| 9 August 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in Japan July 2000
The seismic activity in the area from Miyakejima to Niijima-Kozushima, which began on the night of June 26th accompanying the volcanic eruption on Miyakejima, has shifted its active zones and continues to rise and fall. This seismic activity has included four events of M6.0 or greater, all of which registered maximum seismic intensities of 6 lower. There has also been damage, including fatalities, attendant to this cluster of seismic activity.
On July 21st, there was an M6.0 event at a depth of about 50 km, with a maximum seismic intensity of 5 lower and attendant damage, in the sea off Ibaraki Prefecture. to supplementary reports
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
*On July 21st, there was an M6.0 event at a depth of about 50 km, with a maximum seismic intensity of 5 lower and attendant damage, in the sea off Ibaraki Prefecture. There are no changes to the evaluation of this event made on July 21st.
It is believed that this event occurred near the juncture of the Continental Plate and the subducting Pacific Plate. The focal mechanism was a reverse fault type with a northwest-southeast pressure axis, which is similar to that of other events occurring in this area in the past. The results of GPS observation of the area prior to and following the event showed no marked changes. Aftershocks have been diminishing in frequency since the event.
*The seismic activity in the area from Miyakejima to Niijima-Kozushima, which began at the end of June, has continued in a shifting zone of activity. The results of GPS observation of the area show continuing crustal deformation.
There are now three seismically active zones: the area from the sea west of Miyakejima to the sea east of Niijima-Kozushima, the area from the sea west of Toshima to the sea near Kozushima, and the area from the sea southwest of Miyakejima to the sea southwest of Mikurajima. There were a total of four M6.0 or greater events in July, on the 1st, 9th, 15th, and 30th, all of which registered maximum seismic intensities of 6 lower. In addition, there were five events registering maximum seismic intensities of 5 upper, and seven events registering maximum seismic intensities of 5 lower (see supplement).
*Seismic activity in the subducting Philippine Sea Plate in central Shizuoka Prefecture has continued at lower than average levels since August of last year. The results of GPS observation and leveling surveys of the Tokai region do not show deviations from the trends observed to date. to supplementary reports
*On July 17th, there was an M4.3 event at a depth of 15 km in western Tottori Prefecture. The seismic activity is decreasing. There was seismic activity in this area of maximum M5.1 in this area on September 4th, 1997.
*On July 23rd, there was an M4.0 event at a depth of about 10 km in southern Tokushima Prefecture. The seismic activity is now decreasing.This area experienced seismic activity of maximum M6.4 on July 27th 1955. to supplementary reports
There was no marked activity. to supplementary reports
*On August 3rd, there was an M5.1 event at a depth of about 40 km in the sea near Tanegashima Island (in the sea southeast of the Osumi Peninsula).
*On August 5th, there was an M7.0 event in central Sakhalin.
*On August 6th, there was a deep M7.3 event at a depth of about 430 km in the sea near Torishima. to supplementary reports
*As of August 2nd, the seismic activity in the area from Miyakejima to Niijima-Kozushima continued to rise and fall. In particular, during the period from August 3rd to August 5th, there was increased seismic activity in the sea east of Kozushima including three events of M5.0 or greater, with two events registering maximum seismic intensities of 5 upper, and six events registering maximum seismic intensities of 5 lower. To the present date of August 9th, the seismic activity has been divided into three zones: the area from the sea west of Miyakejima to the sea east of Niijima-Kozushima (Sea Zone 1), the area from the sea west of Toshima to the vicinity of Kozushima (Sea Zone 2), and the area from the sea southwest of Miyakejima to the sea southwest of Mikurajima (Sea Zone 3).
*The results of GPS observation of the area showed that crustal movements in the area from Niijima, Kozushima, Toshima and the Izu Oshima to the southern tip of the Boso Peninsula have continued since August 2nd. In particular, during the period from August 3rd to August 5th, crustal movements intensified in the area of Niijima, Shikinejima, and Kozushima before returning to the movement levels seen in mid-July.
*The characteristics of the focal mechanisms of individual events, the distribution of events within the zone of activity, and the relationships between crustal deformation and seismic activity in Sea Zone 1 are still believed to be associated with dike magma activity beneath the sea floor in the area east of Kozushima, as was stated in the August 2nd report. Similarly, the seismic activity in Sea Zones 2 and 3 is also believed to be triggered largely by the magma and seismic activity in Sea Zone 1.
