Earthquake Outlook for the San Francisco Bay Region, 2014–2043

Known active faults of the Bay Area. Each circle gives the chance of a magnitude 6.7 or greater quake on that fault system by 2043; the lesser-known faults together have a 13 percent chance. USGS.

Using recent earthquakes, better fault mapping and a new probability model, the 2014 Working Group on California Earthquake Probabilities updated California's 30-year forecast. It found a 72 percent chance of at least one earthquake of magnitude 6.7 or greater striking somewhere in the San Francisco Bay region before 2043. Quakes that size can cause widespread damage, so simple steps to reduce injury, damage and disruption — and to speed recovery — are worth taking now.

The odds, 2014–2043

These include the major faults, lesser-known faults and unknown faults. The chance on any single fault is lower: the highest are on the Hayward, Rodgers Creek, Calaveras and San Andreas faults — for magnitude 6.7 or larger, 33 percent on the Hayward or Rodgers Creek faults and 22 percent on the San Andreas. Individual fault sections are lower still, but a magnitude 6.7 anywhere shakes a broad area — which is why the regional number matters.

Preparedness pays: South Napa, 2014

Early on Sunday, August 24, 2014, a magnitude 6.0 earthquake jolted Napa awake. Within 30 minutes, staff of Becoming Independent, a nonprofit that helps adults with intellectual disabilities live independently, were calling the people they serve, then visiting everyone who needed help cleaning up and making their homes safe. It went smoothly because staff and clients had trained for disasters and had emergency kits ready.

The quake shifted houses off their foundations, damaged chimneys, started fires and broke water mains, causing hundreds of millions of dollars in losses and damaging many historic masonry buildings downtown. It was the region's largest since the 1989 magnitude 6.9 Loma Prieta earthquake — a clear reminder of the Bay Area's vulnerability.

A building damaged in the 2014 South Napa earthquake. Erol Kalkan, USGS.

Why the Bay Area has earthquakes

The same process that shaped the region's coastlines, bays, hills and valleys drives its earthquakes. The Bay Area sits on the boundary between the Pacific and North American plates, where the Pacific plate slides slowly northwest.

• San Andreas Fault: the fastest-slipping fault on the boundary, source of two magnitude 7.8–7.9 earthquakes in historical time, including the 1906 San Francisco earthquake.
• Other major faults: Hayward, Rodgers Creek, Calaveras, Maacama, San Gregorio, Concord, Green Valley and Greenville.

Earthquakes larger than magnitude 2.0, 1985–2014. Some sections of major faults, notably the San Andreas, have few small quakes but will produce large ones. Northern California Seismic Network; USGS.

How the odds are calculated

• Measuring strain: GPS and other surveying and geologic methods show how plate motion — 1.6 inches a year across the region — loads each fault. Balancing that motion against slip in large quakes and slow creep gives average earthquake rates over hundreds to thousands of years.
• Digging for history: a trench across the Hayward Fault in Fremont revealed 12 large earthquakes in the past 1,900 years, 100 to 210 years apart; the latest was in 1868. Elsewhere the record is thin: seismographs go back only to about 1900, written accounts are sparse before 1850, and few places can be trenched.
• A short window: 30 years is convenient for people but short compared with the hundreds to thousands of years between large quakes on these faults.
• The post-1906 lull: large quakes were frequent in the late 1800s — 13 of magnitude 6 to 7 in the 50 years before 1906 — but only 6 in the 110 years since, because the San Andreas's huge slip in 1906 relieved stress on many faults. Plate motion has been reloading them; quakes over magnitude 5.5 resumed in the second half of the 20th century, and the rate is expected to rise.
• The 2014 model: UCERF3 (the Uniform California Earthquake Rupture Forecast, version 3) accounts for how fast stress builds on each fault and how long since its last big quake. It added 32 smaller faults to the 5 major fault systems used in 2008, and more ways for faults to rupture together.

Bay Area earthquakes of magnitude 5.5 and larger, 1850–2014. USGS.

Will it affect you?

Probability is only part of the hazard.

• Size matters: bigger quakes shake larger areas harder. South Napa's magnitude 6.0 damage was localized; Loma Prieta's magnitude 6.9 damaged a region nearly 100 miles long.
• Ground matters: local soils, geology, building quality and flooding from dam or levee failure all shape damage — Loma Prieta devastated vulnerable parts of Oakland and San Francisco more than 50 miles from the rupture.

Shaking in the 1989 magnitude 6.9 Loma Prieta earthquake: the stronger the quake, the wider the area of strong shaking. USGS.

Road damage from the Loma Prieta earthquake. H.G. Wilshire, USGS.

Protect yourself and your family

Before the next earthquake:

• Check home and workplace for hazards and secure movable items.
• Plan: make an emergency plan and gather supplies to last your household 72 hours or longer.
• Practice "Drop, Cover, and Hold On" at home, work and school.
• Repeat: reassess every year and join the Great California ShakeOut drill on the third Thursday in October.

More: USGS earthquakes, the UCERF3 fact sheet, Putting Down Roots in Earthquake Country, the ShakeAlert early warning fact sheet, Did You Feel It?, Earthquake Country Alliance and ShakeOut.

Sources

Based on Earthquake Outlook for the San Francisco Bay Region 2014–2043, by Brad T. Aagaard, James Luke Blair, John Boatwright, Susan H. Garcia, Ruth A. Harris, Andrew J. Michael, David P. Schwartz and Jeanne S. DiLeo, USGS Fact Sheet 2016–3020, U.S. Geological Survey (DOI); a work of the United States government in the public domain. The maps, timeline, probability table and USGS photographs come from the fact sheet's PDF; its "Seven Steps to Earthquake Safety" panel, adapted from the Earthquake Country Alliance, is left out.