Hawaii, the New York Times, and Triggered Seismicity
The New York Times has an astonishingly confusing article on the idea that one earthquake could trigger others, thousands of miles away. The real meat of the answer is hidden in the last paragraph:
While strong earthquakes trigger nearby aftershocks (in Hawaii a shallower 5.8 quake occurred within minutes of the first one), they don’t affect seismic zones hundreds of miles away. So why does it sometimes seem like there’s a string of strong quakes throughout a region?
That, Dr. Wolfe said, has more to do with psychology than seismology. The United States Geological Survey estimates that every two and a half days, on average, there is a quake of magnitude 6.0 or higher somewhere around the world. Many of these take place under the oceans or in remote land areas and are little noticed. But then one occurs in a place like Hawaii and earthquakes are in the news. So for a few days or weeks, every Tom, Dick and Harry of a quake is reported, and it feels as if there must be a connection. Even though there isn’t.
But to get there, you first must wade through some improperly contextualized information about plate tectonics:
Generally, no [one earthquake doesn’t lead to another], said Cecily J. Wolfe, a seismologist at the University of Hawaii at Manoa. More specifically, the Hawaiian quake, which damaged roads, buildings and bridges and knocked out power, was a different kind of earthquake than most of those that occur around the Pacific Rim.
Earthquakes around the rim, like the Peruvian quake, result from the release of stresses that build up at the boundaries of tectonic plates. “Out here in Hawaii, we’re in the middle of the interior,” Dr. Wolfe said. There are no plates sliding past each other.
The fact that the Hawaiian earthquake was an intraplate event has absolutely nothing to do with the implausibility of it triggering a subsequent event in Peru.
Long-distance triggered earthquakes are a real (well, controversially real) phenomenon. It’s thought that both the 1992 M7.4 Landers earthquake in the Mojave Desert and the 2002 M7.9 Denali earthquake in Alaska triggered, not big earthquakes, but an increase in the rate of small earthquakes throughout the Western U.S. and Canada. No one quite understands how this happens.
The occurrence of triggered seismicity does seem to be correlated with the direction in which the triggering earthquake radiates most of its energy. It also occurs at such large distances that the changes in stress observed when the earthquake has finished (static stress) are far, far, far too small to plausibly be important; they are much smaller than the stress changes observed as the seismic waves pass by (dynamic stress). So the triggered events are most likely due to something that happens as the seismic waves pass through. Favored explanations begin with “probably these areas were close to failure anyway” and end with “um, somthing something fluids something” or “nonlinear whoozerwhatsits”.
At any rate, the reason we don’t think the Hawaii earthquake triggered seismicity elsewhere has more to do with the fact that it was small (a magnitude 6.6 earthquake radiates 31 times less energy than a magnitude 7.6 earthquake) than that it was in the middle of a plate. And magnitude 6 events occur all the damn time, especially on the Pacific Rim; there is nothing special about either the Peru or Papua New Guinea events that would make us go looking for a special explanation.