I was going to post about this during my forest trip last month, but the trees kept calling me...and so I set aside the geological discussion for another time. But then this article came out today in The New Yorker, and it was so timely with what I had intended to talk about that I figured I should pick the geology piece back up.
First off, check out the article here.
Throughout my time along the coasts of California and Vancouver Island, tsunamis were never far from my mind. You see, the complicated plate tectonics of western North America and the Pacific Ocean mean that the earthquakes of Southern California and the Bay Area are distinctly different from those of Northern California and the Pacific Northwest. The former, which includes that headliner of faults, the San Andreas, is characterized by two plates that are colliding as they grind past each other (check out the blue arrows of relative movement in the graphic below):
This grinding isn't ongoing or smooth, though; the motion moves in fits and starts, an endless series of pressure building and pressure release. Moreover, the boundary between plates -- the point of pressure -- isn't a single fault. The San Andreas is the main boundary, to be sure, but it is really a system of hundreds of faults, some of them quite large in their own right, with thousands of points of pressure and release happening all the time. This is why anybody who lives in the region experiences small earthquakes all the time: the whole system is constantly shifting, binding up in one place and loosening up in another. This is actually good news, because it means that the total stress of the system is kept in an uneasy balance: the "big ones" are relatively rare (at least in human terms).
But notice how north of San Francisco the line representing the San Andreas takes a sharp left and becomes the Mendocino fracture zone. At that junction (a so-called "triple junction"), another dynamic takes place. North of the junction is a subduction zone, where the dense, heavy oceanic crust of the Juan de Fuca plate is sliding ('subducting') beneath the lighter, less-dense continental crust of the North American plate. A close-up of that looks like this:
Subducting plates result in lots of things, but two biggest are volcanoes and tsunami-causing earthquakes. The diagram above clearly illustrates how volcanoes arise from a subducting plate: once the subducting slab reaches a sufficient depth, it begins to melt into the mantle, releasing molten material that rises to the surface and punctuates the crust above it. Most (but by no means all) of the world's volcanoes result in this type of activity -- hence the term "Ring of Fire" for the countless volcanoes that mark subducting boundaries around the edges of the vast Pacific plate and its various spin-offs (like the Juan de Fuca).
The other phenomenon associated with these kinds of plate boundaries -- tsunami-causing earthquakes -- requires a little more visualization. Try to imagine putting the above graphic into motion: as the ocean crust moves down and to the right, it doesn't do so in a smooth, gliding fashion. Instead, it pushes up against the continental plate above it, building pressure until the pressure is suddenly released and the subducting plate lurches forward (down and to the right). Can you picture that? Now, try to picture what happens with the overriding plate: as pressure builds, the leading edge of the upper plate gets pulled down slightly...and when the pressure is released it pops back up. This is the source of the tsunami, which results when the ocean floor is suddenly lifted up, displacing massive amounts of water. A tsunami is, therefore, not really a wave like we tend to think of them. It's a mountain of displaced water surging outward in every direction. This is the same dynamic that created the recent catastrophic tsunamis in Japan and Indonesia. And because these kinds of plate boundaries are not as splintered and fractured as the San Andreas-type boundaries, there are far fewer of the small earthquakes relieving pressure on the system. With a subducting plate, when there's an earthquake it's almost always a significant one.
So while the popular imagination likes to think (or worry) about "The Big One" that will someday hit either Los Angeles or San Francisco, a far bigger disaster awaits the coast from Northern California to Canada. This is because, a) earthquakes along "strike-slip" faults like the San Andreas, though they can still be tremendously powerful, are generally less extreme than earthquakes occurring in subduction zones, and, b) subduction zone earthquakes produce tsunamis, and tsunamis pose a unique kind of widespread destruction. As the article above states, "There aren't many injuries from a tsunami -- because everyone in its path is dead."
All along the Pacific coast, from Redwood National and State Parks all the way to Vancouver Island, signs mark the points that are designated as tsunami hazard zones and the path of quickest escape. Sometimes these zones were surprisingly far inland -- far up a river valley that would act as a funnel for an incoming tsunami, some of which roar inland for miles. Higher ground is the only defense.
The article I linked to was prompted by what is perhaps one of the most insane laws ever passed by any government: an Oregon law that would eliminate previous restrictions on using public money to build things like schools, hospitals, jails, and first responder facilities within a tsunami zone. It's hard to imagine a more short-sighted and ignorant disregard for the realities of geology, and yet we seem to be finding all-to-many examples of this kind of willful ignorance these days. Whether we're talking about climate change, earthquakes caused by fracking, the risk of catastrophic flash flooding (a big issue here along the Colorado Front Range), the alarming increase in rates of extinction, or the very real danger of tsunamis along our west coast, ignorance of science will lead to our downfall. And there's simply no excuse for it. Lawmakers (and the voters who elect them) don't need to be scientists themselves. The Lawmakers in Oregon don't need to be geologists, or even understand the particulars of the timebomb that is the Juan de Fuca plate (past-due for a major event). What they do need to do is listen to those who do understand the science. And the science is not ambiguous in this case; there is no room for reasonable interpretation: the Juan de Fuca plate continues to subduct beneath North America, historical analysis proves that it's past due for another pressure-releasing event, and observations of other similar plates around the world -- as well as evidence of past events on the coast itself -- tell us what will result. To ignore this, regardless of the rationale, is recklessly irresponsible. It will also prove, sooner or later, to be deadly.
