8.9 earthquake: The science behind Japan's quake
8.9 earthquake: A series of large foreshocks preceded Japan's earthquake, the fifth largest ever recorded.
(Page 2 of 2)
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.Skip to next paragraph
In Pictures Japan's 9.0 earthquake
Subscribe Today to the Monitor
"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.
Earthquakes in the region are common, because Japan lies along the volatile Pacific Ring of Fire— a narrow zone around the Pacific Ocean where a large chunk of Earth's earthquakes and volcanic eruptions occur. Roughly 90 percent of all the world's earthquakes, and 80 percent of the largest ones, strike along the Ring of Fire.
The Japan Trench has seen nine events of magnitude 7 or greater since 1973. The largest of these was a magnitude 7.8 earthquake in December 1994, which caused 3 fatalities and almost 700 injuries, approximately 160 miles (260 km) to the north of today's quake. In June of 1978, a magnitude 7.7 earthquake about 22 miles (35 km) to the southwest caused 22 fatalities and over 400 injuries.
- Natural Disasters: Top 10 U.S. Threats
- Why Do Some Earthquakes Cause Tsunamis But Others Don't?
- Earthquakes and Tsunamis: How They Work