Distant Quakes May Affect Area Temblors : Geophysics: Study finds link between seismic activity thousands of miles away and movement of Southland faults several years later. But data is not precise enough to predict tremors.
Two eminent scientists report evidence that seismic activity--or the lack of it--hundreds or even thousands of miles from Southern California can influence the occurrence of subsequent earthquakes in the Southland.
Frank Press, former White House science adviser and head of the National Academy of Sciences, and Clarence Allen, geology professor emeritus at Caltech, say that clusters of sizable earthquakes in the Great Basin, between the Sierra Nevada and Utah’s Wasatch range, or in the Gulf of California are followed within a few years by quakes on the San Andreas Fault. Their report appeared in the current Journal of Geophysical Research.
In an interview Thursday, Press also said quakes in the Aleutian trench in the North Pacific or at other points along the Pacific tectonic plate boundaries could send similar slow “strain waves”--a kind of ground deformation front--toward Southern California at a rate of about 100 kilometers, or 62 miles, a year.
When such waves arrive, Press said, they readjust stress levels in the region, which appears to contribute to San Andreas system quakes. He and Allen said a rough historic calculation shows that a series of earthquakes in the Great Basin has historically been followed by San Andreas quakes in the next four years, while a series of quakes in the Gulf of California is usually followed by such quakes in the next 12 years.
On the other hand, the Great Basin and the Gulf of California “tend to be quiet in the periods before typical non-San Andreas earthquakes,” such as last year’s Northridge earthquake, Press and Allen reported. The exact nature of this apparent connection between distant quiescence and later local quakes is not known, they said.
“Essentially, we are saying that distant events can affect the nature of seismic release in Southern California,” Press said.
The two scientists said their conclusions were based on a study of 46 earthquakes of at least magnitude 5.5 in Southern California between 1915 and 1994.
Earthquakes on the San Andreas predominated from 1915 to 1946. Non-San Andreas earthquakes predominated from 1971 to 1994, and the years between 1946 and 1971 saw a mixed pattern.
Press and Allen said that their hypothesis of a continuing relationship between quakes in Southern California and the Great Basin or the Gulf of California could be verified in the future through measurements by the instrumental array of the Global Positioning System developed by the U.S. military.
But, Allen cautioned in an interview, “it is not in the cards” for the information to be precise enough to allow predictions of earthquakes within a narrow time frame or at a highly specific location along a fault.
Press said the ground deformation caused by the strain waves could be measured, as it proceeded, through the Global Positioning System.
“It takes years for the strain waves to arrive in Southern California,” he said.
When they do arrive, they cause “a readjustment of the stress system,” which can contribute to a San Andreas Fault quake.
“Exactly why (local quakes have) a connection with the Great Basin and the Gulf of California, one can only speculate,” Press said. He said all the dynamics are not known.
But, he said, it seems clear that when there have not been many earthquakes in the distant areas, the San Andreas tends to be quiet, and the non-San Andreas faults tend to be active, as they have been along the San Gabriel and Santa Susana mountains in recent years.
“When one is active, the other tends to be quiet,” Press summarized. “This division by the two kinds of seismic release is affected by what has happened in distant places, and the stress change can only be communicated to Southern California by means of these strain waves.”
This long-term, long-distance process is different from what happened after the 1992 Landers quake, they said. The Landers earthquake appeared to have triggered quakes within days along the Sierra up to a few hundred miles away, and even farther away in Yellowstone National Park.
