Early this morning, a significant chunk of the Bay Area's population was shaken out of bed by some seismic action, a window-rattling 4.0 magnitude quake. This is barely a week after another earthquake struck a few miles away, just offshore of the Golden Gate Bridge. Should you be hella scared?
No and yes. No, because these quakes are probably unrelated. But the Bay Area still sits astride two relatively fast-moving plates. If you're waiting for the Big One, the seismic screws are ever-tightening.
Last week's 3.3 magnitude quake originated a few miles offshore, centered straight out from the Golden Gate Bridge—on the San Andreas fault. Today, the shaking began deep below Piedmont, a city that sits sort of between Oakland and Berkeley, along the Hayward fault. Fewer than 30 miles separated the two epicenters, and their correlation made more than a few eyebrows raise this morning.
But because they lie along different faults, there's a very low chance that their rumblings were related. "It doesn’t make the hairs on the back of my neck tingle," says Richard Allen, the director of UC Berkeley's Seismology Laboratory.
Now, there is a way that two earthquakes could be related: If the energy released by one of them was great enough to impact its neighboring fault. A large quake can cast what's called a stress shadow, where it affects another fault's stress level. But these two earthquakes weren't strong enough to make a stress shadow plausible. There were too many miles of rock to move through for either quake's energy to reach the neighboring fault. "In either case we didn’t see a lot of aftershocks in those locations, so it’s telling us that the amount of stress transferred was pretty small," says Tom Brocher, a seismologist with the USGS Earthquake Science Center in Menlo Park, CA.
But that's not always the case. The thing is, there's no way to predict where the stress will transfer, or at all. The underlying geology of a fault impacts the stress that it can transfer and receive. Rigid bedrock might absorb some of the shaking, while waves roll right through sandy soil beneath bodies of water like the bay. "The 1906 San Francisco earthquake on the San Andreas didn't seem to cause aftershocks on the Hayward fault," Brocher says. "And in fact, the 1989 Loma Prieta earthquake on the San Andreas seemed to actually relieve the southern part of the Hayward fault."
Seismologically, the Bay Area is complicated. The San Andreas may be the marquee fault, but there are plenty of other credit-worthy rifts. The Hayward was today's main source of action—and it's arguably the region's second most famous. But there's also the Concord fault, the Calaveras fault, the Green Valley fault, the Rogers fault, and plenty more scattered from Santa Cruz to Sonoma County. "When I was in undergraduate many years ago, we were taught that the San Andreas was very simple, and you could stand over it with one foot on the Pacific plate and the other on the North American plate," says Brocher. "But now we know it's actually a broad zone of deformation."
The motion between these faults, primarily the San Andreas and Hayward, creates the gap that is the San Francisco Bay. That body of water is one of the best natural harbor on the West Coast, and ironically the reason why so many people live in such intimate earthquake danger.
How great is that danger? The past pair of quakes notwithstanding (both scientists I spoke with hadn't heard about the August 9 quake because they'd been on vacation—not together), the Big One is still a big risk. "The long term estimate is that there is a two in three chance of a major damaging earthquake in the next 30 years," says Allen. Why 30 years? "Because that's the normal duration of a home mortgage." In the meantime, Allen recommends you do little things every year to prepare. "Update your emergency plan, do retrofits, upgrade your earthquake kit," he says.
So yeah, maybe today's quake woke you up, but don't let it leave you shook. Earthquakes can be an effective alarm clock, but thank goodness they aren't very reliable.
