Bay Area earthquake guide: Where could the big one hit?

The fault lies not in our stars but miles below our feet—which, of course, is a problem for anyone living and working in the seismically unstable Bay Area with its ever-ticking geological clock.

The South Bay rang in the new year with an earthquake near Morgan Hill late on New Year’s Day, which the U.S. Geological Survey (USGS) initially measured at a respectable, but not particularly worrisome, 3.9 in magnitude.

What Bay Area residents dread is the next proverbial big one, the next major regional earthquake that seismologists say will statistically strike by the year 2043. But understanding where and when it will most likely happen gets complicated—and not just because (brace yourselves) nobody can predict earthquakes, but because of the intricacy of the fault systems that cause quakes.

To understand the risks, every Bay Area resident should have at least a working layperson’s understanding of what’s going on just under the surface of our thriving but seismically imperiled communities.

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First of all, what is a “fault”?

USGS defines a fault as “a fracture or zone of fractures between two blocks of rock”—in this case most notably between the enormous tectonic plates that make up the surface of the Earth—a stress area that allows the Earth’s crust to shift.

Usually this movement is slow and gradual; geologists call this movement “creep,” and every fault has a normal amount of creep every year.

Earthquakes happen when the faults move suddenly and violently after years (or decades, or centuries) of plates pressing against each other—the planet’s crust will put up with a lot, but it has a breaking point.

A dizzying network of faults laces the Bay Area, some of them so obscure that they don’t have names. But for the most part seismologists only worry about the potential for a few of them to cause enormous damage.

Where things get weird—and potentially more difficult to predict—is where faults meet. As it turns out, just like communities, the Bay Area’s faults have relationships with each other, which can prove worrisome for those living nearby.

The faults

• San Andreas: The largest and most infamous of the faults in the Bay Area, USGS says it’s the most extensively studied fault in California, and perhaps in the world—but possibly not the most dangerous.

In 2014, USGS projected a 22 percent chance of this fault causing a 6.7 or greater quake by 2043—which is not exactly a comforting number, but also not the worst odds in the neighborhood.

The USGS fault database measures the San Andreas stretching over 672 miles, from the King Range Mountains in Humboldt County down to the Gulf of California.

How big of a quake the San Andreas can produce depends on which part of it we’re talking about: Some sections, including the one closest to SF, can shake up to an 8.1 or 8.2 magnitude, roughly three times more powerful than the 7.9 shaker in 1906.

Note that the San Andreas is not a single fault; it’s a “fault zone,” a term that refers to many faults crammed together. These are so tightly connected that for practical purposes they’re usually treated as one, which accounts for the San Andreas’ tremendous size (it extends ten miles deep in some places). The fault zone is part of the larger San Andreas fault system, which also includes many of the other major Bay Area faults on this list.

The main fault does not actually run directly beneath San Francisco, instead it travels up the Peninsula, and then strays off beneath the Pacific Ocean north of Daly City’s Mussel Rock Park. By the time it gets as far north as Lands End, it reaches over three miles west of SF.

But that’s not enough distance to provide any kind of insulation from tremors—in fact, in temblor terms, three miles is essentially right in the next room. The epicenter of the 1989 Loma Prieta Earthquake that wreaked havoc on San Francisco and Oakland was over 50 miles away from SF.

• Hayward: Alarm bells have rung in recent years about the Hayward Fault zone, which stretches over 66 miles from San Pablo Bay down to Fremont, running directly beneath parts of Oakland, Berkeley, San Leandro, and Hayward, among other East Bay cities.

This is the regional fault that’s gone the longest without a major quake in recent history, and thus could likely cause one in the next few decades.

The Hayward Fault, in general, is weaker than its San Andreas neighbor, topping out at quakes of about 7.5 in magnitude. The maximum magnitude a fault can produce depends on its size, but since, as previously noted, fault zones are technically many faults rather than one, their apparent lengths can be deceptive, with some areas stronger than others.

In 2015, UC Berkeley researchers suggested that the Hayward Fault may be connected to the nearby Calaveras Fault, which, if true, would mean the potential for a more powerful quake.

In any case, USGS pegs the Hayward fault at a 33 percent chance of a major quake in the next 25 years, the highest in the region. A 2018 USGS model predicted that 7.0 magnitude quake centered under Oakland would cause hundreds of deaths and more than $80 billion in damages—even in a best-case scenario.

• Rodgers Creek: This is Santa Rosa’s major fault. Note that the Rodgers Creek name designates a specific fault rather than a larger fault zone, but Rodgers Creek does have a potentially dangerous relationship with the Hayward Fault.

Although researchers long knew that the fault ran beneath Santa Rosa, it wasn’t until 2016 that they pinpointed the precise area where the fault rises to the surface (the “active surface trace”), largely because the urban development made the fault harder to study.

Whereas the southern part of the Hayward Fault can connect to the Calaveras Fault with frightening regional implications (see below), the northern section is more likely to synch with Rodgers Creek; this simulation of a 7.2 magnitude quake affecting both faults shows major shaking extending as far as San Jose in only 40 seconds.

Fortunately, the same research that produced that simulation quake map notes that although earthquakes on multiple faults do occur, they are much rarer.

• Calaveras: This is the nearly 97-mile fault zone east of Hayward that spans the distance between Alamo and the Gabilan Mountains in Monterey County.

Although generally more quiet than its immediate neighbors, this fault has caused several quakes of 6.0 or greater around Moraga Hill in the past 200 years. And there’s a 26 percent likelihood that it will cause another big one relatively soon.

As mentioned above, the Calaveras Fault became a lot more interesting—that is to say, terrifying—in 2015 when UC Berkeley found what it believed was conclusive evidence that this fault connected directly to the Hayward Fault, a possibility that was suspected for a long time but never previously illustrated.

If true, that would mean that a major East Bay earthquake could span from around Richmond down to Gilroy—about a 7.3 magnitude. Even worse, the lead researcher on that project, Estelle Chaussard, noted that the Calaveras Fault also connects to the San Andreas zone. So if the quake happened closer to, say, Hollister, it could gain force from that link as well—see what we said before about how these connections complicate things?

• Concord: The relatively short Concord Fault (less than 12.5 miles) runs from around Walnut Creek to the Suisun Bay. It’s also sometimes referred to as the Concord/Green Valley Fault for its connection to the 24-miles Green Valley fault zone, which runs all the way up to near Lake Berryessa.

Along with the Greenville Fault Zone to the south, which runs from around Concord all the way down to Isabel Valley, the Concord Fault has about a 16 percent chance of triggering the big one in the near future.

One USGS scientist, David Schwartz, worries that the Concord Fault, because of its proximity to so many densely populated cities over broad areas, poses a particularly potent danger, calling it “the most urban fault” in the East Bay.

• Maacama: For such an obscure geological detail, the Maacama Fault Zone is actually large, measured over 99 miles from around Laytonville in Mendocino County to Mark West Creek in Sonoma County. It may even extend further north than that, but its exact terminus remains a bit fuzzy.

It’s quite the stretch compared to its bigger neighbors, with only about an eight percent chance to blow by 2043. The Maacama is part of the San Andreas system and has been called its “sister fault”—hopefully, that remains an estranged relationship.

• San Gregorio: The quiet one of the family, at least for now, with only a projected six percent chance of producing anything in the range of a 6.7 anytime soon. This huge fault zone runs 130 miles, from southern Marin County down to around the town of Lucia in Monterey County, almost all of that distance under the ocean.

Although it’s certainly capable of producing a large and disastrous quake, this fault zone hasn’t made waves in a long time; the last big one here was prior to 1775, and possibly as far back as 1200 CE.