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The first earthquake is usually the worst, but aftershocks, or more earthquakes, may follow. Aftershocks usually occur within the next two days, but have also been recorded to happen for years. Their size, strength, and frequency usually diminish with time. (Incidentally, there is also such a thing as a foreshock, a small tremor that comes before a major earthquake.)

Why do aftershocks occur? Even though the major strain between two plates is released by the initial quake, their touching edges still need to adjust to new positions. The edges may not be able to pass each other smoothly, and this additional realignment creates the smaller shocks.

When the earth’s plates finally stop grinding against each other, the surface landscape may have changed over huge areas around the earthquake’s center. This happened after the 1906 earthquake in San Francisco, California (United States), when a piece of land 430 kilometers (267 miles) long shifted north by 6 meters (20 feet).

Aftershocks are able to do serious damage, so don’t look them over lightly! They have the power to crumble buildings, roads, and bridges already weakened by the initial earth movement.

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