Foreshocks, Mainshocks and Aftershocks
April 6, 2026
When you set the map to a single year and land on 1964 or 2011, one region lights up with a dense cloud of dots rather than a single point. That cloud is an earthquake <em>sequence</em>: one large mainshock and the hundreds or thousands of smaller earthquakes that follow it.
The three labels
The labels are relative. If an aftershock turns out to be larger than the original earthquake, seismologists relabel the sequence: the first shock becomes a foreshock and the new one becomes the mainshock.
- Mainshock — the largest earthquake in a sequence.
- Aftershocks — smaller earthquakes on and around the same fault, triggered by the stress the mainshock redistributed. They begin immediately and can continue for months to years.
- Foreshocks — earthquakes that precede the mainshock in the same area. They exist only in hindsight: at the time, a foreshock looks exactly like an ordinary small earthquake.
Aftershocks follow simple rules
Two empirical laws, both more than a century old, describe aftershock sequences remarkably well:
- Omori’s law (1894): the rate of aftershocks falls off roughly in proportion to 1 divided by the time since the mainshock. There are many in the first day, far fewer after a week, and a slow tail that can last years.
- Båth’s law: the largest aftershock is on average about 1.2 magnitude units smaller than the mainshock, regardless of the mainshock’s size. A magnitude 7 mainshock typically has a magnitude ~5.8 largest aftershock.
Because these laws are reliable, the USGS and other agencies issue aftershock forecasts after a large earthquake: not a prediction of any single event, but a probability, such as “a 15% chance of a magnitude 6 or larger within the next week.” These forecasts are updated as the sequence evolves.
Why foreshocks don’t help prediction
Roughly half of all large earthquakes are preceded by at least one foreshock in the days or weeks before. That sounds useful — but the problem is the false-alarm rate. The vast majority of small earthquakes are not foreshocks; they are just small earthquakes. There is currently no way to look at a magnitude 4 and tell whether it is an isolated event or the last warning before a magnitude 7.
The 2011 Tōhoku earthquake was preceded by a magnitude 7.3 foreshock two days earlier. It was recognised as a foreshock only after the magnitude 9.1 mainshock struck.
On the map, aftershock clouds are one of the clearest features. Try 1960 (Chile), 1964 (Alaska), 2004 (Sumatra) and 2011 (Japan) with “Show” set to that single year.
Swarms: sequences with no mainshock
Some clusters have no dominant earthquake at all — dozens of similar-sized events over days or weeks, then quiet. These swarms are common in volcanic areas and geothermal fields, where moving magma or fluid, rather than a single locked fault, is driving the activity.
Sources
- U.S. Geological Survey — “Aftershocks”, “Foreshocks”, “What is the difference between foreshocks, aftershocks and mainshocks?”, Aftershock Forecast product documentation.
- Utsu, T., Ogata, Y. & Matsu’ura, R. S. (1995). “The centenary of the Omori formula for a decay law of aftershock activity.” J. Phys. Earth 43, 1–33.
- Båth, M. (1965). “Lateral inhomogeneities of the upper mantle.” Tectonophysics 2, 483–514.