This is the first of two posts on this blog that consist of asking Austin Elliott, who’s working on a Ph.D in geology at UC-Davis focused on seismology, a few questions about the nature of earthquakes in the Northwest and California and the general science of seismology. (Part two is The Work of Seismology: Explaining a Few Issues and Changes in the State of the Art.) Austin also has a blog, The Trembling Earth, which is connected to his Ph.D work and looks at various topics in the earthquake field. In this post, Austin explains current thinking about subduction zone risks along the West Coast and the overall earthquake risk for the three coastal states.
What’s your sense of how a subduction zone quake would develop off the Northwest coast? Will the whole Juan de Fuca fault unzip, from Northern California up to B.C., or will only segments of it move?
There’s been a lot of research about this lately, ever since the recognition around a decade ago that the Cascadia subduction zone has produced these monster earthquakes routinely. Some theoretical models suggest that the quakes alternate in length from half-subduction-zone to full-subduction-zone. The possibility of a Cascadia rupture inducing rupture of the northern San Andreas fault–apocalyptic as that scenario may sound–has been seriously discussed by seismologists, and existing data about the timing of past earthquakes can’t rule it out. I view such a scenario as plausible but unlikely. However, I take a rather dire view of earthquake potential: knowing that an earthquake of a certain nature has happened in the past is the best evidence we have that it will probably happen again in the future. I don’t know whether the Cascadia megathrust will slip in parts or as a whole, but it’s interesting and sobering to consider which would be worse: a single magnitude 9 earthquake that’s felt from San Francisco to Vancouver, or a months- to decades long series of magnitude 8s up and down the U.S. west coast.
Why does the subduction zone disappear along most of the California coast, only to reappear off the Mexican coast and continue down along Chile’s coast?
This is a great question, with a relatively straight-forward answer. Have a look at this figure of global tectonic plates as a guide. The tectonic plate boundary in California separates the North American (green) and Pacific (beige) Plates, which are sliding past each other like ships passing in the night, only with more crumpling and earthquakes, Pacific to the northwest, North America to the southeast. To the north and south of California, a different plate (or rather, remnants of a different plate, which have now been named individual plates, and are colored blue) is squeezed between the Pacific and North American plates. These blue plates are moving toward North America, diving beneath it in subduction. The portion of that intervening plate that used to occupy the coast of California has long since been swallowed by the subduction zone, and what’s left of it to the north (the Juan de Fuca plate) doesn’t have much time left… geologically, as you can tell from its measly proportions on this map.
On the whole, how do you compare the earthquake threat in Northern California to the earthquake threat in Oregon and Washington? In terms of both severity and what types of earthquakes happen in the two regions.
Another good question, since the difference in the type of plate boundary leads to a dramatic difference in the size, frequency, and relative locations of earthquakes that happen along the west coast. In general I consider California’s earthquake risk to be much more acute, and much more lethal. The Pacific Northwest can expect to see infrequent, very large earthquakes like Japan’s, with minutes and minutes of heavy shaking and a dangerous tsunami. However, as in Japan, the earthquake source will largely be offshore, making it intrinsically more distant from the population than the quakes that any of California’s densely inhabited faults could produce. The shaking from a Cascadia megaquake will reach farther and last longer than a strike-slip quake in California, but dampening with distance will be a mitigating factor in any death toll, and the low frequency of the waves will spare people’s homes heavy structural damage. In California’s big cities, however, the shaking from a local earthquake–even one of modest size–will be very severe because the cities lie literally atop the state’s most active fault lines. This presents a real hazard of extreme ground motion and is likely to cause heavy structural damage to buildings of all sizes throughout the cities, much like we’ve seen in Christchurch, New Zealand over the past year. I consider this a much more dire situation, as the quake itself would likely induce a great deal more casualties than an offshore megaquake.