The or Okushiri earthquake occurred at 13:17:12 UTC on 12 July 1993 in the

Sea of Japan The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it h ...

near the island of

Hokkaido is Japan's second largest island and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by the undersea railway Seikan Tunnel. The la ...

.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
It had a magnitude of 7.7 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

and a maximum felt intensity of VIII (''Severe'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

. It triggered a major

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...

that caused deaths on Hokkaidō and in southeastern

Russia Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...

, with a total of 230 fatalities recorded. The island of Okushiri was hardest hit, with 165 casualties from the

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

, the tsunami and a large

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...

Tectonic setting

The northwestern side of Honshu lies on the southeastern margin of the

, an area of

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

created by

back-arc spreading A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

associated with the

convergent boundary A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more Plate tectonics, lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can ...

where the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

subducted Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

beneath the

Okhotsk Plate The Okhotsk Plate is a minor tectonic plate covering the Kamchatka Peninsula, Magadan Oblast, and Sakhalin Island of Russia; Hokkaido, Kantō and Tōhoku regions of Japan; the Sea of Okhotsk, as well as the disputed Kuril Islands. It was ...

. The spreading was active from the

late Oligocene The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/ Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stage ...

middle Miocene The Middle Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0.05 Ma to 11.608 ± 0.005 Ma (million y ...

. The

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on ...

associated with the spreading formed a series of N-S trending

extensional fault An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create a ...

s and associated

sedimentary basins Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsiden ...

. Currently the area is being deformed by contractional tectonics, causing

inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...

of these earlier basins, forming anticlinal structures. It has been suggested that the northwestern coast of Honshu represents an incipient subduction zone, but there remain significant uncertainties about the existence of the Okhotsk Plate and the nature and precise location of its boundary in the Sea of Japan, if it does exist. This region has been the location for several historical earthquakes, such as the 1964 Niigata and 1983 Sea of Japan earthquakes, with reverse fault mechanisms, on faults trending approximately north–south.

Damage

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.

Characteristics

Earthquake

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.

Tsunami

The tsunami inundated large parts of Okushiri, despite its tsunami defences. Okushiri had been struck by another

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

with a run-up of 3.5 m at

Akita is a Japanese name and may refer to: Places * 8182 Akita, a main-belt asteroid * Akita Castle, a Nara period fortified settlement in Akita, Japan * Akita Domain, also known as Kubota Domain, feudal domain in Edo period Japan * Akita, Kumamoto ...

in northern

Honshu , historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separ ...

, up to 4.0 m in southeastern Russia and up to 2.6 m on the coast of

South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korea, Korean Peninsula and sharing a Korean Demilitarized Zone, land border with North Korea. Its western border is formed ...

Landslide

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 A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...

. The slide occurred in two phases that may match the two separate shocks recorded for the earthquake.

Aftermath

The destructive power of this tsunami led to an overhaul of the sea defences on Okushiri involving the construction of tsunami

sluice Sluice ( ) is a word for a channel controlled at its head by a movable gate which is called a sluice gate. A sluice gate is traditionally a wood or metal barrier sliding in grooves that are set in the sides of the waterway and can be considered ...

s on a river and strengthened embankments. New escape routes were also provided and help was given for households to purchase emergency broadcast receivers.

References

External links

* {{DEFAULTSORT:1993 southwest-off Hokkaido earthquake Earthquakes of the Heisei period Tsunamis in Japan History of Hokkaido

1993 tsunamis Hokkaidō earthquake 1993 Hokkaidō earthquake July 1993 events in Asia