San Francisco’s Next “Big One” Could More Likely be “Big ONES”

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Bay Region faults

Bay Region faults and their associated 30 year earthquake probability. Image courtesy USGS.

The San Francisco Bay Area’s next big seismic event could be another great quake like the infamous 1906 temblor, or perhaps more likely, the region could possibly be struck by a series of earthquakes, according to a new study published in the Bulletin of Seismological Society of America.

The Bay Area has a long seismic history, as its location straddles the both the northwest-moving Pacific Plate and the southeast-moving North American plate.

Stresses on these plates accumulate as their progress is impeded by their slow grind against each other.

Once built up to capacity, energy is released in the form of an earthquake along one of the area’s main faults.

While the San Andreas may be the most well known, several other faults are present and equally as dangerous: the Calaveras, Concord-Green Valley, Greenville, Hayward-Rodgers Creek and San Gregorio faults.

San Francisco Bay Area’s Earthquake History

Written earthquake history of the San Francisco Bay region began in 1776, when the Mission Dolores and the Presidio began keeping records of felt earthquakes and their ensuing damage.

This new study provides a window of insight of nearly 200 additional years worth of earthquakes, obtaining data from as early as the year 1600. Researchers created trenches across known faults to both observe previous recent surface ruptures as well as to date paleoearthquakes. They dated the ancient quakes by using radiocarbon dating of detrital charcoal and determining the presence of non-native pollen grains.

Researchers discovered that between 1690 and 1776, when record keeping began, that a cluster of earthquakes ranging from M6.6 to M7.8 occurred on the Calaveras fault (north segment), Hayward fault (north and south segments), Rodgers Creek fault, San Andreas fault (North Coast and San Juan Bautista segments) and the San Gregorio fault.

These earthquakes likely released an amount of energy equivalent to the release of the giant 1906 quake.

Recent California Quake Activity

Based on this most recent study, earthquake activity occurring in clusters appears to be the norm for this region’s fault system rather than occurrences of a larger, single event – the devastating 1906 earthquake in retrospect appears to have been an anomaly in the area’s seismic history.

The high amount of stress released from the crust during this event led to an uncharacteristic lull in earthquake activity in the region. During this quiet seismic period, the population boomed – the city of San Francisco in 1906 before the earthquake had around 400,000 residents; today’s entire Bay Area is now home to around 7 million.

What Does the Bay Area’s Seismic Future Hold?

With multiple faults literally crossing directly through this huge population center, even a moderate sized quake would impact thousands of people and cause extensive damage.

As far as probabilities are concerned, a 2008 study conducted by the US Geological Survey, the California Geological Survey and the Southern California Earthquake Center estimates the Bay Region faces a 63 percent probability for one or more M6.7 or greater quakes through 2036.

The faults most likely to be affected are the well-overdue Hayward-Rodgers Creek system and the San Andreas. The Hayward-Rodgers Creek Fault has a 31 percent chance of producing an earthquake in the next 30 years – it hasn’t seen one since 1868. The San Andreas Fault system’s chances of producing a quake in the next 30 years are slightly less, at 21 percent – the most recent earthquake this fault produced was the M6.9 1989 Loma Prieta event.

When Will We Enter the New Earthquake Era – or Have We Already?

After the 1906 earthquake, a M6.5 struck the city in 1911, and no other activity was recorded until 1979 when a M6.0 was recorded, followed by another M6.0 the next year. A M6.3 then occurred in 1984, with the largest recent earthquake, the M7.1 Loma Prieta event occurring in 1989. A M5.1 in 2001 was then followed in 2007 by a M5.6.

Decoded Science asked the study’s lead author David Schwartz, an earthquake geologist with the US Geological Survey, about the area’s history and potential for future seismic activity. He tells us,

“When activity does resume it is likely to be distributed on faults across the Bay Area and be more similar to what we saw in the prehistoric cluster than to a single great earthquake like 1906 because all of the faults are moving the plates and have gone a long period of time without producing large earthquakes. This is consistent with our 63 percent 30-year probability of one or more M≥6.7 earthquakes. This probability is distributed across all of the major regional faults. However, we can’t say when this will actually start or how long this potential sequence will last.”

Earthquake Prediction

Although we can’t definitively predict earthquakes, we can assume their likelihood of occurring again based on past occurrences. With this prolonged stretch of inactivity, followed by the recent seismic events beginning in 1979, perhaps this activity is a sign of things to come in the near future.

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