The short answer to why California is so prone to earthquakes is, of course, location. California is particularly vulnerable to earthquakes because it sits atop an unlucky place where two of the Earth's tectonic plates meet: The Pacific plate and North American plate meet below California, thus creating a number of faults. The two plates usually just grind against each other, one moving northwest and the other southeast. However, when the friction becomes too strong, the two plates lock; this means that the plates no longer move, but the energy within them is still present. As the energy builds, the force of the energy eventually overcomes the lock and the plates snap back into place. The energy that gets released causes one of California's many earthquakes.
The boundary area between these two major plates is the San Andreas fault, which is a continuous break in the Earth's crust that runs some 800 miles (1,287 kilometers) from northern California south toward San Bernardino. The San Andreas is a "major" fault line that has smaller branch faults along it. The fault line runs at least 10 miles (16 kilometers) beneath the Earth's surface. Scientists believe that during the San Andreas fault line's geologic existence, which extends back about 15-20 million years, the total amount of "spread" of the land along the fault is about 350 miles [source: U.S. Geological Survey].
Not surprisingly, given the destruction and loss of life earthquakes can bring about, scientists try their best to monitor plate activity and also try to gauge how earthquakes in one place might increase the risk of another one someplace else. California scientists in the spring of 2010 thought an Easter Sunday quake in Mexicali did not bode well for pressures along two other fault lines -- the San Jacinto and Elsinore. They thought the Mexicali earthquake increased pressure on those two lines and, sure enough, just a few months later southern California got a 5.4 scale earthquake along the San Jacinto fault [source: Los Angeles Times]. Hopefully, if the feared "big one" ever hits somewhere, someday in California, scientists might be able to give citizens at least some advanced warning.
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