We regret to inform you but the Big One is no myth.
Like tornadoes and hurricanes, earthquakes strike with baffling randomness. They roll through some neighborhoods with brutish force while leaving others relatively spared. Consider the Northridge earthquake of 1994, which crippled the town of Fillmore in Ventura County but did much less damage to several closer communities in the San Fernando Valley. As recently as 30 years ago, seismologists had tremendous difficulty understanding the capriciousness of earthquakes. They did not know when quakes would strike and could make only rudimentary forecasts as to where the worst damage would occur. The dream of predicting earthquakes still eludes them, but the second goal—calculating where the damage will occur—is within their grasp. Southern California scientists are leading this research, using networks of supercomputers to marry vast troves of geological information with the actions of different earthquakes. A picture is emerging of how earthquake shocks travel through the deep strata of the region and which areas will be hit the hardest. What’s more, this knowledge is being translated into maps that identify the more hazardous communities.
At Caltech, civil engineering professor Swaminathan Krishnan is expanding on earthquake knowledge by locating and mapping zones where high-rise buildings of 10 to 30 stories could collapse in a 7.9 magnitude earthquake on the San Andreas Fault, the fabled Big One. Perhaps the most ambitious project is being carried out at USC’s Southern California Earthquake Center, which is creating animated simulations and maps using supercomputers provided by the National Science Foundation. Directed by seismologist Tom Jordan, SCEC’s work is yielding some of the clearest pictures yet of shaking patterns from a variety of earthquakes (pick up the October issue of Los Angeles magazine on newsstands now to see one of these pictures).
The calculations involved in these projects can be enormously complex, but the underlying principles are disarmingly simple. Scientists have long known that deep sedimentary soil, whether moisture laden or dry, amplifies earthquake shock waves, making them more dangerous than those that travel through bedrock. Thus, communities sitting on deep sediments, such as those in the central Los Angeles basin, face a higher risk of intense shaking. Some of the most perilous neighborhoods are those perched on deep sediments where the water tables are high, leaving them doubly susceptible to high-level shaking and the phenomenon of liquefaction, in which water-saturated soils begin to slide or collapse under intense shaking.
At the opposite end are neighborhoods built on bedrock, most of which are located in hillside areas. They are vulnerable to landslides, but the bedrock compresses the shock waves, making those waves shorter and less dangerous. True enough, if a big quake occurs underneath your house, you will be hit hard, no matter the soil or water table, and living in an unreinforced brick building is dangerous anywhere in Southern California. But assuming two neighborhoods are equally distant from an earthquake and in the same direction from the quake, we can now make much better predictions as to which will fare better. Such information may prove crucial in helping us prepare. Because one thing we know: The southern section of the mother of all faults, the San Andreas, historically has produced great earthquakes every 150 years or so. The last great earthquake on the southern section occurred in 1857. You do the math.