Where the Quakes in Chile Struck

This map of topography and water depth along the Chilean coast shows quake locations and magnitudes (black circles), with lighter colors indicating higher elevation on land and shallower depth in the water. The boundary where the two tectonic plates converge is marked by a red line. Also there is a trench where the Nazca Plate begins to dive beneath the South America Plate. When these plates get locked together for any time the pressure will eventually break, resulting in an earthquake like the 8.8 magnitude temblor on Feb. 27.

(Image credit: NASA Earth Observatory.)

The 8.8-magnitude earthquake that devastated parts of Chile was the result of a collision between two giant slabs of Earth.

The jolt occurred along a so-called subduction zone, where one tectonic plate dives beneath another. In this case, the Nazca Plate is plowing under the South America Plate at an average rate of 3 inches (80 millimeters) a year. In addition to the Feb. 27 earthquake and others, the plate collision gives rise to the spectacular Andes Mountains.

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The boundary where the two plates converge is marked by a red line. But even without the line, a trench about 62 miles (100 km) offshore would mark the boundary. The trench occurs where the Nazca Plate begins its descent beneath the South America Plate. The trench is most sharply defined on the eastern (continental) side, where depth plunges rapidly from a few hundred meters (light blue) to several thousand meters (deep blue).

In places along their boundary, the two plates may slide easily past each other, but in other locations, they get locked together for a time. Eventually the pressure is too great for the rocks to withstand, and they break. The plates lurch past each other violently. The result: an earthquake. When large quakes occur underwater, the seafloor can heave or sink. This ground movement is what triggers a tsunami.

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