美国有线新闻 CNN 墨西哥强震伤亡惨重 总统宣布国家进入紧急状态（在线收听）

AZUZ: A new national emergency is what Mexico's president is calling an earthquake that struck his country on Tuesday. It was the second tremor to shake Mexico in less than two weeks.

This one was a magnitude 7.1 earthquake. Its epicenter, the point on the earth's surface directly above the quake was in central Mexico, about 100 miles southeast of Mexico City and it occurred in a depth of 32 miles, which is considered a shallow earthquake. That's significant because shallow tremors are usually more destructive.

Damage and deaths were reported across hundreds of miles. At least 225 people were killed in this disaster, across Mexico. But the number could go higher. It's just too soon to know. The country declared three days of mourning for the lives lost.

This quake struck in the afternoon when people are at work and school. In fact, Mexico's government says more than 2,000 schools were damaged by the quake — some of them seriously.

Millions of people have no electricity, and many have moved into shelters with their homes either destroyed or uninhabitable.

The Mexican government says its priorities are rescuing people who are trapped and getting medical help for those who were injured. Residents are helping officials in the search for survivors. That's going on around the clock. Water and canned foods are being collected at soccer stadiums and other venues.

SUBTITLE: Five things you should know about earthquakes.

MYERS: Number one, an earthquake occurs when two blocks of the Earth slip past each other. Now, for most of the time, those blocks are together with friction. But they are building up energy because they are moving in different directions. When one block decides to slip, all of a sudden, that energy is released by seismic waves, kind of like ripples on a pond, creating the earthquake.

Number two, an earthquake can occur very near the surface of the Earth. Those earthquakes are typically very destructive, or as deep as 400 miles down into the crust. Now, where the shaking actually happens, that's called the hypocenter. But directly above it, on the surface, that's called the epicenter.

Number three, the power of an earthquake is called magnitude. Now, the intensity of he shaking can vary depending on the geography, the typography, or even the depth of the quake.

Now, the USGS says there are 500,000 detectable quakes every year. One hundred thousand can be felt and 100 will create damage.

Number four, earthquake themselves actually don't kill that many people. It's the natural and manmade structures that fall to the ground during the shaking that injure and kill.

Number five, the majority of all earthquakes and volcanoes happen along plate boundaries. The largest is the Pacific plate and its series of boundaries all along the Pacific Ocean known as the Ring of Fire.