The Seattle Times, Sunday edition had an interesting article about the impact of a major quake off the Pacific Northwest's coast line.

http://seattletimes.nwsource.com/html/localnews/2017551958_sciencemeeting20m.html

Blake

www.outdoorquest.biz

This story got a lot of play up here as well. We have major population centres (and an increasing number of oil/LNG facilities) along this coast. And I have family on Vancouver Island. Yikes.

At least we know a bit more about what we don't know.

I keep an eye on the quake patterns off the West Coast because I live in L.A. There is a theory now that the San Andreas and the Cascadia zone could be linked.

Just recently, in the last week or so, there have been two fairly large undersea quakes on the southern Cascadia fault zone. That struck me as a little unusual, because I've been watching these things for a while and don't recall that activity there.

However, you know how this stuff goes. It's got a lot of random unpredictability to it. So you can't get worked up by a couple of small events. The main thing is to be prepared in case the Cascadia fault zone does rupture. The impact along the coast of Oregon and northern CA will be big. And the chances of a subsequent rupture on the San Andreas would be increased.

Pete2

As a Californian, that concerns me. By "linked" do you mean physically connected as one giant fault? Or just that earthquakes in one could precipitate quakes in the other fault?

After the 2011 Japan quake, they say that all that stress that was released was transferred further south to the Tokyo region. Unfortunately, we don't know if that stress will release in another megaquake tomorrow or 50 years from now.

As a fellow denizen of earthquake country, the best thing we can do is prepare. There will be quakes, most of them will be rather minor, but clearly, eventually the Big One (or one that is close enough for government work) will hit....

Maybe I have a simplistic understanding of tectonics, but don't the North American and Pacific plates grind against each other up and down the west coast, with a large area to the north pushing under North America creating volcanoes in the north and earthquakes everywhere?

They do not join each other directly. They are, however, linked together by another fualt, called the Medicino Fault Zone. See Where the San Andreas Fault Ends for a general view. As I understand the thinking, the idea is that a major earthquake on one might transfer some stress to the other. This in turn might make an earthquake on the other more likely. Note that there is a lot of rethinking going on in the earthquake science community. Partly just the general onward progress of research, and partly a result of new data from the big quakes around the world, particularly Japan. Our understanding is changing very rapidly.

It isn't that your understanding is simplistic. Rather that the interactions of the plates are rather complex, and have evolved through time. "Triple Junctions", which is what we are talking about here, are sometimes tough to visualize in a simple diagram, let alone describe in a short text message.

I knew I'd seen a nice animation somewhere, and after a bit of googling, I found it. Check out "An Animated Tectonic History of Western North America and Southern California" by Tanya Atwater. It shows how over time the situation changed from one long subduction zone, into a subduction zone linked to a major fault (the San Andreas). The Mendicino Fault is not labeled in the animation, but it is the prominent East West trending line that eventually colides with N America. There is a bit of text below the video panel that helps explain it.

(Back when people were just beginning to wrestle with the implications of Plate Tectonics, Tanya Atwater was one of those who had a "Eureka Moment" about what it meant for California.)

Looks like you guys have answered your own questions, and that was a nice pix of the plate boundaries by the way.

In So Cal the San Andreas is moving laterally. Hence the slip is sidewards along the main earthquake fault. Geologists generally believe that this type of motion is a little more benign, compared to the big quake in Japan. The San Andreas is hung up on the southern section, running from (just north of) Palm Springs down past the Salton Sea. Expected quake magnitude might be an 8-something. This quake is overdue. Places like Palm Springs and Banning will be toast when this quake goes down.

In Oregon they have got a real problem with the small plate boundary called the Juan de Fuca. That is the type of motion where one plate slides under the other. Exact same motion that generated the big Japan quake. The southern section of Oregon is now overdue to trigger on that fault line. Very likely it will trigger a big tsunami along the OR coast and the coast of northern CA.

There is a geological theory that the activity on the San Andreas and the Cascadia fault (OR) is linked. This comes from studies looking at when quakes (historically) coincided. Hence, if the OR fault system triggers, that could mean that the San Andreas is at higher risk. I don't think that geologists are saying that everything would all rupture at the same time. That would be a nightmare.

However, that worst-case event cannot be ruled out. Needless to say, if there's a rupture along hundreds of miles of fault line from southern CA all the way to OR, the west coast of the USA would be a basket case. It's not worth thinking about :-(

Pete2

Tectonics we can live with....
http://www.youtube.com/watch?v=s1TVrNFFQaE&feature=related