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Geodynamics

Japan earthquake 11 March 2011, M=8.9 (11.03.2011)

An earthquake with magnitude 8.9 (reported by USGS) occurred of the east coast of Honshu, Japan, at 05:46 UTC on 11 March 2011. The earthquake was located about 400 km NE of Tokyo in the subduction zone between the Pacific and Eurasian plates. The convergence rate between the two plates is about 8 cm/year. This relative motion leads to a build-up of stresses when the plates are locked; the release of the stress takes place during sudden rupture - the earthquake. Due to its shallow size (hypocenter at 24 km depth) and thrust mechanism it generated a large tsunami with wave heights of about 10 m on the Japanese coast, with the potential to sperad across the Pacific. There was a magnitude 7.2 foreshock on 9 March 2011, and three more foreshocks greater than magnitude 6. Both the earthquake and tsunami have caused considerable damage in Japan. However, it will probably take some days before a clear picture of the damage emerges. A large number of aftershocks are recorded, the largest so far with a magnitude of 7.1, 50 minutes after the mainshock. Aftershocks are expected to follow, possibly for several months.

This was one of the ten largest earthquakes to occur in the last century, these are called great earthquakes. The previous most recent great earthquakes struck Indonesia in 2004 and Chile in 2010. These earthquakes involve rupture over several hundred kilometres with slip of more than 10 metres. Compared to the recent Christchurch (New Zealand) earthquake, the 2011 Japan earthquake was more than 1000 times larger.

Earthquakes of this size are recorded on all seismic stations around the world, so also in Norway. It took about 10 minutes before the first seismic waves had reached Norway, where the stations are between 6600 and 8500 km from the source. At this distance and for earthquakes of this size, the magnitude is measured from surface waves. Ground motions measured from this earthquake in Norway are of the order of 5 mm at low frequencies.

Links:

IRIS - teaching material: http://www.iris.edu/hq/retm/#1328

USGS: http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usc0001xgp.php

Pacific tsunami warning: http://www.weather.gov/ptwc

US West Coast and Alaska tsunami warning: http://wcatwc.arh.noaa.gov