Have you ever wondered why all other states of the United States are not so often visited by earthquakes than California? Arguably, many major earthquakes in the world happened in Los Angeles and San Francisco area, such as the 1906 big quake that left 3000 casualties. Barely a month ago, Los Angeles was hit by a relatively slight shaking and that bigger earthquakes are always a constant threat to Californians; their properties and their lives.
These large quakes are a direct consequence of the geology of the region. The famous San Andreas Fault that geologically borders Los Angeles from mainland United States is one of the most active faults in the world. The Los Angeles region is actually in motion past the mainland and after a thousand years or so; San Francisco will eventually come abreast with Los Angeles (Fig.1). The motion is caused by a flowing semi-fluid region underground that geologists call asthenosphere (Fig 2.); serving like a conveyor belt that moves overlying land masses, albeit at a very slow rate of 2 cm per year.
How this motion causes sudden and enormously strong movements of the ground during an earthquake? According to the Elastic Rebound Theory put forward by Henry Fielding Reid, the faults as they slide against each other gradually pick and store up elastic potential energy, and upon reaching a significantly strong level the stored energy is suddenly released, shaking the ground. An earthquake has occurred. The situation is pretty much similar to a spring gradually given a stretch than when released causes a quick and strong movements.
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After a major earthquake, the faults resume their usual business of accumulating elastic potential energy, resulting to thousands of tiny and unnoticeable earthquakes everyday within regions at the vicinity of a fault line like California, and because of the slow rate of the built up due to slow motion of the sliding plates, the rate of accumulation is significantly slow, thus the occurrence of major earthquakes once, twice or none at all in a lifetime. And we know for a fact that a major earthquake can exterminate a civilization that our masonry toiled to establish in years in matter of minutes or even seconds. And much to our helplessness, the whole of science is still far from predicting this paramount catastrophe on the planet. And because earthquakes will occur again and again, Californians and all others residing near a fault line are really in continues danger.
