The Japanese earthquake ruptured near the boundary between the Pacific and North American tectonic plates — huge, moving slabs of the Earth's crust. The quake was a megathrust earthquake, where the Pacific plate dove underneath Japan at the Japan Trench. The seafloor was pushed away from Japan sending waves roaring toward Hawaii and the West Coast of the United States.
The epicenter of today's quake was about 15.2 miles (24.4 kilometers) deep, according to the USGS, which is near enough to the surface to set off a tsunami.
"Generally we don't get a tsunami unless we have a shallow quake, and that's exactly what happened," Caruso said.
Foreshocks, not forewarning
Today's earthquake was preceded by a series of large foreshocks over the previous two days, beginning on March 9 with a magnitude 7.2 quake about 25 miles (40 km) away, and continuing with three other earthquakes greater than magnitude 6, according to the USGS.
Japan's latest national seismic risk map gave a 99 percent chance of at least a magnitude 7.5 quake hiting the region in the next 30 years, Robert Geller, University of Tokyo geophysicist, told Science Magazine, but today's quake was more than 100 times powerful.