Three years ago, I wrote here at Skeptophilia about the scary Cascadia Subduction Zone, which is capable of enormous earthquakes and tsunamis -- and which, unfortunately, lies right off the coast of British Columbia, Washington, and Oregon. A subduction zone is a region along which two plates are coming together, forcing one underneath the other. Because rocks experience a high degree of friction, the two plates often get stuck, sometimes for centuries, and then can give suddenly. This lurch is what causes big earthquakes.
The motive forces here are convection and drag. Rising plumes of magma underneath ridges diverge, and the friction between the magma plume and the underside of the plates forces them apart. Where the leading edge of the plate strikes another, something's got to give. In this case, the oceanic Juan de Fuca Plate, made of (relatively) thin, brittle basaltic rock, hits the old, thicker and colder North American Plate. The Juan de Fuca Plate jams up and eventually plunges underneath. The downward drag produces a trench, and inland from the trench you often find volcanoes, created as the subducted plate melts and the molten rock pushes its way to the surface. (This is how the Cascade Volcanoes, most famously Mount Rainier, Mount Shasta, Mount Hood, and Mount Saint Helens, formed.)
What hasn't been clear until now is how exactly subduction happens. We know that the process usually isn't smooth (as I described, it often goes by fits and starts rather than releasing the compressional force gradually). But what happens to the plate itself as it descends and is destroyed in the upper mantle?
Thanks to a new study out of Louisiana State University, we now have our first good picture of how this process occurs.
It turns out that the destruction of the last piece of a plate, such as Juan de Fuca -- which is one of the only remaining fragments of the Farallon Plate, that once underlay most of the northeastern Pacific Ocean -- is anything but orderly. The (relatively) small slab of solid rock beneath the ocean off the coast of the Pacific Northwest is being bent as its eastern edge is pulled downward, creating multiple fractures and dozens of "microplates." "Getting a subduction zone started is like trying to push a train uphill -- it takes a huge effort," said geologist Brandon Shuck, lead author of the study, which appeared in Science Advances. "But once it's moving, it's like the train is racing downhill, impossible to stop. Ending it requires something dramatic -- basically, a train wreck... This is the first time we have a clear picture of a subduction zone caught in the act of dying. Rather than shutting down all at once, the plate is ripping apart piece by piece, creating smaller microplates and new boundaries. So instead of a big train wreck, it's like watching a train slowly derail, one car at a time."
Which, if you think about it, makes sense. Picture shoving together two saltine crackers. One will likely push underneath the other, but the leading edges are going to crumble, and what you'll be left with will probably be a disordered pile of cracker crumbs.
This process doesn't really change the picture with regards to earthquake risk; just because the plate is shattering into smaller chunks doesn't mean the effects will be small when the breaks occur. One example -- the Shuck et al. research found a major, 75-kilometer long fault where pieces of it have dropped by five kilometers. The scary part is that despite the fault collapse, it's not done separating. "This is a very large fault that's actively breaking the [subducting] plate," Shuck said. "It's not one hundred percent torn off yet, but it's close."
Further reinforcing my assessment that while I dearly love the Pacific Northwest for some of the most beautiful scenery in the world and the absolute best gardening climate in the United States, I'd never live there again.
It bears mention, however, that it may be that the fault won't rupture for another two hundred years; on the other hand, it could happen tomorrow. While our ability to analyze plate tectonics is light years beyond what it was even thirty years ago, when the situation in the Northwest first began to come clear, we still don't have any way to determine when the earthquake will happen with any kind of precision. At the moment, all we know is that it will rupture, sooner or later.And I don't want to be anywhere near it when it does.
