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foreshock
This term is used primarily in seismology to describe the precursor activity of a larger seismic event. It carries a connotation of warning or anticipation, though in practice, foreshocks are often only identified as such after the mainshock has occurred, as they are indistinguishable from ordinary small earthquakes at the time of occurrence.
In technical discourse, it is strictly contrasted with the mainshock and the subsequent aftershocks. While a mainshock is the largest event in a sequence, the foreshock is defined by its temporal position before that peak event.
Meanings
A relatively small earthquake that precedes a larger, more powerful earthquake in the same area.
The seismologists noted a series of foreshocks before the main event occurred.