| 9 February 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan January 2000
A magnitude (M)6.8 earthquake occurred on 28 January offshore of eastern Hokkaido, causing damage. to supplementary report
A magnitude (M)6.8 earthquake occurred on 28 January offshore of eastern Hokkaido at a depth of about 60km. The hypocenter was in the lower plane of the double-planed seismic zone adjacent to the southwestern edge of the aftershock area of the "1994 Earthquake offshore of Eastern Hokkaido" (M8.1). Moreover, the focal mechanism was a reverse-fault type showing a roughly WNW-ESE compressional axis, closely corresponding to that of the 1994 event. Immediately following the recent earthquake there were frequent aftershocks, but they are gradually tailing off. to supplementary report
An M5.0 earthquake occurred on 9 January offshore of Fukushima Prefecture (Pref.) at a depth of about 50km. to supplementary report
* An M4.4 earthquake occurred on 12 January in northern Ibaraki Pref. at a depth of about 60km.
* Earthquake activity in the Philippine Plate in central Shizuoka Pref. has been quiet since August of last year. Moreover, the results of GPS observation in the Tokai (Shizuoka-Aichi-Gifu) area and Kakegawa-Omaezaki (Hamaoka) water-level measurements showed no marked variation. to supplementary report
An M4.0 earthquake occurred on 17 January in the Suo Sea at a depth of about 15km. to supplementary report
From mid- to late January numerous earthquakes occurred in the sea near Tanegashima Island. Including an M5.2 event on 23 January, there were 190 earthquakes. to supplementary report
An M4.1 earthquake occurred on 6 February in northern Tochigi Pref. at a depth of about 10km.
| February 9, 2000 |
| Earthquake Research Committee |
"M6.8 earthquake offshore of eastern Hokkaido"
This is the first M6.0 or greater earthquake in Japan or its environs to also record a seismic intensity of 4 or greater since the fairly deep-focus M6.4 earthquake on 13 May 1999 in Kushiro Administrative District at a depth of about 100km.
Also, in Japan or its environs, 44 earthquakes of M4.0 or greater were recorded during this month (44 in December as well) and 6 events of M5.0 or greater (2 in December). Therefore, from the standpoint of the number of events of M4.0 or greater, the level of activity this month was the same as last month. Furthermore, the following were major areas of seismic activity in the past year; in January, however, among these areas of seismic activity and their environs, only the Sea near Tanegashima Island and the Central Inland Sea were active.
- Sea near Tanegashima Island 24 Jan. 1999 M6.2 (about 50km)
- Central Nagano Pref. 28 Jan. 1999 M4.7 (about 10km)
- Off the southern coast of Akita Pref. 26 Feb. 1999 M5.1 (about 20km)
- Sea near Niijima and Kozushima Islands 14 Mar. 1999 M4.7 (10km or less)
28 Mar. 1999 M5.0 (20km or less)
- Kushiro Administrative District 13 May 1999 M6.4 (about 100km)
- Northern Wakayama Pref. 21 Aug. 1999 M5.4 (about 70km)
- Taiwan 21 Sep. 1999 M7.7 (per USGS)
- Central Inland Sea 30 Oct. 1999 M4.5 (about 15km)
- In November 1999, M4.0 or greater earthquakes were recorded in the Kumamoto District of Kumamoto Pref. (about 10km), offshore of Fukui Pref. (about 15km), and western Aichi Pref. (about 50km).
"reverse-fault type showing a roughly WNW-ESE compressional axis"
This focal mechanism is similar to that of the 1994 event. Moreover, the 1994 event occurred inside the subducting Pacific Plate (interplate earthquake). Also, on the southwest side of the current earthquake there occurred the "17 June 1973 earthquake offshore of Nemuro Peninsula" (M7.4) which caused damage due to a tsunami as well as through seismic motion. The 1973 event occurred between the subducting Pacific Plate and the continental plate (interplate earthquake).Two people were slightly hurt as a result of the current earthquake (as reported by the Fire Defense Agency of the Ministry of Home Affairs, 31 January, 2000).
In addition, an M5.0 deep-focus earthquake occurred on 6 January in the sea south of Hokkaido at a depth of 400km. In February 1999, 100km northeast of this event's hypocenter, there had previously occurred an M5.9 earthquake (depth:430km), and the vicinity was recognized as being subject to seismic activity.
"Offshore of Fukushima Prefecture (Pref.) at a depth of about 50km"
Near this hypocenter an M5.2 earthquake occurred in August 1998, and the area is prone to frequent seismic activity.
In addition, the Tohoku area was subject, in January, to the following seismic activity.
- On the east side of the ocean trench offshore of the Sanriku (Akita-Iwate-Miyagi) area seismic activity intensified from mid-January, and on the 19th there was an M4.0 and on the 21st an M4.1 earthquake. By the end of January activity had largely terminated. This area of activity and its environs have previously been the site of seismic activity.
- On 10 January in Northern Miyagi Pref. (near the border of Akita, Miyagi, and Yamagata Prefs.) there were 18 earthquakes, including an M4.2 event at a depth of about 10km, and activity lasted only one day. This area of seismic activity coincides with the Onikobe geothermal system, where previously there has been intensified microearthquake activity.
"Northern Ibaraki Pref. at a depth of about 60km"
This hypocenter is in the vicinity of the upper plane of the subducting Pacific Plate's double-planed seismic zone, and it has previously been the site of intense seismic activity.
"Earthquake activity in the Philippine Plate in central Shizuoka Pref. has been quiet since August of last year"
According to earthquake frequency statistics kept since June 1986, the frequency of M1.5-or-greater earthquakes in the subducting Philippine plate has averaged 6 per month. However, since August 1999 this figure has dropped to 2 or 3, where it remains.
"Showed no marked variation"
The results of water-level and other measurements show that the tendency toward subsidence of the Omaezaki side as opposed to the inland side continues unabated, but no sharp change was noted.
In addition, in the Kanto-Chubu region an M3.6 earthquake occurred in January in western Nagano Prefecture and recorded a seismic intensity of 4. The vicinity of the hypocenter has previously been the site of intense microearthquake activity.
"Suo Sea at a depth of about 15km"
Including the current earthquake there have, since 1926, been less than ten M4.0 earthquakes in the Suo Sea. It is an area of low earthquake frequency. The largest earthquake since 1926 was an M6.0 event recorded on 28 October, 1991. Numerous active faults have been noted in the seabed here.
In addition, in the Kinki-Chugoku-Shikoku region microearthquake activity has been recorded in the central Inland Sea and continues with the same frequency as in December.
"Seismic activity in the sea near Tanegashima Island"
The sea near Tanegashima has, from the past, been the site of relatively intense seismic activity. Moreover, some 50km north of the area of current activity at a depth of some 50km (the interior of the subducting Philippine Plate) there occurred on 24 January of last year an M6.2 earthquake. However, the depth of this event differs from the current earthquake.
In addition, in the Kyushu-Okinawa area in January there was the following seismic activity.
- An M5.9 somewhat deep-focus earthquake occurred on 29 January in the East China Sea at a depth of about 160km. Earthquake activity has been previously recorded in the environs of the current seismic activity.