1994 offshore Sanriku earthquake
Encyclopedia
The 1994 offshore Sanriku earthquake (Japanese
: 1994年三陸はるか沖地震 Senkyūhyakukyūjūyo-nen Sanriku-haruka-oki Jishin) occurred on December 28, 1994 at 12:19 UTC (21:19 local time). This was a magnitude Mw 7.7 earthquake with epicenter located in the Pacific Ocean
at about 180 km east of Hachinohe
, Aomori
(haruka-oki means "far offshore"). Three people were reported dead and more than 200 injured. 48 houses were completely destroyed. Road damage and power outages were reported. Liquefaction occurred in the Hachinohe Port area. The intensity reached shindo
6 in Hachinohe, Aomori, about 187.6 km from epicenter. It could be felt in Tokyo
, about 632.9 km from epicenter, with shindo 2. The Japanese Meteorological Agency put the magnitude at Mjma 7.5. Slip associated with this earthquake continued for more than a year and it has been termed an 'ultra-slow earthquake'.
and Hokkaido
lie above the convergent plate boundary
, where the Pacific Plate
is subducting
beneath the Okhotsk Plate
. 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
. This part of the plate boundary has been the location of many powerful historical earthquakes, dating back to the 869 Sanriku earthquake
and most recently of the devastating 2011 Tōhoku earthquake
. Most of these events relate to rupture along the plate interface, but some, such as the 1933 Sanriku earthquake
, involved deformation within the subducting plate. According to the studies of past great earthquakes, a weak seismic coupling is suggested in the offshore Sanriku
region. From the distribution of past seismicity, the width of coupling at 40°N was assumed to be about 150 km.
, with a low-angle reverse fault focal mechanism
, 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 '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
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 singificantly 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 Mw 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 Mw 7.84. The seismic moment of the earthquake was put at 4.3×1020 Nm by the USGS
. 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.
and Ayukawa
in Iwate
and Hachinohe in Aomori the tsunami was about 50 cm high. There was no report of damage caused by the tsunami.
The strongest aftershock occurred on January 6, 1995 at 22:37 UTC (January 7, 1995 at 07:37 local time) with magnitude Mw 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.
, Shindo 5 at Aomori
, Morioka
and Mutsu
and Shindo 4 at Miyako, Ofunato
, Hakodate
, Obihiro
, Tomakomai
and Urakawa
. 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.
Japanese language
is a language spoken by over 130 million people in Japan and in Japanese emigrant communities. It is a member of the Japonic language family, which has a number of proposed relationships with other languages, none of which has gained wide acceptance among historical linguists .Japanese is an...
: 1994年三陸はるか沖地震 Senkyūhyakukyūjūyo-nen Sanriku-haruka-oki Jishin) occurred on December 28, 1994 at 12:19 UTC (21:19 local time). This was a magnitude Mw 7.7 earthquake with epicenter located in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
at about 180 km east of Hachinohe
Hachinohe, Aomori
- Neighbouring municipalities :Aomori Prefecture*Sannohe District**Hashikami**Gonohe**Nanbu*Kamikita District**OiraseIwate Prefecture*Karumai-History:...
, Aomori
Aomori Prefecture
is a prefecture of Japan located in the Tōhoku Region. The capital is the city of Aomori.- History :Until the Meiji Restoration, the area of Aomori prefecture was known as Mutsu Province....
(haruka-oki means "far offshore"). Three people were reported dead and more than 200 injured. 48 houses were completely destroyed. Road damage and power outages were reported. Liquefaction occurred in the Hachinohe Port area. The intensity reached shindo
Japan Meteorological Agency seismic intensity scale
The Japan Meteorological Agency seismic intensity scale is a seismic scale used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of...
6 in Hachinohe, Aomori, about 187.6 km from epicenter. It could be felt in Tokyo
Tokyo
, ; officially , is one of the 47 prefectures of Japan. Tokyo is the capital of Japan, the center of the Greater Tokyo Area, and the largest metropolitan area of Japan. It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family...
, about 632.9 km from epicenter, with shindo 2. The Japanese Meteorological Agency put the magnitude at Mjma 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 HonshuHonshu
is the largest island of Japan. The nation's main island, it is south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Strait...
and Hokkaido
Hokkaido
, formerly known as Ezo, Yezo, Yeso, or Yesso, is Japan's second largest island; it is also the largest and northernmost of Japan's 47 prefectural-level subdivisions. The Tsugaru Strait separates Hokkaido from Honshu, although the two islands are connected by the underwater railway Seikan Tunnel...
lie above the convergent plate boundary
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
, where the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
is subducting
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
beneath the Okhotsk Plate
Okhotsk Plate
The Okhotsk Plate is a tectonic plate covering the Sea of Okhotsk, the Kamchatka Peninsula, Sakhalin Island and Tōhoku and Hokkaidō in Japan. It was formerly considered a part of the North American Plate, but recent studies indicate that it is an independent plate, bounded on the north by the...
. 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
Japan Trench
__notoc__The Japan Trench is an oceanic trench, a part of the Pacific Ring of Fire, in the floor of the northern Pacific Ocean off northeast Japan. It extends from the Kuril Islands to the Bonin Islands and is at its deepest. It is an extension of the Kuril-Kamchatka Trench to the north and the...
