USGS · sequence names

Foreshocks are labeled afterward

The naming rule that stops a small earthquake on the live map from being treated as a prediction of a bigger one.

Not a forecast. This page explains published records. It does not say when or where the next earthquake or eruption will happen.

The words are relative

USGS says “foreshock” and “aftershock” are relative terms. A foreshock is an earthquake that precedes a larger earthquake in the same location. An aftershock is a smaller earthquake in the same general area during the days to years after a larger event, the mainshock. Aftershocks occur within about 1–2 fault lengths and while the rate is still above the background. Their frequency decreases with time. USGS also says deep earthquakes, deeper than 30 km, have historically been much less likely to be followed by aftershocks than shallow ones.

You cannot apply the foreshock label in advance

The same FAQ states the operational point: an earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs. A dot on the live map is an earthquake that has happened. Calling it a foreshock before a larger one has happened is a forecast, and this site does not do that.

A published rate, not a city forecast

USGS reports that observations show the chance of an earthquake being followed by a larger one nearby and within a week is about 5%. That is a statement about sequences in general. It is not a chance for a city, a fault, or tonight. USGS also publishes aftershock forecasts for larger earthquakes in the United States and its territories. This website does not compute those forecasts and does not display them. The aftershock page is about the decay after a mainshock. The swarm page is about a cluster with no single mainshock. Neither page, and not this one, names the next earthquake.

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