4 (March 1999)
| 14 April 1999 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan March 1999
* A magnitude (M)4.7 earthquake occurred on 14 March and an M5.0 earthquake on 28 March in the sea near Niijima and Kozushima Islands. Both events were accompanied by a temporary increase in seismic activity.
* There was no marked activity.
* An M4.2 earthquake occurred on 8 March off the coast of southern Akita Prefecture (Pref.) at a depth(d) of about 20km. This event occurred within the aftershock zone of the M5.1 earthquake of 26 February, and is thought to be an aftershock. Aftershock activity of the 26 February earthquake has ebbed and, largely, come to an end.
* An M4.6 earthquake occurred on 11 March off the coast of northern Iwate Pref. at a depth of about 30km. Its focal mechanism was the reverse-fault type showing a N-S compressional axis. In the past (5 June 1996) an M4.1 earthquake with the same focal mechanism occurred about 30km to the southwest.
* An M5.7 earthquake occurred offshore of eastern Aomori Pref. at the plate boundary.
* An M6.2 earthquake occurred on 2 March offshore of eastern Kanto. On 20 February M5.8 and M5.7 earthquakes occurred.
* An M4.7 earthquake occurred on 14 March in the sea near Niijima and Kozushima Islands (northwest Kozushima), and there was a temporary increase in seismic activity; however activity ceased after about one day. Its focal mechanism showed a roughly E-W tension axis. From the GPS observation point in northwest Kozushima, west-facing crustal deformation was observed. Similar seismic activity occurred in roughly the same vicinity on 14 February and 28 March (largest: M5.0).
In this area seismic activity increased from about 1991. Tidal observations indicate that Kozushima has been upheaving since about 1990, and the results of recent GPS observation confirm that Kozushima is dilating.
* An M4.9 earthquake occurred on 26 March in northern Ibaraki Pref. at a depth of about 60km, and caused damage. It was a low-angle thrust-fault-type earthquake occurred at the plate boundary, accompanying subduction of the Pacific plate.
* The seismic activity that began in central Mie Pref. at the end of January had, from mid-March, shifted toward a reduced number of seismic events.
* The results of GPS observation in the Tokai (Shizuoka-Aichi-Gifu) area showed no marked variation.
3.0, d=30km or less)
* An M4.9 earthquake occurred on 16 March in northern Shiga Pref. at a depth of about 10km. The focal mechanism showed a reverse fault with an E-W compressional axis. Furthermore, results of observation near the hypocenter showed variation in undergroundwater levels and strain prior to the earthquake.
* An M4.5 earthquake occurred on 9 March in the Aso district of Kumamoto Pref. at a depth of about 10km, and caused damage. Immediately after the event seismic activity increased, but later it decreased.
* An M6.0 earthquake occurred on 24 March in the sea near Amami Oshima Island.
* An M4.1 earthquake occurred on 3 April in the Satsuma district of Kagoshima Pref. at a depth of about 10km.
* An M7.2 earthquake occurred on 8 April in the vicinity of Vladivostok, Russia at a depth of about 600km (deep-focus earthquake).
