The 1994 offshore Sanriku earthquake (

Japanese Japanese may refer to: * Something from or related to Japan, an island country in East Asia * Japanese language, spoken mainly in Japan * Japanese people, the ethnic group that identifies with Japan through ancestry or culture ** Japanese diaspor ...

: 三陸はるか沖地震 ''Sanriku Haruka Okijishin'') occurred on December 28, 1994, at 12:19 UTC (21:19 local time). This was a magnitude 7.7 earthquake with epicenter located in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

at about 180 km east of

Hachinohe is a city located in Aomori Prefecture, Japan. , the city had an estimated population of 213,453 in 110,121 households, and a population density of . As of October 2020, Hachinohe is Aomori Prefecture's second largest city by population. The c ...

Aomori , officially Aomori City (, ), is the capital Cities of Japan, city of Aomori Prefecture, in the Tōhoku region of Japan. , the city had an estimated population of 264,945 in 136,781 households, and a population density of 321 people per squa ...

(''haruka-oki'' means "far offshore"). The intensity reached shindo 6 in Hachinohe, Aomori, about 187.6 km from epicenter. It could be felt in

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, about 632.9 km from epicenter, with shindo 2. The

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put the magnitude at 7.5. Slip associated with this earthquake continued for more than a year and it has been termed an 'ultra-slow earthquake'.

Geology

The northern part of

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 ...

and

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lie above the

convergent plate 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

subducting Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second pla ...

beneath the Okhotsk microplate. The convergence rate across this boundary lies in the range of 7.9–9.2 cm per year. The plate interface in the area of the earthquake epicenter shows an abrupt increase in dip from about 5° to about 15°, 80 km landward of the

Japan Trench The Japan Trench is an oceanic trench part of the Pacific Ring of Fire off northeast Japan. It extends from the Kuril Islands to the northern end of the Izu Islands, and is at its deepest. It links the Kuril–Kamchatka Trench to the north and ...

. This part of the plate boundary has been the location of many powerful historical earthquakes, dating back to the

869 Sanriku earthquake __NOTOC__ Year 869 ( DCCCLXIX) was a common year starting on Saturday of the Julian calendar. Events By place Byzantine Empire * Summer – Emperor Basil I allies with the Frankish emperor Louis II against the Saracens. He sends ...

and most recently of the devastating

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. Most of these events relate to rupture along the plate interface, but some, such as the

1933 Sanriku earthquake The occurred on the Sanriku coast of the Tōhoku region of Honshū, Japan on March 2 with a moment magnitude of 8.4. The associated tsunami caused widespread devastation. Earthquake The epicenter was located offshore, east of the city of ...

, involved deformation within the subducting plate. According to the studies of past great earthquakes, a weak seismic coupling is suggested in the offshore

Sanriku , sometimes known as , lies on the northeastern side of the island of Honshu, corresponding to today's Aomori, Iwate and parts of Miyagi Prefecture and has a long history. The 36 bays of this irregular coastline tend to amplify the destructivenes ...

region. From the distribution of past seismicity, the width of coupling at 40°N was assumed to be about 150 km. In 1999, an investigation was conducted in the source regions of the 1968 offshore Tokachi earthquake and the 1994 offshore Sanriku earthquake. It was found that the structures of crust are heterogeneous in the north and south of 40°10' N. The thickness of the crust is 21 km in the north and 15 km in the south. The P wave velocity in the north is 7% slower than that in the south.

Earthquake

This was an

interplate earthquake An interplate earthquake occurs at the boundary between two tectonic plates. Earthquakes of this type account for more than 90 percent of the total seismic energy released around the world. If one plate is trying to move past the other, they will ...

, with a low-angle

reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic f ...

focal mechanism The focal mechanism of an earthquake describes the Fault (geology)#Slip.2C heave.2C throw, deformation in the Hypocenter, source region that generates the seismic waves. In the case of a Fault (geology), fault-related event, it refers to the ori ...

