Subduction zones and earthquakes

I am still waiting to hear from a couple friends in Japan, and I hope to hear that they and their families are safe. There are also extensive fears about nuclear radiation, as well as problems with evacuations and shelter for those displaced, throughout the Tohoku region. There are many good resources for information on these factors, so I will not focus on them.

Instead I will focus on the geological effects of this earthquake, since I understand these a bit better.

The 8.9 magnitude earthquake has shifted the Earth of its access by about 16.5 cm. This will cause the planet to rotate faster and shorten the length of the Earth's day by 1.8 millionths of a second or so. It has also moved the coastline of Japan in the Tohoku region by about 4 m to the east in some regions.
The cause of this earthquake is due to the subduction zone which lies to the east of Japan (the thick line to the east of the epicenter on the image below). The Pacific Plate is being subducted, or pulled under, the North American Plate, on which Japan lies. The motion of the scraping of the two plates together builds up strain energy which then must be released at some indeterminable period. The release of the strain results in an earthquake, in this case a very large earthquake.

When the movement of the Earth occurred underwater, it displaced a large bit sea water and thus created the large tsunami which hit the coastal regions of the Tohoku region. The Pacific Plate's maximum westward movement was about 20 m, with the movement along the fault reducing as you go away from the epicenter. This is also why the amount of shaking and damage decreased away from the epicenter and fault.





BBC - How the quake has moved Japan

8.9 Magnitude Earthquake Offshore Japan

An 8.9 magnitude earthquake occurred offshore of Honshu, Japan, on Friday, March 11, 2011, at 14:26 local time. This is the largest earthquake the country has ever experienced, and the seventh largest ever recorded. It occurred 382 km northeast of Tokyo.

The towns along a 2,100-km range of eastern coastline of Honshu were affected by the earthquake and its 19 aftershocks, most of which were greater than 6.0 magnitude. A 7-metre tsunami was launched which carried cars, boats, and even planes inland. At least 60 people have been killed.

Tsunami warnings have been issued for the west coast of South America, the west coast of the U.S., Hawaii, New Zealand, and other areas in the Pacific: Pacific Tsunami Warning Center.

More News:
Earthquake: Japan Hit by 8.9 Earthquake/Japan Tsunami Warning (Sydney Morning Herald)