Last September’s seismic eruption has left people frightened at the slightest tremor, and the tears in the faces of those who endured the last major quake were easy to spot on the streets.

Many could be heard repeating the words “Oh God, not again.”

Video footage from inside the Mexico City newsroom of a daily newspaper, Milenio, showed employees ducking underneath desks as light fixtures swung wildly.

The initial 7.2-magnitude shock was followed 57 minutes later by a magnitude-5.8 aftershock.

The epicenter of Friday’s earthquake was between those of a magnitude-8.2 quake on Sept. 8 and the 7.1-magnitude quake on Sept. 19. But from a geological standpoint, all three occurred in the same general area — a so-called subduction zone, where one piece of the earth’s crust, in this case the Cocos Plate, is slowly sliding under another, the North American.

Like other subduction zones around the Pacific and elsewhere, this region is the source of many earthquakes, some of them very strong and destructive. The movement of the two plates relative to each other is very slow — about two to three inches a year — but it causes stresses to build, either at the boundary between the two plates or, as was the case with the September quakes, within one of them. At some point the stresses become too much and the rock formations slip, releasing energy as an earthquake.

Shortly after Friday’s quake, the United States Geological Survey released a brief initial analysis, saying that it occurred “on or near” the boundary between the two plates, and about 55 miles north of the Middle America Trench, where the Cocos begins its slide beneath the North America plate.

In addition to local destruction, strong Mexican earthquakes often cause damage in Mexico City — even if, as in this case, the capital is miles away. Mexico City was built on an ancient lake bed, and the sediments of sand and clay amplify the seismic waves as they arrive from the epicenter.

Depending on the amount of energy released, the depth of the epicenter and its distance from Mexico City, the seismic waves from a quake can affect some buildings in the capital more than others. In the Sept. 19 quake, mostly shorter buildings were knocked down. But in a 1985 quake that killed 10,000 people, most of the buildings that were severely damaged or destroyed were six to 16 stories tall.