SOUTHLAND 'BIG ONE' COULD BE BIGGER ONE.

Byline: Staff and Wire Services

Earthquakes far more massive than previously predicted could strike the Los Angeles region, which has fault structures similar to those that caused a huge temblor in Alaska last year, according to experts and studies published Thursday in the journal Science.

The studies of the magnitude-7.9 Denali Quake that rocked Alaska last year determined that it was actually a composite of three smaller quakes across three faults.

That suggests that a similar situation could arise in seismically analogous Southern California, where a rupture on the Sierra Madre thrust fault close to downtown Los Angeles could trigger a quake on the strike-slip San Andreas Fault of magnitude-8.0 or greater.

``It is the worst-case scenario for Los Angeles, but it puts us in the same position as San Francisco, where the San Andreas Fault runs right through the middle of the city,'' said Lucy Jones, scientist in charge of the U.S. Geological Survey Office in Pasadena and one of the authors of the study published in Science.

``Here's a way in which we could have a really big earthquake coming into the city. This is probably something we should include in our planning scenarios,'' Jones said.

The studies suggest that the Alaska fault system is similar to Southern California's and that the Southland could produce a similar event - a rupture on the Sierra Madre thrust fault that could trigger an earthquake on the strike-slip San Andreas Fault.

An earthquake that struck simultaneously on both faults could produce a quake of at least a magnitude-8.0 just a few miles from downtown Los Angeles. Such a quake would be eight to 10 times stronger than the magnitude 7.0 or 7.2 quake that experts had traditionally figured would be the big one in the region.

James Dolan, an earthquake geologist at the University of Southern California, said the new studies confirm what many had suspected about the simultaneous movement of multiple faults.

``What we have now seen in the Denali Earthquake is that indeed moderate earthquakes on a thrust fault can trigger, if the geometry is correct, very large earthquakes on adjacent strike-slip faults,'' said Dolan, who was not connected with the studies.

In strike-slip faults, two blocks move past each other in a horizontal fashion to produce earthquakes. Thrust faults do so when one block pushes upward over another, as if moving up a ramp.

The Nov. 3 Denali Quake also produced shaking of a longer duration and period than would a smaller quake, increasing the likelihood of damage to tall buildings and bridges, scientists said.

Also, the energy released by the earthquake was focused in the same direction as the southeasterly progression of the rupture of the three faults. Were a similar quake to occur on some segments of the San Andreas, its energy could be focused directly at the Los Angeles region.

Together, that has ``big implications for Los Angeles,'' Jones said.

Jones also noted that the Trans-Alaska oil pipeline did not break despite the fault in the Denali Quake moving 18 feet. The pipeline was built to sustain a magnitude-8 earthquake with 20 feet of movement, according to the USGS.

That should demonstrate to Angelenos that they can lessen or prevent serious damage in a major earthquake, Jones said.

``Everybody in the city has something they can do to make their house safer.''