Audio Transcript:

Lisa Mullins: Mexico City is still evaluating the damage from the 7.4 earthquake that struck on Tuesday. There was some damage, especially near the epicenter in the coastal state of Guerrero, but there were no deaths and that is a huge difference from the quake that devastated Mexico City in 1985. That one killed some 10,000 people. Building codes have been strengthened since then, but the quake's location helped as well, according to Paul Caruso. He's a geophysicist with the US Geological Survey's National Earthquake Information Center.

Paul Caruso: The reason it didn't cause much damage is because the epicenter was in a fairly remote area. In the past we've had large quakes far away from Mexico City that have killed people in Mexico City because a lot of Mexico City is built on hills, which is like sand and mud. And when seismic waves get into that stuff they cause liquefaction where the ground just basically turns into a liquid, and buildings can actually sink into the ground. In California we're very concerned about that also because in the World Series quake in 1989 most of the damage was done in areas where buildings were built on soil that had been dredged out of the bay, and those soils liquefied during the quake. And I have a picture of a building where as a third floor of the building had crushed a car in San Francisco because the ground had liquefied under that building and it sank right into the ground.

Mullins: So if a large part of this equation in terms of the damage caused by earthquakes is dependent on of course where the epicenter is, the nature of the quake itself, how much do things like building codes matter then?

Caruso: They make a huge difference. In Chile when we had that magnitude 8.8 quake back in 2010, because they were so well prepared because of their history of earthquakes, only 500 people were killed, and that's amazing to me.

Mullins: But then we look at Haiti where 300,000 people died.

Caruso: Yes, only a magnitude 7 earthquake, but again, there were no building codes in Haiti and that's why so many people were killed.

Mullins: So it's unusual that we have good news stories out of massive earthquakes, this was a pretty massive one, 7.4 in Mexico on Tuesday, but you're saying that past experience paid off.

Caruso: Yes, we were surprised also that no one was killed, but another thing that might have factored into that was the fact that on Tuesday they had scheduled an earthquake drill for Chiapas, and the earthquake drill was scheduled to start at noon, so they had just started their earthquake drill and two minutes later at 12:02 was when this earthquake hit. So that could've had something to do with why there were no casualties.

Mullins: Paul Caruso is a geophysicist with the US Geological Survey at the National Earthquake Information Center in Golden, Colorado. Thanks.

Caruso: Thanks, Lisa.