Definition of Big One Shifts; Timing Remains a Mystery : Quakes: Temblor could be an 8 on San Andreas Fault or a 7 nearer L.A. Either way, massive damage is a given.

The Big One. That mammoth earthquake that scientists have been predicting for Southern California has been on just about everyone’s minds and lips for nearly three weeks.

Where and when will it come? What will it be?

The answers remain a mystery, in part because of lack of scientific knowledge and in part because scientists’ definitions of the Big One have tended to change over the years.

To qualify for the most feared of labels, an earthquake should probably strike on the San Andreas Fault, but it does not have to; it should be magnitude 8 or larger, but not necessarily, and it should cause massive destruction and damage (no “buts” about this one).

Today, when scientists talk about the Big One, they are usually talking about segments of the southern San Andreas Fault between Ft. Tejon and the Salton Sea, and particularly the Coachella Valley segment between Salton Sea and Desert Hot Springs.

Scientists interviewed this week said that if all the southern segments were involved at once, it would probably be a magnitude 8 quake. If only one segment went, it would be a 7.5.

“Whenever you talk about the Big One, I always assume it’s the San Andreas,” said Lucile M. Jones of the U.S. Geological Survey’s Pasadena office. “It’s the longest fault in Southern California and you need a real long fault. It is the only one here that is capable of generating an 8.”

But alternatively, the scientists said, the Big One could be a magnitude 7 to 7.5 earthquake on a fault within the Los Angeles-Orange County metropolitan area.

The fault most often mentioned is the Newport-Inglewood, the one responsible for the magnitude 6.3 Long Beach earthquake in 1933 that killed 120 people.

But it could be any number of other surface or subterranean faults that are believed capable of generating a magnitude 7 earthquake.

The closer a fault is to a metropolitan area, the more damage it seems likely to cause. State and federal authorities who have periodically issued casualty estimates for the Big One calculate that a temblor in the magnitude 7 range inside the metropolitan area would prove to be more devastating than a magnitude 8 on the San Andreas Fault.

In 1982, when the state Division of Mines and Geology prepared assessments of probable casualties and damage from an 8.3 quake on the southern San Andreas Fault, it predicted 3,000 to 14,000 deaths.

But in a 1988 report, it said that deaths would range from 4,000 to 21,000 in a magnitude 7.0 quake on the Newport-Inglewood Fault. As the range indicates, such estimates--as scientists are quick to note--are imprecise and may be unreliable, as is the science of quake prediction in general.

Regardless, it is clear that the distance of the Big One’s epicenter from populated areas would have a good deal to do with the number of casualties and amount of damage.

Aside from proximity to the epicenter, populated areas could also be affected by the direction of the rupture.

If a southern San Andreas rupture began near Indio and ripped north to San Bernardino, its direction and the intensity of shaking would almost certainly be greater in the Los Angeles area than if the rupture began in San Bernardino and went in the other direction. This is demonstrated by strong-motion instruments in the recent earthquakes that showed shaking was much stronger in the direction that ruptures moved.

Thomas Heaton of the USGS said last week that had the 7.5 Landers earthquake on June 28 “happened in a populated area, rather than way out in the desert and pointed further out, we might well have considered it the Big One.”

But, he continued, even if it had occurred in a more populated area, such as the vicinity of Palm Springs or San Bernardino, it still might not have been considered the Big One.

“Without widespread catastrophic shaking in the Los Angeles area,” he said, “I doubt most people would accept this as the Big One, even if it had a serious impact on all lifelines coming in from the east: railroads, roads, aqueducts and electric transmission lines.”

Caltech seismologist Egill Hauksson makes a similar point.

“Preparedness materials released by the state Office of Emergency Services speak of people having to rely on their own resources, without aid from police, fire or medical agencies, for 72 hours,” he said. “So if it was all right in 24 hours, maybe people wouldn’t think that the Big One had struck.”

Heaton’s and Hauksson’s statements point up that popular conceptions are also an important element in considering definitions of the Big One.

Though scientists might call it the Big One, there would have to be popular agreement as well. And by nature of its size and population, it is likely that the Los Angeles area would have to be significantly affected to meet this criterion.

In recent years, the locale and nature of the Big One have undergone some alteration.

