It's as certain as sunrise: the moment there's a sizeable earthquake anywhere near the Cascadia subduction zone, nearly everyone starts wondering if it means The Big One is next. The M6.3 earthquake that struck on the Blanco Fault Zone on Thursday, August 29th was no different. Almost as soon as the mild shaking along the southern Oregon Coast stopped, worried folks were asking if Cascadia might rip. Worries will probably increase as aftershocks, some of them hefty like yesterday's 5.9, continue.

The verdict of seismologists: there's no increased risk. If Cascadia does rupture soon, it won't be because this latest earthquake triggered it. The fault zones are connected to the same plate, but they're very different beasts. (And no, despite the words placed in a professor's mouth by one eager reporter, the Cascadia subduction zone is most certainly not "overdue" for a megathrust earthquake. We'll get to that in a bit.)

Lots of people have written very serious words about this event, so I'm going to take a much breezier tone with this post. The BFZ is seriously awesome and a lot of fun to get to know!

The Blanco Fracture Zone: Seriously Not Even Close to Cascadia

The BFZ is a nice transform fault zone that's a bit like the San Andreas, only underwater and much less dangerous to humans. It forms the boundary between the Juan de Fuca and Pacific plates, and is around 200 kilometers west of the Cascadia subduction zone, where the Juan de Fuca dives under the North American plate.

This may not seem like a great distance on a global scale, but it's pretty darned far when it comes to separation between fault zones. It would take a much bigger earthquake to have any effect on far away faults. All the effects of this earthquake, I'm assured by Dr. Paul Bodin of the University of Washington, will be strictly local. That's good news for Cascadia, which won't be feeling any strain from this event.

Map of the Blanco Fault Zone
The Blanco Fracture Zone. Credit: Pszczolak Wikimedia (CC BY-SA 4.0)

Earthquakes of This Magnitude? Pfft. They Happen Here All the Time!

As Dr. Jackie Caplan-Auerbach of Western Washington University said when discussing this earthquake in the Pacific Northwest Earthquake Discussion group, "Business as usual on the Blanco." And it really is. The Blanco isn't nearly as prone to large earthquakes as the San Andreas, but it still hosts some fairly hefty ones each year. In fact, there was an earthquake nearly identical to this one almost exactly one year ago!

(Cascadia stayed quiet through that one, too.)

And in between medium-large events, the fault zone fairly hums with small earthquakes. It's got about 49mm per year of motion to accommodate, as the Juan de Fuca plate travels past the Pacific plate towards the North American plate. Some of that motion happens during larger events, some during earthquakes of quite minor magnitude, and some probably happens quietly and continuously without causing detectable earthquakes at all.

Why Isn't the BFZ Prone to San Andreas-Style Large, Damaging Quakes?

This is where it gets really neat, at least to me and hopefully to you.

The Juan de Fuca plate is born at the Juan de Fuca Ridge. This ridge is a wee cousin of massive mid-ocean ridges like the Mid-Atlantic Ridge, which erupts lovely mid-ocean ridge basalts and creates fine new ocean floor. The plate is only about 442 kilometers (275 miles) wide from its birth at the ridge to its death at the subduction zone. That means it's young and warm, much more pliant than the old, cold continental crust that makes up the majority of the San Andreas. So it tends to break in much smaller events, sort of like a cookie that hasn't been long out of the oven, compared to the sudden and brittle crack of a cookie that's been sitting on the counter for several days. Some seismologists think the BFZ might possibly be capable of an M7 or maybe a bit bigger, but a catastrophic quake like an M8 is quite unlikely there.

Also, seismic waves don't travel as readily through young, warm crust as they do through old, cold crust. So that, plus the considerable distance from population centers along the coast, means folks living along the Oregon and California coasts don't have to worry about the Blanco murdering them in their sleep.

But It's Underwater! Doesn't That Mean Tsunamis Are a Possibility?

Probably not. The BFZ's got the kind of transform faults that don't really lend themselves to creating tsunamis, and most of the quakes it produces aren't big enough to generate them anyway. It might produce the occasional wee tiny tsunami in exceptional circumstances, but it's pretty doubtful it'd do more than get your ankles extra-wet at the beach. You're in more danger from poorly-anchored shelves and such at home in the case of the largest earthquake it's likely to generate, and even that danger is quite negligible. (You should probably go ahead and get anchoring, anyway, if you haven't already. The Pacific Northwest is absolutely riddled with faults that are much closer to population centers, and hence likely to knock stuff over on you.)

So, Like, If It Isn't Dangerous, Why Is It So Exciting?

It is an ocean transform fault that is close enough to civilization so as to be easy to study,* which basically makes it the Mount St. Helens of ocean transform faults. We can intensively study it without having to spend gigantic amounts of money on just getting there and back again. The entire Juan de Fuca system, from mid-ocean ridge to subduction zone, is a fantastically convenient plate tectonics lab plunked down within easy reach of some of the finest universities in the country.

Heck yeah, that's exciting!

Okay, Fine. Now Let's Talk About Cascadia Being "Overdue" for a Major Earthquake

If you read the CBS article about the August 29th earthquake, you may have raised your eyebrows at this indirect quote:

The quake was caused by slipping along an offshore fault called the Blanco Fracture Zone and has nothing to do with the more well-known Cascadia fault, which is believed to be overdue for a major earthquake in the Pacific Northwest, said Paul Bodin, manager of the Pacific Northwest Seismic Network at the University of Washington.

Note well the lack of quotation marks. It appears the reporter slipped in an interpretation of his own while paraphrasing Dr. Bodin. When I reached out to Dr. Bodin for clarification, he advised that was not a direct quote.

This, however, is: "Like many seismologists (the majority?), I don't like the word "overdue" because I (we?) don't really buy the underlying concept."

The Earth can be regular, but it's not abiding by human clocks and calendars. It doesn't have "Cascadia Rupture" penciled in for precisely X number of years since the last one. And while we can infer the general frequency and pattern from past events, we are working with pretty large plus-or-minus numbers. There's the average, and then there's the actual, and there's hundreds of years of wiggle room there. That's an eyeblink geologically, but entire generations in human time.

When you know the fault ruptures every five hundred years on average, but sometimes there's 300 years between events and sometimes 700, give or take many decades, talking about Cascadia being "overdue" only 320 years after the last major earthquake becomes meaningless. As Dr. Bodin said, "We are entering an interval of time that geologists think has seen recurrences in the past, but they also wouldn't be surprised if we had to wait another 300 years, based on their interpretation of our past earthquake history."

So, y'know, rumors of Cascadia's being overdue have been greatly exaggerated. That doesn't mean we can stop worrying and start ignoring seismic building codes, though - far from it! It's not like Cascadia couldn't produce a catastrophic earthquake right now, and even if it keeps snoozing for another few hundred years, the dizzying variety of other faults under and around major population centers will absolutely certainly make it worth our whiles to invest in earthquake-resistant infrastructure.

One thing's for sure, though: the Blanco Fracture Zone will remain more friend than foe for a very long time to come.

*That link is another example of people desperately wanting to link the BFZ to the Cascadia subduction zone. I read the paper this press release is talking about. The only mentions of Cascadia are referring to the Cascadia Depression, which is a far, far different structure than the subduction zone.