*The average daily incidence of M3.5 or greater events in Sea Zone 1 from June 26th to July 13th was 60 events, but this dropped to around 30 events per day in the period from July14th to August 8th. Also, the crustal movements in the area of Niijima, Shikinejima, and Kozushima which suggest magma activity to the east of Kozushima temporarily intensified during the period from August 3rd to August 5th, and have continued without major deviations from the trend since then.
*Considering the continued crustal movements to present, the possibility of the occurrence of relatively strong seismic activity concentrated in a brief time span as with the period from August 3rd to August 5th cannot be ruled out. Similarly, should another event of similar magnitude (maximum near M6.0) occur, depending on the location, the possibility of seismic intensities of 6 lower cannot be ruled out at present.
*Taking all the above factors into consideration, we would like to re-evaluate the situation in one or two weeks following observation of the movement of crustal deformations and seismic activity.
| August 9, 2000 |
| Earthquake Research Committee |
There were 431 seismic events of M4.0 or greater (compared to 148 in June), or 63 such events excluding earthquakes occurring in the area from Miyakejima to Niijima-Kozushima (as of the end of last year, the monthly average for the past thirty years was 43 such events). Of that number, there were 39 M5.0 or greater events (compared to 17 in June), or 11 such events excluding earthquakes occurring in the area from Miyakejima to Niijima-Kozushima. In July there were a large number of earthquakes in the area from Miyakejima to Niijima-Kozushima, but even when those events are not considered, the incidence for the rest of the region was higher than average. For events of M6.0 and greater, the yearly average for the area in the period from 1997 to 1999 was eleven events (or eight, if events in the Taiwan region are excluded), but there have already been thirteen such events as of the end of July this year (there were four M6.0 or greater events in the area from Miyakejima to Niijima-Kozushima).
There were the following major events in the period from March of last year to June of this year:
| -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 relatively deep 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, all M4.0 or greater earthquakes | |
| -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), 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) |
There was the following seismic activity in the Hokkaido region:
-On July 10th, there was an M5.7 deep event in the southern Sea of Okhotsk
There was the following seismic activity in the Tohoku region:
-On July 8th there was an M4.8 event on the outer edge of the trench axis off the Sanriku coast, on July 20th there was an M5.2 event off the Sanriku coast (in the sea off Miyagi Prefecture), and on July 31st there was an M5.2 event off the Sanriku coast (in the sea off Iwate Prefecture).
-On July 1st there was an M5.1 event off the coast of Fukushima Prefecture.
"Seismic activity in the subducting Philippine Sea Plate in central Shizuoka Prefecture has continued at lower than average levels since August of last year."
The average incidence of earthquakes (M1.5 and greater) in the subducting Philippine Sea Plate in central Shizuoka Prefecture was six events per month, but since August of 1999 the incidence dropped to from two to four events per month, and activity has continued at this reduced level since that time.
((This coincides with the views (see reference below) expressed at the procedural meeting of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters, which were published on August 1st.)
(reference)Recent seismic activity and crustal movement in the Tokai region and its surrounding area (August 1st, 2000, Meteorological Agency, Department of Seismology and Volcanology)
"The Tokai region continues to be quiet and there has been no remarkable seismic activity within the crust or subducting slab. It is acknowledged that seismic activity in the Suruga Bay and the western coastal region is on a recovery trend. However, the activity level of the area as a whole continues to be low. There has been a small amount of volumetric strain observed in the Tokai region in association with the volcanic eruption on Miyakejima and seismic activity in the Niijima-Kozushima area, but at present it is not believed that that activity is having effects on seismic activity or crustal movement in the Tokai region."
-Following the June 3rd event (M6.0) in northeastern Chiba Prefecture, seismic activity took the form of aftershocks to that event, and on July 19th the largest aftershock to date occurred at M4.2. The aftershocks are gradually declining.
-Following the June 7th event (M6.1) in the sea off western Ishikawa Prefecture, seismic activity took the form of aftershocks, which are gradually declining.
There are no supplementary items for the Kinki-Chugoku-Shikoku region.
-Following the June 8th event (M4.8) in the Kumamoto region of Kumamoto Prefecture, seismic activity took the form of aftershocks, which are gradually declining.
| 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. |