Did Earth give clues prior to Bay quake?

DidEarth give clues prior to Bay quake?

For residents of the San Francisco Bay area, last year's Loma Prieta earthquake hit without warning. But researchers reported last week that the Earth may have provided hints of the deadly jolt months and days before it actually struck -- renewing the hope that scientists will someday accurately predict impending seismic shocks.

"It's a little strong to say that [Loma Prieta] has given us the key to earthquake prediction. But it certainly looks like this will really invograte the field," says seismologist Paul G. Silver of the Carnegie Institution of Washington (D.C.), one of the researchers who discussed quake precursors at the American Geophysical Union meeting in San Francisco.

The strongest report of pre-quake clues comes from records of crustal warping near the San Andreas fault, studied by Malcolm J.S. Johnston of the U.S. Geological Survey (USGS) in Menlo Lark, Calif., and Alan T. Linde of the Carnegie Institution. The researchers report that a strainmeter 35 kilometers south of the Loma Prieta epicenter began measuring a change in the rigidity of the fault zone in early 1989, long before the Oct. 17 quake.

The strainmeter is sensitive enough to detect extremely subtle deformations in the ground due to tidal forces. Normally the tides affect the crust in a regular manner, but in the months prior to the quake, the strainmeter measured a 10 percent increase in the effect of tides on the region. Johnston now speculates that microfracturing might have occurred along the fault during the months leading up to the earthquake, weakening the ground and amplifying tidal warping in the area. Japanese researchers have reported detecting similar effects before quakes in Japan, he says.

Johnston also found increased tidal deformation in records from a second type of strainmeter, located only a few kilometers away from the other one he studied. But Michael T. Gladwin from the University of Queensland in St. Lucia, Australia, did not detect any exaggerated tidal signal when he analyzed data from the second strainmeter. The two researchers met this week to discuss the discrepancy.

Although Gladwin did not see the changes in tidal warping, he and his colleagues report the second strainmeter detected another pre-quake change -- a significant speedup in warping along the San Andreas fault. The acceleration occurred in August 1989, at about the time of a magnitude 5.2 quake that preceded the magnitude 7.1 Loma Prieta shock.

Gladwin and Johnston remain cautious about their findings because they have information from only two instruments, which sit fairly distant from the earthquake's epicenter. Their observations could reflect instrument error or tectonic changes unrelated to the Loma Prieta quake, says James Savage of the USGS in Menlo Park.

The strainmeters did not detect any precursors immediately prior to the quake. But Silver and colleagues report that the Old Faithful geyser in Calistoga, Calif., acted strangely days before the disaster. Just 60 hours in advance of the main stock, the geyser began erupting less frequently. Silver suggests pre-quake tectonic changes could have altered the plumbing system of the geyser, located 180 km from the epicenter. Though other researchers have observed earthquakes altering geyser behavior, this is the first report of geyser changes right before a quake, Silver says.