Seismic Activity in Japan January 1999

1 Major Seismic Activity

* A magnitude (M)6.2 earthquake occurred on 24 January in the sea near Tanegashima Island. An M4.7 earthquake occurred on 28 January in central Nagano Prefecture (pref.) and caused damage.

• The whole of Japan M4 (January, 1999)

2 Seismic Activity in each Region

(1) Hokkaido Region

* An M5.6 earthquake occurred on 9 January in the vicinity of Etorofu Island at a depth of about 120km.

• Hokkaido Region
• The earthquake in the vicinity of Etofofu Island (9 Jan. 1999, 12h05m, M5.6, depth(d) = 121km)
• The earthquake in southern Tokachi Administrative District (9 Feb. 1999, 09h19m, M4.7, d = 57km)

(2) Tohoku Region

* There was no marked activity.

• Tohoku Region
• Seismic activity off the coast of Fukushima Pref.

(3) Kanto-Chubu Region

* An M5.0 earthquake occurred on 2 January off the coast of Ibaraki Pref. at a depth of about 50km.

* An M4.3 earthquake occurred on 11 January in the Reihoku district of Fukui Pref. at a depth of about 10km.

* An M6.0 earthquake occurred on 12 January in the sea near Chichijima Island at a depth of about 470km.

* An M4.7 earthquake occurred on 28 January in central Nagano Pref. at a depth of about 10km and caused damage. The focal mechanism of this earthquake showed the NW-SE compressional axis. The focal region was situated about 7km east of the center of the Itoigawa-Shizuoka-kozosen fault zone with the NNW-SSE strike(Matsumoto-bonchi-toen fault), and there is a possibility that the activity is related to this fault zone.

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

• Kanto-Chubu Rregion
• Seismic activity off the coast of Ibaraki Prefecture (Pref.)
• Seismic activity in the Reihoku district of Fukui Pref.
• Seismic activity in central Nagano Pref. (1)
• Seismic activity in central Nagano Pref. (2)
• The earthquake in the sea near Chichijima Island(12 Jan. 1999, 11h32m, M6.0, d=466km)
• Seismic activity in central Mie Pref. (M1.6 and greater)
• GPS continuous observation base-line chart (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

* There was no unusual activity.

• Kinki-Chugoku-Shikoku region
• Seismic activity in eastern Yamaguchi Pref.

(5) Kyushu-Okinawa Region

* An M6.2 earthquake occurred on 24 January in the sea near Tanegashima Island at a depth of about 50km. The focal mechanism showed the normal fault with the E-W tension axis. In the vicinity of this hypocenter an M6.2 earthquake occurred on 18 October 1996. The focus of the 1996 earthquake was at a depth of about 40 km and its focal mechanism was of the reverse-fault with the E-W compressional axis. As their depths and focal mechanism differ, it is thought that this recent earthquake was in the interior of the subducting Philippine Sea plate and that the 1996 event occurred near the plate boundary.

• Kyushu Region
• Seismic activity in the sea near Tanegashima Island (1)
• Seismic activity in the sea near Tanegashima Island (2)
• Seismic activity in the sea near Tanegashima Island (3)
• Okinawa Region

3 Addenda

* An M4.7 earthquake occurred on 9 February in southern Tokachi Administrative District at a depth of 60km.

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

* Microearthquakes less than M2 began to occur on 24 January in central Mie Pref. (near the border with Nara Pref.) at a shallow depth of about 10km. From 1 February M2-class microearthquakes began to occur; from 3 February they increased in number; and from 8 February they gradually decreased in number, accompanied by a reduction in magnitude. The largest earthquake (M3.1) occurred on 5 February.

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