Seismic Activity in Japan September 1999

1 Major Seismic Activity

A Magnitude (M) 7.7 (per U.S. Geological Survey (USGS)) earthquake occurred on 21 September in Taiwan, which neighbors on Japan, and registered a seismic intensity 2 (JMA) in southwestern Okinawa Prefecture (Pref.). Other than this there was no marked activity; all was quiet.

• The whole of Japan M4 (September 1999)

2 Seismic Activity in each Region

(1) Hokkaido Region

Since July there has been no marked activity.

• Hokkaido Region

(2) Tohoku Region

An M4.0 earthquake occurred in the Sanpachikamikita area of Aomori Pref. on 13 September at a depth of about 15km.

• Tohoku Region
• Seismic activity in the Sanpachikamikita area of Aomori Pref.
• Seismic activity offshore of Iwate Pref.

(3) Kanto-Chubu Region

* An M5.0 earthquake occurred on 13 September in northwestern Chiba Pref. at a depth of about 80km. The focal mechanism was a reverse-fault type showing an E-W compressional axis. From the focal mechanism and the depth of the hypocenter it is thought that this earthquake occurred near the upper plane of the subducting Pacific Plate.

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

• Kanto-Chubu Region
• Seismic activity in northwestern Chiba Pref.
• GPS continuous observation base-line chart (the vicinity of Kakegawa and Omaezaki)
• Base-line-length variation graph
• Height-difference variation graph

(4) Kinki-Chugoku-Shikoku Region

There was no marked activity during this month. Moreover, no marked aftershock activity was observed following the M5.4 earthquake in northern Wakayama Pref. during the previous month (August), which registered a seismic intensity 5 lower(JMA).

• Kinki-Chugoku-Shikoku Region
• Seismic activity in northern Wakayama Pref.

(5) Kyushu-Okinawa Region

Neither during the previous month (August) nor this month was there marked activity.

• Kyushu Region
• Okinawa Region

3 Other Regions

A Magnitude (M) 7.7 (per USGS) earthquake occurred on 21 September in Taiwan, which neighbors on Japan, and registered a seismic intensity 2 (JMA) in southwestern Okinawa Pref. Also, in southwestern Okinawa Pref., some of the larger aftershocks registered a seismic intensity 1(JMA). The focal mechanism of this earthquake was a reverse-fault type showing an E-W compressional axis. It has been estimated, through waveform analysis, that there is an average slip of 2 meters on the earthquake fault which is 80km long and 40km wide.

Furthermore, reports from multiple on-the-scene surveys made following the earthquake show that a surface earthquake fault extending N-S for more than 50km has appeared, and that the eastern side is elevated.

Taiwan lies in a region where the Eurasian Plate, on which most of Taiwan rides, and the Philippine Sea Plate, which is advancing in a northwesterly direction, are pushing against each other. The recent earthquake occurred on the active fault at the boundary area between the Philippine Sea Plate and the Eurasian Plate - an area that is closely linked to earthquakes occuring in southwestern Japan; it is thought, however, that the event's influence upon seismic activity in southwestern Japan is extremely small: neither before nor after the recent earthquake was there marked variation in seismic activity in the Okinawa region.

Addendum

An M5.6 earthquake occurred on 3 October offshore of Iwate Pref. at a depth of about 10km.

• Hypocenter distribution of the Taiwan earthquake
• Focal mechanism solution for the Taiwan earthquake (1)
• Focal mechanism solution for the Taiwan earthquake (2)
• Emergency Survey Report of the Chichi Taiwan Earthquake by Disaster Prevention Research Institute, Kyoto Univ.
• Seismic activity in the area ranging from the Yaeyama Islands to Taiwan