, which ruptured part of the plate interface. The region of the largest slip in this earthquake coincides with the region of low seismic moment release in the 1968 Offshore Tokachi earthquake on May 16, 1968, implying that the 1994 Offshore Sanriku earthquake completed the incompleted fault slip in the source region of the 1968 Offshore Tokachi earthquake. The average slip of the fault was estimated to be 0.4 m and the maximal slip about 1.2 m. The slip caused by this earthquake can be divided into a ''coseismic'' slip that occurred immediately before, during and immediately after the event, and a long period of slow post-seismic slip that continued for more than a year after the original event. The slow post-seismic slip following this earthquake had a large

seismic moment Seismic moment is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment M_0 is defined by the equation M_0=\mu AD, where *\mu is the shear modulus of the rocks involved in the earthquake (in pascal (unit) ...

compared with the coseismic slip associated with the main event. The post-seismic slip occurred in two phases; an initial faster short-term phase that lasted for about ten days, terminating with the largest aftershock, followed by a long-term slower phase. The slip area of the initial phase matches the rupture area for the main event. The second phase migrated significantly further down-dip and to the south, along the plate boundary. The main event released a seismic moment of about 3×10²⁰ Nm, corresponding to magnitude 7.59. However, the total seismic moment released by the main event and the following slow slip events up to about 1 year thereafter was about 8×10²⁰ Nm, which corresponds to magnitude 7.84. The seismic moment of the earthquake was put at 4.3×10²⁰ Nm by the

USGS The United States Geological Survey (USGS), founded as the Geological Survey, is an government agency, agency of the United States Department of the Interior, U.S. Department of the Interior whose work spans the disciplines of biology, geograp ...

. The slow slip in the landward side of the source region was found to be larger than that in the trenchward side of the source region. The maximum accelerations recorded in Hachinohe were 602.3 cm/s² in NS, 488.4 cm/s² in EW, and 94.1 cm/s² in UD.

Tsunami

A local tsunami was triggered by this earthquake. The recorded maximum height of the tsunami was in Miyako, which was about 55 cm. In Ayukawa, Miyagi Prefecture and in Hachinohe, Aomori Prefecture, the tsunami was about 50 cm high. There was no report of damage caused by the tsunami.

Aftershocks

The locations of the most part of the aftershocks were within a 170 km × 84 km region west of the initial break of the main shock. Most of the aftershocks occurred in the region of small slip in the main rupture. The strongest aftershock occurred on January 6, 1995, at 22:37 UTC (January 7, 1995, at 07:37 local time) with magnitude 6.9, leaving 29 people injured. It was located at 40.246 N, 142.175 E with a depth of 27 km. The intensity reached shindo 5 in Hachinohe, Aomori. This aftershock could be felt in Tokyo with shindo 2.

Damage

The felt intensity of the earthquake reached Shindo 6 in Hachinohe and Misawa, Shindo 5 at

Morioka is the capital Cities of Japan, city of Iwate Prefecture located in the Tōhoku region of northern Japan. On 1 August 2023, the city had an estimated population of 283,981 in 132,719 households, and a population density of . The total area of t ...

and Mutsu and Shindo 4 at Miyako, Ofunato,

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Obihiro is a Cities of Japan, city in Tokachi Subprefecture, Hokkaido, Japan. Obihiro is the only designated city in the Tokachi Subprefecture, Tokachi area. As of July 31, 2023, the city had an estimated population of 163,084. The next most populou ...

Tomakomai is a city and port in Iburi Subprefecture, Hokkaido, Japan. It is the largest city in the Iburi Subprefecture, and the fourth largest city in Hokkaido. As of 31 July 2023, it had an estimated population of 167,372, with 83,836 households, an ...

and

Urakawa is a List of towns in Japan, town located in Hidaka Subprefecture, Hokkaido, Japan. As of 1 October 2020, the town has an estimated population of 12,074 and a population density, density of 17 persons per km2. The total area is 694.24 km2. ...

. In the prefectures of Aomori and Iwate, about 69,000 households had no electricity. A total of 6,229 buildings were affected, with 48 completely destroyed, 378 partially destroyed and another 5,803 suffering some damage. The total damage was estimated at 170.4 million US dollars. Three people were reported dead and more than 200 injured; 48 houses were completely destroyed. Road damage and power outages were reported.

Liquefaction In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics. It occurs both naturally and artificially. As an example of t ...

occurred in the Hachinohe Port area.

References

External links

* {{JapanTrenchMegathrust Offshore Sanriku earthquake Offshore Sanriku Offshore Sanriku earthquake Offshore Sanriku earthquake Earthquakes of the Heisei era Tsunamis in Japan Offshore Sanriku earthquake