The earthquake occurred in one of the most seismically active regions of California, a place seismologists call the Mendocino Triple Junction. This part of California sees more energy release from earthquakes than just about anywhere else, and although there is little in the way of population nearby, the earthquake hazards here are significant. Had an EQW system been in place here, the residents of Eureka could have received nearly 10 seconds of warning before they felt any shaking, and residents of more distant towns would have received even more warning. Considering this region is at risk of experiencing earthquakes up to magnitude 9, this warning could be very helpful in protecting the residents of the North Coast.
Triple junctions are places where three tectonic plates come together at a point, kind of like the “Four Corners” where Utah, Colorado, Arizona and New Mexico come together. Only instead of state lines converging at a single point, it’s three major faults. The result, predictably, is a tectonic mess. At the Mendocino Triple Junction, the three plates are the North American, Pacific and Gorda Plates, and the three faults are the San Andreas Fault, the Mendocino Fracture Zone, and the Cascadia Subduction Zone. The San Andreas Fault, as you know by now, is a strike-slip fault, and it produces earthquakes that are right-lateral with a northwesterly strike (alignment). Right-lateral means that if you stand on one side of the fault and look across, in an earthquake the ground on the other side of the fault moves to your right. The Mendocino Fracture Zone is also a right-lateral strike-slip fault, but its earthquakes strike east-west, in line with the fault itself. Finally, the Cascadia Subduction Zone generates thrust earthquakes, and in fact is capable of the sort of megathrust earthquakes we saw in Japan 3 years ago, and in Sumatra about a decade ago. A topic for another day…
OK, so northwest-southeast trending strike-slip, east-west trending strike-slip, and roughly north-south trending thrust earthquakes. Complicated, but hardly a tectonic mess. And I promised you a tectonic mess, didn’t I? Well… take a look at the plate motions in the map, represented by the fat green arrows. Notice how the Pacific Plate is moving NW relative to North America, and the Gorda Plate is moving directly east. The Juan de Fuca plate, north of the Gorda Plate, is moving sort of ENE, but there isn’t enough northward motion there to match the swiftly moving Pacific Plate. The result is that the Gorda Plate is getting crushed, slowly, agonizingly, and relentlessly, between the Pacific and Juan de Fuca Plates. It’s a pretty young piece of crust (notice how close the Gorda Ridge is, where the crust is constantly being newly formed), so it’s relatively weak and pliable compared to its neighbors to the north and south. As it gets crushed, it relieves the stress by slipping on relatively small “intraplate” strike-slip faults, some trending NW-SE and others trending NE-SW, within the Gorda Plate itself. This crisscross pattern of strike-slip faults is called “conjugate” faulting.
It is exactly this sort of fault, in this case near the triple junction but not on any of the major faults, which slipped in this week’s earthquake. As you can see from the “beach ball” diagram to the right, the orientation of this earthquake is either NE-SW or NW-SE (it’s hard to tell just from the beach ball which is the real orientation), and the shading is consistent with the Gorda Plate being squished. The shaded areas represent outward motion and the white areas represent inward motion, so imagine taking a beach ball and squishing it from the north and the south, this shading pattern is more or less what you’d get. The good news here is that these conjugate faults are not oriented particularly favorably to encourage events on any of the major faults, so there’s no reason to think this could be a precursor to an even larger event (though as I’ve said before, I never say never). The unfortunate thing is that earthquakes on these conjugate faults tend to have higher “stress drop,” meaning they are felt more strongly and sharply for a given magnitude, relative to a comparable earthquake on one of the major faults. I don’t know whether that is the case for this particular earthquake, but it’s a good bet.