11 days after a major quake, Mexico battered again

Mexico City was rocked Tuesday afternoon by a 7.1-magnitude earthquake that collapsed buildings, damaged roads and killed scores of people — 32 years to the day that another quake in 1985 devastated the city and left thousands dead.

[Mexico City residents search through the night for survivors of quake that killed more than 200]

Earthquakes from Sept. 4-19

In seismically active regions like much of Mexico, strong earthquakes are often followed or preceded by many smaller tremors. But this event is somewhat unusual in that it closely follows but seems unlikely to be related to an 8.1-magnitude quake, which occurred several hundred miles away less than two weeks earlier on Sept. 8.

The country saw at least 206 seismic events of magnitude 4 or greater in the two weeks leading up to Tuesday’s earthquake, many of which seemed to be aftershocks of the larger Sept. 8 event, which occurred south of Mexico’s Pacific coast near the border with Guatemala.

Perceived shaking of Mexico’s two recent major quakes

The two epicenters were several hundred miles apart, but many areas felt moderate to severe shaking in both earthquakes. The Sept. 8 magnitude-8.1 quake was roughly 10 times stronger, but Tuesday’s earthquake was located closer to more heavily populated areas, mainly Mexico City and the surrounding states of Puebla and Morelos.

Tuesday’s quake was also closer to the surface — 31 miles down, compared to 43 miles down — than the Sept. 8 quake. Shallower quakes cause more shaking at the surface than deeper ones of the same magnitude because seismic waves lose energy as they travel.