However, a few hours later, the fault shifted again, just south of Parkfield, and this time it didn't stop short. The break raced southeastward at 6,000 mph all the way to the Cajon Pass near San Bernardino, generating the largest earthquake in the state's written history. Fortunately, the rolling waves of solid earth spread across Southern California without doing much damage to its sparse settlements.
The 1857 earthquake was the last Big One in Southern California — an estimated magnitude of 7.9. Its 150th anniversary won't be marked with the hullabaloo witnessed during last April's centenary of the more famous 1906 earthquake that left San Francisco a smoking ruin. Yet today's milestone is more significant to seismologists like me because we know from the geologic record that such seismic monsters have recurred along the 1857 break with an average spacing of about 150 years. If that doesn't give you the willies, consider the southernmost section of the fault, from the Cajon Pass to the Salton Sea, which hasn't slipped since about 1680.
We can't predict when the southern San Andreas fault will rumble next, but the probability of a large earthquake during the next 30 years is thought to lie between 30% and 70%, depending on how you interpret the geologic constraints on the regularity of the seismic cycle.
The sedimentary basins of California have become highly urbanized since 1857. These regions are strung along the San Andreas fault system in a natural but unfortunate geometry that can funnel energy from large earthquakes into very intense, long-duration waves. New computer simulations indicate that the shaking — and the damage — in the urban basins could be substantially worse than previously predicted.
Not everyone appears to be worried, however. As Sharon Bernstein reported in this paper, "efforts to bolster earthquake safety in California have hit roadblocks at the state and local levels as memories of major temblors fade and lawmakers and business owners balk at the cost of retrofitting structures." Last year, the governor vetoed funding for the California Seismic Safety Commission.
The sad experience of Hurricane Katrina taught us how large disasters can break the system. Don't count on the folks with white hats to ride to your rescue. Act in advance. Identify problems in your home. Strap top-heavy furniture and appliances to walls, secure televisions and other heavy objects that can topple. Homes and other buildings should be retrofitted, if necessary. If your house was built before 1997, it's probably not as strong as it should be. Stock your supplies, and create a family disaster plan.
If you are unsure what steps can be taken, consult the free guides published by the Seismic Safety Commission, the Red Cross or the Southern California Earthquake Center, or visit http://www.daretoprepare.org .
But more must be done at all levels of society. California has good building codes, but no code is retroactive. Many schools built before 1975 — thousands statewide — still need reinforcing. The state should appropriate substantial funding to accelerate the use of new scientific knowledge in promoting seismic safety. And federal agencies should provide more resources to scientists and engineers to help forecast large earthquakes and mitigate their effects. If we dare to prepare, perhaps the next Big One won't catch us with our guard down.