The or Okushiri earthquake occurred at 13:17:12

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

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 ...

near the island of

Hokkaido is the list of islands of Japan by area, second-largest island of Japan and comprises the largest and northernmost prefectures of Japan, prefecture, making up its own list of regions of Japan, region. The Tsugaru Strait separates Hokkaidō fr ...

.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 Mwg, 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. was defined in a 1979 paper ...

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

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or ...

. It triggered a major

tsunami A tsunami ( ; from , ) 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 underwater explosions (including detonations, ...

that caused deaths on Hokkaidō and in southeastern

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

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

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

, the tsunami and a large

landslide Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...

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 ultramaf ...

created by back-arc spreading associated with the

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

where the Pacific plate is subducted beneath the Okhotsk microplate. The spreading was active from the

late Oligocene The Chattian is, in the geologic timescale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the pro ...

to the

middle Miocene The Middle Miocene is a sub-epoch of the Miocene epoch (geology), epoch made up of two Stage (stratigraphy), stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0. ...

. The

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the Tectonics, tectonic processes associated with, the stretching of a planetary body's Crust (geology), crust or lithosphere. Deformation styles The types of structure and the ...

associated with the spreading formed a series of N–S trending extensional faults and associated sedimentary basins. Currently the area is being deformed by contractional tectonics, causing inversion of these earlier basins, forming

anticlinal Anticlinal may refer to: *Anticline, in structural geology, an anticline is a fold that is convex up and has its oldest beds at its core *Anticlinal, in stereochemistry, a torsion angle between 90° to 150°, and –90° to –150°; see Alkane_st ...

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 resulting 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 in northern

Honshu , historically known as , is the largest of the four main islands of Japan. It lies between the Pacific Ocean (east) and the Sea of Japan (west). It is the list of islands by area, seventh-largest island in the world, and the list of islands by ...

, 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. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...

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 commonly defined as a vent or fissure 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 fo ...

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

bed. 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 A sluice ( ) is a water channel containing a sluice gate, a type of lock to manage the water flow and water level. There are various types of sluice gates, including flap sluice gates and fan gates. Different depths are calculated when design s ...

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 era Tsunamis in Japan Earthquakes in Hokkaido

1990s tsunamis Hokkaidō earthquake 1993 1990s landslides 1993 natural disasters July 1993 in Asia Tsunamis in South Korea July 1993 in Japan