by Hai Shunxi

Created on: December 23, 2009

Geologists are not the only people concerned about the San Andreas Fault – they just understand it a little better than most people. To appreciate why this major geological feature is so worrisome to so many people, it helps to know what the San Andreas Fault is and where it is. This knowledge allows scientists to guess what kind of havoc an earthquake on the San Andreas is capable of causing.

WHAT IS THE SAN ANDREAS FAULT?

The San Andreas Fault isn’t an ordinary fault: instead, it’s a boundary between two of the giant plates that make up the earth’s crust. This sort of plate boundary is known as a “transform fault.” Anything on the ground west of the San Andreas is actually on the Pacific plate; east of the fault is 100% North America. All this might be meaningless, except that the plates are in motion. North America creeps along a little north of west, and the Pacific plate moves northwest at a slightly faster pace. The difference in their movement amounts to only about 30-40mm (1.2-1.4 inches) per year, but over many years it adds up.

Both plates consist of hard, brittle rocks; so the boundary between them is expressed as a break in the earth’s surface. That break is the San Andreas System of faults; actually many faults that are braided like a loosely-plaited rope. The entire system has strike-slip motion, which simply means that movement along the line of the fault is sideways instead of vertical.

WHERE IS THE SAN ANDREAS FAULT?

The San Andreas Fault System can be traced from the Salton Sea in southeastern California, where it joins the Pacific spreading center separating Baja California from mainland Mexico. The system curves left (west) between the San Gabriel and San Bernardino Mountain Ranges just east of Los Angeles; then bends back to the right to track along the Coast Ranges. The fault system continues through the San Francisco Bay area as several semi-parallel fault strands before ending a few miles offshore of Cape Mendocino. Beyond the system’s northern end lie an oceanic spreading center and a small remnant of the eastern half of the Pacific plate, which is being actively consumed beneath the Cascade volcanic range.

The location of the San Andreas is key to its potential for causing a natural disaster. Two major US metropolitan areas lie not just near the fault, but actually straddle it. Combined, the Los Angeles and San Francisco metropolitan areas are home to twenty million people and trillions

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