EARTHQUAKE / THE LONG ROAD BACK : Aftershock Data Points to Thrust Fault : Seismology: Oak Ridge and Frew systems are not to blame, scientists say. A rift at a shallow angle is new culprit.

Based on new data about the alignment and depth of the aftershocks, Caltech seismologists have changed their view about what fault was responsible for the deadly Northridge earthquake.

The Oak Ridge fault system and the steeply dipping Frew Fault were not involved, the scientists said Thursday. They now believe they have traced the source of the magnitude 6.6 temblor to a thrust fault dipping southerly at a more shallow angle, which they hope to name soon, when the analysis is complete.

Scientists said the significance of precisely identifying the culprit fault is that it may help them to calculate future seismic hazards as they continue to digest the mountains of information that have accumulated at Caltech.

Seismologist Egill Hauksson said it became evident that the Frew Fault could not have been involved after the depth and position of hundreds of aftershocks was determined through study of the seismographic evidence.

The aftershocks simply have been occurring at too shallow an angle, he said. “The Frew Fault dips at 60 degrees, and the angle (of the aftershocks) was only 30 degrees,” he explained. “No aftershocks have been following the path of the Frew Fault.”

As for the Oak Ridge system, Hauksson said flatly, “It is way off in Ventura County.”

Hauksson said a number of his colleagues had agreed with him, and it turned out that this included Kerry Sieh, the Caltech seismologist who on Monday and Tuesday had been the most outspoken exponent of the Oak Ridge system and the Frew Fault.

Sieh said the aftershock data had made it clear that those two faults probably were not involved, especially considering that the Frew had not been active for 500,000 years. The newly suspected fault, he added, appears to have been recently active.

Some scientists, including Hauksson, at least partially attributed the change of mind about the faults involved to the considerable pressure on the experts to provide the public with fast answers. Both the U.S. Geological Survey and Caltech give as frequent as hourly briefings at the Caltech earthquake media center.

Jim Mori, director of the Geological Survey’s Pasadena field office, said Thursday night: “There’s pressure, but on the other hand I think we should say what we want to say and not be intimidated or controlled by the question. To say that we don’t know is fine, and I say that often. It’s part of our responsibility when we do give out information to give the right information.”

One of the most frequent briefers, Caltech’s Kate Hutton, often says plainly that she doesn’t know. Another, Lucile M. Jones of the U.S. Geological Survey, sometimes gives a tentative answer, usually with an emphasis that it is “very preliminary.”

Even Sieh had cautioned Tuesday that the early hypothesis, which has now been abandoned, had to be confirmed later.

Hauksson said Thursday that the aftershock pattern indicates the primary underground rupture zone of the Northridge earthquake trends about 10 degrees east of due north from the epicenter near the intersection of Roscoe and Reseda boulevards in Northridge. The fault runs under Granada Hills, the Santa Susana Mountains and very likely extends into the Newhall Pass and the Santa Clarita Valley.

Scientists flying over the region have noticed signs of surface disturbance, such as ground cracking, in the Newhall Pass and the southern edge of the Santa Clarita Valley, he said. Such deformations--like the half-mile surface disturbance discovered Tuesday by Caltech seismologist James Dolan near Balboa Avenue and the Simi Valley Freeway--is a strong indication of the likely path of the main rupture.

Another is a series of pipeline ruptures in the north San Fernando Valley, Hauksson said.

In addition, he said, some scientists have reported seeing surface evidence of a secondary rupture zone in the Simi Valley.

The existence of a secondary rupture demonstrates that the main quake caused movement on a nearby fault, scientists said. Often, zones of heavy quake damage follow the alignment of such secondary ruptures, as seemed to occur in this case. This was also the case in the 1992 Landers quake south and east of Yucca Valley, a few miles from the primary rupture.

Hauksson said there may be more than one such zone in the Northridge quake, but he listed no further specific examples.

After Wednesday’s magnitude 5 aftershocks, the intensity of aftershocks tailed off Thursday, but Hauksson reported there was a minor continuing migration of aftershocks toward Ventura County.

He had reported five miles of migration toward the northwest on Wednesday. Thursday, he said the migration was “six or seven miles.”

“It is a concern,” he said, “but as time progresses you expect the aftershock zone to expand. We will have to follow this some more to assess its significance.”

Migration of Joshua Tree aftershocks two years ago may have triggered the larger Landers quake. But that phenomenon, and the abnormally large number of aftershocks following the Joshua Tree quake took several weeks to develop.

Hauksson said the probability of another magnitude 5 aftershock of the Northridge quake was 1 in 3 for the coming week and there may be seven in the next 12 months.