Aside from Mid-Ocean Ridges, subduction zones, and collision zones, there are areas where the stretching or compression of plates causes slight fractures in the earth’s crust. These areas of crustal stress are called faults and there are essentially two types. Normal faults occur where tension within the Earth stretches the crust to form a basin, or range, with fault-block mountains flanking the basin. The southern Rockies include a basin and range area formed as a result of a normal fault. Reverse faults occur where compression squeezes the crust together as one block of land slides over another forming overthrust mountains. Impressive examples of overthrust formations can be seen in Montana’s ‘Glacier National Park.’

Another type of fault occurs where plates are sliding, shearing, or grinding past each other, folding mountains and producing earthquakes in the process. These plate boundaries are called Lateral, or Transform faults, and they are found where significant movement occurs along a fracture in the earth’s crust.

The San Andreas Fault
Clearly, the most famous and most visible transform fault in the world is the San Andreas Fault. This enormous fault stretches for over 1,000 miles from northern California, through western California, to the East Pacific Rise beneath the waters of the Gulf of California.

The Pacific Plate lies to the west of the San Andreas Fault and the North American Plate lies to the east. Scientists have determined that the Pacific Plate moves northwest at the rate of about two inches every year relative to the North American Plate. The western half of California lies on the Pacific Plate while the eastern half of California lies on the North American Plate. Besides the short term effects of being an earthquake ‘hot zone’, western California will, in about one million years, be part of Alaska (assuming Alaska stays put), as the Pacific Plate continues its northwesterly trek. Much crushing and grinding takes place as these two enormous plates move past each other. When sections of the plates become locked, stress builds up until the friction is relieved by a minor tremor, or major earthquake.

The relative amount of energy released by an earthquake, its magnitude, can be measured by an instrument called a seismograph. An earthquake’s magnitude is translated into a measurement on a numerical scale, called the Richter scale. Major earthquakes usually measure between 6.0 and 9.1 (the highest recorded) on the Richter scale. Each increase of one unit on the Richter scale represents a 32-fold increase in the intensity of the earthquake. For example, a magnitude 8.5 earthquake is 32 times more intense than an earthquake with a measured magnitude of 7.5. Earthquakes are quite a common occurrence on our planet. Several per day are detected by seismologists, but because of their relative weakness, they are not made known to the global public.
   
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