Mainshock

The largest in a series of earthquakes occurring closely in time and space. The mainshock may be preceded by foreshocks or followed by aftershocks.
Found on http://www.encyclo.co.uk/local/20129

Mainshock

The largest earthquake in a "cluster" of earthquakes. Mainshocks are sometimes precede by "foreshocks", and generally followed by aftershocks.
Found on http://earthquakescanada.nrcan.gc.ca/info-gen/glossa-eng.php

mainshock

mainshock. The largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.
Found on http://www.sciencemaster.com/physical/item/earthquake_glossary.php
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