Seismic Activity in Japan April 1999

1 Major Seismic Activity

* There was no marked activity.

• The whole of Japan M4 (April 1999)

2 Seismic Activity in each Region

(1) Hokkaido Region

* There was no marked activity.

• Hokkaido Region

(2) Tohoku Region

* A magnitude (M)4.3 earthquake occurred on 19 April in the southwestern Iwate Prefecture (Pref.) at a depth of about 10km.

• Tohoku Region
• Seismic activity in the southwestern Iwate Pref.

(3) Kanto-Chubu Region

* An M5.1 earthquake occurred on 25 April in northern Ibaraki Pref. at a depth of about 60km. It was a low-angle thrust-fault-type earthquake occurred near the plate boundary, accompanying subduction of the Pacific plate. An M4.9 earthquake occurred on 26 March in about the same place.

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

• Kanto-Chubu Region
• Seismic activity in northern Ibaraki Pref.
• Seismic activity in central Shizuoka Pref. (1)
• GPS continuous observation Base-line chart Western Shizuoka Pref.
• Base-line-length variation graph
• Base-line-length variation graph
• GPS continuous observation base-line chart (Vicinity of Kakegawa and Omaezaki)
• Base-line-length variation graph
• Height-difference variation graph
• Secular variation at bench mark 2595 (Hamaoka-cho)

(4) Kinki-Chugoku-Shikoku Region

* There was no marked activity.

• Kinki-Chugoku-Shikoku Region

(5) Kyushu-Okinawa Region

* An M4.1 earthquake occurred on 3 April in the Satsuma region of Kagoshima Pref. at a depth of about 10 km. It occurred in the aftershock zones of the M6.5 earthquake of March 1997 and the M6.3 event of May 1997.

* An M5.2 earthquake occurred on 29 April in the sea near Amami Oshima Island. The focal mechanism of this earthquake was a normal-fault type showing a roughly E-W tension axis.

• Kyushu Region
• Seismic activity (M2.5 and greater) in northwest Kagoshima Pref. (Satsuma region)
• Seismic activity in the sea northeast of Amami Oshima Island
• Okinawa Region

3 Others

* An M7.2 earthquake occurred on 8 April in the vicinity of Vladivostok, Russia at a depth of about 600km (deep-focus earthquake). This event occurred inside the subducting Pacific plate, and the compressional axis of its focal mechanism was in the direction of plate subduction. The earthquake was felt widely in a belt along the Pacific coast extending from the Kanto region to Hokkaido, and showed a distribution of seismic intensity (zone of abnormal seismic intensity) characteristic of a deep-focus earthquake.

Addenda

* An M4.7 earthquake occurred on 7 May in central Shizuoka Pref. at a depth of about 20km.

• The earthquake in the vicinity of Vladivostok (8 Apr. 1999, 22h10m, M7.2, d=about 600km)
• Seismic-intensity distribution of the earthquake in the vicinity of Vladivostok on 8 April 1999