April 18 marked the 102nd anniversary of the tragic 1906 San Francisco earthquake, one of the most significant earthquakes of all time. Today, Golden State residents seem to believe the benefits of living where they live outweigh the risks. Decades ago, when the risks were imprecise, people could only consider the number of people who died or were injured per past event.

In 2003, the United States Geological Survey (USGS) forecast a 62 percent chance of an earthquake with a moment magnitude of 6.7 before 2032.

In 2005, Simon Winchester, a geologist who trained at Oxford, explained in A Crack at the Edge of the World:

It is not a question of whether a big earthquake will occur, nor even a question of precisely where it will hit. There will be a quake, and it will be considerable, it will be somewhere in the vicinity of San Francisco, it will more than likely affect the San Andreas Fault or one of its cadet branches — and it will take place, most probably before 2032.

Today, thanks to various sciences and investment in research, the risks are far more precise.

In a comprehensive study recently released by the USGS, the California Geological Survey and the Southern California Earthquake Center, scientists report new statewide probabilities of earthquakes in California as a result of a first-of-its-kind model of earthquake fault rupture. The consensus of the scientific community allows, for the first time, meaningful comparisons of the hazards in Los Angeles and the San Francisco Bay Area, as well as comparisons among several large faults.

According to the new report, Californians will experience one or more earthquakes with a magnitude of 6.7 or greater, potentially causing extensive damage and loss of life. The probability of this occurring exceeds 99 percent in the next 30 years.

These probabilities do not include the Cascadia subduction zone, a 750-mile long offshore source of quakes that extends south about 150 miles into California.

The USGS even breaks these probabilities down even more by looking at Northern California versus Southern California and the San Francisco region versus the Los Angeles region (not including Cascadia subduction zone):

These new statewide probabilities are the result of a model that comprehensively combines information from seismology, earthquake geology, geodesy and paleoseismology. Considerable resources and time spent by scientists and engineers have gone into creating these probabilities.

The official earthquake forecasts, known as the Uniform California Earthquake Rupture Forecast (UCERF), were developed by a multidisciplinary group of scientists and engineers known as the Working Group on California Earthquake Probabilities. The organizations sponsoring the Working Group include the USGS, the California Geological Survey and the Southern California Earthquake Center. An independent scientific review panel, as well as the California and National Earthquake Prediction Evaluation Councils, have evaluated the new UCERF study.

These new probabilities enable planners, engineers, builders, code inspectors, health care providers, zoning specialists and emergency preparedness professionals to minimize the damage that will occur when the earthquake strikes. Also, the data help insurers to offer rates that can permit rebuilding while still allowing the company to earn a profit.

The data allows for creation of seismic-hazard maps too. They use a time-independent forecast in which the probability of each earthquake rupture is completely independent of the timing of all others.

In the 1906 earthquake (magnitude 7.9 to 8.3), more than 700 San Franciscans died and 250,000 became homeless. Winchester wrote that if an earthquake of that magnitude were to occur again, "some 30,000 would be dislocated and [now more than] some $24.7 billion in direct economic damage would result."

Investment in studies, research and new building and infrastructure designs makes preparation to minimize damage possible and necessary.

Resources

Forecasting California's Earthquakes — What Can We Expect in the Next 30 Years
by Edward Field, Kevin R. Milner and the 2007 Working Group on California Earthquake Probabilities USGS, Southern California Earthquake Center, California Geological Survey
California Earthquake Authority, 2008

The Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2)
by Edward H. Field et. al.
U.S. Geological Survey, California Geological Survey, 2008

New Study Shows Odds High for Big California Quakes
USGS, April 14, 2008

A Crack at the Edge of the World
by Simon Winchester
Harper Collins Publishers, p. 363, 2005