Seismic Activity in Japan October 1999

1 Major Seismic Activity

An M4.5 earthquake occurred on 30 October in the central Inland Sea area where microearthquakes have continued since March of this year; activity temporarily intensified.

• The whole of Japan M4 (October 1999)

2 Seismic Activity in each Region

(1) Hokkaido Region

* Since July there has been no marked activity.

• Hokkaido Region

(2) Tohoku Region

* An M5.6 shallow earthquake occurred on 3 October offshore of Iwate Pref. The focal mechanism was a low-angle thrust fault type showing an ENE-WSW compressional axis. It is thought that this event occurred at the boundary of the Pacific Plate and the continental plate.

• Tohoku Region
• Seismic activity offshore of Iwate Pref.
• Seismic activity in the Hamadori region of Fukushima Pref.

(3) Kanto-Chubu Region

* An M5.0 earthquake occurred on 16 October offshore of Ibaraki Pref. at a depth of about 30km.

* An M4.0 earthquake occurred on 29 October in northern Nagano Pref. at a depth of about 10km.

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

• Kanto-Chubu Region
• Seismic activity offshore of Ibaraki Pref.
• Seismic activity in northern Nagano Pref.
• GPS continuous observation base-line chart (the vicinity of Kakegawa and Omaezaki)
• Base-line-length variation graph
• Height-difference variation graph
• Secular variation of benchmark 2595 (Hamaoka Town)

(4) Kinki-Chugoku-Shikoku Region

An M4.5 earthquake occurred on 30 October in the central Inland Sea area (about 10km offshore of the border between Ehime and Kagawa Prefs.) at a depth of about 15km. This region has been the site of microearthquake activity since March 1999, and, during October, the M4.5 event was preceded by four earthquakes larger than M3. The focal mechanism was a strike-slip type showing a NW-SE compressional axis.

• Kinki-Chugoku-Shikoku Region
• Seismic activity in the central Inland Sea area (offshore of the border between Ehime and Kagawa Prefs.) (1)
• Seismic activity in the central Inland Sea area (offshore of the border between Ehime and Kagawa Prefs.) (2)
• Seismic activity in the central Inland Sea area (offshore of the border between Ehime and Kagawa Prefs.)

(5) Kyushu-Okinawa Region

An M4.0 earthquake occurred on 31 October in the Kumamoto region of Kumamoto Pref. at a depth of about 10km.

Furthermore, although it is outside of our time frame, an M4.3 earthquake occurred on 10 November in roughly the same area. The focal mechanism showed a north-south tension axis. This event occurred in the northern part of the Hinagu fault.

• Kyushu Region
• Seismic activity in the Kumamoto region of Kumamoto Pref.
• Okinawa Region

3 Addenda

An M4.4 earthquake occurred on 7 November in the Hamadori region of Fukushima Pref. at a depth of about 80km.