| 12 September 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in August 2001
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
On August 14th, there was an M6.2 earthquake at a depth of approximately 40 km in the sea off Aomori Prefecture. Then, on the 24th, there was an M5.2 earthquake at a depth of approximately 40 km in about the same location. The focal mechanism of both events was a reverse fault type on a WNW-ESE pressure axis. These events occurred in conjunction with the subduction of the Pacific Plate. The M5.2 earthquake was an aftershock to the M6.2 event. to supplementary reports
*Seismic activity in the area from Miyakejima to Niijima-Kodushima is declining, but continues. The results of GPS observation of the area also show that the recent crustal movements have not yet entirely stopped.
*Crustal seismic activity in Suruga Bay and the surrounding area had been at a slightly low level, but now appears to be returning to normal activity.
The slight changes in activity centered in western Shizuoka Prefecture, which were seen in the results of GPS observation of the Tokai region, have continued since that time. to supplementary reports
On August 25th, there was an M5.1 earthquake at a depth of approximately 10 km in southern Kyoto. The focal mechanism was a reverse fault type on an east-west pressure axis. Subsequent seismic activity has followed a main shock-aftershock pattern, with the M5.1 event as the main shock. to supplementary reports
On August 18th, there was a shallow M6.3 earthquake in the sea near the main island of Okinawa (about 80 km southwest of Kumejima). Then, on the 20th, there was an M5.7 earthquake in approximately the same location. The focal mechanism of both events was a normal fault type on a northeast-southwest tension axis. Subsequent seismic activity has followed a main shock-aftershock pattern, with the M6.3 event as the main shock. to supplementary reports
On September 4th, there was an M5.2 earthquake at a depth of approximately 50 km in the sea off Ibaraki Prefecture. to supplementary reports
| September 12, 2001 |
| Earthquake Research Committee |
There were 46 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 47 in July and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, 5 events were M5.0 or greater (as compared to 6 in July).
The average number of M6.0 or greater earthquakes in the period from 1998 to 2000 was 16 per year (the 30-year average up to 2000 was also approximately 16 per year). There were 2 such events in August, and there have been eight such events in the period from January 2001 to the present.
The following are evaluations of major seismic activity in the period from August 2000 to the end of July 2001.
| -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 M7.3event (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) | ||
| -Central Shizuoka Prefecture | ||
| April 3rd 2001 M5.1 (at a depth of about 35 km) | ||
There was the following seismic activity in the Hokkaido region:
- The continuing microearthquake activity that began in April near Mt. Yoichidake, declined in mid-August. The maximal event was an M3.0 earthquake on August 11th.
"On August 14th, there was an M6.2 earthquake at a depth of approximately 40 km in the sea off Aomori Prefecture. Then, on the 24th, there was an M5.2 earthquake at a depth of approximately 40 km in about the same location. The focal mechanism of both events was a reverse fault type on a WNW-ESE pressure axis. These events occurred in conjunction with the subduction of the Pacific Plate. The M5.2 earthquake was an aftershock to the M6.2 event.":
Seismic activity has taken the form of main shock-aftershock, with the M6.2 event as the main shock. There was a rise in activity in the two days following the main shock, but activity gradually declined thereafter and had nearly ceased by the end of August.
There was the following additional activity in the Tohoku region:
- On August 4th, there was an M4.9 earthquake at a depth of approximately 40 km in the sea off Fukushima Prefecture.
"Crustal seismic activity in Suruga Bay and the surrounding area had been at a slightly low level, but now appears to be returning to normal activity.
The slight changes in activity centered in western Shizuoka Prefecture, which were seen in the results of GPS observation of the Tokai region, have continued since that time.":
A long-term review of activity shows a somewhat reduced incidence of activity since around 1996 with an even greater reduction beginning around October 2000. But in the past three months, there seems to have been a measurable increase in seismic activity within the crust. This trend seems to have continued in August.
In August, the results of GPS observations in the area from western Shizuoka to southern Gifu Prefecture, continued to show a trend toward moderate change which began in April 2001 and involving movement at a fixed rate to the WNW in conjunction with the subduction of the Philippine Sea Plate to the northwest, on the Pacific side of the Tokai-Chubu region.
There was the following additional activity in the Kanto-Chubu region:
- The microearthquake activity in the area of Hakone, which began in the latter half of June, had markedly declined by late August. The changes in GPS observations of the area have also decreased.
"On August 25th, there was an M5.1 earthquake at a depth of approximately 10 km in southern Kyoto .... Subsequent seismic activity has followed a main shock-aftershock pattern, with the M5.1 event as the main shock.":
The aftershock distribution extends NNE-SSW.
There was the following additional activity in the Kinki-Chugoku-Shikoku region:
- On August 10th, there was an M4.3 earthquake at a depth of approximately 10 km in the Kii Suido (near the city of Wakayama).
- On August 24th, there was an M4.3 earthquake at a depth of approximately 50 km in the Aki Nada. This event was an aftershock to the 2001 Geiyo Earthquake.
- Since the latter half of May, there had been continued microearthquake activity (maximum M3.6) at a depth of between 10 and 15 km near the border between Wakayama and Nara Prefectures, but from mid-August new activity began in an area 2 to 3 km to the northeast, and as of the end of August, activity has been concentrated in this region (with a maximal M3.4 event), while other regions have grown quiet.
"On August 18th, there was a shallow M6.3 earthquake in the sea near the main island of Okinawa ... Subsequent seismic activity has followed a main shock-aftershock pattern, with the M6.3 event as the main shock.":
Aftershocks have been occurring at the rate of 1 to 2 M3.0 or greater events per day.
There was an error in the report, "Seismic activity in July 2001."
error: involving movement at a fixed rate to the WNW in conjunction with the subduction of the Philippine Sea Plate to the northwest
correct: involving movement at a fixed rate to the WNW in conjunction with factors such as the subduction of the Philippine Sea Plate to the northwest
| 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. |