Mainshock
The largest earthquake in a sequence of related seismic events. It is often followed by aftershocks and may be preceded by foreshocks.
Earthquake rupture: The rapid propagation of fault slip that releases stored elastic energy during an earthquake. A mainshock occurs when a major rupture releases accumulated stress along a fault.
Earthquake sequence: A group of earthquakes linked by location, timing, and related seismic processes. A mainshock is defined by its place within a sequence of related events.
2011 Tōhoku earthquake: A magnitude 9.0 megathrust earthquake off northeastern Japan on March 11, 2011, followed by a destructive tsunami. Its rupture and aftershock sequence illustrate the scale of a major subduction-zone mainshock.
Earthquake prediction: The attempt to specify an earthquake's time, location, and magnitude in advance with useful precision. A sequence's early events rarely establish with certainty whether a larger mainshock will follow.
Aftershock: An earthquake occurring after a larger event in the same region as the crust adjusts to changed stresses. Aftershocks cluster around the mainshock rupture as nearby faults respond to stress changes.
Moment magnitude scale: A logarithmic scale estimating earthquake size from seismic moment, including fault area, slip, and rock rigidity. Comparing magnitudes helps identify the largest event in a sequence.
2004 Indian Ocean earthquake and tsunami: A magnitude 9.1–9.3 megathrust earthquake off Sumatra on December 26, 2004, that generated a devastating Indian Ocean tsunami. Its exceptionally large rupture produced a long, consequential sequence of aftershocks.
Earthquake forecasting: The estimation of earthquake probabilities over defined regions and time periods. Forecasts can describe changing aftershock risk without claiming to predict the next mainshock precisely.
Foreshock: An earthquake that precedes a larger earthquake in the same region and is identified as a foreshock only afterward. A smaller preceding event receives this label only if a later mainshock follows.
Seismic moment: A measure of earthquake size equal to fault area multiplied by average slip and rock rigidity. It captures the physical scale of the rupture that produces a mainshock.