People aren’t doodling sea monsters over blank spots on the map anymore, but that doesn’t mean scientists have Earth all figured out. Plenty of geological features have yet to be discovered and understood---especially where things get watery, or subterranean, or both.
Case in point: a team of seismologists recently discovered what they believe is a fault line running along the the eastern shore of the Salton Sea in southern California. According to their (provisional, and somewhat speculative) data, the fault is many thousands of years old, and lies roughly parallel to, and is only kilometers from, the famous San Andreas fault. Sounds like something people should have known about, right?
The team that found the fault wasn't even looking for it. They were part of a National Science Foundation-funded project to get some seismic images of the Salton Sea (which has a seriously weird history—early 20th century engineers accidentally made it while rerouting the Colorado River). Given it's proximity to the Gulf of California rift zone and the San Andreas fault, they were looking for some prime geological weirdness in the salt rock basin. But rather than focus on the salt layers piled up in the sea's belly, the team's seismic imagers detected deformation and asymmetry in the sediment beneath the surface---evidence of a fault. "We were expecting to see some little faults within the profile, but not something like this," says Valerie Sahakian, lead author of the study and a seismologist at USGS’ Earthquake Science Center. "Because it runs so close to the San Andreas, they could interact, but we don’t know how."
Though California is densely populated, notoriously quake-prone, and boasts several seismological laboratories, faults like this can easily go undetected. Even active faults can be tricky to locate, and these fissures criss-cross the Earth's crust in complicated, interconnected ways. But, despite growing alarm about what the Salton Trough fault means for the San Andreas' late-coming next big quake, finding a new fault nearby doesn't increase earthquake risk. It's really a reminder that there's still a lot of science to be done in our own backyard.
Plate tectonics is tricky stuff, but the basic idea is that the earth is covered by a fractured shell of gigantic plates. These things are always moving—oh so slightly—and the stress between them builds up, eventually causing earthquakes.
Scientists know the Pacific plate moves 48 millimeters northwest relative to the North American plate every year, which loads stress onto faults like the San Andreas and the Imperial and all the little faults in between. "We know the overall motion---that’s the raw material for the next earthquake---but it's tricky to attribute it to one fault or another," Beroza says. "The earthquake gets resolved wherever the stress is highest, but it could be on any fault." So it's impossible to know if Salton Trough fault is taking the San Andrea's share of the strain.
When fault lines aren't jumping around making tsunamis for Dwayne "The Rock" Johnson to punch (Is that not what happens in San Andreas?), they can be hard to spot. “Large faults like the San Andreas are often found because they have a very obvious surface expression---it looks like the ground is all cracked,” Sahakian says. "But moderately-sized and really small faults, hairline fractures a couple of kilometers long, are not as easily detected." So you have to rely on technology. Getting data becomes more expensive, and the knowledge becomes patchier. (Not quite "here be dragons" patchy, but "where be fault lines?" is maybe scarier than dragons.) The Salton Trough fault is particularly well hidden: It's underwater, in a coastal area with active erosion, and not prone to microseismicity. In that case, you kind of have to wait for someone to get lucky ... or for it to quake.
People have been surprised and concerned by the news of the Salton Trough fault. Especially because there was just a swarm of 200+ baby quakes in the area---though those quakes ran perpendicular to the Salton Trough fault, so they're probably not directly related. Quake-watchers have been expecting the San Andreas fault to deliver a mammoth magnitude-7 quake every 175 years or so. It's about 160 years overdue. And some seismologists are now speculating that this fault is maybe the reason the San Andrea's has been so quiet for that last 300 years.
Although that's not quite settled. Seismologists aren't too clear on how the Salton Trough fault fits into its neighboring fault system. The ends of the San Andreas and Imperial faults fall a little to the side of each other, leaving an inconvenient chunk of Earth between them. "When you get to an offset, the relative motion of plates has to be accounted in another complicated way, with lots of interactions with little faults," says Richard Allen, director of the seismological lab at Berkeley. "The discovery of this new fault forces the scientific community to probe what we do and don’t understand understand about the accommodation of strain between San Andreas and Imperial fault."
What that means is scientists need to update their earthquake modeling. What it does not mean is that the region is any more or less likely to get a quake. "In Southern California, it's not about one fault. This just adds a layer of complexity," says Gregory Beroza, a seismologist at Stanford and director of the Southern California Earthquake Center. Based on their current data---including that recent earthquake swarm---scientists expected to find small faults running perpendicular to the San Andreas, not parallel. So it's a bit a head-scratcher.
Sure, neither of those scenarios is particularly comforting. California is earthquake country, and you just have to be prepared. But if this fault proves to be genuine, it will bring seismologists a little closer to understanding the complex web of stressed out cracks that lie underneath California. The late discovery of the Salton Trough fault isn't all that surprising. Even though scientists might seem a little high-minded, there's no legion of smug fault finders just waiting to point out every flaw in the Earth's crust.
