Crustal Plates

Our planet’s surface crust is relatively thin and extends to a depth of about 70 kilometers (40 miles) beneath the oceans to about 150 kilometers (90 miles) beneath the continents. The crust is now known to be discontinuous - that is, it is broken into a number of large fragments, called plates, varying in width from a few hundred to many thousands of miles.

Man-made Quakes

Seismologists believe that both man-made reservoirs and deep wells lead to an increase in the pore-water pressure of underlying rocks, lessening their tensile strength. Also, an increase in water content may lubricate already existing fracture zones and may lead to active faulting.

Seismic Waves

Seismic waves are sound waves traveling through and across the earth that are produced by earthquakes. Some waves travel down through the earth and other waves travel over the surface of the ground. The surface waves travel faster than the interior waves. The waves from a large earthquake can be recorded on instruments on the opposite side of the world, having taken about 21 minutes to pass right through the earth.

Earthquake Predictions

Statistical Data

From the frequency of earlier recorded quakes in a given region, general statistical statements about future probabilities can be made. An earthquake of magnitude 6 can be expected in California every 10 months (from historical records). The average interval between major earthquakes across the San Andreas fault in Southern California has been 140 years over the last 1,200 years (from geological studies).

Seismic Gaps

From the amount of seismic activity registered at monitoring stations along plate boundaries, scientists can determine which regions have been slipping and therefore releasing built-up strains, and which regions have been quiet - seismic gaps that point to deficits in motion and indicate that stresses are building up. The seismic gaps are thus the most likely places for earthquakes to occur.

Earthquake Precursors Phenomena

These are the things that happen or rock properties that change prior to an earthquake. From these, it may be possible to identify warning signs of an earthquake before it occurs. Some precursor phenomena might be explained in terms of a dilatancy model in which cracks open in stressed rock, allowing fluids to seep in before an earthquake occurs, after which the rocks snap back elastically to their unstressed condition.

Earthquake Precursors

Seismicity - A swarm of small tremors called foreshock frequently, but not always, precede a major earthquake.

Subsidence and uplift - A vertical movement of the surface of the land indicates a build-up of strains in the crust.

Wave speeds - Russian seismologists have successfully predicted several major earthquakes shortly after a change in the ratio of P and S waves. Velocity changes the strain of the rocks, as well as water content and other factors. This method holds a lot of promise but may not be applicable to all earthquake-prone areas.

Water levels in wells - Underground water levels often rise or fall before earthquakes. Water levels in wells fell noticeably just before the successful prediction of the 1975 quake in China.

Geomagnetism - Changes in the earth’s magnetic field have been measured prior to earthquakes in several places.

Geoelectricity - As water content in rock changes, so does the electrical resistance. When rocks become strained and microscopic cracks appear, more water can enter, lowering electrical resistance.

Strains - The build-up of strain along fault lines can be measured by determining the relative displacement between two points. Changes in rates of displacement could indicate a coming earthquake.

Radon content of groundwater - Radon is a slightly radioactive gas, found naturally in ground and well water. An initial build-up spanning 10 years, followed by a sudden drop in the radon content of ground water were observed in Russia shortly before a major quake.

Animal behavior - Some animals have been claimed to show unusual and restless behavior just before an earthquake. The validity of this claim is not yet established.