I've posted several different explanations here describing how earthquakes and tsunamis work. This week, though, the Miller-McCune Curiouser and Curiouser podcast takes that generalized information and does a nice job of applying it specifically to Japan. First, they talk about the general stuff—plate tectonics and why earthquakes happen. Then they talk about the Tohoku earthquake and tsunami—and how the general facts play out into real-world disasters.
On Wednesday, March 9, two days before the main event, there was a 7.2 magnitude earthquake on the plate boundary just East of Japan under the Pacific Ocean. Not the main thing but still a very big earthquake, the kind this part of Japan gets every few decades. This time, though, very bad. Part of the Pacific Plate came unstuck and suddenly shifted underneath Japan. The part that moved put even more strain on the much larger part of the Pacific Plate that was still really, really stuck—and two days later, the strain was too much. On March 11, a huge stretch of the Pacific Plate, maybe 300 miles long, broke free and surged westward under Japan—as much as 120 ft of movement, all at once. At the same time, the plate Japan sits on moved east by 9 feet.
If you want to skip ahead to the Tohoku tsunami description, it begins at about -2:50 in the podcast
The image above is a detail from a really informative poster that the USGS has put together.
Via Kerri Wachter