In 1976, scientists tended to see it in the Mojave segment of the San Andreas Fault, in the neighborhood of what became known as the “Palmdale bulge,” which seemed to be uplifted as much as 10 inches over a wide area. Some scientists thought the bulge indicated an impending large quake.

Later, however, instrumental miscalculations were discovered, discounting the idea that there had been a bulge.

Further research pointed scientists southeastward along the San Andreas toward the Coachella Valley, where studies indicated there had been no big earthquake since about 1680.

With the San Andreas’ last Big One in Southern California having occurred in 1857, a magnitude 8 giant rupturing the fault from Parkfield through Ft. Tejon to the Cajon Pass, scientists noted that the great fault had been locked or jammed much longer in its Coachella Valley and San Bernardino Mountains segments than in its Mojave and other segments farther north.

Following the 1987 Whittier Narrows earthquake, there has also been some change in speculation about where the Big One could strike if it were centered in the Los Angeles-Orange County metropolitan area.

The Whittier Narrows 5.9 quake brought new scientific focus on the existence of deeply buried faults crisscrossing the area.

A geologist who attempted to map them, Thom L. Davis, said in a report prepared for the American Geophysical Union that “deep-level thrusts that do not reach the surface . . . suggest the earthquake potential of the Los Angeles Basin has probably been underestimated.” He said one such fault under downtown Los Angeles “is certainly capable of quakes in the 6 to 7 range.”

This might not be the Big One, but the potential is there to keep some scientists from thinking solely about the Newport-Inglewood Fault.

Probably the most contentious issue among scientists, especially since the Landers and Big Bear earthquakes, has been when the Big One will come. The recent temblors were close enough to the southern San Andreas to engender concern that it could happen soon.

Some scientists, such as the Geological Survey’s Western regional branch chief, Allan Lindh, characterized the danger as immediate.

“There is evidence we’re near the jumping off place for a big one on (the San Andreas),” Lindh said at a Caltech briefing June 29. A few days later, he characterized the Landers quake to the New York Times as “like a final warning” and advised Southern Californians to “act as if the damn thing will happen tomorrow.”

Some other scientists said privately they felt Lindh was an alarmist. At a public briefing at Caltech last Thursday, Heaton stated flatly, “You can’t predict earthquakes.” He dismissed as speculation any talk of an imminent Big One.

Hauksson, meanwhile, said he had been placing aftershocks of the Landers and Big Bear earthquakes on graphs showing what days they had occurred and where they were in terms of distance from the San Andreas. The graphs clearly show that the number of aftershocks has been decreasing, and that they are, on balance, migrating away from the San Andreas.

So, despite a few aftershocks happening on or very close to the San Andreas near Yucaipa and Indio--where extensions of the faults involved in the Big Bear and Landers earthquakes may intersect the San Andreas--Hauksson said he found some reasons to be optimistic that there would be no immediate Big One.

Four years ago, a task force of scientists, which included both Lindh and Jones, estimated there was a 60% chance of a big earthquake occurring on the southern San Andreas Fault by the year 2018. Lindh and Jones have said in recent days that they think it may happen quite soon. But Heaton contended at Thursday’s briefing that there is no credible basis for such a conclusion.

“Some say the recent quakes were the last straw on the camel’s back,” Heaton said last week. “But we have no way of knowing how many straws are on the camel’s back, so how can we say this is the last one?”

Scenarios for the Big One

The idea of the Big One earthquake has taken hold in the imaginations of many Southern Californians, but its definition has slowly been altered by earthquake scientists.

Scientists now believe that the Big One will either be a magnitude 7.5 to 8.3 earthquake on one or more segments of the southern San Andreas Fault or a temblor of magnitude 7.0 to 7.5 on a fault in the Los Angeles-Orange County metropolitan area, perhaps the Newport-Inglewood Fault. (Segments shown on map.)

Disaster experts say that a 7.0 quake centered in the metropolitan area would probably cause more damage and casualties than an 8.0 temblor on the San Andreas Fault outside Los Angeles. NOTE: To cause a magnitude 8.0 quake, the rupture would have to extend through all three San Andreas Fault segments shown above. A quake limited to one segment would probably not exceed magnitude 7.5, but could still be considered the Big One if it caused very substantial damage in populated areas.