Like a skinny grade schooler wondering which of the playground bullies will jump him for his lunch money tomorrow, southern California has always eyed its network of earthquake faults with a mixture of dread and resignation.
For decades, Californians have worried about "the big one," and the hugely destructive, but seismologically moderate (magnitude 6.6) Northridge quake of 1994 only heightened uneasiness over what a more powerful quake might do. Yet even as scientists last week announced the discovery of another active fault under Los Angeles, there is mounting evidence that the situation may not be as dire as many seismologists had previously thought.
Two groups of scientists from the United States Geological Survey (USGS) and the University of Southern California and University of California at Los Angeles now say that the deficit - a key indicator of future seismic activity - may be nonexistent or much less than originally theorized.
The notion of a deficit was first suggested in two studies published shortly after the Northridge event. It holds that the stress that builds up along fault lines must be periodically released by earthquakes. If the quakes don't relieve all the stress, then it begins to accumulate.
For this reason, many scientists believed that Los Angeles and its surrounding counties were facing a significantly increased rate of small to moderate earthquakes or the possibility of a Hollywood-style superquake in excess of magnitude 8.0 when the faults finally gave way.
But the new evaluation of the data by the USGS as well as USC and UCLA suggests that the network of fault lines from the Mexican border to just north of Bakersfield may be closer to equilibrium that the earlier interpretations posited.
Part of the difficulty with the earlier efforts, says Ross Stein of the USGS, was reliance on newspaper records to determine the level of quake activity during the 1800s. "Newspapers," says Dr. Stein, "were the seismometers of the 19th century."
But reporting of quakes was anecdotal at best and further limited by the small number of papers in southern California. Today, more comprehensive and sophisticated data, beginning with the work of Charles Richter in 1903, indicate that, "as far as we can tell, the system is in balance," Stein adds.
Yet for quake-weary southern Californians, the reevaluation is not a get-out-of-jail-free card. While the likelihood of greater-than-usual seismic activity may now all but be eliminated, the area is anything but earthquake free - as last week's announcement about the active Los Angeles fault indicates. But many scientists worry that the public may be getting the wrong idea.
"I've had people ask how come we're not going to have earthquakes any more," says James Dolan, professor of geology at USC.
While scientists are reevaluating the previous quake forecasts, they say that these predictions were not grievously wrong and shouldn't be just thrown out. "[The past] model is not very far off and is still the best basis for planning on earthquake hazards," says UCLA geophysicist David Jackson.
What scientists are learning from the current research is that the network of fault throughout the Southwest are more intricate than they ever imagined.
"These regional fault networks may be hundreds of kilometers apart, [yet] they seem to be able to communicate with one another," says Dr. Dolan. "The way to think of it is that all these faults, and there are hundreds of them ... seem to be behaving like a single mechanical unit, a single entity working in some way we don't understand yet."
The real challenge, he says, is "trying to understand how these faults 'talk' to one another, why they talk ... and how their talking ... influences future earthquake behavior."