[fawr-shok, fohr-]
noun Geology.
a relatively small earthquake that precedes a greater one by a few days or weeks and originates at or near the focus of the larger earthquake.

1900–05; fore- + shock1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
foreshock (ˈfɔːˌʃɒk)
Compare aftershock a relatively small earthquake heralding the arrival of a much larger one. Some large earthquakes are preceded by a series of foreshocks

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
These maps graphically illustrate the change in earthquake probability during
  aftershock and possible foreshock sequences.
The increase in foreshock activity triggered the evacuation warning.
Scientists can't tell that an earthquake is a foreshock until the larger
  earthquake happens.
The tremor was preceded by a light foreshock the previous day.
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