• Tsunami Alerts: Correspondent Bennett Richardson has lived in Japan for eight years, so he's had firsthand experience with the earthquake and tsunami alert system (see story). "They flash the location and strength of earthquakes on the TV fairly regularly. Within a few minutes a second alert flashes that lets people know about the possibility of a tsunami. Usually, the alert says that there isn't one coming."
The last major tsunami to hit Japan occurred in 1983. It hit the island of Hokkaido, killing nearly 300 people. "That event caused a reexamination of the Japanese alert system. The quake was so close to the shore that there was little time to warn people of the coming tsunami. That prompted calls for improvements so that people would know in less than 10 minutes."
• How Quakes Are Measured: Charles F. Richter developed a scale in 1935 that is often called the Richter Scale. Today, variations of the original Richter formulas are used.
Seismologists indicate the size of an earthquake in units of magnitude, a logarithmic measure of earthquake size.
In simple terms, this means that at the same distance from the earthquake, the shaking will be 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake. The total amount of energy released by the earthquake, however, goes up by a factor of 32. "What it amounts to is that there are a number of different formulas and they are applied in specific cases to get the best representation of the size of the earthquake," Don Blakeman of US Geographical Survey told Reuters.
Sunday's earthquake was the largest in 40 years and had a moment magnitude of 9.0, the USGS said.
The formula for measuring moment magnitude (Mw) is equal to the rigidity of the earth multiplied by the average amount of slip on the geological fault multiplied by the amount of fault area that has slipped, the USGS Web site said. For more information visit: http//earthquake.usgs.gov/recenteqsww/