Glossary term
Foreshock
Sequences & patterns · earthquake & volcano glossary
What is Foreshock?
A smaller quake that precedes a larger one on the same fault. Foreshocks can only be identified with certainty after the mainshock, so they aren't a reliable warning on their own.
Related terms
MainshockThe largest earthquake in a sequence. If a later event is bigger, the labels are reassigned and the earlier mainshock becomes a foreshock.AftershockSmaller quakes following a mainshock as the crust readjusts. They are most frequent in the first hours and days and taper off over weeks to years, following Omori's law.SwarmA cluster of earthquakes in one area over a short period with no single dominant mainshock. Swarms are common near volcanoes as magma moves underground.b-valueA statistic describing the ratio of small to large earthquakes in a region. A drop in b-value can indicate rising stress, which is one of the precursors QuakeGlobe's volcano model watches.Moment magnitude (Mw)The modern measure of an earthquake's size, based on the area of the fault that slipped and how far it moved. It replaced the original Richter scale for medium and large events because Richter saturates (stops increasing) for the biggest quakes. The scale is logarithmic: each whole number is about 10× more shaking amplitude and ~31× more energy.Richter scale (ML)The original 1935 local-magnitude scale. Still widely cited in the media, but agencies now report moment magnitude for most events because it stays accurate for large earthquakes.
← All glossary terms