noun. A minor tremor of the earth that precedes a larger earthquake originating at approximately the same location.

noun. A small earth tremor which precedes the mainshock in an earthquake sequence. Not all mainshocks have foreshocks.

noun. a tremor preceding an earthquake

"If the team's results are correct, then this swarm is probably not a foreshock of something larger on the Hayward Fault."

earth tremor  microseism  tremor