Magnitude 8.9 earthquake hits off coast of Tokyo, Sendai, Japan; tsunami warnings out

Earthquake and aftershocks from NOAA's weather.gov

A shallow earthquake, with a preliminary magnitude of 8.9 has hit off the coast of Japan.  Friends in Japan, our thoughts are with you.
     The islands of the Japanese archipelago lie on the Pacific Ring of Fire, a swath of tectonic activity that surrounds the Pacific ocean basin.  Earthquakes and volcanic eruptions occur here where large plates on the earth's surface collide.  In the western Pacific, the Pacific Ocean plate is forced down (subducted) beneath the Eurasian Plate.  Sediment and water are scraped off the subducted plate, melt, and rise up to form islands, such as Japan. Subduction is not a smooth process, but occurs in fits and starts as stresses build up between the plates and are then released in fault motions and earthquakes. In detail, Japan's tectonic setting is very complicated with many different faults, and interaction with a third plate, the Philippine plate (see map below from Wiki that illustrates this. At least two articles in the New York Times have said erroneously that the tectonic activity arises from a collision between the Pacific and North American plates!)
     The earthquake was offshore, about 230 miles from Tokyo. This is the largest earthquake to hit Japan in recorded history, and larger than expected by geologists.  Susan Hough of the USGS said that it was bigger than the 8-8.5 that head been expected for the area. A tsunami warning is posted for around the Pacific, with some estimates of waves 5 meters (17 feet) in height. There are four nuclear power plants in the vicinity of the maximum damage and people had been evacuated as a precaution because there were some troubles cooling one reactor. It appeared that all four had been safely shut down, but it has proven difficult or impossible to cool two of them.  There is confusing information about whether or not there have been small releases of nuclear materials. Here is a March 11 (evening, EDT) New York Times article on the situation with two of the reactors.

Tectonic setting of Japan, simplified
From Wiki. Satellite view is public domain, NASA
More information on aftershocks as of March 15 is at
this USGS link.

     Cnn.com is carrying live news, and it's repetitious to repeat it here. Aftershocks greater than M6 are frequent and at least 2of magnitude 7.1 have been recorded. A rule of thumb is that aftershocks can be as large as the earthquake minus one magnitude (8.91=7.9), so more magnitude 7 quakes are likely. The world stock markets are dropping because Tokyo, a financial hub of the world, is badly hit.
     The best documentation that I have ever seen of the size and power of a tsunami is this video from CNN.com of the events in Sendai, Nitori City and several other places.  The opening shot of white foam-crested tsunami waves approaching the shore is stunning. Initially, relatively clear water moves ships around in a slow-motion ballet, and then as it picks up more cars and boats, they are driven chaotically driven inland, under bridges that still carry cars and people over the chaotic waters below.  Ugly lobes of black water and mud carry mats of cars, buildings, and crushed buildings across highways and farm fields, sweeping all in its path away.  Burning buildings float on the mat as if unaware of the watery conveyor belt.  Frothy white water covers fields and airplane runways.  In this video, cars from a manufacturing plant (?) have caught fire, an eerie scene that looks like the Boy Scouts gathered cars for kindling instead of wood. These videos have several parts separated by commercials, and may change with time as CNN edits their footage.
   Here an oil refinery or chemical plant has caught fire. This is a video that shows a longer clip of the ships from the port of Kamaishi being swept under a bridge. It is an especially interesting video because at the beginning the tsunami is not very deep (a few meters?) and there is an obvious hydraulic jump (white wave) on the left side of the bridge where water pours over a lip.  Within seconds, the water level rises and the hydraulic jump disappears under the bridge, and then emerges as white water and waves on the right side of the bridge.  Amazing that people remain standing and driving on the bridge!
    Here's a different view of the tsunami advancing. Note the lobate fronts and the moving burning buildings. This video of the tsunami advancing onto Rikuzentataka gives a new meaning to the word inexorable. Again, this video may be linked to some others in a sequence, so there may be two or three scenes. With some redundancy, there is a good helicopter shot of different parts of the advancing tsunami front here, as well as shots that show the big whirlpool circulation.
      The Big Picture, a Boston.com site devoted to telling news stories in photographs, has a collection of striking pictures is here. Picture #10 is especially interesting if anyone can help tell why such a large whirlpool formed! #36 is an amazing shot of a fault running right down the center stripe of a highway. This NYTimes site has good footage of boats being swept away at Brookings, Oregon, of the tsunami coming onshore at Crescent City, California (a place known for tsunami hazards).  It had a video of the tsunami engulfing the nuclear power plant that is having problems in Japan, but now "it has been removed by the user." Wonder what that means?

Google runs a Google Crisis Response page that seems to have good and reliable technical and logistic information. Other information is available at NOAA's West Coast and Alaska Tsunami Warning Center.