| 14 February 2001 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in January 2001
On January 2nd, there was an M4.4 earthquake at a depth of about 10 km in the Chuetsu region of Niigata Prefecture. Then, on January 4th, there was an M5.1 earthquake at a depth of about 10 km approximately 40 km SSE of the January 2nd event. Both events registered maximum seismic intensities of 5 lower. On January 12th, there was an M5.4 earthquake at a depth of about 10 km in northern Hyogo Prefecture, registering a maximum seismic intensity of 4, with attendant damage. to supplementary reports
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
*On January 2nd, there was an M4.4 earthquake at a depth of about 10 km in the Chuetsu region of Niigata Prefecture. The focal mechanism was a reverse fault type with a NW-SE pressure axis.
Also, on January 4th, there was an M5.1 earthquake at a depth of about 10 km in the Chuetsu region of Niigata Prefecture (about 40 km SSE of the epicenter of the January 2nd event). The focal mechanism was a reverse fault type with an east-west pressure axis. These two events are not thought to be directly connected. Following this activity, there was microearthquake activity from the middle of January including a maximal M2.9 event in the area of the January 2nd earthquake. In the vicinity of the January 4th event, there was an M3.9 event in mid-January, and a brief period of activity with 3 microearthquakes in the latter half of the month.
*On January 6th, there was an M4.6 earthquake at a depth of 50km in the eastern Mino region of Gifu Prefecture (at the border of Gifu and Aichi Prefectures This earthquake occurred in the area of the tip of the subducting Philippine Sea Plate.
*The seismic activity that began at the end of June 2000 in the area from Miyakejima to Niijima-Kodushima, while declining, continues. On January 3rd, there was an M3.6 earthquake 25 km SSW of Miyakejima. The results of GPS observation of the area also indicate that the activity has not yet entirely ceased.
*Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which had been at lower-than-average levels since August 1999, but began to show indications of a resumption of activity in December 2000, continues to rise and fall, although it appears to be returning to a regular activity level. However, the results of GPS observation of the Tokaido region show no marked changes in the trends observed to present. to supplementary reports
*On January 12th, there was an M5.4 earthquake at a depth of about 10 km in northern Hyogo Prefecture (near the border with Tottori Prefecture). The focal mechanism was strike slip type with a NNW-SSE pressure axis. On January 20th, there was an M4.7 earthquake approximately 4 km NNW of the epicenter of the M5.4 event. This activity is part of microearthquake activity that began in December 2000, in which the activity level has increased following the M5.4 event and is following a general pattern of foreshock - main shock - aftershock.
The current region of activity has expanded approximately 6 km to the east and west, and 7 km to the northwest and southeast. However, the earthquakes have been distributed in a complex pattern, and the activity is believed to include some events triggered by the M5.4 earthquake. No changed were observed in the results of GPS observation at the time of the M5.4 event, and no definitive changes have been observed since that time. From the above, while there is the potential for the occurrence of further M4 class earthquakes (maximum seismic intensity of 4), it is believed that the probability will decrease over time.
*On January 25th, there was an M4.4 event at a depth of about 50 km in northern Tokushima Prefecture. The earthquake occurred within the subducting Philippine Sea Plate.
*In the region of the 2000 Western Tottori Earthquake (M7.3, tentative), which occurred on October 6th, 2000, since the beginning of January there was an M3.7 aftershock in the northern part of the aftershock zone on 22nd, but in general aftershock activity has dropped to below its level in December. Activity in the regions of activity triggered by the M7.3 (tentative) event 25 km WSW and 30 km ENE of the aftershock zone is similarly diminishing. to supplementary reports
There was no marked activity. to supplementary reports
On January 14th, there was an M6.2 earthquake (moderately deep) approximately 600 km south of Chichijima. There was an M7.6 earthquake on March 28th, 2000 in the same area.
*On February 2nd, there was an M4.2 earthquake at a depth of about 20 km in western Kanagawa Prefecture.
*On February 8th, there was an M4.2 earthquake at a depth of about 15 km in southern Tokushima Prefecture.
*On February 8th, there was an M5.8 earthquake in the sea near Miyakojima.
*On February 11th, there was an M4.3 aftershock to the 2000 Western Tottori Earthquake at a depth of about 15 km in eastern Shimane Prefecture.
