| 8 March 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan February 2000
There was no marked activity. to supplementary reports
An M4.6 earthquake occurred on 19 February in southern Tokachi Administrative District at a depth of about 55km. The focal mechanism was a reverse-fault type, showing a NW-SE compressional axis. to supplementary reports
* An M4.7 earthquake occurred on 11 February in southern Iwate Prefecture (near the border of Iwate, Miyagi, and Akita Prefs.) at a depth of about 10km. Immediately after this event, there were frequent microearthquakes, totalling some 100 in February. However by the end of February the activity had almost ceased. The focal mechanism of the M4.7 event was a reverse-fault type, showing a WNW-ESE compressional axis.
* An M5.1 earthquake occurred on 16 February offshore of Fukushima Pref. on the east side of the Japan Sea Trench. to supplementary reports
* An M4.1 earthquake occurred on 6 February in northern Tochigi Pref. at a depth of about 10km.
* An M4.2 earthquake occurred on 11 February in eastern Yamanashi Pref. at a depth of about 20km.
* An M4.3 earthquake occurred on 29 February offshore of Niigata Pref. at a depth of about 25km.
* Earthquake activity in the subducting Philippine Sea Plate in central Shizuoka Pref. continues at a level below the average. Moreover, the results of GPS observation in the Tokai (Shizuoka-Aichi-Gifu) area showed no deviation from the previous tendency. to supplementary reports
There was no marked activity. to supplementary reports
An M4.1 earthquake occurred on 28 February in the Aso region of Kumamoto Pref. at a depth of about 10km. to supplementary reports
An M6.3 deep-focus earthquake occurred on 13 February in the vicinity of Vladivostok at a depth of about 600km. to supplementary reports
| March 8, 2000 |
| Earthquake Research Committee |
"There was no marked activity":
In and around Japan, 42 earthquakes of M4.0 or greater were recorded during this month (44 in January) and 5 events of M5.0 or greater (6 in January). Therefore, from the standpoint of the number of events of M4.0 or greater, the level of activity this month was much the same as last month. Furthermore, the following were major areas of seismic activity in the last year. However, there were no marked activities in February in and near these areas of activities.
- Off the southern coast of Akita Pref. 26 Feb. 1999 M5.1 (D=about 20km)
- Sea near Niijima and Kozushima Islands . 14 Mar. 1999 M4.7 (D=10km or less)
- ditto 28 Mar. 1999 M5.0 (D=20km or less)
- Kushiro Administrative District . 13 May 1999 M6.4 (D=about 100km)
- Northern Wakayama Pref. 21 Aug. 1999 M5.4 (D=about 70km)
- Taiwan . 21 Sep. 1999 M7.7 [according to USGS]
- Central Inland Sea 30 Oct. 1999 M4.5 (D=about 15km)
- The Kumamoto District of Kumamoto Pref. (D=about 10km), November 1999, M4.0 or greater
offshore of Fukui Pref. (D=about 15km),
and western Aichi Pref. (D=about 50km).
- Southeast offshore of Nemuro Peninsula . 28 Jan. 2000 M6.8 (D=about 60km)
"Southern Tokachi Administrative District at a depth of about 55km":
The vicinity of this hypocenter lies between the subducting Pacific Plate and the continental plate; the area has previously been subject to seismic activity.
"The focal mechanism was a reverse-fault type, showing a NW-SE compressional axis":
The focal mechanism is the same as those of the December 1998 M4.6 and February 1999 M4.7 events, which occurred in roughly the same area; the direction of the compressional axis is consistent with of the Pacific Plate's subduction.
Also, aftershock activity following the M6.8 earthquake on 28 January is steadily diminishing despite a maximum 4.3 event in February.
"Southern Iwate Prefecture at a depth of about 10km":
In the last 20 years, M4.0 or greater earthquakes have occurred at the rate of one every 3 or 4 years in the vicinity of this hypocenter.
"The focal mechanism was a reverse-fault type, showing a WNW-ESE compressional axis":
The direction of this focal mechanism's compressional axis corresponds to the direction of the maximum compression derived from the GPS data in the vicinity of the hypocenter (see a supplement to "Seismic Activity in Japan").
"Offshore of Fukushima Pref. on the east side of the Japan Sea Trench":
This event, where M >= 50, is the first in 50 years to occur in the vicinity of its hypocenter. The focal mechanism was of a normal-fault type. Regarding this earthquake, no aftershock of M3.0 or greater was recorded.
"Northern Tochigi Pref. at a depth of about 10km":
The hypocenter was in an area where there occurs an M4.0 earthquake or greater once in every 2 or 3 years
"Eastern Yamanashi Pref. at a depth of about 20km":
The hypocenter of this earthquake was in the area of the border between the subducted Philippine Sea Plate and the continental plate.
"Offshore of Niigata Pref. at a depth of about 25km":
Within a radius of about 20km of the vicinity of this hypocenter there occurred M4.0 or greater earthquakes in November 1997 (M4.0) and April 1998 (M4.5).
"Earthquake activity in the subducting Philippine Sea Plate in central Shizuoka Pref. continues at a level below the average":
The average frequency of earthquakes occurrence (M1.5 or greater) in the Philippine Sea Plate in central Shizuoka Pref. was some 6 events per month, but from August 1999 the frequency was 2 to 4 per month; the lower-than-average condition continued in February. (This coincides with the views expressed at the arrangement meeting on 28 February, by the committee members of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
In addition, in the Kanto-Chubu region, there was the following seismic activity.
- The seismic activity in central Mie Pref. that began in late January 1999 had been diminishing since November; however this month activity temporarily escalated and the area of activity spread out some 1 to 2km northward. Nevertheless, the largest magnitude was less than M3.0.
- An M5.5 earthquake occurred on 8 February in the sea near Torishima Island at a depth of about 20km.
In the Kinki-Chugoku-Shikoku Region there was the following seismic activity.
- An M4.0 earthquake occurred on 12 February in central Kochi Pref. at a depth of about 45km.
- Microearthquake activity continued at the same frequency as in January in the central Inland Sea. Moreover, in early March, the activity increased somewhat remarkably in an area 2 to 3km west of the previous main area of activity.
"The Aso region of Kumamoto Pref. at a depth of about 10km":
An M6.1 earthquake occurred in January 1975 in the vicinity of this hypocenter.
Also in the Kyushu-Okinawa Region, a somewhat deep-focus M5.2 earthquake occurred on 25 February in the sea near Ishigakijima Island at a depth of about 130km. Moreover, intense seismic activity from mid- to late January in the sea near Tanegashima Island at a depth of about 20km diminished at the end of January, and in February the activity was sporadic.
"The vicinity of Vladivostok at a depth of about 600km":
The vicinity of this hypocenter is the tip of the subducted Pacific Plate. It is thought that earthquakes in this vicinity occur inside the plate, and that such was the case in this instance. This event showed the distinctive distribution of seismic intensity (zone of abnormal seismic intensity) of deep-focus earthquakes: on the Japan Sea side, which is nearer the epicenter, it was unfelt, while on the Pacific Ocean side (Hokkaido), farther from the epicenter, a seismic intensity of 1 was registered.