22 November, 2010

Great Scarp! Seattle Has Faults!

You know how I promised you a tour of one of Seattle's most prominent fault scarps a few months ago?  You'd probably given up hope I'd ever get round to it.  But here we are at last, taking a trip through the southern end of the Seattle Fault Zone, and seeing some pretty dramatic evidence of what happens when the ground rips.

Wacky Trees on Ginormous (Presumed) Fault Scarp
We'll begin with a bit o' scarp that I've been unable to confirm or deny as a fault scarp.  It certainly looks like a fault scarp.  In fact, the only thing that it doesn't have in common with the known fault scarp at Seward Park is the fact it's enormous and easily accessible.  There you are, walking the nice, wide, paved path around the boat launch area and Andrews Bay, and all of a sudden the hill gives way to a cliff.  It looks like the end of the drumlin was just sliced right off by a giant's meat cleaver.  It's dramatic and a little shocking.

By now, the non-geologists in the audience are probably wailing, "But Dana - what the fuck is a fault scarp?"  It is, quite simply put, what happens when, during an earthquake, one bit of earth either goes zipping up or another goes down (or possibly both at once, I suppose).  Here, you can easily create your own right there at home.  Hold your hands together like you're praying.  Then aim them like you're about to shoot somebody.  Let your left hand slide toward the floor a good inch or two, and you will see your right hand become a fault scarp justlikethat.  Neat, huh?

Here's a nice, simple illustration showing you all the relevant bits:

So, you wanna know why there's a ginormous fault scarp or two hanging about one of Seattle's city parks?

I thought so.

Welcome to the Seattle Fault Zone, my darlings.  You can read about my first adventures in it here.  On January 30th, 2009, it gifted us with a 4.5 magnitude earthquake, reminding us that we live in a very tectonically active area.  I mean, practically all of Seattle's getting crunched and squished and tossed about like a cowboy on the back of a bronco standing on a wild bull standing on the deck of a ship in a hurricane.  People living near the San Andreas Fault don't know how good they've got it.  All they've got to deal with is a nasty transverse boundary.  We get a subduction zone, and we're dealing with no less than three tectonic plates all jockeying for position up here.  It leads to subduction zone earthquakes and volcanoes and tsunamis, oh my.

This enormous fault zone:

means that Seattle's getting crushed in a north-south vice.  Nobody explains it better than Paul Talbert:
Why is western Washington being compressed? Although the part of the Pacific Ocean that lies offshore from western. Washington rests on the Juan de Fuca Plate, most of the Pacific lies over the Pacific plate, which is slowly moving northwest relative to the North American plate. As the Pacific Plate slides northward along the San Andreas Fault in California, it drags the edge of California northward, rotating western Oregon and squeezing western Washington up against the more stationary rocks of British Columbia. Combined with the northeast movement of the Juan de Fuca Plate, this motion causes compression and thrust -faulting in the Seattle area in the north-south direction.
And that, dear readers, causes things like this:

Probable Fault Scarp Dappled in Sunshine
The Seattle area's prone to three different types of earthquakes, in fact.  The Big One the teevee yammers about every time they go on about megaquakes in the Pacific Northwest are the subduction zone or interplate earthquakes.  They're the quakes you get when a subducting plate comes unstuck suddenly and causes a hellacious quake.  They're called "megathrust" for a reason, and the last one we had hereabouts just over 300 years ago was powerful enough to cause a tsunami in Japan.  Then you've got your basic intraplate or Benioff zone earthquake, which occur within the subducting plate.  In this case, it's the Juan de Fuca plate breaking at depths of around 25-100 kilometers (15-60ish miles).  We'll discuss those in more detail at some future point, but if you'd like an example of one, look no further than the 2001 Nisqually earthquake that caused so much angst up here.  Locals still love to yammer about that one.

