SAN FRANCISCO — A 6.2 earthquake that rattled across the Imperial Valley on Nov. 23 struck so close to the southern end of the San Andreas Fault that scientists feared that the mighty fault was about to break loose.
"I thought, oh God, here we go," said U.S. Geological Survey seismologist Lucile Jones.
Jones was one of several experts who spent that night at the Caltech seismological laboratory, trying to decide whether to initiate a full-scale alert of emergency response officials.
Likely Starting Point
The 6.2 earthquake had hit about 6 p.m. near the southern end of the San Andreas, "a likely starting point" for a major quake that would rip northward along the great fault, Jones said. Officials have warned that a catastrophic earthquake should hit somewhere along the southern San Andreas within the next few decades.
The night was marked by a series of conference-call meetings among officials of the U.S. Geological Survey in California and the agency's Reston, Va., headquarters; representatives of California's Office of Emergency Services, and scientists on the California Earthquake Prediction Evaluation Council, Jones said.
After worrying through the night, "we decided there wasn't enough evidence" to call the alert, Jones said.
Second Quake Hits
They knew they were right the next morning, shortly after 5 a.m., when a 6.6 quake hit, shaking up the Southland but showing that the action was on the less-threatening Superstition Hills Fault instead of the San Andreas.
Still, the back-to-back earthquakes are "different than anything we've ever seen before," said Tom Hanks, an engineering seismologist in the Geological Survey's Menlo Park, Calif., office.
The ground just south of the Salton Sea appears to have virtually shattered with scores of small fractures leading in different directions, rather than along a single fault. That most likely occurred with the first earthquake, because the latter quake left a sharply delineated scar across the desert floor, "just like your garden variety California fault," Hanks said.
Oddly, enough, the energy from the first quake propagated in a northeasterly direction toward the San Andreas, while the second moved in a northwesterly direction. Normally, faults that close in proximity would move in the same direction. That extremely rare pattern, plus the multitude of fractures, have given scientists something special to work with.
"It's unique," Hanks said.
Furthermore, the area was heavily instrumented, giving scientists the kind of data they have never had before, he added.
For the first time, instruments were in place at the time of an earthquake to study liquefaction--ground failure that occurs when the soil assumes the characteristics of a liquid.
Liquefaction, which Jones described as something like "quicksand," is of major concern to engineers because most structures would collapse if the ground on which they are built suddenly fails.
Many areas of Southern California are considered prime candidates for liquefaction. That includes coastal areas with a high water table and other regions, such as ancient lake beds where the water table is high and the soils are loose.
After a 5.7 quake near Westmoreland in 1981, instruments were placed in a 30-foot well midway between Brawley and the Salton Sea to determine exactly how the ground behaved. As it turned out, the quakes hit nearby and for the first time scientists were there, at least through their instruments, when it hit.
"We've never had this kind of data before," Jones said.
The data is still being analyzed, but so far it has already produced some surprises.
Reasons Not Clear
The first quake did not cause liquefaction, but the second one, which lasted about four times longer than the first, did. The reasons for that are not clear, Hanks said.
Astonishingly, liquefaction occurred a full 90 seconds after the ground stopped moving, giving the scientists another mystery.
That, plus the bizarre pattern of small fractures throughout a 100-square-mile area, has left scientists baffled, excited, and loaded with data to study.