Seismicity of the Sanriku coast

The Sanriku coast has a well-documented history of significant seismic activity. A major earthquake in the 19th century caused more than 20,000 deaths, and another in the 20th century caused thousands more. The recurrence of major seismic activity continues in the 21st century.

There is geological evidence{{cite web|url=http://mdn.mainichi.jp/mdnnews/news/20110822p2a00m0na021000c.html |title=Signs of 6 massive tsunami over past 6,000 years found in disaster-hit city – the Mainichi Daily News |accessdate=2011-09-04 |url-status=dead |archiveurl=https://web.archive.org/web/20110824004129/http://mdn.mainichi.jp/mdnnews/news/20110822p2a00m0na021000c.html |archivedate=2011-08-24 }}{{cite web|url=http://web.bureau.tohoku.ac.jp/manabi/manabi16/mm16-45.html|script-title=ja:津波災害は繰り返す|language=ja|first=Koji|last=Minoura|publisher=Tohoku University|date=30 June 2001|access-date=14 March 2011}} of six catastrophic tsunamis hitting the Sanriku coast within 6000 years. Among them are:

• A catastrophic tsunami about 5400–6000 years ago, predating the Towada eruption.
• A catastrophic tsunami about 2000 years ago.
• 869 AD, Jogan earthquake and tsunami.
• 1611 AD, Keicho Sanriku earthquake.

Some 22,000 people were killed in the Meiji Sanriku earthquake of 1896. Most of the deaths were caused by tsunami. The disaster struck at 7:32{{nbsp}}pm on the afternoon of June 15.[http://www.ngdc.noaa.gov Online "NOAA/Japan: Significant Earthquake Database,"] U.S. National Oceanic and Atmospheric Administration (NOAA), National Geophysical Data Center (NGDC) The epicenter was determined to have been located at 39.5 Latitude/140.6 Longitude, and the earthquake had an estimated magnitude of {{M|w|8.5|link=yes}} .{{cite journal|last=Nishimura|first=T.|author2=Miura S.|author3=Tachibana K.|author4=Hashimoto K.|author5=Sato T.|author6=Hori S.|author7=Murakami E.|author8=Kono T.|author9=Nid K.|author10=Mishina M.|author11=Hirasawa T.|author12=Miyazaki S.|name-list-style=amp|year=2000|title=Distribution of seismic coupling on the subducting plate boundary in northeastern Japan inferred from GPS observations|journal=Tectonophysics|volume=323|issue=3–4|pages=217–238|doi=10.1016/S0040-1951(00)00108-6|bibcode = 2000Tectp.323..217N }}

Major seismic activity on the Sanriku coast during the 20th century includes:

• 1933 Sanriku earthquake- More than 3,000 people died; the casualties were caused by the subsequent tsunami. This quake/tsunami was not on the scale of the March 2011 disaster, nor earlier disasters noted above.
• 1978 Miyagi earthquake. Damage was greatest around Sendai. The earthquake triggered widespread landslides.{{cite web|url=https://earthquake.usgs.gov/earthquakes/eqarchives/significant/sig_1978.php|title=Significant Earthquakes of the World 1978|publisher=United States Geological Survey|accessdate=6 July 2010|url-status=dead|archiveurl=https://web.archive.org/web/20100123173040/http://earthquake.usgs.gov/earthquakes/eqarchives/significant/sig_1978.php|archivedate=23 January 2010}}Sidle, R.C. et al. (1985). {{Google books|2CDHFi2AVDoC|Hillslope stability and land use, pp. 1–9.|page=1}}
• 1994 offshore Sanriku earthquake. Damage in Hachinohe, Aomori.Nakayama and Takeo. [http://bssa.geoscienceworld.org/cgi/content/abstract/87/4/918 "Slip history of the 1994 Sanriku-Haruka-Oki, Japan, earthquake deduced from strong-motion data,"] Bulletin of the Seismological Society of America, Vol. 87, No. 4, p. 918.

Significant seismic events which devastated Sanriku coastal communities in the 21st century include:

