Surprise quake spurs new ideas.

Surprise quake spurs new ideas 

Last week's earthquake in the Aleutian Islands intrigues seismologists more for where it didn't happen than for where it did. The earthquake, an aftershock of a quake on May 7, failed to break through a seismic gap -- an area where no earthquakes have occured for many years and where scientists have been expecting accumulated seismic strain to trigger a major rupture. Scientists are using such unexpected events to refine their models for understanding and predicting the movement of the earth's crust.

The June 18, magnitude 6.3 aftershock stopped at the western edge of the earlier, magnitude 7.7 rupture, ending at the eastern wall of submerged Adak Canyon. Carl Kisslinger of the University of Colorado in Boulder says both the main earthquake and the recent aftershock seem to have run up against an asperity--a hard sport along a fault that holds strong when the rest of the faul breaks, like a knot in a pine board being sawed in two.

The recent earthquakes in unexpected places, the increased volcanic activity in the Pacific basin (SN: 5/17/86, p. 309) and the existence of at least two large seismic gaps in the Gulf of Alaska (SN: 2/15/86, p. 104) tantalize seismologists but elude attempts to establish a clear pattern that would explain the upsurge in activity. According to Klaus Jacob of Lamont-Doherty Geological Observatory in Palisades, N.Y., an active background of ordinary seismic movement in the area makes pinpointing significant events difficult. "We're not sure whether we can call it a pattern," he says of the recent events.

They do, however, give scientists an opportunity to refine crude models of plate tectonics to better explain why earthquakes continue to occur in unexpected places. Jacob speculates that, although seismologists used to view plate boundaries as uniform, the boundaries may actually be thinner under oceans than under continents. The oceanic part of the North America-Pacific plate boundary, where the Adak quakes occurred, may accumulate strain more quickly than regions under the continent. If this is the case, "the time between major earthquakes would be shorter on the average than previously through," Jacob says. Events at the edges of these boundaries could forebode a future rupture, he concludes.

Kisslinger, too, expects more movement in the Adak region. "Based on what we know about this region," he says, "I would expect eventually that the region out to the west [of Adak Island] will break in one or two substantial earthquakes, but we have no way of knowing when."