California is in an Earthquake Drought | Big One Almost Certain in next 30-Years

Steven Agar |
California, earthquake, drought
Fires break out across San Francisco after the April 18, 1906 earthquake. Credit: USGS

California earthquakes are a geologic inevitability. The state sits across the North American and Pacific tectonic plates and is crisscrossed by the San Andreas and other active fault systems. The magnitude 7.9 earthquake that struck off Alaska’s Kodiak Island on Jan. 23, 2018 was just the latest reminder of major seismic activity along the Pacific Rim, writes Richard Aster at

The earthquake situation in California is actually more dire than people who aren’t seismologists may realize. Although many Californians can recount experiencing an earthquake, most have never personally experienced a strong one.

For major events, with magnitudes of 7 or greater, California is actually in an earthquake drought.

Experts are aware that multiple segments of the expansive San Andreas Fault system are now sufficiently stressed to produce large and damaging events.

Perspective view of California’s major faults, showing forecast probabilities estimated by the third Uniform California Earthquake Rupture Forecast. Credit: NSGS

The 1906 San Francisco earthquake was the last quake greater than magnitude 7 to occur on the San Andreas Fault system. The inexorable motions of plate tectonics mean that every year, strands of the fault system accumulate stresses that correspond to a seismic slip of millimeters to centimeters. Eventually, these stresses will be released suddenly in earthquakes.

But the central-southern stretch of the San Andreas Fault has not slipped since 1857, and the southernmost segment may not have ruptured since 1680. The highly urbanized Hayward Fault in the East Bay region has not generated a major earthquake since 1868.

Reflecting this deficit, the Uniform California Earthquake Rupture Forecast estimates that there is a 93 percent probability of a 7.0 or larger earthquake occurring in the Golden State region by 2045, with the highest probabilities occurring along the San Andreas Fault system.

No California earthquake since the 1933 Long Beach event (6.4) has killed more than 100 people. Quakes in 1971 (San Fernando, 6.7); 1989 (Loma Prieta; 6.9); 1994 (Northridge; 6.7); and 2014 (South Napa; 6.0) each caused more than US$1 billion in property damage, but fatalities in each event were, remarkably, dozens or less. Strong and proactive implementation of seismically informed building codes and other preparations and emergency planning in California saved scores of lives in these medium-sized earthquakes. Any of them could have been disastrous in less-prepared nations.

California, earthquake, drought
Collapsed Santa Monica Freeway bridge across La Cienega Boulevard, Los Angeles after the Northridge earthquake, Jan. 17, 1994. Credit: Robert A. Eplett/FEMA

The good news is that earthquake readiness is part of the state’s culture, and earthquake science is advancing – including much improved simulations of large quake effects and development of an early warning system for the Pacific coast.

To read the article in full, recommended, head over to

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