• 2003 Miyagi earthquake. There were two major earthquakes in 2003 in Miyagi Prefecture. The first quake in May injured 171 and caused $97.3 million in damage.Disaster Control Research Center, Tohoku University: [http://www.disaster.archi.tohoku.ac.jp/eng/topicse/030526htm/030526e.htm May 26, 2003 Miyagi-Oki earthquake] {{webarchive|url=https://archive.today/20120907130512/http://www.disaster.archi.tohoku.ac.jp/eng/topicse/030526htm/030526e.htm |date=September 7, 2012 }} Another quake in July injured 676. More than 11,000 buildings were affected, causing an estimated $195.4 million in damage.Disaster Control Research Center: [http://www.disaster.archi.tohoku.ac.jp/eng/topicse/030726htm/030726e.htm July 26, 2003 Northern Miyagi earthquake] {{webarchive|url=https://archive.today/20120804223023/http://www.disaster.archi.tohoku.ac.jp/eng/topicse/030726htm/030726e.htm |date=August 4, 2012 }}
• 2005 Miyagi earthquake. The seismic event was originally recorded by the United States Geological Survey as a 7.2-magnitude earthquake on the Moment magnitude scale, but the Japan Meteorological Agency called it a magnitude 6.9 earthquake. The slippage in the seabed subduction zone was located about {{convert|330|mi|km|abbr=off}} east-northeast of Tokyo about {{convert|24|mi|km|abbr=off}} below the surface of the Pacific Ocean. The Japan Meteorological Agency issued a tsunami warning almost immediately; and warnings were also issued for the northwest coast of the United States
• 2011 Tōhoku earthquake and tsunami. In March 2011, the largest earthquake ever recorded in Japan struck off the Sanriku coast, setting off a {{convert|10|m|ft|abbr=off}} tsunami. The 9.0-magnitude quake near Tohoku was comparable in scale to undersea seismic events near Indonesia in 2004 (3rd largest on record) and near Chile in 2010 (6th largest). It caused 15,889 deaths, 6,152 injured, and 2,609 people missing across twenty prefectures, with an estimated $235 billion in property damage.

In the list of 45 most powerful earthquakes recorded worldwide since 1500, there are five that appear to be clustered in a 8-year span (in 2004, 2005, 2010, 2011, 2012) with the largest being the 2004 Indian Ocean earthquake of magnitude 9.1. Prior to this, no sizable earthquake occurred. There was another apparent cluster (in 1960, 1963, 1964, 1965) which contained the largest recorded earthquake, the 1960 Valdivia earthquake of magnitude 9.4. However, experts construe this as a statistical anomaly or random chance.Pappas, Stephanie. [http://www.livescience.com/13196-earthquakes-worse.html "Sumatra, Japan, Chile: Are Earthquakes Getting Worse?"] LiveScience, 11 March 2011.Brahic, Catherine. [https://www.newscientist.com/article/mg20928043.000-the-megaquake-connection-are-huge-earthquakes-linked.html "The megaquake connection: Are huge earthquakes linked?"] New Scientist (UK), 16 March 2011.

The phenomenon of comparably large quakes that happen on the same or neighbouring faults within months of each other—for example, the Miyagi quakes in 2003—can be explained by a sound geological mechanism. This does not fully demonstrate a relationship between events separated by longer periods and greater distances.

Earthquakes occur where the Pacific plate meets the plate beneath northern Honshu in a subduction zone.Sample, Ian. [https://www.theguardian.com/world/2011/mar/11/japan-earthquake-tsunami-questions-answers "Japan earthquake and tsunami: what happened and why,"] The Guardian (UK). 11 March 2011; retrieved 14 Mar 2011 The Pacific plate, which moves at a rate of {{convert|8|to|9|cm|abbr=on}} per year, dips under Honshu's underlying plate releasing large amounts of energy. This motion pulls the upper plate down until the stress builds up enough to cause a seismic event.

Quakes of large magnitudes in the Sanriku region can have a rupture length of hundreds of kilometers; and this generally requires a long, relatively straight fault surface. However, the plate boundary and subduction zone in this area is not very straight.Maugh, Thomas H. [http://www.latimes.com/news/nationworld/nation/la-sci-japan-earthquake-20110310,0,7154967.story "Size of Japan's quake surprises seismologists,"] Los Angeles Times (US). 11 March 2011; retrieved 11 Mar 2011 The "epicentral region" of an earthquake is defined as an elliptical area which encompasses the location of highest felt intensity of an earthquake. This term is used for identifying the likely epicenter for earthquakes and tsunami in the history of the Sanriku coat.

The hypocentral region of Sanriku coastal earthquakes can occur in locations extending from offshore Aomori Prefecture to offshore Ibaraki Prefecture.

• Sanriku earthquake (Sanriku tsunami)
• List of earthquakes in Japan

{{Wikinews | Quake measuring 6.9 off Japan's Honshū coast }}

{{Reflist}}

• Sidle, R.C.; Pearce, A.J.; O'Loughlin, C.L. (1985) Hillslope stability and land use. Washington, D.C.: American Geophysical Union. {{ISBN|978-0-87590-315-6}}

• {{cite news | url = https://news.yahoo.com/s/ap/20051114/ap_on_re_as/japan_quake | title = Strong Earthquake Shakes Northern Japan | date = November 14, 2005 | agency = Associated Press }}
• {{cite news | url = http://www.cnn.com/2005/WORLD/asiapcf/11/14/japan.quake/index.html | title = Japan quake prompts tsunami alert | publisher = CNN| date = November 14, 2005 }}
• {{cite web | url = https://earthquake.usgs.gov/recenteqsww/Quakes/usfkbr.htm | title = Technical information| publisher = United States Geological Survey}}
• [https://www.usatoday.com/news/world/2005-11-14-japan-earthquake_x.htm Earthquake shakes Japan, small tsunamis measured]

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Category:Earthquakes in Iwate Prefecture

Category:Earthquakes in Fukushima Prefecture

Category:Earthquakes in Miyagi Prefecture