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 of dollars of infrastructure. That means that anyone whose home, business, or commerce involve California needs to be prepared for what Californians half-jokingly (and half-fearfully) call “the big one.”
WHAT CAN THE SAN ANDREAS FAULT DO?
As the Pacific plate slides past the North American plate, friction and the rugged plate edges combine to force a jerky rhythm onto their motion. The two plates stick together, then release; stick, then release. They may stick for days, years, or millennia; and one stretch of the fault may be moving smoothly while three hundred miles away the two sides are stuck tight. When and where the two sides are stuck, a force called “stress” accumulates in the crust. When the two sides suddenly start slipping again, this stress is released in the form of shock waves. The shock waves are an earthquake.
The size, or magnitude, of an earthquake relates to the amount of stress that has built up during the “stuck” period. That means that the length of time between periods of movement may be an indicator of how large the next release will be. This fault and its strands have been responsible for many major earthquakes within recorded history, including the San Francisco Earthquake of 1906 and the 1989 Loma Prieta Earthquake (the World Series Earthquake) in northern California. Near Los Angeles, major quakes include the Northridge earthquake of 1994 and 1971’s San Fernando Earthquake. Medium-sized seismic events occur annually along the system, while “microseismic” activity occurs daily.
The amount of damage an earthquake causes is controlled by many variables. The magnitude of the shock is most familiar. The Loma Prieta event, for instance, had a magnitude of 6.9; while the Northridge event was measured at 6.7. Scientists estimate that the 1906 San Francisco Earthquake had a magnitude of 7.8, which (since the magnitude scale is logarithmic) is more than ten times as powerful as the Northridge quake. The distance to the earthquake’s epicenter (the point on the surface straight above the center of the shock wave) is another control on the amount of damage. The depth of that central point, or focus, is also a variable. With so many unknowns, no one can predict what will happen in the case of a seismic shift. Any prediction would be only a guess because there are also variables related to the kind of bedrock, construction quality, and time of day.
From history, we know that when – not if, when – “the big one” hits, buildings will crumble and bridges will fall. Fires will break out as gas mains rupture, and firefighters will find water mains drained by their own breaks. The streets will be dotted with rubble and jammed with traffic as electric signals dangle blankly. In some areas soils may “liquefy,” causing massive damage to anything on the surface. Hospitals, if open at all, will be jammed with citizens injured by falling objects or caught in falling buildings. Hundreds, perhaps thousands, will perish either during the temblor or in its aftermath. Commerce and industry will grind to a halt. And then the Californians will pick themselves up, dust off the seat of their pants, and build it all back up again – figuring that now that the big one has occurred, everything is safe again.
MORE INFORMATION:
U. S. Geological Survey publication: http://pubs.usgs.gov/gip/earthq3/contents.html
Stanford University, Palo Alto, CA: http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html
University of California at Santa Cruz: http://www.es.ucsc.edu/~es10/fieldtripEarthQ/EarthQWelcome.html
Southern California Earthquake Center: http://www.data.scec.org/fault_index/sanandre.html
