Study Shakes Up Earthquake Theory

Soon after last year’s big Landers earthquake, scientists knew that many areas of the West were being shaken much more than normal, and within a week they issued a report that Landers had unexpectedly triggered “sympathetic” quakes as far north as Mt. Shasta.

Now, after months of studying reams of data, researchers report that sympathetic quakes were recorded in Yellowstone National Park, 800 miles from the Landers epicenter. The quakes were felt over six states--California, Nevada, Utah, Oregon, Idaho and Wyoming--with the strongest a magnitude 5.6 at Little Skull Mountain, Nev.

It is only now that the full dimensions of the triggering have been explored and the first theories presented explaining possible mechanics for such distant temblors, which began as soon as 30 seconds after the main shock and in some cases continued to occur for days or weeks.

Although there were also a considerable number of magnitude 3 and 4 earthquakes, most of the jolts recorded were tiny, in the magnitude 1 and 2 range.

A team of 31 scientists, including some from California, reported today in the journal Science that their findings “challenge a longstanding skepticism regarding the reality of triggered seismic activity at great distances.”

They added that they are rethinking whether other large earthquakes also triggered quakes a long distance away. Perhaps, they wrote, a magnitude 6.2 temblor that struck the Imperial Valley 11 hours after the San Francisco earthquake of 1906 was related to that large quake.

Last year’s sympathetic earthquakes--ranging from a magnitude 3.9 in Pasadena the day after the Landers temblor to quakes at Yellowstone; Cascade, Ida.; Cedar City, Utah, and Mammoth Lakes, Calif.--most often took place “in areas of geothermal activity or young volcanism (of the) last 1 million years,” the scientists noted.

From this, the team theorized that heated subsurface liquids, or even the molten rock known as magma, might be particularly susceptible to changes in pressure generated by earthquake waves coming from long distances; that new pressure could trigger the earthquakes.

But why did the onset of the tremors vary from as little as 30 seconds to as long as 33 hours? Lucile M. Jones of the U.S. Geological Survey’s Pasadena Field Office, one of the study’s authors, explained it in an interview in terms of pressure on a stick of gum.

“Some sort of strain step is involved,” Jones said. “A surge of pressure will break a stick of gum at once, but lesser pressure will only stretch it slowly to a point where it finally breaks off later.”

What is puzzling, said Geological Survey scientist Paul A. Reasenberg of Menlo Park, is that while strains are always transmitted after earthquakes, causing aftershocks from stress buildups, these are usually restricted to within 100 miles of the quake epicenters.

“We expect to see such strains,” Reasenberg said. “The unexpected thing here is that the strains we calculate are quite small at the great distances involved and really much too small to trigger earthquakes.”

So, he said, other processes must be at work. “No one really has the answer yet,” he said.

But in today’s article, the researchers proposed several theories:

* Two kinds of surface waves, the Rayleigh and Love waves, may play a key role. Traveling along the Earth’s surface like ocean waves, they produce the strongest shaking at distant points and temporarily add to stresses in seismically active areas where local faults are about to become active anyway.

* The triggered earthquakes continue for some time as a result of changes in water pressure, weakening faults and making them more susceptible to movement.

* Dynamic stresses caused by the tremors may shake loose bubbles in magma, increasing pressure in magma chambers, expanding them and straining the rocks above, thus triggering the earthquakes.

None of these explanations would individually account for all the earthquakes that occurred after Landers, but in combination the scientists contend that they may explain them.

Sympathetic Quakes

The magnitude 7.3 Landers earthquake last June 28 triggered temblors throughout the West from 30 seconds to several hours afterward. Greater than normal numbers of quakes occurred for days, and the strongest took place at Little Skull Mountain, Nev. until Landers, most scientists had discounted such long-distance triggering. Major quake clusters are shown here.