Stress
Many minute cracks develop within a rock under stress, increasing its volume and allowing the inward precolation of water and gases. The microcracks and water change the properties of the rock in ways which may be detectable.

Changes in ground levels: the area around the focus may tilt due to deformations and swell slightly due to microcracks. Tiltmeters, using laser technology and GPS, measure the slope of the ground level vbery accurately. Strain gauges in boreholes measure deformation and therefore any increase in stress.
Measurements of gases: radon, a radioactive decay product of uranium in granite percolats up through the microcracks. As a heavy gas, radon accumulates in water well in which it is easily detecte by its radioactivity. If the amount of radon increases, new pathways are opening for the gas and an earthquake may be imminent.
Changes in water levels in wells: groundwater percolates into the microcracks, lowering the level of water in wells. The levels return to normal as the water is replenished, before the earthquake occurs.
Physical properties: the number of foreshocks increases beofre the main event. P wave velocites decrease the increase again before the quake. Water increases the electrical conductivity of the ground, lowering its resistivity. Coloured lights in the sky immediately before an eathquake may be caused by changes in the electrical properties of quartz and other minerals under stress.
Magnetism and animal behaviour: research in China confirms that animals show disturbed behaviour just before an earthquake. Animals may be able to detect the slight changes in the Earth's magnetic field or feel the very small vibrations in the foreshocks.

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Undergraduate Geology - Part 1 (Earthquakes) Notiz am Predicting Earthquakes, erstellt von siobhan.quirk am 08/05/2013.

Predicting Earthquakes