PASADENA, CALIF — PASADENA, Calif. -- Seismologists had depressing news for frightened Southern Californians in the wake of yesterday's two powerful earthquakes.
Instead of relieving tension on the San Andreas fault, the two earthquakes on adjacent faults near Landers and Big Bear Lake probably increased seismic strain in the region. Thus the dreaded "Big One," a catastrophic magnitude 8-plus earthquake that is sure to strike some day, may hit sooner rather than later.
"There is nothing to suggest stress has been relieved on the San Andreas," said Dr. Lucile M. Jones, a seismologist at the U.S. Geological Survey office in Pasadena. "If anything, this a sign of increased stress."
Early in the day, Paul Flores, deputy director of the State Office of Emergency Services, said "there is a greater than 50 percent chance of magnitude 6 aftershocks" occurring in the next few days. People should be prepared and check for structural damage to their homes, he said.
Mr. Flores noted that weakened structures are prone to further damage. At least 50 aftershocks, ranging from magnitude 3.7 to 5.3, occurred yesterday on both faults, he said.
An aftershock is a smaller earthquake following a greater one on the same fault.
It was the second time that state officials, acting on advice from scientists, told the public to stand by for more earthquakes. The first advisory, on April 22, followed a magnitude 6.1 earthquake near Joshua Tree National Park.
Advisories are issued whenever a moderate earthquake occurs within 10 miles of the San Andreas because such events can be precursors to a much larger earthquake, Dr. Jones said.
Yesterday's second earthquake met that criterion, she said. The earlier 7.4 magnitude earthquake near Landers, east of Los Angeles, was about 20 miles from the San Andreas fault and would not by itself have prompted the advisory.
The Landers earthquake, which opened a 12-foot crack in the ground, probably occurred on a different length of the same fault that broke in April, Dr. Jones said. It was a relatively shallow quake that began one to two miles below the surface, she said.
Does this mean the so-called Big One is imminent?
"We don't know," Dr. Jones said. "Things are obviously more complicated with two large earthquakes just 20 miles miles apart. We are not dealing with a simple magnitude 7." The two earthquakes are unquestionably related "in ways we do not understand," she said.
The San Andreas is California's largest fault, stretching 600 miles from the Mexican border to Cape Mendocino. Although it is one system, the fault has various segments with distinct characteristics and historical patterns of rupturing.