2nd Fault Eyed in Northridge Quake : Geology: Researchers meeting in Texas note there was unexpected seismic activity in an east-west direction.

Data gathered after last year’s Northridge earthquake suggests a second fault may have been involved in the killer temblor, which caused $20 billion in damages in Southern California, a federal government researcher said Wednesday.

To date, only one north-south fault in the San Fernando Valley beneath Northridge is thought to have been involved in the Jan. 17, 1994 quake, which killed 61 people.

But unexpected seismic activity observed nearly 4 miles from the main shock epicenter has raised suspicions among U.S. Geological Survey researchers that another fault was active during the quake, although they have noted there is no conclusive evidence there is a second fault.

“I would say it’s speculative, because the data that we have cannot be very easily explained by any single fault model,” said Paul Spudich, a USGS geophysicist in Menlo Park, Calif.

The USGS data, which was discussed in El Paso during a meeting of the Seismological Society of America, shows that ground in the southern San Fernando Valley area between the Tarzana and Encino areas shook in an east-to-west direction.

Yet, the known existing fault involved in the earthquake suggests that the major ground motion should have had a north-south orientation.

“Since we expected the north-south direction to be big, we were surprised to see the east-west direction were the big motions,” Spudich said in an interview from California. “That’s why we considered the idea that another fault may be involved.”

Seismic readings taken during Northridge aftershocks also showed that ground motion was unexpectedly six times greater at the measurement site in Tarzana than it was at the site some 1.2 miles away in Encino.

The difference can only partially be explained by the fact that the Tarzana site is on a hill, which amplifies ground velocity, and also is on softer rock, which shakes harder, said USGS geophysicist Peggy Hellweg.

“There is still some other difference between the two sites that we haven’t found yet,” said Hellweg, who attended the El Paso meeting.

Spudich noted that USGS researchers are not suggesting the existence of another fault is responsible for the differences recorded in Tarzana and Encino. “Because if another fault caused (the readings at) Tarzana to be big, it would also cause the Encino site two kilometers away to be big,” he said.

Spudich said he will probably attempt more detailed modeling of the ground response at the Tarzana site.

Several other groups are also looking at the area. “That may confirm or refute the hypothesis of a second fault being involved,” said Spudich.

The seismological society meeting will continue in El Paso through Friday and will also include discussions on this year’s devastating earthquake in Kobe, Japan.