LOS ANGELES (AP) – The magnitude-7.9 earthquake that rocked Alaska Nov. 3 has lent new credence to the theory that large temblors can trigger seismic activity even thousands of miles away.
Immediately following the earthquake, the largest to strike on land since the 1906 quake that leveled most of San Francisco, seismic instruments recorded increased activity as far away as California. There, swarms of tiny temblors shook The Geysers, north of San Francisco, and Long Valley, in the Eastern Sierra.
Similar minor quakes were recorded at Washington’s Mount Rainier and in Yellowstone National Park in Wyoming, all volcanic areas. Most seismologists agree the quakes were triggered by the quake on Alaska’s Denali fault.
“There can’t be any question at this point, that the phenomenon does occur and probably more frequently than we thought before,” said David Hill, a U.S. Geological Survey seismologist.
It was 1992’s magnitude-7.3 Landers earthquake in the desert east of Los Angeles that first suggested to seismologists that remotely triggered quakes were even possible. In the minutes, hours and days following that quake, instruments recorded upticks in seismic activity near Lassen Peak, Long Valley, The Geysers and Little Skull Mountain in Nevada.
Before Landers, which was among the first large earthquakes to strike in the era of modern seismic instruments, seismologists could not definitively link temblors that struck in close succession but were separated by large distances.
In places like California, earthquakes come nearly every minute of every day; that any two should strike in different locations within the state at the same time is usually no more than mere coincidence.
“There’s one problem we face, and that’s people like to correlate things,” said USGS seismologist David Wald. “People like to see patterns.”
Now, with more and better instruments deployed in the field, seismologists can discern any change in the background levels of seismic activity that might indicate earthquakes triggered by distant, larger quakes.
Some still do not accept the theory that they trigger one another, however.
“So what, there are earthquakes all the time, right? You’d be hard pressed to tie it to that,” said Christopher Scholz, of the Lamont-Doherty Earth Observatory of Columbia University.
Scholz does allow that earthquakes can trigger seismic activity in volcanic zones, as was the case with the Denali quake.
Since Landers, seismologists have pored over historical records, looking for accounts of past earthquakes that may have triggered other temblors, even in nonvolcanic areas.
USGS seismologist Susan Hough said the three strong earthquakes that hit New Madrid, Mo., in the winter of 1811-12 triggered quakes as far away as northern Kentucky _ hardly a volcanic region.
“There is compelling evidence that it’s happened in areas that are not volcanic,” Hough said.
More recently, Wald and student Aron Meltzner spent more than a year studying newspaper accounts from the time of the April 18, 1906, San Francisco earthquake. They found what they called a “marked clustering” of earthquakes in the West in the 48 hours after the estimated magnitude-7.8 quake. They attribute the change to aftershocks and triggered quakes. (The difference between the two is largely one of distance – and semantics, seismologists said.)
Among them were earthquakes reported in Oregon, Arizona and Southern California, including an estimated magnitude-5 quake that struck off the Los Angeles coast on April 19, 1906.
“There are clear effects at great distances from these earthquakes,” Wald said of those of large magnitude.
How they trigger other quakes remains unclear, although the apparent tie to volcanic areas is a clue, said USGS seismologist Joan Gomberg.
“These volcanic areas seem to be more sensitive, for whatever reasons,” Gomberg said.
Among the theories is that the shaking redistributes bubbles of gas trapped in the magma underlying volcanic regions, Gomberg said.
Seismologists caution that triggered earthquakes seem to occur only in areas that are already seismically active.
“They’re already primed in some sense for producing earthquakes,” Hill said.