foreshock
A smaller earthquake that precedes a larger one on the same fault — identifiable as a foreshock only in retrospect, after the mainshock arrives. At the time it occurs, a foreshock is indistinguishable from any other small earthquake. The term names a relationship that can only be seen looking backward, which is one of the central frustrations of seismology.
Etymology
English compound — a shock that comes before. The counterpart to aftershock, but without the certainty: you know an aftershock when it happens, but you only know a foreshock after the fact.
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