foreshock n : a tremor preceding an earthquake

This article is about the geological event. For other uses of the term see Aftershock (disambiguation).

An aftershock is an earthquake that occurs after a previous earthquake (the main shock). An aftershock is in the same region of the main shock but is always of smaller magnitude strength. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are smaller earthquakes formed as the displaced plate boundary tries to adjust itself.

Many scientists hope to use foreshocks to predict upcoming earthquakes. In particular, the East Pacific Rise transform faults show foreshock activity before the main seismic event. Reviews of data of past events and their foreshocks showed that they have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. (McGuire et al., 2005)

Aftershocks (and foreshocks) occur with a pattern that follows Omori's law. Omori's law, or more correctly the modified Omori's law, is an empirical relation for the temporal decay of aftershock rates. In 1894, Omori published his work on the aftershocks of earthquakes, in which he stated that aftershock frequency decreases by roughly the reciprocal of time after the main shock.

n(t) = \frac

where:

• n(t) is the number of earthquakes n measured in a certain time t,
• K is the amplitude, and
• c is the "time offset" parameter.

The modified version of Omori's law, now commonly used, was proposed by Utsu in 1961.

n(t) = \frac

where

• p modifies the decay rate and typically falls in the range 0.7–1.5.

According to these equations, the rate of aftershocks decreases quickly with time. The rate of aftershocks is proportional to the inverse of time since the mainshock. Thus whatever the odds of an aftershock are on the first day, the second day will have 1/2 the odds of the first day and the tenth day will have approximately 1/10th the odds of the first day (when p is equal to 1). These patterns describe only the mass behavior of aftershocks; the actual times, numbers and locations of the aftershocks are 'random', while tending to follow these patterns. As this is an empirical law values of the parameters are obtained by fitting to data after the mainshock occurred and they have no physical basis/meaning.

The other main law describing aftershocks is known as Bath's Law and this says that any mainshock typical has an aftershock approximately 1 magnitude (on average 1.2) less than its mainshock. Aftershock sequences also typical follow Gutenberg-Richter scaling.

Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the mainshock. Bigger earthquakes have more and larger aftershocks and the sequences can last for years or even longer especially when a large event occurs in a seismically quiet area; see, for example, the New Madrid Seismic Zone, where events still follow Omori's law from the mainshocks of 1811-1812. An aftershock sequence is deemed to have ended when the rate of seismicity drops back to a background level; i.e., no further decay in the number of events with time can be detected.

See also

• Earthquake storm

Notes

References

External links

• What Are Aftershocks, Foreshocks and Earthquake Clusters? (USGS)
• Parent's Guide to Aftershocks
• Earthquake Aftershocks Not What They Seemed