| 12 July 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan June 2000
There were a number of earthquakes of maximum seismic activity of 5 lower: an M6.0 event at a depth of 50 km in northeastern Chiba Prefecture, an M6.1 event at a depth of less than 20 km in the sea to the west of Ishikawa Prefecture, an M4.8 event at a depth of 10 km in the Kumamoto region of Kumamoto Prefecture, and an M5.2 event at a depth of 10 km in the sea near Kozushima, all of which were accompanied by some damage. All of these events occurred in patterns typical to the regions involved, and are not thought to be directly related to each other. to supplementary reports
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
*On 3 June, there was an M6.0 event at a depth of 50 km in northeastern Chiba Prefecture, with a maximum seismic intensity of 5 lower and attendant damage (see supplement).
*On 5 June, there was an M4.7 event at a depth of 10 km in the Reinan region of Fukui Prefecture. Aftershocks to the event had abated by the middle of June.
*On 7 June, there was an M6.1 event at a depth of less than 20 km in the sea to the west of Ishikawa Prefecture with a maximum seismic intensity of 5 lower and attendant damage (see supplement).
*On 10 June, there were deep events of M6.5 at a depth of about 530 km, and M6.1 at a depth of about 510 km in the sea off Tokaido.
*On 26 June, following the start of volcanic activity on the island of Miyakejima, there has been a high level of seismic activity in a shifting zone in the area from Miyakejima to Niijima-Kozushima.At 12:11 p.m. on 29 June, there was an M5.2 event near Kozushima, with a seismic intensity of 5 lower recorded near the hypocenter and attendant damage. Seismic activity has continued thereafter in a shifting region of activity.
(Activity in July)
Seismic activity has continued in a shifting zone since the beginning of July. At 4:02 p.m. on 1 July, there was an M6.4 event in the sea 5 km east of Kozushima, recording seismic intensities of 6 lower on Kozushima and 5 lower on Niijima, and attendant damage. There was also an M6.1 event at 3:57 a.m. on 9 July about 2 km northeast of the 1 July event, which also registered a seismic intensity of 6 lower on Kozushima, with attendant damage. Since that time, there has been continued activity focused on the area of the hypocenter of the 9 July event (see supplement).
*Seismic activity in the subducting Philippine Sea Plate in central Shizuoka has continued at reduced levels since August of last year. The results of GPS observation of the Tokai region show no changes in the present trend.
*Following a transient reduction in activity, the seismic activity that began in central Mie Prefecture in January of last year resumed a higher degree of activity in February of this year, and the zone of activity has expanded further to the north. Since the beginning of June, the activity has diminished somewhat. to supplementary reports
On 2 June, in northern Wakayama Prefecture, there was an M4.0 event at a depth of 60 km. This event occurred within the subducting Philippine Sea Plate. to supplementary reports
*On 6 June, in the sea near Amami Oshima, there was an M6.1 event. The focal mechanism was a reverse fault type with a northwest-southeast pressure axis. On 15 June, there was also an M5.8 event at a depth of about 10 km approximately 50 km to the east of the June 6th event. Both events occurred in conjunction with the subduction of the Philippine Sea Plate. There were aftershocks to both events, but they had subsided by the end of June.
*On 8 June, there was an M4.8 event at a depth of about 10 km in the Kumamoto region of Kumamoto Prefecture, with a maximum seismic intensity of 5 lower and attendant damage (see supplement).
*On 25 June, there was an M5.9 event at a depth of about 50 km in the sea near Tanegashima (in the sea southeast of the Osumi Peninsula). This event occurred in conjunction with the subduction of the Philippine Sea Plate. Aftershocks to the event are decreasing. The results of GPS observation of the area show no marked changes prior to or following the event. to supplementary reports
*There are no changes to the June 14 evaluation results.
*At 5:54 p.m. on 3 June, there was an M6.0 event at a depth of about 50 km in northeastern Chiba Prefecture, with a seismic intensity of 5 lower recorded near the hypocenter and attendant damage.
*It is believed that this event occurred near the junction of 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 other past events in this area. Subsequent seismic activity has taken the form of aftershocks to the 3 June event, with aftershocks of up to M3.6 having occurred as of 8 July. Aftershock activity has been declining since the end of June.
*The results of GPS observation of the area show no marked changes prior to or following the event.
*Since 1926, M6.0 or higher events have occurred every 10 to 20 years within 40 km of this event's hypocenter, and it should be noted that there is a tendency towards the occurrence of multiple events within a relatively short time frame. Examples showing continuous activity over one year periods include: 1954-5, when there were events on 18 July, 1954 (M6.4) and 24 July, 1955 (M6.0); 1973-4, when there were events on 30 September, 1973 (M5.9), and the next day on 1 October (M5.8), followed by an event on 3 March, 1974 (M6.1); and 1989-90, when there were events on 6 March, 1989 (M6.0) and 1 June, 1990 (M6.0).
*There are no changes to the June 14 evaluation results.
*At approximately 6:16 a.m. on 7 June, there was an M6.1 event at a depth of less than 20 km in the sea to the west of Ishikawa prefecture border, with attendant damage. Seismic intensity of 5 lower associated with this event was recorded in Komatsu City, approximately 100 km southeast of the hypocenter.
*It is believed that this event occurred within the Eurasian Plate, in the sea 80 km northwest of the Ishikawa-Fukui border. The focal mechanism was a reverse fault type with a WNW-ESE pressure axis. Subsequent seismic activity has taken the form of aftershocks to the June 7 event, and is gradually decreasing. Up to 22 June, the maximum aftershock was M4.6, but from that time to the end of June, there were no events of M4.0 or higher.
