Foreshocks, aftershocks - what's the difference?

"Foreshock" and "aftershock" are relative terms.

Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs. 

Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock." They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes. (modified from Univ. of Washington)

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What is the difference between aftershocks and swarms?

Aftershocks are a sequence of earthquakes that happen after a larger mainshock on a fault. Aftershocks occur near the fault zone where the mainshock rupture occurred and are part of the "readjustment process” after the main slip on the fault. Aftershocks become less frequent with time, although they can continue for days, weeks, months, or even...

Do earthquakes occur in Antarctica?

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Where can I find earthquake educational materials?

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What is surface faulting or surface rupture in an earthquake?

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What is an earthquake and what causes them to happen?

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