CNN News:地震夷平意大利小镇 解读地震预警系统

The mayor of Amatrice, which is located in central Italy, says his town is no more. When a powerful 6.2 magnitude earthquake struck early Wednesday, Amatrice was at the epicenter. That's the point in the earth's surface directly above where the earthquake occurred. The town's clock tower, its time stopped at the moment the earthquake hit, was one of the only structures still standing. Rescue efforts have been going on around the clock.
Volunteers have been using their bare hands to dig through rubble. There are several Italian villages that were affected. They're in remote mountainous areas that can be hard to reach.
Italy's prime minister says the quake killed at least 120 people, but warned that toll could increase. The quake was felt as far away as Rome, 100 miles from the epicenter. And dozens of aftershocks have been reported. Pope Francis asked people to pray for those affected.
Earthquakes are entirely uNPRedictable. There are warning systems that can save some lives, but only under certain conditions.

CHAD MYERS, cnn CORRESPONDENT: If you're in an epicenter of an earthquake, you're going to get no warning. There's no time for that.
The warning depends on the distance you are to the earthquake epicenter.
SUBTITLE: Earthquake early warning system.
MYERS: An early warning system for earthquakes is expensive and also complex. Part of the problem is we don't have a system that can predict the earth is going to move. Our warming system is based on the fact that the earth is already moving and then if you're farther away from that epicenter, we can give you some time to prepare. Some minor time but some time.
Unlike a tornado warning where Doppler Radar can see the rotation and issue a warning before the tornado, an earthquake warning happens when the earthquake is already shaking. Another limitation is how close the sensor is to the epicenter. If the sensor is 10 miles away from the epicenter, it takes five seconds to get to that sensor. So, the more sensors we get, the better the lead time we'll get as well.
If we get 10 seconds notice of an earthquake that's happening, especially a big one, you can open up the elevators on the nearest floor. You can stop trains. You can stop all those things that are moving before the shaking gets there.
This entire system works because of the speed of light, or the speed of the warning going down the line, is 186,000 miles per second. The earth as it's wiggling and shaking along is going at 2 miles per second.
So, if you're 30 miles away, your warning could be 15 seconds. It's 15 seconds that could save your life.