What the [insert expletive of choice here] is happening? That’s pretty much the first question that traverses most of our minds when the formerly-solid ground starts rocking and rolling. Those of us in seismically-active areas or who have had the sometimes-dubious pleasure of living in them in the past realize pretty quickly what’s going on. Yep. Definitely an earthquake. And once the shaking stops, a lot of us have another question: “All right, which fault’s fault is it?”
Napa, California and the surrounding communities definitely wanted an answer when a magnitude 6.0 struck, jarring folks severely at 3:20 in the morning Pacific time. USGS says the shaking only lasted about 10-20 seconds, depending on how far you were from the epicenter. Granted, that doesn’t sound like a lot of time. But go ahead and sit there without doing anything for twenty seconds. Now imagine the structure you’re in is shivering and shaking and then feeling like a roller coaster. Imagine stuff is falling down all over the place, and you’re torn between staying frozen in place and ducking for cover (remember the drill: drop, cover, hold on!), things are pretty noisy, and you’re not sure if this is the Big One or just a Big Enough One, which means these may be your last seconds on earth…. Yeah. Those seconds can take hours.
And it could be days before we know for certain which fault is at fault. But we have a prime suspect.
Instruments at UC Berkeley and the USGS in Menlo Park were able to find the epicenter and focus right quick, down to the tenth of a mile. Great! You’d think that would tell us the fault, but it’s not so simple round those parts. And to understand why, first we need to understand a little something about faults in general. By lucky coincidence, my geoteacher, Lockwood, had posted a little something on just that subject this month:
We often draw or visualize faults as a line, or single break. But actually most larger faults have multiple, interweaving strands, and are more properly referred to as fault zones. When we map an area, a single line will often cover the entire zone at the map scale, but on the ground, as with most things geological, it’s a little more complicated than just a single, simple line.
Now, we’ll still talk about faults, but we’re dealing with more of a fault zone. And the San Andreas isn’t a fault so much as a fault system: an entire network of related faults, all caused by the Pacific plate grinding against the North American plate as it moves ever-so-slowly northwest in relation to the continent. The forces are titanic. We get a sense of them when the strain builds to the point where faults rupture, and that energy is unleashed.
This earthquake happened about 6 km (4 mi) northwest of American Canyon and 8 km (5 mi) south-southwest of Napa, near the eastern shore of San Pablo Bay. This is within the San Andreas Fault system, and it’s right between two other active systems: the Hayward-Rodgers Creek system to the west, and the Concord-Green Valley system to the east. There’s a couple of faults there: the West Napa and Carneros-Franklin faults. The whole zone is about 70 km (44 mi) wide. This was a shallow-focus earthquake, meaning it didn’t happen very deep underground – only about 11 km (7 mi) down. It was pretty easy to pinpoint, but figuring out which fault is a bit more difficult because of the same thing that makes the ground there so dangerous to structures when earthquakes happen: sediment.
When you have nice, crispy, crunchy rocks, they break. You can see the break. If you visit that link to Lockwood’s blog, you’ll see one of the possible results: a lovely slickensided cliff. And I think everybody’s familiar with the San Andreas Fault’s surface expression further south, but I’ll show you it just in case:
So that’s what that mighty fault looks like, running through the desert. But that’s pretty rocky ground with just a splash of sediment. Get into the rich Napa Valley further north, and suddenly you’re dealing with all sorts of sediment dumped by the Napa River and its tributaries, and you’ve got alluvial fans bringing more sediment down from the hills, and then you factor in the bay, where you’ve got salt marshes galore. Now consider the difference between carving in stone and writing in sand. Now you understand why a fault just can’t express itself.
So it’s hard to tell if there’s a fault zone under all that soft stuff, but we did know from various traces that one was there. It was named the West Napa Fault Zone, and USGS geologists looking in to it divided it into two sections: a southern one called the Napa County Airport Section, and a northern one called the Browns Valley section. There was enough evidence to tell it was a northwest-striking (remember strike and dip?) set of dextral faults. What’s dextral? Basically, it means that whichever side of the fault you’re on, when you’re looking at the fault, it’ll look like the other side’s moved to the right. There’s a great photo of that taken on Oak Rock Drive, one of the roads cracked by the South Napa quake. That’s 24 cm (about 9.5 inches) of movement. No wonder the buildings shook!
And, of course, shaking’s worse on loose sediment. You can try this at home: here’s a rather delicious recipe for geology in your kitchen.
That fault zone has been monitored for creep ever since the last big earthquake in that area, Loma Prieta, but none was seen. So you might think it’s fairly quiet, but the USGS knew it was still active, and by six in the ay-em, they’d decided it was probably the culprit. But you don’t know these things until you get boots on the ground, and get a bit more certainty. We get a little closer to certainty with Dr. Michael Oskin and PhD students Alex Morelan and Chad Trexler from UC-Davis. Dr. Oskin was in southern California, but his students were on the ground and mapping soon after the quake, and they were able to determine that, yep, it’s probably the West Napa Fault, Brown’s Valley section. And remember how we were talking about zones rather than neat, clean, single faults? The rupture zone they found is right about half a kilometer (.3 mi) wide, 16 km (10 mi) long, and had a maximum slip of 25 centimeters (9 inches). Not bad for the first day’s work!
What does the future hold? Well, we’ll probably see more detail, and adjustments to what we know as more data comes in. For residents in and around the Napa Valley, there’s the cleanup, and aftershocks, and a very slight chance this wasn’t the biggest quake they’ll endure. We know there will be even larger earthquakes in the area in the future. We don’t know how soon. We don’t know how big. There’s no way to predict exactly what will happen. So build to code, retrofit your buildings, and make sure you’re prepared!
South Napa Earthquake Links:
Geology in Motion: South Napa Earthquake today, M 6.0-6.1–geologic context.
Outside the Interzone: Geo 730: August 11, Day 588: Now That’s Slick!
UC-Berkeley Seismo Blog: Which fault is it?
San Jose Mercury News: Napa earthquake: Hundreds hurt, dozens of buildings condemned after 6.0 jolt.
Dr. Mike Oskin on Twitter. Just his whole stream today has been amazing.
Chris Rowan on Twitter: Shakemap for CA quake: some very strong shaking at epicentre (rupture only 10 km deep).