Seismic Activity in Japan February 1999

1 Major Seismic Activity

* A magnitude (M)5.1 earthquake occurred on 26 February off the southern coast of Akita Prefecture (pref.) and caused damage.

• The whole of Japan M4 (February, 1999)

2 Seismic Activity in each Region

(1) Hokkaido Region

* An M4.7 earthquake occurred on 9 February in southern Tokachi Administrative District at a depth of about 55km.

• Hokkaido Region
• An earthquake in southern Tokachi Administrative District (9 Feb. 1999, 09h19m, M4.7, d=57km)

(2) Tohoku Region

* An M5.1 earthquake occurred on 1 February off the coast of Fukushima Pref. at a depth of about 50km.

* An M4.0 earthquake occurred on 21 February in the Aizu region of Fukushima Pref. at a depth of about 15km.

* An M5.1 earthquake occurred on 26 February off the southern coast of Akita Pref. at a depth of about 20km, and caused damage. A maximum intensity of 5 lower was recorded for this event in both Kisakata-machi, Akita Pref. and Yuza-machi, Yamagata Pref. An M4.6 aftershock was recorded a minute later, and M3.9 and M3.7 aftershocks on 28 February. However the number of subsequent aftershocks is decreasing.
The focal mechanism was reverse-fault with a WNW-ESE compressional axis. This corresponds to the direction of compressional force extending E-W over a wide area of the Tohoku Region.
GPS observation in the vicinity showed no variation due to the earthquakes.

• Tohoku Region
• Seismic activity off the coast of Fukushima Pref.
• Seismic activity in the Aizu region of Fukushima Pref.
• Seismic activity off the southern coast of Akita Pref.
• GPS continuous observation base-line chart (vicinity of Kisakata)
• Base-line-length variation graph(1)
• Base-line-length variation graph(2)
• Base-line-length variation graph(3)

(3) Kanto-Chubu Region

* Seismic activity that began in late January in central Mie Pref. at a depth of about 10km continued in February. In light of past examples of seismic activity in the vicinity, the fact that current seismic activity has persisted for more than a month indicates a possibility that the activity will continue for several months.

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

• Kanto-Chubu Region
• Seismic activity in the sea off the eastern shore of the Kanto Region
• Seismic activity in central Mie Pref. (M1.6 and greater)
• Mie Pref., 1 January 1997 - 8 March 1999, 60km and shallower
• GPS continuous observation base-line chart (Vicinity of Kakegawa and Omaezaki)
• Base-line-length variation graph
• Height-difference variation graph

(4) Kinki-Chugoku-Shikoku Region

* An M4.0 earthquake occurred on 7 February in eastern Yamaguchi Pref. at a depth of about 10km.

* An M4.0 earthquake occurred on 12 February in southern Kyoto Pref. at a depth of about 15km.

• Kinki-Chugoku-Shikoku Region
• Seismic activity in southern Kyoto Pref. (1)
• Seismic activity in southern Kyoto Pref. (2)
• Seismic activity in eastern Yamaguchi Pref.

(5) Kyushu-Okinawa Region

* There was no marked activity.

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

3 Addenda

* An M4.2 aftershock occurred on 8 March in the focal region of the earthquake that occurred on 26 February off the southern coast of Akita Pref.

* An M6.2 earthquake occurred on 2 March offshore of the eastern coast of the Kanto Region at a depth of about 60km.

* An M4.5 earthquake occurred on 9 March in the Aso region of Kumamoto Pref. at a depth of about 10km.