Mexico's back-to-back earthquakes, explained by a seismologist

The second major earthquake to strike Mexico in less than two weeks has caused catastrophic damage in the country's capital. The magnitude 7.1 temblor started at around 1:15PM -- cracking highways, collapsing buildings, and, so far, killing more than 50 people. Less than two weeks ago on September 7th (local time), a magnitude 8.1 quake struck roughly 400 miles southeast from today's. It's not common to hear of such strong earthquakes happening back-to-back so close to one another, says John Bel...