1989 Quake Eased Stress on Dangerous Fault, Experts Say

Quake researchers report that the 1989 Loma Prieta earthquake relaxed stress on the dangerous Hayward fault in the East Bay, reducing creeping movements along the fault to virtually nil for seven years and possibly delaying the advent of another big quake on the fault, which has not ruptured since 1868.

The measurements, reported in today’s issue of Science, buttress findings that in some areas big quakes create “stress shadows,” turning off seismic activity for lengthy periods. A stress shadow persists in some areas from the 1906 San Francisco quake.

This new research finds that Loma Prieta created a less persistent shadow, but still a noticeable one. And scientists say the same phenomena could be at work in Southern California from the Northridge quake.

Creep is a term referring to small, usually steady movements that take place along a fault without earthquakes occurring. About half of fault movement may occur in this way, while the other half results from earthquakes.

Whether creep reduces or increases the likelihood of an earthquake depends on the type of fault, scientists say. On the Hayward fault, the creep that scientists are studying is a relatively shallow phenomenon. In such cases, the reduction of creep suggests that stress is being relieved. In contrast, when the creep occurs through the entire depth of a fault--such as on portions of the San Andreas--its absence may increase chances of a quake.

When the Hayward fault last ruptured in a big way 130 years ago, a 6.8 temblor killed 30 people. Scientists say that the magnitude 7.1 Loma Prieta quake of Oct. 17, 1989, “acted to delay [a new] large earthquake on the Hayward fault by a few years.”

A similar delay in a rupture may have occurred at Parkfield on the San Andreas fault in Central California as a result of a relaxation of stress caused by the Coalinga quake of 1983. Scientists had predicted that a magnitude 6 quake was 95% likely in Parkfield by 1993, but none has taken place.

Bay Area earthquake scientists have said that the probability of a magnitude 7.0 or larger quake on the Hayward fault in the next 30 years is 45%. The new findings could reduce those chances.

There is particular concern about a big quake on the northern segment of the fault underlying Oakland and Berkeley. That part of the fault has not ruptured since 1836, or possibly before, because new research indicates that the 1836 quake may have been centered farther south, near San Jose.

Today’s report by U.S. Geological Survey scientists James J. Lienkaemper and Robert W. Simpson and San Francisco State geoscientist Jon S. Galehouse details measurements taken at numerous survey markers.

At each measuring site, markers have been installed in an L-shaped array on both sides of the fault, along with a marker parallel to the fault. By periodically measuring the angles between markers, movement is recorded.

Galehouse says that average creep on the Hayward fault “has historically been between 4.5 and 5 millimeters per year. But after Loma Prieta, for seven years, measurements showed it was nil, and then, in 1996, there was a sudden movement of 18 millimeters.”

In the 70 years since the construction of the football stadium at UC Berkeley, which is right over the fault, creep has amounted to over a foot, Lienkaemper said. The stress on concrete in parts of the stadium is evident.

Galehouse said the 1996 measurements of new movement indicated that whatever stress relaxation had occurred as a result of Loma Prieta on the fault has now lapsed.

Simpson also noted that although Loma Prieta seems to have reduced stress for a while on the Hayward fault, it appears to have increased it on the San Andreas both north and south of the 1989 epicenter. “Creep has more than doubled near San Juan Bautista,” he said.

Other scientists have postulated that in Southern California, the 1994 Northridge quake may have relaxed stress in some areas and increased it in others, such as the Ventura basin.