Sierras' sway raises queries of new tremor

It's a whole lot of shaking -- but what does it mean? Seismologists are trying to assess the meaning of three strong earthquakes and a series of smaller shocks that hit much of California and parts of Nevada earlier this week.

As tourists streamed out of the Mammoth Lakes resort area in the Sierra Nevada mountains, scores of earth scientists descended on the region to study the activity. A number of experts contacted by the Monitor said they did not know how long it would last or whether the quakes portend a more powerful temblor.

"I wouldn't be surprised if there were a large quake coming," says Alan Ryall , a University of Nevada geophysicist who has closely followed seismic activity in the region for several years. "Then again, I wouldn't be surprised if there weren't."

Seismologists point out that two similar series of shakes in the quake-prone California-Nevada region in 1927 and 1941 were not precursors of large-scale earthquakes.

But according to Professor Ryall, the present series of quakes follows a similar pattern of seismic sequences in 1952 and 1954 that preceded earthquakes registering 7 or more on the Richter scale. (Each step on the scale indicates a degree of severity approximately 60 times greater than the one below it.)

A lull in seismic activity occurred in 1977 and most of 1978. Beginning in the fall of 1978, there has been a series of moderate-to-strong temblors in the quake-prone region -- including this week's shocks, says Professor Ryall.

The three major shocks in the Mammoth Lakes area near Yosemite National Park in the past week reached at least 6.0 on the Richter scale.

A large quake -- which may not happen for six months to a year, if it happens at all -- could register as high as 7.5 to 8.0 and involve a series of relatively short faults over a 75-mile stretch, according to Professor Ryall.

Although such a shock could set skyscrapers swaying as far away as Los Angeles and San Francisco, it would not cause damage to those cities or increase the likelihood of earthquakes there, the experts say.

Nor would it be likely to ravage the mountainous Mammoth Lakes area, which is not densely populated. In addition, buildings in the resort area are particularly sturdy, Professor Ryall notes, because they have been built to withstand the weight of heavy snowfalls.

Scientists have poured into quake area with what one observer described as "instrument on top of instrument" to register the seismic activity. They say they expect to reap some valuable information from California's latest upheavals.