*The results of GPS observation of the coastal regions around the hypocenter showed no marked changes.
*Since 1926, there have been no M6 class or higher events recorded within 50 km of the hypocenter of this event.
*There are no changes to the June 14 evaluation results.
*At 9:32 a.m. on 8 June, there was an M4.8 event at a depth of about 10 km in the Kumamoto region of Kumamoto Prefecture, with attendant damage. The maximum seismic intensity associated with this event was 5 lower.
*This 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.
The aftershocks to this event were distributed in a 5 km area along the strike of the Hinagu fault. Subsequent seismic activity has taken the form of aftershocks to the June 8 event, and is gradually diminishing. As of 12 July, the maximum aftershock was M3.9 (two such events on 10 June).
*The results of GPS observation of the area around the hypocenter showed no marked changes.
*The most recent occurrences of M4 class or higher earthquakes in the vicinity of this event's hypocenter were an M4.0 event on 31 October, 1999 and an M4.1 event on 10 November of the same year. Both occurred approximately 5 km northeast of the hypocenter of this event. Since 1926, there have been two M4.5 or higher events within 10 km of the hypocenter of this event: an M5.0 event in 1928, and an M5.1 event in 1937.
*Seismic activity in the area from Miyakejima to Niijima-Kozushima began on the night of 26 June in association with the volcanic activity on Miyakejima, and continued after moving west into the sea. On 1 July at 4:02 p.m., there was an M6.4 event with its hypocenter to the east of Kozushima, and on 9 July at 3:57 a.m., there was an M6.1 event with its hypocenter 10 km east of Kozushima. Both of these events recorded seismic intensities of 6 lower on Kozushima. It is believed that both of these events, as with the M5.2 event that occurred on the northern coast of Kozushima on 29 June, were triggered by the effects of the Miyakejima volcanic activity on the earth's crust.
*Compared to the 1962 seismic activity in the area from Miyakejima to Niijima-Kozushima, the current activity has a larger zone of activity and a higher incidence of M4.0 or greater events. The results of GPS observation of the area from Mikurajima to Niijima-Kozushima also show continuing crustal deformations associated with the M6.4 and M6.1 events, or the serial seismic activity. Seismic activity continues to rise and fall at the time of writing -the overall level remains high, with periods of activity and quiet- and it is thought that the present trend will continue for the next one or two weeks.
*The current seismic activity is more intense than that in 1962 in the sea near Miyakejima, and the expanded zone of activity has resulted in events in the vicinities of Kozushima, Miyakejima, Niijima, and Shikinejima, with strong vibrations (5 lower) associated with events of about M5 in such cases. Since 1926, there has been no incidence of three M6.0 or higher events occurring in a cluster, but as the current activity is more intense than that in the past, that possibility cannot be ruled out.
| July 12, 2000 |
| Earthquake Research Committee |
There were 148 M4.0 or higher events recorded in Japan and its surrounding area (compared with 44 in May, and a monthly average of 43 over the past thirty years), and of those, seventeen were M5.0 or higher (compared with four in May). June was more active than average, including seismic activity in northeastern Chiba Prefecture, in the sea to the west of Ishikawa Prefecture (six events of M4.0 or higher), and the activity from Miyakejima to Niijima-Kozushima (eight M5.0 or higher events; 89 M4.0 or higher). In the period from 1997 to 1999, the average annual incidence of M6.0 or greater earthquakes was eleven (or eight if events in the area of Taiwan are excluded), but there have already been nine such events as of the end of June this year.
Among the major seismic events since March of last year were the following. In this group, the zones of activity and areas of events in the Kumamoto region of Kumamoto Prefecture since the beginning of June saw activity including seismic events of up to M4.8.
| -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 | |
There was the following seismic activity in the Hokkaido region:
-There has been no event of M3.0 or higher associated with the volcanic activity of Mt. Usuzan since the M3.0 earthquake on 14 May. The incidence of seismic events has been declining since the beginning of May.
-On 13 June, there was an M4.6 event in the sea off Kushiro.
-On 9 June, there was an M3.8 event in the sea off Matsumae.
There are no supplements in the Tohoku region.
"On 5 June, there was an M4.7 event at a depth of 10 km in the Reinan region of Fukui Prefecture."
The focal mechanism was a strike-slip fault type with a WNW-ESE pressure axis, similar to other earthquakes occurring in this area in the past.
"On 10 June, there were deep events of M6.5 at a depth of about 530 km."
The focal mechanism had a pressure axis in the direction of the subduction of the Pacific Plate.
"Seismic activity in the subducting Philippine Sea Plate in central Shizuoka has continued at reduced levels since August of last year."
The frequency of occurrence of seismic events (M1.5 and higher) within the Pacific Plate in central Shizuoka Prefecture averages about six per month, however in August of 1999 the frequency dropped to a lower-than-average two to four events per month, and continued at this reduced pace through March of this year. In April, there was a moderate increase in activity with seven events, and in May there were four. Since then, there have been six events, and the state of reduced activity continues.
(This corresponds 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 July 3.)
(reference)
There has been no remarkable activity within the crust or subducting slab in the Tokai region, and the area is quiet on the whole. It is acknowledged that there appears to be a trend toward resurgence of seismic activity in Suruga Bay and the coastal region to the west since last month, but it is believed that the overall activity level continues to be low. The area near Miyakejima has been experiencing increased seismic activity associated with the volcanic activity around the island, but it is not believed that this has affected seismic activity or crustal deformation in the Tokai region.
There are no supplements in the Kinki-Chugoku-Shikoku region
There are no supplements in the Kyushu-Okinawa region
There was the following seismic activity in other regions.
-On 11 June, there was an M6.2 event near Taiwan.
| 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. |