of Ishikawa and Fukui Prefectures. The focal mechanism was a reverse fault type, with a WNW-ESE pressure axis. The subsequent seismic activity is believed to be aftershock activity, and is gradually attenuating. The activity level and attenuation pattern of the aftershock activity are about average compared with past earthquakes in Japan and the surrounding region, and it is believed that aftershocks will continue to decrease.
*The results of GPS observation of the coast surrounding the epicenter showed no marked changes.
*No M6 class events have been recorded within 50 km of the epicenter of this event since 1926.
*This earthquake occurred near an upheaval zone stretching northeast-southwest on the ocean floor. The results of sonic profiling show active fault distribution along the upheaval zone, but it is not thought to be very active.
*At 9:32 a.m. on 8 June, an M4.8 earthquake occurred at a depth of about 10 km in the Kumamoto Region of Kumamoto Prefecture and caused damage. Seismic intensity of 5 lower was recorded in the vicinity of the epicenter.
*The earthquake occurred in the northern part of the Hinagu fault. The focal mechanism was a right lateral strike-slip fault type, with a northwest-southeast tension axis. As of 14 June, the subsequent seismic activity is believed to be aftershock activity, and is gradually attenuating. The aftershock activity has been distributed in a 5 km area along the Hinagu fault. The aftershock activity has been more active than the average level of past earthquakes in Japan and the surrounding area. That is, occurrence rate of larger aftershocks is higher than the average. The probability of an M4.5 or greater aftershock occurring within the next three days from 14 June 2000 was estimated as about 20%. If such an aftershock does occur, there is a possibility that seismic intensity in the vicinity of the hypocenter could be about 4.
*The results of local GPS observation showed no marked changes.
*The most recent M4 class or greater events recorded in the vicinity of this epicenter were an M4.0 event on 31 October, 1999, and an M4.1 event on 10 November of the same year. Both occurred about 5 km northeast of this epicenter. Since 1926 there have been the following M4.5 or greater events within an area of 10 km of this epicenter: an M5.0 event in 1928, an M5.1 event in 1937, and an M4.5 event in 1970.
| June 14, 2000 |
| Earthquake Research Committee |
"There was no marked activity."
Japan and the surrounding region recorded 44 M4.0 or greater seismic events in May. (There were 50 in April. The monthly average for the past thirty years as of the end of last year was 43.) Of that number, four were M5.0 or greater (compared to six in April). From this point of view, seismic activity in May is an average. Additionally, although the yearly average of M6.0 or greater events in the 1997-1999 period was about 11 (about 8 if events in the Taiwan area are not included), this year there had been only three such events as of the end of May.
The following is a list of significant seismic activity since March of last year, up to the end of May. No increase in seismic activity had been noted in the zones of activity associated with these events.
| -The sea near Niijima-Kozushima | 14 March 1999, an M4.7 event (at a depth of less than 10 km) |
| 28 March 1999, an M5.0 event (at a depth of less than 20 km) | |
| -Southern central Kushiro administrative district | 13 May 1999, a intermediate-depth M6.4 event(at a depth of about 100 km) |
| -Northern Wakayama Prefecture | 21 August 1999, an M5.4 event (at a depth of about 70 km) |
| -Taiwan | 21 September, 1999, an M7.7 event (source:US Geological Survey) |
| -Central Inland Sea | 30 October 1999, an M4.5 event (at a depth of about 15 km) |
| -Kumamoto region of Kumamoto Prefecture (at a depth of about 10 km), off the coast of Fukui Prefecture (at a depth of about 15km) | |
| and western Aichi Prefecture (at a depth of about 50 km), in November 1999, M>4.0 events | |
| -Off the eastern coast of Hokkaido | 28 January 2000, an M6.8 event (at a depth of about 60 km) |
| -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), and other seismic activity associated with the volcanic activity | |
"The seismic activity associated with the volcanic activity at Mt. Usuzan decreased in April and continued to lessen in May."
