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. CONTENTS * 1 Distribution of aftershocks * 2
* 2.1 Omori\'s law
* 2.2 Båth\'s law
* 2.3
* 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 and the
AFTERSHOCK SIZE AND FREQUENCY WITH TIME Aftershocks rates and magnitudes follow several well-established empirical laws. OMORI\'S LAW The frequency of aftershocks decreases roughly with the reciprocal of
time after the main shock. This empirical relation was first described
by
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 ) = k ( c + t ) p {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
Where: * N {displaystyle N} is the number of events greater or equal to M {displaystyle M} * M {displaystyle M} is magnitude * a {displaystyle a} and b {displaystyle b} are constants In summary, there are more small aftershocks and fewer large aftershocks. 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
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
FORESHOCKS Main article:
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
MODELING Seismologists use tools such as the Epidemic-Type
PSYCHOLOGY Following a large earthquake and aftershocks, many people have reported feeling "phantom earthquakes" when in fact no earthquake was taking place. This condition, known as "earthquake sickness" is thought to be related to motion sickness , and usually goes away as seismic activity tails off. REFERENCES * ^ Omori, F. (1894). "On the aftershocks of earthquakes" (PDF).
Journal of the College of Science, Imperial University of Tokyo. 7:
111–200.
* ^ 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) on 2015-07-16.
* ^ Quigley, M. "New Science update on 2011 Christchirch Earthquake
for press and public: Seismic fearmongering or time to jump ship".
Christchurch
EXTERNAL LINKS * Earthquake |