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| Date | July 12, 1993 |
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| Magnitude | 7.7 Mw |
| Depth | 17 km |
| Countries or regions | Japan, Hokkaido |
| Tsunami | yes |
| Casualties | 230 |
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.[1] It had a magnitude of 7.7 on the moment magnitude scale and a maximum felt intensity of VIII (Destructive) 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.[2]
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The earthquake occurred in the backarc region of the convergent boundary where the Pacific Plate subducts beneath the Eurasian Plate.
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.[2]
The earthquake had two distinct shocks. The first lasted for 20 seconds, while the second lasted 35 seconds.[3]
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.[2]
The tsunami inundated large parts of Okushiri, despite its tsunami defenses. 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.[4]
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.[3]
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.[5]