Alaskan great quake: ready or not?

Alaskan great quake: Ready or not?

The Aleutian islands of Alaska have more than adequately demonstrated a propensity for earthquakes. Since 1938, great earthquakes--with magnitudes exceeding 8 -- have rattled different segments of the island chain as a result of the Pacific plate subducting, or sliding beneath, the North American plate. But there are three "gaps" or segments along the Aleutian subduction boundary where no great earthquakes have occurred since the turn of the century; these gaps have been targeted as the most likely Aleutian sites of great earthquakes in the near future by a number of seismologists. Now, however, a group of researchers at the U.S Geological Survey in Menlo Park, Calif., argues that one Aleutian gap is far from ready to rupture.

Geophysicists Jim Savage, Mike Lisowski and Will Prescott have measured the strain accumulating since 1980 at two Aleutian gaps: the Shumagin gap, which ruptured in 1788, 1847 and possibly 1903; and the Yakataga gap, which last ruptured in 1899. According to one line of thinking, the amount of strain building up along a fault is an indicator of how ripe that fault is for an earthquake. The theory holds that the relative motion of the plates steadily builds up strain along a locked fault until the strain reaches a critical level and the fault slips, suddenly releasing all the strain in the form of an earthquake. Then strain begins to build up again and the stick-slip cycle continues. At the Yakataga gap, Savage's group measured an accumulation of strain consistent with the rate at which the North American and Pacific plates are converging. But at the Shumagin gap, they detected no buildup of strain, the researchers report in the Feb. 7 SCIENCE. "This finding appears to be inconsistent with the imminent occurrence of a great earthquake in that area," says Savage.

One explanation for the lack of strain, according to the researchers, is that the way in which the two plates slide past one another has changed fundamentally. Instead of sticking and then slipping as they had done before the last great earthquakes, the two blocks are now sliding continuously and stably, in a process alled aseismic subduction. Because reliable strain measurements have been made for only a relatively short period of time, it's not possible to tell if the subduction along the gap is now permanently aseismic or if is is episodic, with long periods of aseismic subduction being punctuated by strain- and earthquake-generating stick-slip cycles.

Alternatively, says Savage, "our models of how strain accumulates ay be wrong." It's possible that just before a large quake, the strain rate falls below a detectable level, he notes.

Last fall the state seismologist of Alaska, in conjunction with Klaus Jacob and other researchers at Lamont-Doherty Geological Observatory in Palisades, N.Y., issued an earthquake alert for the Shumagin gap because the occurrence rate of small earthquakes during the months May through October 1985 had increased from 30 to 80 percent above the average rate measured in previous years. Moreover, a sequence of magnitude 5 to 6.4 earthquakes had rattled the islands in October, and because the seismic pattern of this sequence was not typical, seismologists were concerned that these were precursory signals of a great earthquake. Now seismologists have to reconcile those seismic signals with Savage's strain results.

If aseismic slip is indeed occurring in the Shumagin gap, says Jacob, "then the activity we saw in October [could mean] that while most of it slips aseismically, there are patches that rupture seismically." Jacob, however, thinks permanent aseismic subduction, while possible, is the least likely explanation for the lack of strain. The alternative, and the most tantalizing of the ideas, he says, is that the patches are only the beginning of a larger rupture to come, and that, as Savage notes, the absence of strain is itself a precursor of a great quake.

"So right now we are in somewhat of a dilemma," says Jacob. "From a research point of view it's an extremely crucial time." Ironically, in spite of the Shumagin gap's heavy instrumentation and puzzling behavior, Jacob worries that it may not be the best place for catching the precursory signals of a great quake. In the course of reanalyzing the seismic hazards in Alaska and parts of Canada, Jacob and his co-workers recently identified a 1,000-kilometer segment -- stretching from Kodiak Island to Unalaska Island and including the 350-km-long Shumagin gap -- that has a high probability of rupture. It is possible that a great earthquake and its precursory signs could start somewhere near Kodiak and outside of the Shumagin gap, which would be only minimally involved in the quake. "If that is the case," says Jacob, "we may miss our chance to do really good earthquake prediction."