The Tohoku earthquake that struck Japan on 11th March was one of the biggest earthquakes recorded in the last years and caused shaking at the surface that lasted 6 minutes. It was originally read as an 8. 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 aftershocks, the largest reaching magnitude 7. 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. 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. 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! Northern Japan is largely on top of the western tip of the North American plate. Southern Japan sits mostly above the Eurasian plate.
Image credit: USGS. The areas where the arcs meet each other are called collision zones, and feature more complex geography than other parts of the country. Currently, the Pacific Plate is undergoing the process of subduction under the Okhotsk Plate albeit at a slow speed of a few centimeters per year.
The process of subduction leads to the recycling of the continental crust, which is why most of the rocks in the Japanese archipelago date back the Permian era or beyond. These faults accumulate strain over the years as two plates butt heads. The resulting earthquake was about 1, times stronger than the 6.
In fact, the earthquake generated roughly the amount of energy that the U. Displacement of the fault by a magnitude 7. The upward thrusting of the seafloor contained enough energy to displace an enormous amount of water, churning up huge waves that swept across swaths of the Japan coast and tore across the open ocean at the speed of a jet plane.
They move as fast as miles per hour, he added. Tsunamis are tracked using sensors out in the ocean, such as Deep-ocean Assessment and Reporting of Tsunamis DART buoys, which track pressure and movement and send information by satellite back to the Pacific Tsunami Warning Center. Unlike a normal surfing wave, which curls over from the top, but is quiet underneath, a tsunami moves from top to bottom toward the shore, bulldozing everything in its path. As it moves toward the land, it shortens in length, but rises in height.
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