The Tohoku earthquake that struck Japan on 11th March 2011 was one of the biggest earthquakes recorded in the last 100 years and caused shaking at the surface that lasted 6 minutes. It was originally read as an 8.9 magnitude quake but was recalculated as a magnitude 9 as more data became available. It was centered on the seafloor 72 km east of Tohoku, at a depth of 24 km below the surface. The main quake was followed by over 5000 aftershocks, the largest reaching magnitude 7.9.
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Tohoku earthquake mechanism: © Pekachu
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Honshu, Japan’s main island, lies at the intersection between three tectonic plates: the Eurasian, Philippine and North American. While the geometry of the plates in Japan is quite complicated, broadly speaking the oceanic crust of the Pacific Plate is converging on the Eurasian and Philippine plate and is subducting beneath the continental crust of Japan. As these plates converge and jostle for position at the boundaries, compressional stress builds up along the moving plate boundaries, and it is the release of this stress that causes the earthquakes in Japan. In the case of Tohoku, the earthquake is linked to the subduction of the Pacific plate below the Okhotsk plate. The Pacific plate moves towards the Okhotsk at around 8-9 cm/year which is pretty speedy for a tectonic plate and as the two moved past each other, the stresses in the crust built up until they were released in the earthquake.
A magnitude 9 earthquake is very large. So large, in fact, that GPS data from the Geospatial Information Authority of Japan shows that the north-east coast of Japan moved east by up to 4 metres. At the same time, the coastline subsided by about 0.5 metres. Earthquakes of this size, where there are significant slip distances, often result in the formation of pseudotachylytes – fine grained, glassy rocks that form during frictional melting - as seen at the Outer Isles Fault in the UK. During rapid movement along the fault plane, frictional melting of the wall rocks along the fault can occur causing the formation of pseudotachylytes. The thickness of the pseudotachylyte zone is an indication of the magnitude of the event and the displacement distance. We would expect significant pseudotachylyte deposits to be found in associated with the Tohoku earthquake, it remains to be seen if we find a way to observe these but they could provide fossilsed evidence of the earthquake for future generations! Drilling into the subduction zone has led to scientists discovering a thin, slippery clay layer lining the fault. The clay layer is less than 5 m thick and would have reduced friction along the fault, allowing the two plates to slide such a long distance (50 meters).
In terms of the impact above land, as many readers will remember, the earthquake triggered a 39 m high tsunami wave which hit northern Honshu, causing devastation to people, buildings and to the Fukushima nuclear power station. Now, tsunami experts from around the world have been asked to assess the history of past tsunamis in Japan, to better predict the country's future earthquake risk. The direct financial damage from the disaster is estimated to be about $199 billion dollars (about 16.9 trillion yen), according to the Japanese government. The total economic cost could reach up to $235 billion, the World Bank estimated, making it the costliest natural disaster in world history.
Further reading:
British Geological Survey -Tohoku FAQs
Japan Earthquake & Tsunami of 2011: Facts and information