Eastern Honshu, Japan's largest island, was devastated by a great (M 8.9) earthquake at 2:45 pm Friday local time. It was a classic subduction thrust event, and a 4-meter tsunami washed over the Japanese coast. Tsunami waves have affected most of the Pacific rim. [Update: Chris Rowan discusses the scientific details of this typical subduction-zone event. And Erik Klemetti addresses the persistent claim that the Moon influenced this quake. And the US Geological Survey has issued new estimates putting the magnitude as high as 9.1, making it the fifth largest ever recorded by seismographs.]
The USGS notes that this quake had foreshocks starting two days beforehand. The first of these was a M 7.1, coincidentally the same size as the largest aftershock so far. Of course, no one could definitively say that they were foreshocks at the time, because there's no way known to distinguish them from ordinary earthquakes.
The USGS' automated PAGER system estimated that more than 4 million people were subjected to very strong (intensity VII) shaking, and that Tokyo and Yokohama, with their 12 million inhabitants, felt strong (VI) shaking. The model estimated that there have been on the order of 100 deaths from the earthquake; this does not include the tsunami.
Japan has been preparing for events like this for more than 30 years. Its "design quake," called the Tokai Earthquake, is one that appears to recur regularly farther south near Tokyo. But the lessons learned from Tokai planning are applied all over the country. In America we can anticipate a similar earthquake, in size and effects, somewhere in the Cascadia subduction zone on the century scale.