1. 
   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 op http://www.geophys.washington.edu/SEIS/PNSN/INFO_GENERAL/NQT/glossary.html
   
   
2. 
   Mainshock
   The largest earthquake in a "cluster" of earthquakes. Mainshocks are sometimes precede by "foreshocks", and generally followed by aftershocks.
   Found op http://earthquakescanada.nrcan.gc.ca/info-gen/glossa-eng.php
   
   
3. 
   mainshock
   mainshock. The largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.
   Found op http://www.sciencemaster.com/physical/item/earthquake_glossary.php
   
   

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