`BIG ONE' THREAT SMALLER, SCIENTIST SAYS.

Byline: Tony Knight Daily News Staff Writer

One of the earthquake scientists who said Southern California was due for the ``Big One'' now says the chances of such a great quake on the San Andreas Fault in anyone's lifetime could be very small.

The bad news is that when the quake does hit, it could be much larger than anyone expected, in the range of magnitude 8.5 or greater, according to UCLA seismologist David Jackson. That's the equivalent of more than 50 magnitude-7 quakes.

The new theory by one of the Southland's leading earthquake scientists has sparked a lively debate among geological scientists over just how big an earthquake could be.

The answer is important, because many seismologists have predicted that Southern California is experiencing an ``earthquake deficit'' and is due for a spate of Northridge-size quakes.

But if Jackson is right, there probably is no deficit and everyone could breathe a qualified sigh of relief - at least until the really big one hits.

In 1988, Jackson was part of a team of scientists that calculated there was a 60 percent to 70 percent chance of a magnitude-7.5 earthquake on the San Andreas Fault - the fabled ``Big One'' - within 30 years.

The calculations were based on a theory that said the maximum size of an earthquake is determined by the length of the fault, and therefore the absolute maximum quake that could occur on the San Andreas was about magnitude 8.

Looking at fault lengths and the known slip rate between the North American and Pacific continental plates, scientists also concluded that strain is building up in Southern California that will have to be released by a spate of moderate to large quakes - hence the earthquake deficit.

Jackson maintains that fault lengths might not limit earthquake sizes, a concept that challenges the accepted theories. He postulates that the strain could be released in huge magnitude-8.5 quakes that happen every 1,000 years or more.

``A big quake takes the place of smaller quakes,'' Jackson said in a telephone interview from St. Louis, where he presented a paper on his theory at the Seismological Society of America annual meeting.

``A magnitude 8 takes the place of 30 magnitude 7s. A magnitude 8.2 would take the place of something like 50 magnitude 7s,'' he said.

But could we have a quake that big? Jackson says yes, if the length of the earthquake fault doesn't limit the magnitude as most scientists now believe.

``Earthquakes can create new faults,'' Jackson said. ``They can occur on more than one fault at once. The (1992 magnitude 7.2) Landers Earthquake was a good example of that. It occurred on a collection of faults that was mapped as five separate faults.''

Jackson, who is also science director of the Southern California Earthquake Center, has caused a stir with his new theory, because most earthquake scientists have accepted the earthquake-deficit model. The center is housed at USC and includes a consortium of scientists.

``This is a discussion that has been going on within the geological community in Southern California for almost a year now,'' said SCEC geologist Jim Dolan. ``The problem that many of us in the community have with this interpretation is that we don't see any possible way that an 8.5 earthquake could physically happen.''

U.S. Geological Survey seismologist Lucy Jones said most scientists accepted that a quake can jump from one fault to another after Landers, but added that doesn't mean the area shouldn't expect its share of moderate to large quakes.

``It's a theory,'' she said. ``And it's possible that he's right. It's at the extreme of the range of models that people think are likely.''

Does Jackson have a revised estimate on when the area is due for a ``Big One''? Not really.

``We don't have new calculations, because we don't know the size of the largest earthquake,'' he said. ``We have to solve that problem first.''