Contact: Robert Irion (408/459-2495)

STUDY UNVEILS A WAY TO PROBE FAULT ZONES BEFORE AN EARTHQUAKE HITS

* This press release is embargoed until 5 p.m. EST Thursday, November 23, 1995. The research will be published in the November 24 issue of the journal Science.

SANTA CRUZ, CA--A new technique may let seismologists estimate which parts of a fault are likely to rupture most severely during an earthquake, even if the fault hasn't broken for a century or more.

The technique relies upon an apparent connection between the pre-earthquake geology of a fault zone and the amount of motion that a quake triggers along different segments of the fault. A similar relationship exists between fault-zone geology and the pattern of aftershocks that strike after the main earthquake.

Seismologist Justin Revenaugh of the University of California, Santa Cruz, who devised the method, says it cannot predict when an earthquake will happen or how large it will be. However, the method may help researchers refine their maps of seismic hazard by hinting in advance which fault segments will pack the biggest wallop. That information could prove especially useful for the many mysteriously "locked" segments in southern California and elsewhere.

"It's difficult to figure out how much slip might occur on locked faults," says Revenaugh, a member of the UC Santa Cruz Institute of Tectonics. "This scheme begins to give us a rational way of looking at each fault and dividing it up into segments. Those segments are the building blocks that could break in one large earthquake or several smaller ones."

Revenaugh bases his conclusions on an analysis of the magnitude 7.3 Landers temblor and two other sizable quakes that ripped across California's Mojave Desert region in 1992. His study appears in the November 24 issue of the journal Science.

Revenaugh adapted a technique used by researchers to monitor nuclear tests. Effectively, waves from earthquakes can serve as sonar-like "pings" to probe the earth's crust. Revenaugh's method models the rocks within the crust as a swarm of points that scatter energy, like fish in the water scatter sonar. Quakes from across the globe send waves through the swarm. Seismographs receive the echoes; their intensity and timing tells Revenaugh which points scatter the energy most strongly. Pronounced scattering exposes an abrupt change in the earth's structure, such as a dense network of cracks along a fault.

For his latest work, Revenaugh drew upon detailed recordings of 81 earthquakes around the Pacific Rim between 1982 and 1992. The quakes were big enough to rattle dozens of stations in the Southern California Seismic Network. Revenaugh scoured the records for a special type of seismic echo, reflecting sudden changes in geologic properties about three to ten miles underground.

His study focused on a patch of land around the Landers fault zone, which knifed a 50-mile scar across the Mojave Desert on June 28, 1992. The study area also encompassed a magnitude 6.1 foreshock ("Joshua Tree" earthquake), a magnitude 6.2 aftershock ("Big Bear" earthquake), and thousands of smaller aftershocks. Revenaugh used seismic scattering to illuminate patterns in the geology of the three fault zones in the decade prior to the quakes, then checked to see whether those patterns somehow were evident in the quakes themselves.

He found several convincing ties. Most notably, he linked the degree of pre-earthquake scattering to two measurements: the amount of motion on the faults during the quakes and the pattern of aftershocks in the months that followed. Extreme transitions in seismic scattering--from low scattering to high or vice versa within short stretches of a fault--usually pointed to areas where the fault moved most severely and where aftershocks clustered most tightly.

Based on differences in the way the seismic waves "lit up" the fault zones, Revenaugh concluded that the scattering must come from clusters of cracks or other imperfections that align in the direction of each fault. The research did not reveal what might cause such flaws in the rock. However, Revenaugh thinks they are related to stresses that build up to different degrees along each segment of a fault before an earthquake occurs.

"If I had published this paper in early 1992," he says, "I couldn't have told you when these earthquakes would happen or how big they would be. But I could have told you how the faults were segmented and where the most and least slip would occur." For Landers in particular, Revenaugh states, "This would have pinpointed fault segments which, before 1992, we could only guess at."

Revenaugh now is extending his technique to other fault zones in southern California. He is checking his approach against existing seismic records and will try to apply it to poorly understood areas. He notes that the method will not unravel the tectonic complexities of the Los Angeles Basin, because most faults there rupture in a different way and are too close together for the method to resolve. But he feels confident it will shed light on locked segments of major strike-slip faults--such as a long stretch of the San Andreas from Cholame to San Bernardino, which unleashed a huge earthquake (bigger than magnitude 8) in 1857 but hasn't budged since.

Revenaugh will present early results from these studies at a poster session on December 14 at a meeting of the American Geophysical Union in San Francisco.

Editor's note: You may reach Revenaugh at jsr@earthsci.ucsc.edu or (408) 459-3055. He will not be available on Thursday, November 23. For assistance or for a copy of his paper, call Robert Irion at (408) 459-2495 or (408) 335-4185 (home).

Press Releases Home | Search Press Releases | Press Release Archive | Services for Journalists

Maintained by:pioweb@cats.ucsc.edu