What Causes Earthquakes?
Most of us have read about earthquakes, or seen pictures of the damage they can cause. Fortunately, few of us have actually experienced them. The study of earthquakes is called seismology (from the Greek seismos meaning a shaking) and the shock waves produced by them are recorded on an apparatus called a seismograph.
There are many centers throughout the world recording and studying earthquakes. The instruments used are able to detect many very small earthquakes which could not be felt by man. Such waves prove that our earth is not the solid and unmoving mass we might imagine.
Earthquakes originate where rock structures beneath the surface of the land break up and move along lines known as faults.
Earth tremors can occur anywhere, but real earthquakes, during which whole cities or towns may be shaken to pieces and thousands of people killed, are almost all limited to certain regions.
These regions correspond to the major mountain ranges, volcanoes, and deep sea trenches where land is still being built up and where subterranean, or underground, activity is therefore most frequent.
The most immediate effects of an earthquake are landslides and the creation of large crevices in the surface of the earth. If earthquakes originate under the sea, they can also produce huge ocean waves, and these may travel for thousands of miles. When such a wave reaches land it causes flooding and a great loss of life and property. Tidal wave is the name sometimes given to such a wave although, of course, it has nothing to do with the tides. Its correct name is a tsunami.
The intensity of the shock waves recorded on the seismograph are measured by the various earthquake stations.
By studying these waves scientists can find out much about the interior of the earth. A comparison of the time taken for the waves to reach different stations, and their position in relation to each other, enables the exact location of an earthquake to be worked out wherever it may have occurred. This location is called the epicenter. The point directly beneath this where the earthquake originated in the earth's crust is called the focus.
The various layers of the earth's crust have all been determined from the study of shock waves. It has also led to the discovery of a definite boundary line between the crust and the mantle, called the Mohorovicic Discontinuity after the Yugoslav scientist Andrija Mohorovicic who first identified it in 1909.
The shallow quakes are thought to be the rapid release of slowly accumulated strain along fault lines extending over a wide region. The rupture of a fault and the friction between faulting rock surfaces produces the elastic waves. The origin of the slowly accumulated strain is not known. However, areas of weakness may be placed under stress by continental drift. Scientists are accumulating data for successful prediction of earthquakes, including changes in local magnetism in rocks and water level change in deep wells, the occurrence of foreshocks or a lull in small-scale tremors, and changes in animal behavior.