In April, there were five M3.0 or greater earthquakes associated with the volcanic activity of Mt. Usuzan, and in May there was one (an M3.0 event on 14 May).
There was the following additional activity in the Hokkaido region:
-On 15 May, in the Soya Kaikyo Straits, at a depth of about 10 km, there was seismic activity with a maximal magnitude of M4.3.
There was the following seismic activity in the Tohoku region:
-On 2 May, two M4.8 earthquakes occurred in approximately the same area as the M5 class earthquake that occurred off the shore of the Sanriku region on 26 April. Also, on 24 May, there was an M4.5 event 150 km northeast of this seismic activity (in approximately the same area as the strongest aftershock of the 1968 earthquake off the shore of Tokachi).
"M4.6 earthquake at a depth of about 50 km in the Kashima Nada sea"
In the northern and coastal regions (Kashima Nada sea) of Ibaraki Prefecture, there has been seismic activity constituting a single group at a depth of 50 to 60 km. This earthquake is included in that group. On 8 May, an M4.4 event occurred further offshore, at a depth of about 50 km.
"Seismic activity in the subducting Philippine Sea Plate of central Shizuoka Prefecture has continued to be lower than average levels since
August of last year."
There are an average of six M1.5 or greater earthquakes in the Philippine Sea Plate in central Shizuoka Prefecture per month, but beginning in August of 1999, the frequency dropped to 2 to 4 events per month, and this continued until March of this year. In April, there was a moderate
increase, with seven such events, but the activity continues to be lower than average frequency, with four events in May, and six thereafter.
(This coincides with the views (see reference below) expressed at the procedural meeting, on 29 May, of the Assessment Committee for Area Under Intensified Measures Against Earthquake Disaster.)
(Reference) Recent seismic and crustal activity in the Tokai (Shizuoka-Aichi-Gifu) area and vicinity (Meteorological Agency Earthquake and Volcano Office, 29 May 2000)
"There was no marked seismic activity in the crust and subducting slab throughout the Tokai region, and the area remains quiet. Recently, there has been a trend towards a slight return of activity, but it seems the overall activity level continues to be low in Suruga Bay and the western coastal region. No significant changes were observed in the surrounding area such as the Izu Peninsula."
"Seismic activity in central Mie Prefecture has continued with a slight increase in activity and northward expansion of the zone of activity since February of this year."
Since February 2000, 60 to 100 events have been recorded per month, but no M>3.0 events occurred in the period between 26 March (M3.1) and 11 June (M3.3).
"There was seismic activity in three locations in the area between southern Kyoto Prefecture and northern Osaka Prefecture."
In February and March of 1999, there was seismic activity of maximal magnitude M4.0 in an area 10 km east-northeast of the event that occurred at the Kyoto-Osaka Prefecture border. Also, in March of 1999, there was seismic activity of maximum magnitude M4.9 in an area 30 km north-northeast of the earthquake that occurred at the border of Kyoto and Shiga Prefectures.
Since the southern Hyogo earthquake in 1995, there has been increased seismic activity in the area from the aftershock region of the Hyogo event to Lake Biwa, which continues to the present.
Additionally, there was the following activity in the Kinki-Chugoku-Shikoku region:
-On 4 May, in the Iyo Nada sea, there was an M4.0 event, at a depth of about 60 km.
"The seismic activity in central Oita Prefecture (near Beppu Bay), which began on 29 April, had diminished considerably by the second half of May."
Seismic events occurred concentratedly within a two-day period from 29 April, and a slight transient increase in activity on 5 May. Since the second half of May, only one M2.1 event, and seven M<2.0 events have been recorded. A total of 800 events, including M<2.0 events, were recorded in the period from 29 April to the end of May.
Additionally, there was the following seismic activity in the Kyushu-Okinawa region:
-On 19 and 23 May, in the Hyuga Nada sea, at a depth of about 20 km, there were M4 class events (both M4.4)
-On 23 May, in the East China Sea, at a depth of about 220 km, there was an M5.1 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.