|UTC time||1993-07-12 13:17:11|
|Local date||July 12, 1993|
|Areas affected||Japan, Hokkaido|
|Max. intensity||VIII (Severe)|
The 1993 southwest-off Hokkaido earthquake (北海道南西沖地震 Hokkaidō Nansei Oki Jishin) occurred at 13:17:12 UTC on 12 July 1993 in the Sea of Japan near the island of Hokkaido. It had a magnitude of 7.7 on the moment magnitude scale and a maximum felt intensity of VIII (Severe) on the Mercalli intensity scale. It triggered a major tsunami that caused deaths on Hokkaidō and in southeastern Russia, with a total of 230 fatalities recorded. The island of Okushiri was hardest hit, with 165 casualties from the earthquake, the tsunami and a large landslide.
The earthquake shaking caused moderately severe damage, VIII on the Mercalli scale. The tsunami reached Okushiri between 2 and 7 minutes after the earthquake. A tsunami warning was given 5 minutes after the earthquake by the JMA. However, this was too late for the inhabitants of Okushiri. The quake caused fires to start in the town of Okushiri, adding greatly to the total damage.
The earthquake had two distinct shocks. The first lasted for 20 seconds, while the second lasted 35 seconds.
The rupture occurred on a fault that dipped at 24 degrees to the east. It had an estimated length of 150 km with a displacement of 2.5 m. The island of Okushiri subsided by 5–80 cm.
The tsunami inundated large parts of Okushiri, despite its tsunami defences. Okushiri had been struck by another tsunami 10 years earlier. A maximum run-up of 32 m was recorded on the western part of the island near Monai. A tsunami was widely observed in the Sea of Japan with a run-up of 3.5 m at Akita in northern Honshu, up to 4.0 m in southeastern Russia and up to 2.6 m on the coast of South Korea.
The Okushiri-port landslide involved a volume of 1.5 x 105 m³ of rock. The slide failure occurred at the base of a volcanic breccia bed. The slide occurred in two phases that may match the two separate shocks recorded for the earthquake.
The destructive power of this tsunami led to an overhaul of the sea defences on Okushiri involving the construction of tsunami sluices on a river and strengthened embankments. New escape routes were also provided and help was given for households to purchase emergency broadcast receivers.
- Japan Meteorological Agency officially named this earthquake 平成5年(1993年)北海道南西沖地震 (Heisei 5 nen (1993 nen) Hokkaidō nansei-oki jishin, literally the 1993 Southwest-off Hokkaido Earthquake). 気象庁が命名した気象及び地震火山現象 Archived 2017-10-29 at the Wayback Machine
- NGDC. "Comments for the Significant Earthquake". Retrieved 6 November 2010.
- Yamagishi, H. (2000). "Recent Landslides in Western Hokkaido, Japan". Pure and Applied Geophysics. 157 (6–8): 1115–1134. Bibcode:2000PApGe.157.1115Y. doi:10.1007/s000240050020. Retrieved 6 November 2010.
- NGDC. "Comments for the Tsunami Event". Retrieved 6 November 2010.
- Nakao, M. "Okushiri Tsunami Generated by southwest-off Hokkaido earthquake". Failure Knowledge Database. Retrieved 6 November 2010.[permanent dead link]