# Dictionary Definition

foreshock n : a tremor preceding an
earthquake

# Extensive Definition

- 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

## Notes

## References

## External links

foreshock in Spanish: Réplica (sismología)

foreshock in French: Réplique (sismologie)

foreshock in Korean: 여진 (지진)

foreshock in Indonesian: Gempa susulan

foreshock in Icelandic: Eftirskjálfti

foreshock in Latvian: Pēcgrūdiens

foreshock in Malay (macrolanguage): Gempa
susulan

foreshock in Japanese: 余震

foreshock in Norwegian: Etterskjelv

foreshock in Polish: Wstrząs wtórny

foreshock in Russian: Афтершок