Caltech Question of the Month: What is really happening when we have aftershocks after an earthquake?

Submitted by Gloria Hughes, Pasadena, California, and answered by Lucile M. Jones, Seismologist, U.S. Geological Survey/Visiting Research Associate, Caltech.

Earthquakes occur in clusters. In any cluster, the earthquake with the largest magnitude is called the mainshock, anything before it is a foreshock, and anything after it is an aftershock. A mainshock will be redefined as a foreshock if a subsequent event has a larger magnitude. Aftershock sequences follow predictable patterns as a group, although the individual earthquakes are random and unpredictable.

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