. This part of the plate boundary has been the location of many powerful historical earthquakes, dating back to the 869 Sanriku earthquake
869 Sanriku earthquake and tsunami
The struck the area around Sendai in the northern part of Honshu on 9 July 869 . The earthquake had an estimated magnitude of 8.6 on the surface wave magnitude scale...
and most recently of the devastating 2011 Tōhoku earthquake
2011 Tōhoku earthquake and tsunami
The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
. Most of these events relate to rupture along the plate interface, but some, such as the 1933 Sanriku earthquake
1933 Sanriku earthquake
The was a major earthquake whose associated tsunami caused widespread damage to towns on the Sanriku coast of the Tōhoku region of Honshū, Japan on March 2, 1933.-History:The epicenter of the 1933 Sanriku earthquake was located offshore,...
, involved deformation within the subducting plate. According to the studies of past great earthquakes, a weak seismic coupling is suggested in the offshore Sanriku
Sanriku
is a historical region of Japan on the northeastern side of the island of Honshu, corresponding to today's Aomori, Iwate and parts of Miyagi Prefecture...
region. From the distribution of past seismicity, the width of coupling at 40°N was assumed to be about 150 km.
Earthquake
This was an interplate earthquakeInterplate earthquake
An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. If one plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates to slip relative to each other...
, with a low-angle reverse fault focal mechanism
Focal mechanism
The focal mechanism of an earthquake describes the inelastic deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is also known as a fault-plane solution...
, 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 '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
Seismic moment is a quantity used by earthquake seismologists to measure the size of an earthquake. The scalar seismic moment M_0 is defined by the equationM_0=\mu AD, where*\mu is the shear modulus of the rocks involved in the earthquake...
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 singificantly 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 Mw 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 Mw 7.84. The seismic moment of the earthquake was put at 4.3×1020 Nm by the USGS
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
. 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.
Tsunami
A local tsunami was triggered by this earthquake. In MiyakoMiyako, Iwate
is a city located in Iwate, Japan.It was founded on February 11, 1941.The city lies along the coast where the flows into the Pacific Ocean. It is connected to Morioka by an east-west train line and highway and the coastal highway also goes through the town...
and Ayukawa
Ayukawa
-People:*Ami Ayukawa, AV Idol*Yoshisuke Aikawa, entrepreneur, businessman, and politician*Makoto Ayukawa, guitarist, composer, member of the Japanese rock band Sheena & The Rokkets*Mami Ayukawa, singer-songwriter*Yoshifumi Ayukawa, baseball player-Places:...
in Iwate
Iwate Prefecture
is the second largest prefecture of Japan after Hokkaido. It is located in the Tōhoku region of Honshū island and contains the island's easternmost point. The capital is Morioka. Iwate has the lowest population density of any prefecture outside Hokkaido...
and Hachinohe in Aomori 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 Mw 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 MisawaMisawa, Aomori
is a city located in eastern Aomori Prefecture in the Tōhoku region of Japan. As of 2009, the city had an estimated population of 42,399 and a density of 353 persons per km²...
, Shindo 5 at Aomori
Aomori, Aomori
is the capital city of Aomori Prefecture, in the northern Tōhoku region of Japan. As of 2009, the city had an estimated population of 302,068 and a density of 366 persons per km². Its total area was 824.52 km².- History :...
, Morioka
Morioka, Iwate
is the capital city of Iwate Prefecture, Japan.As of 2005, the city has an estimated population of 300,740 and a population density of 588.11 persons per km². The total area is 489.15 km²....
and Mutsu
Mutsu, Aomori
is a city located in northeastern Aomori in the Tōhoku region of Japan. As of 2009, the city had an estimated population of 61,205 and a density of 70.09 persons per km²...
and Shindo 4 at Miyako, Ofunato
Ofunato, Iwate
is a coastal city located in Iwate, Japan.It was founded on April 1, 1952.On November 15, 2001, the town of Sanriku, from Kesen District, was merged into Ōfunato....
, Hakodate
Hakodate, Hokkaido
is a city and port located in Oshima Subprefecture, Hokkaido, Japan. It is the capital city of Oshima Subprefecture.Hakodate was Japan's first city whose port was opened to foreign trade in 1854 as a result of Convention of Kanagawa, and used to be the most important port in northern Japan...
, Obihiro
Obihiro, Hokkaido
is a city located in Tokachi, Hokkaidō, Japan. Obihiro is the only city in the Tokachi area. The next most populous municipality in Tokachi is the town of Otofuke, with less than a third of Obihiro's population. The city has approximately 500 foreign residents . The city contains the headquarters...
, Tomakomai
Tomakomai, Hokkaido
is a city and port located in Iburi, Hokkaido, Japan. Though its name implies a small, idyllic town, it is the largest city in the Iburi Subprefecture. As of 2008, it had an estimated population of 173,504 and a population density of 310 persons per km²...
and Urakawa
Urakawa, Hokkaido
is a town located in Urakawa District, Hidaka, Hokkaidō, Japan.As of 2008, the town has an estimated population of 14,814 and a density of 21.3 persons per km². The total area is 694.24 km².-External links:* in Japanese...
. 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.