But it's the third kind o' quake that concerns us here, your standard-issue shallow crustal earthquake.  These go only skin-deep - roughly down to 30km (18mi), but don't think shallow means gentle.  Washington State's largest historic earthquake, the North Cascades earthquake, was probably shallow.  That didn't stop it from being felt in four states and two Canadian provinces, now, did it?  And, being so close to the surface, they leave quite a mark.

Fault Scarp Looming Over Park-Goers

About 1,100 years ago, something in the Seattle Fault Zone let rip, and sent bits of land careening up by 20 feet.  No less than three substantial tracts of forest went slip-sliding down right into Lake Washington, where they evaded logging for a thousand years before being discovered when the lake got lowered, and enterprising businessmen said, "Oooo, timber!"

I haven't got pictures of any sunken forests, considering they succumbed to the saw mill, and more importantly, I haven't got an underwater camera, scuba gear, or any idea how to use any of the above.  But I have got a neat photo of roots growing in the suspected scarp:

I have no idea how old these scarps are - they might be from 1,100 years ago, they could be in their mere hundreds.  It's hard to find specific info on these very scarps.  But I can tell you that they're on the side of the fault zone that's going up while the other goes down (which means that bits of Seattle north of the I-90 have
more to worry about than just global warming as far as sea level goes - the northerly bits have dropped 6 miles since the Seattle Fault Zone became active, so the next earthquake could cause parts of the city to become unexpectedly aquatic).  And I can't tell you much about the formations the scarps are cut in to, but I can show you they have some pretty patterns:


There's some bedrock I'll babble about in our next installment, and we've got a bit on glacial erratics coming up, but for now, I'll just show you some of what leads me to suspect we're looking at either glacial deposits or old lahars.  It's the great big boulders popping out of nowhere:

Boulder with Lichens
Up till now, we've dealt with a suspected scarp.  And you'll see why it's a good suspect once I show you a known offender:

Undoubted, Indubitable Fault Scarp

Now, the folks who designed the path leading to it weren't thinking clearly, and planted a whole bunch of vegetation between the old social trail and the new official trail to encourage people to stay put.  The only thing that accomplished was to make the geology nearly impossible to see because of all the damned biology.  But we can catch some good glimpses:

Closer Look
In fact, with a 10x optical zoom, we can practically touch the scarp face:

Even Closer Look

And then we can zoom out to take in the whole scene:

Geology Through the Biology

The thing's at least 20, maybe 30 feet high.  Now, I have a few things for you to consider: 1. The Seattle Basin is shaped just right for containing seismic waves, ensuring we get shaken harder than one might expect.  2.  A fault crosses the bottom of Lake Washington from Seward Park to Mercer Island.  3.  The above photos show you that a lot of ground can move in a quake round these parts.  And 4. Have I mentioned the fault under Lake Washington yet?  The big, deep, filled-with-lotsa-water lake?  You can see what I'm getting at: those of us in the East Sound who laugh at all those tsunami-prone areas on the coasts should probably stop laughing just about now.  In fact, when that fault under the lake goes, we could be talking waves up to 18 feet high.  And the fact that it'll technically be a seiche rather than a tsunami is no comfort at all.  We shall get very wet either way.

So.  Fault scarps in city parks, underwater forests, and inland tsunamis - anyone who thinks Seattle is a quiet place to live hasn't paid attention to geology at all.  That's why it's a good idea for everyone to get their arses out to Seward Park, where they can stand dwarfed by a rather imposing example of just what happens when the ground round here lets rip.

And just think, we haven't even talked about the ice and the implications of exposed bedrock yet....


Lyle said...

Washington state is as much a pile of different mini plates as California. In fact geologically Washington might provide a view of what California looked like before the Trench docked and caused the San Andreas fault system.

Karen said...

Dana, your cartoon shows a normal fault, where one side of the ground drops (you can tell by the arrows on the STRESS line); but your text implies that the faults creating these scarps are thrust faults, where one side of the fault gets pushed up. You end up with a fault scarp either way, but it's a matter of downdropping vs. uplifting.

But as a resident of the San Andreas System, I don't envy you the violence of your geology. :-)