Quake Boosts Stress on Faults in L.A. Basin

A team of scientists said Thursday that there is evidence the Northridge earthquake increased geological stress under a wide area of the Los Angeles Basin, but particularly in Glendale, Burbank, North Hollywood, Hollywood and parts of the Westside.

The scientists from the U.S. Geological Survey and institutions in Woods Hole, Mass., and Strasbourg, France, said the stress increases were small but possibly significant.

“The amount of stress is not enough to cause an earthquake, but could trigger one if a fault were already close to failure,” said Ross Stein of the Geological Survey.

Knowledge about earthquakes, however, is usually not sufficient to tell with reliability when a fault is close to failure, he and other scientists acknowledged Thursday.

At the same time, Stein told a four-hour seminar of about 200 earthquake experts organized by the Southern California Earthquake Center at USC that stress appears to have decreased in areas of Malibu, Topanga Canyon, Agoura, the San Gabriel Mountains and large parts of the Santa Clarita Valley.

Santa Monica was said to be in a neutral zone where stress had neither increased or decreased as a result of the magnitude 6.8 Northridge earthquake.

Stein, speaking for the group of five scientists, characterized their findings as preliminary, but said the results are in accord with a 61-year pattern of stress transfers in the Los Angeles area dating to the 1933 Long Beach earthquake.

He said calculations have shown that the Long Beach temblor increased stress in the San Fernando and Northridge areas where earthquakes occurred in 1971 and 1994, respectively. Similarly, he said, the San Fernando earthquake increased stress in Northridge and in the Whittier Narrows area, where a quake occurred in 1987.

“A tentacle of San Fernando aftershocks went straight into Northridge,” Stein said.

The geophysicist added that he and Ruth Harris and Bob Simpson of the Geological Survey, Jian Lin of the Woods Hole Oceanographic Institution and Geoffrey King of the Institut de Physique du Globe in Strasbourg, believe the Jan. 17 Northridge quake has increased stress on the Elysian Park fault system extending from Whittier past Downtown Los Angeles and along the Hollywood Hills and Santa Monica Mountains.

The Northridge quake probably also transferred stress to the northern segment of the Newport-Inglewood Fault--beneath parts of Inglewood and the Westside of Los Angeles--as well as the Oak Ridge Fault system in the Santa Susana Mountains and parts of Ventura County and the Simi Valley Fault. Stein said stress rose on a portion of the San Andreas Fault between Palmdale and the Cajon Pass.

The scientists present at the meeting at the University Hilton--constituting the elite of earthquake scientists in California--listened with rapt attention and peppered a series of speakers with questions.

Among other presentations at the meeting:

* Dan Ponti of the USGS, assigned to study ground deformations caused by the Northridge quake, questioned whether houses should be rebuilt in an area of Granada Hills near Balboa Boulevard and Rinaldi Street where substantial deformation was found. “What could be done to mitigate future deformations of the kind we have seen here?” he asked.

* Randy Jibson of the USGS, assigned to study landslides generated by the earthquake, said that while publicity has centered on landslides in Malibu, there were far more pervasive and spectacular slides in the Santa Susanas. One involved the movement of 1 million square meters of dirt, he said, and he noted that urban development “is creeping into canyons where such landslides occurred.”

* Jim Dewey of the USGS released the first maps using the modified Mercalli scale to show shaking intensities across Southern California. The scale estimates shaking intensities on a Roman numeral scale of I to XII, based on observations of damage and the way people in various areas felt the quake.

According to Dewey, the highest Mercalli readings in the Jan. 17 earthquake reached IX in areas of Reseda, Northridge, the Newhall Pass, Sherman Oaks, Santa Monica and the West Adams district of Los Angeles south of Downtown. All of those areas suffered partial collapses of modern store buildings or medical buildings, destruction of wood-frame apartment buildings or freeway collapses.