A foreshock is an

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

that occurs before a larger seismic eventthe

mainshock In seismology, the mainshock is the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks. Foreshock A foreshock is an earthquake that occurs before a larger seismic ev ...

and is related to it in both time and space. The designation of an earthquake as ''foreshock'', ''mainshock'' or

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in Epicenter, the same area of the Mainshock, main shock, caused as the displaced Crust (geology), crust adjusts to the effects of the main shock. Large earthq ...

is only possible after the full sequence of events has happened.

Occurrence

Foreshock activity has been detected for about 40% of all moderate to large earthquakes, and about 70% for events of M>7.0. They occur from a matter of minutes to days or even longer before the main shock; for example, the

2002 Sumatra earthquake The 2002 Sumatra earthquake occurred at 08:26:10 WIB (01:26 UTC) on 2 November. It had a magnitude of 7.2–7.4 with an epicenter just north of Simeulue island, and resulted in three fatalities. It is regarded as a foreshock of the 2004 India ...

is regarded as a foreshock of the

2004 Indian Ocean earthquake On 26 December 2004, at 07:58:53 local time (UTC+07:00, UTC+7), a major earthquake with a magnitude of 9.2–9.3 struck with an epicenter, epicentre off the west coast of Aceh in northern Sumatra, Indonesia. The Submarine earthquake, undersea ...

with a delay of more than two years between the two events. Some great earthquakes (M>8.0) show no foreshock activity at all, such as the M8.6 1950 India–China earthquake. The increase in foreshock activity is difficult to quantify for individual earthquakes but becomes apparent when combining the results of many different events. From such combined observations, the increase before the mainshock is observed to be of inverse power law type. This may either indicate that foreshocks cause stress changes resulting in the mainshock or that the increase is related to a general increase in stress in the region.

Mechanics

The observation of foreshocks associated with many earthquakes suggests that they are part of a preparation process prior to

nucleation In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...

. In one model of earthquake rupture, the process forms as a cascade, starting with a very small event that triggers a larger one, continuing until the main shock rupture is triggered. However, analysis of some foreshocks has shown that they tend to relieve stress around the fault. In this view, foreshocks and aftershocks are part of the same process. This is supported by an observed relationship between the rate of foreshocks and the rate of aftershocks for an event. In practice, there are two main conflicting theories about foreshocks: earthquake triggering process (described in SOC models and ETAS-like models) and the loading process by aseismic slip (nucleation models). This debate about the prognostic value of foreshocks is well known as Foreshock Hypothesis.

Earthquake prediction

An increase in seismic activity in an area has been used as a method of predicting earthquakes, most notably in the case of the

1975 Haicheng earthquake On February 4, 1975, at 19:36 Chinese Standard Time, CST, an earthquake of 7.5 and Modified Mercalli intensity scale, intensity (MMI) IX hit the city of Haicheng, Liaoning, China. Much of the city was evacuated before the earthquake, so few die ...

in China, where an evacuation was triggered by an increase in activity. However, most earthquakes lack obvious foreshock patterns and this method has not proven useful, as most small earthquakes are not foreshocks, leading to probable false alarms. Earthquakes along oceanic

transform fault A transform fault or transform boundary, is a fault (geology), fault along a plate boundary where the motion (physics), motion is predominantly Horizontal plane, horizontal. It ends abruptly where it connects to another plate boundary, either an ...

s do show repeatable foreshock behaviour, allowing the prediction of both the location and timing of such earthquakes.

Examples of earthquakes with foreshock events

* The strongest recorded mainshock that followed a foreshock is the

1960 Valdivia earthquake The 1960 Valdivia earthquake and tsunami () or the Great Chilean earthquake (''Gran terremoto de Chile'') occurred on 22 May 1960. Most studies have placed it at 9.5–9.6 on the moment magnitude scale, while some studies have placed the magnitu ...

, which had a magnitude of 9.5 M_W. * Note: dates are in local time

References

{{Reflist Seismology Types of earthquake