*On February 13th, there was an M3.9 earthquake with a maximum seismic intensity of 5 lower in a shallow area of the sea near Niijima-Kodushima (approximately 4 km west of Shikinejima). This earthquake is part of the activity in the area from Miyakejima to Niijima-Kodushima, which has diminished, but continues, and nor marked changes have been observed in that activity since the occurrence of this event.
| February 14, 2001 |
| Earthquake Research Committee |
There were 44 M4 or greater earthquakes in Japan and the surrounding area (compared with 43 in December, and a monthly average of 46 in the 30 years up to the end of 2000). Of that number, 5 were M5 or greater (compared with 4 in December).
The average annual occurrence of M6 or greater events in the years from 1998-2000 was 16 (the average over the past 30 years was also approximately 16). In January 2001, there was one such event, in the sea near Chichijima.
There was the following major seismic activity in the period from January 2000 to the end of December 2000:
| -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) |
| -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 M6.4 event (at a depth of about 10 km) on July 30th. | |
| -The sea off Ibaraki Prefecture | July 21st 2000, M6.0 (at a depth of 50 km) |
| -Western Tottori Prefecture | On October 6th, there was an M7.3 (tentative) 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) |
*On January 3rd, there was an M5.6 earthquake in the sea east of Hokkaido. The epicenter was located in the eastern tip of the aftershock region of the 1994 Earthquake in the Offshore of Eastern Hokkaido.
There are no supplements in the Tohoku Region.
The two events in the Chuetsu region of Niigata Prefecture "...are not thought to be directly connected":
As was stated in the supplement to the December 2000 report, this conclusion was reached as these were both M4-5 class events, with epicenters separated by approximately 40 km. On February 10th, there was an M3.6 earthquake in the aftershock zone of the January 2nd event.
"Seismic activity within the subducting Philippine Sea Plate in central Shizuoka Prefecture, which had been at lower-than-average levels since August 1999, but began to show indications of a resumption of activity in December 2000, continues to rise and fall, although it appears to be returning to a regular activity level.":
Although there were three months since August 1999 where the occurrence 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. However, beginning on October 15th, there were three successive 30-day periods in which the occurrence was 9, 10 and 7 such events. There were 5 such events in the 30 days from January 13th to February 11th.
(This coincides with the views (see reference below) expressed at the procedural meeting, on January 29th, of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(reference) Recent earthquakes and crustal movement in the Tokai region and surrounding area (January 29, 2001; Department of Earthquakes and Volcanoes, Meteorological Agency)
"A decrease in activity had been observed in the subducting slab in the Tokai region since the year before last, but it appears that the activity, with some fluctuations, is returning to a regular level. It is also thought that the crustal seismic activity in Suruga Bay and the western coastal area is continuing at a somewhat low level. In addition, no noteworthy changes in crustal movement have been observed in the Tokai region."
There was the following seismic activity:
*In November and December there was a dramatic increase in low frequency earthquakes at a depth of about 15 km in the area of Mt Fuji, but this activity diminished considerably in January.
(Regarding the earthquake in northern Hyogo Prefecture)
*Since 1977, there was microearthquake activity including M3.0 or greater events in 1977, 1994 and last year (moderate activity including M3.1 and M3.0 events in December) in the region of activity of the present event.
*The results of GPS observation of the region of activity and surrounding area of the present event show trends of WNW-ESE and east-west contraction.
*Since 1926, there have been 3 M5.0 or greater earthquakes within an area of about 20 km of the present region of activity (of that group, there was an M6.3 earthquake in 1949 that occurred at the western edge of the present active zone). All of these events are thought to have been aftershocks to the 1943 Tottori Earthquake (M7.2) that occurred in the western part of the present active zone.
*In 1925 the Northern Tajima Earthquake (M6.8) and in 1927 the Northern Tango Earthquake (M7.3) occurred approximately 30 and 50 km, respectively, east of the present active zone.
There was the following seismic activity:
*On January 9th, there was an M4.6 earthquake in the Iyo Nada.
There was the following seismic activity:
*On January 10th, there was an M3.9 earthquake in the Aso region of Kumamoto Prefecture.
*On January 26th, there was seismic activity including a maximal M4.2 earthquake in the sea near Amami Oshima (near Akusekijima), where there were M5.7 and M5.2 earthquakes on October 2nd, 2000.
*Since January, the seismic activity that began in mid-November 2000 near Iriomotejima, while diminishing, continues.
| 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. |