An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. 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 formed as the crust around the displaced fault plane adjusts to the effects of the main shock.
1 Distribution of aftershocks
2.1 Omori's law 2.2 Båth's law 2.3 Gutenberg–Richter law
3 Effect of aftershocks 4 Foreshocks 5 Modeling 6 Psychology 7 References 8 External links
Distribution of aftershocks
Most aftershocks are located over the full area of fault rupture and
either occur along the fault plane itself or along other faults within
the volume affected by the strain associated with the main shock.
Typically, aftershocks are found up to a distance equal to the rupture
length away from the fault plane.
The pattern of aftershocks helps confirm the size of area that slipped
during the main shock. In the case of the 2004 Indian Ocean earthquake
2008 Sichuan earthquake
n ( t ) =
( c + t )
displaystyle n(t)= frac k (c+t)
where k and c are constants, which vary between earthquake sequences. A modified version of Omori's law, now commonly used, was proposed by Utsu in 1961.
n ( t ) =
( c + t
displaystyle n(t)= frac k (c+t)^ p
where p is a third constant which 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 and this relationship can be used to estimate the probability of future aftershock occurrence. Thus whatever the probability of an aftershock are on the first day, the second day will have 1/2 the probability of the first day and the tenth day will have approximately 1/10 the probability of the first day (when p is equal to 1). These patterns describe only the statistical behavior of aftershocks; the actual times, numbers and locations of the aftershocks are stochastic, while tending to follow these patterns. As this is an empirical law, values of the parameters are obtained by fitting to data after a mainshock has occurred, and they imply no specific physical mechanism in any given case. Båth's law The other main law describing aftershocks is known as Båth's Law and this states that the difference in magnitude between a main shock and its largest aftershock is approximately constant, independent of the main shock magnitude, typically 1.1–1.2 on the Moment magnitude scale. Gutenberg–Richter law
Magnitude of the Central Italy earthquake of August 2016 (red dot) and aftershocks (which continued to occur after the period shown here)
Main article: Gutenberg–Richter law
a − b M
displaystyle !,N=10^ a-bM
is the number of events greater or equal to
In summary, there are more small aftershocks and fewer large
Effect of aftershocks
Aftershocks are dangerous because they are usually unpredictable, can
be of a large magnitude, and can collapse buildings that are damaged
from the main shock. 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 main shocks 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.
Land movement around the New Madrid is reported to be no more than
0.2 mm (0.0079 in) a year, in contrast to the San Andreas
Fault which averages up to 37 mm (1.5 in) a year across
California. Aftershocks on the San Andreas are now believed to top
out at 10 years while earthquakes in New Madrid are considered
aftershocks nearly 200 years after the 1812 New Madrid earthquake.
Main article: Foreshock
Some scientists have tried to use foreshocks to help predict upcoming
earthquakes, having one of their few successes with the 1975 Haicheng
earthquake in China. On the
East Pacific Rise
^ Omori, F. (1894). "On the aftershocks of earthquakes" (PDF). Journal
of the College of Science, Imperial University of Tokyo. 7:
^ Utsu, T. (1961). "A statistical study of the occurrence of
aftershocks". Geophysical Magazine. 30: 521–605.
^ Utsu, T.; Ogata, Y.; Matsu'ura, R.S. (1995). "The centenary of the
Omori formula for a decay law of aftershock activity" (PDF). Journal
of Physics of the Earth. 43: 1–33. Archived from the original (PDF)
^ Quigley, M. "New Science update on 2011 Christchirch