There are all kinds of resources available right now that explain how earthquakes and tsunamis happen. Some are written for experts, some are written for "regular people" and many are written specifically for children. I included a few of these resources on a previous post.
Here are a few more sites you might want to visit:
Satellite Photos of Japan, Before and After the Quake and Tsunami - excellent site from The New York Times. Be sure to click on the "slider" and move it back and forth to reveal the before and after pictures.
Japan Earthquake/Tsunami Infographic - this infographic does a nice job of giving details about the power of the earthquake and tsunami, as well as information on casualties, damages and effects, and ways we can help.
Japan Quake Map - Did you know there were actually 78 earthquakes that happened in and around Japan on Friday, March 11, 2011? (and as of today, there have been 734 quakes since March 11???) This map does a great job of showing exactly when, where, and how big each quake actually was. Use the drop-down menu to select a specific day and then watch. The size of the ring indicates the magnitude and the color indicates how deep in the ocean the quake occurred.
How Earthquakes Trigger Tsunamis - this is an excellent video from BBC News that demonstrates how earthquakes caused the tsunamis. It's in simple enough terms that our elementary students will be able to understand it.