‘Triggered’ Quakes May Be Common, Researchers Find

Beginning just minutes after the 7.3-magnitude Landers earthquake in 1992, a series of temblors occurred hundreds of miles away, at Mammoth Lakes, Mt. Shasta and Yellowstone National Park. Scientists agreed they had been “triggered” by the Landers quake.

After the 1999 Hector Mine earthquake, magnitude 7.1, there was a triggered magnitude 4.6 quake scores of miles away near a geothermal zone alongside the Salton Sea.

The two episodes inspired a California scientist, Sue Hough of the U.S. Geological Survey office in Pasadena, and two New York scientists, Leonard Seeber and John Armbruster, to do research to determine whether triggered earthquakes had followed other major temblors in more stable settings in the East and Midwest.

The Bulletin of the Seismological Society of America reported this week that the search has been successful in relation to both the New Madrid, Mo., quakes of 1811-12 and the Charleston, S.C., quake of 1886.

What the work of Hough, Seeber and Armbruster indicates is that triggered earthquakes far from tectonic plate boundaries may be a common result of major quakes.

According to a paper by Hough, the quakes in the New Madrid fault zone were quickly followed by quakes as powerful as magnitude 5.5 near Cincinnati. That is a size that can do substantial local damage.

Scientists had noticed that a short time after the great San Francisco earthquake of 1906, there was a quake of about 6.25 in the Imperial Valley, 500 miles away. But at first that was widely thought to be coincidental.

Now, it seems it was not coincidence, that the occurrence of such quakes may point up quick relief of stress accumulation in areas far away from the original temblor.

The Seismological Society says the implications may include even areas where stress accumulation is so slow that quakes normally occur hundreds of years apart.

By far the largest numbers of American earthquakes occur in Alaska, the West Coast states of Washington and California and over such volcanic “hot spots” as Yellowstone and Hawaii.

But the findings of triggered earthquakes in the East and Midwest show that other parts of the country cannot discount such quake dangers either, even though they may not occur as frequently.

“Triggered earthquakes are not aftershocks like we commonly feel after earthquakes,” Hough said in an interview Friday.

“Those are usually no more than one rupture length from the original temblor. These may be farther away.”

The triggered, or “sympathetic,” quakes that took place at volcanic Mt. Shasta, for example, in 1992 were 530 miles away from the Landers rupture, which was only about 50 miles long in Southern California’s Mojave Desert.

Hough noted Friday that she had written a paper in 2001 that concluded that even “in areas of low seismic activity, such as Central/Eastern North America, the hazard associated with localized [fault] zones [susceptible to triggered quakes] might be more far reaching than previously recognized.”