Seismic Activity in Japan December 1999

1 Major Seismic Activity

Numerous earthquakes with a seismic intensity of 1 or larger occurred, over a short period of time, in Oita Prefecture (Pref.). However, overall it was quiet.

• The whole of Japan M4 (December 1999)

2 Seismic Activity in each Region

(1) Hokkaido Region

A Magnitude (M)4.0 earthquake and an M4.3 earthquake occurred on 3 December in roughly the same place in Soya Strait. Including these events, there occurred 18 earthquakes of M2.0 or greater.

• Hokkaido Region
• Seismic activity in Soya Strait (1)
• Seismic activity in Soya Strait (2)

(2) Tohoku Region

During December there was no marked activity.

• Tohoku Region

(3) Kanto-Chubu Region

* An M4.8 earthquake occurred on 4 December off the coast of southern Ibaraki Pref. at a depth of about 100km.

* Two M4.1 earthquakes occurred one after the other on 16 December on the border between Tochigi and Gumma Prefs. at a depth of about 10km.

* An M4.0 earthquake occurred on 27 December in southwestern Ibaraki Pref. at a depth of about 50km.

* The results of GPS observation in the Tokai (Shizuoka-Aichi-Gifu) area showed no marked variation.

• Kanto-Chubu Region
• Seismic activity at border between Tochigi and Gumma Prefs.
• Seismic activity in southwestern Ibaraki Pref.
• Seismic activity off the coast of southern Ibaraki Pref.
• GPS continuous observation base-line chart (the vicinity of Kakegawa and Omaezaki)
• Base-line-length variation graph
• Height-difference variation graph

(4) Kinki-Chugoku-Shikoku Region

Microearthquake activity in the central Inland Sea area, which began in March 1999, continues. The level of activity is equivalent to that prior to the M4.5 event in October.

• Kinki-Chugoku-Shikoku Region
• Seismic activity in the central Inland Sea area (offshore of the border between Ehime and Kagawa Prefs.)

(5) Kyushu-Okinawa Region

Forty-four earthquakes of M2.0 or greater occurred between 20 and 22 December in central Oita Prefecture. By 23 December, this activity had largely terminated. The largest event was an M3.3 earthquake. GPS observation prior to and following this activity showed no marked variation.

• Kyushu Region
• Seismic activity in the vicinity of Beppu Bay
• Base-line chart
• Base-line-length variation graph
• Base-line-length variation graph
• Okinawa Region

3 Addenda

An M5.0 earthquake occurred on 9 January offshore of Fukushima Pref. at a depth of about 50km.