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Many different types of short-term activity, ranging from days to several years in length, have been noted before large earthquakes. Seismologists have looked for regular patterns amoung such short-term precursors.
By the one hand, foreshoks, earthquake swarms or quiet periods have all been noted before a large earthquake strikes, although they do not necessarily have to occur, and even animals seem to behave different.
On the other hand, the incredible stress on rocks about to snap can cause them to heat up, deform or expand just before the earthquake. Thus there are a number of changes in the Earth that sometimes precede earthquakes and scientists use a variety of devices in an attempt to measure and record these changes. Some of these changes, and how they be detected, are listed below:
Sometimes the ground may tilt by millimeters or centimetres prior to the earthquake. Tiltmeters, carefully fitted into deep, precisely drilled boreholes can detect this.
Changes have been found in the speed of seismic waves through the stressed rock close to the fault. Microscopic cracks in stressed rock are aligned relative to the direction in which they are being stressed and this can affect the way that shock waves from small earthquakes pass through them.
Radon gas may be emitted from these fresh, tiny cracks in the stressed rock. Water permeating through the rock absorbs chemicals, such as radon, from the rock and this can be picked up by monitoring the chemical content in deep wells.
The groundwater flowing into such cracks may lower the water table of the area.
In some rocks close to their snapping point a change in electical conductivity is also recorded.