Earthquakes are violent movements of the Earth's surface, due to shock waves traveling through Earth. There are over a million Quakes in a year, almost 3000 per day, but so mild that they can be detected only by sensitive instruments. These movements sometimes form large fractures called faults. Waves are also propagated through Earth's interior enabling a study of that region. Seismology is the branch of geophysics that studies these waves to determine Earth's internal structure.
Earth's outer shell is made up of many plates. Most earthquakes occur at plate boundaries. Movements of these plates cause strains on rocks. Since rocks possess elastic properties, energy is stored as elastic deformities. But if the deforming forces are large enough to overcome the friction, then the fault walls move suddenly releasing this stored energy causing earthquakes. If this elastic limit is not reached, then the rock will rebound elastically to its original shape. However, if its elastic limit is exceeded, the rock will not fully recover, rupturing the rock formation.
One of the best known fault is the San Andreas fault system, which lies on the boundary of the Pacific Plate and the American Plate. The Pacific Plate is moving northward relative to the American Plate at a rate of about two inches a year. In about 10 million years, Los Angeles (on the Pacific Plate) would have the same Latitude as San Francisco (on the American Plate). I about 50 million years, the plates would separate completely. There have been frequent Earthquakes along this fault. For a map showing the location click below.
Focus is the point of initial energy release or slippage. It is usually under the surface, a few to several hundred miles. Epicenter is the point on the Earth's surface directly above the focus. The energy released ar the focus propagates outwards as two types of seismic waves. Surface waves travel along the outer layer of Earth and cause most damage, and two types of body waves traveling through Earth. P waves (for primary) are longitudinal compressional waves propagated by particles moving longitudinally back and forth in the same direction as the wave is traveling and the S waves (for secondary) which are transverse waves in which the particles move at right angles to the direction of wave travel. P waves can travel through solids, liquids or gases, while the S waves can only travel through solids. P waves are also faster than S waves. Because of these differences, seismologists can locate the focus and study the Earth's internal structure. For more details, click below.
Seismographs monitor the seismic waves of an earthquake. The recorded seismogram gives the time delay of the arrival of different types of waves as well an indication of their energy. (The greater the energy, the greater the amplitude of the traces.)
The severity of the earthquakes are represented on many scales. The most common is the open ended Richter scale, developed in 1935 by Charles Richter, which gives an absolute measure of the energy released by calculating the energy of seismic waves at a standard distance. Another scale is the modified Mercalli scale, developed by Guiseppe Mercalli in the 1890's (before the advent of seismographs), which describes the severity of an earthquake by its observed effects. For more information of the Richter scale, click below.
The numbers on the Richer scale are logarithmic, that is each whole number step represents 31 times more energy than the preceding whole number step. Hence an earthquake of magnitude 9 is 31x31x31x31 times = 923,521 times one of magnitude 5. This scale does not indicate the damage caused by an earthquake.
For more information of modified Mercalli scale, click below.
Tsunamis erroneously called Tidal waves are huge waves generated when earthquakes occur near or beneath ocean floors. They travel across oceans at speeds of 600 mph. Although they are only 3 feet high in the open ocean, near the coast, in shallow water they may as high as 50 feet. They cause tremendous havoc in coastal areas.
For a pictorial view of the Kobe earthquake and of the earthquake zones, click below and view slides 33 through 36 and slide 39.
For more information about earthquakes, click below.
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