Strange things are happening along the San Andreas, the premier geologic fault in California. * The Mohave Desert has grown by several acres in the last few months, a result of stretching of the earth's crust in that region.
* Radon, an exotic gas that Russian scientists say is a "precursor" of earthquakes, is bubbling out of wells along the San Andreas in increasing quantities.
* Alterations in the earth's magnetic field, and perhaps its gravitational field, in the area suggest deep-seated changes in the geological forces associated with the fault.
These unusual developments have increased scientists' concern about the possibility of a large earthquake on a long-dormant stretch of the San Andreas extending from San Bernardino south to the Salton Sea.
"This part of the San Andreas concerns me the most," says Kerry E. Sieh of the California Institute of Technology. He has been reconstructing the chronology of past earthquakes along several stretches of the fault, which reaches from San Francisco to Mexico.
While extremely suggestive, the recent observations are not sufficient to predict a large earthquake in the near future, explains C. Barry Rayleigh of the US Geological Survey (USGS) after a symposium on the San Andreas at the annual meeting of the American Association for the Advancement of Science here.
"We certainly don't want to scare people unnecessarily, but it might not be a bad idea to take the antique china off the shelf and pack it securely," Dr. Rayleigh comments.
For the past decade seismologists have been searching for precursors: special signs that will tell them the approximate size, center, and time of occurrence of major earthquakes, before they happen. While the experts have drawn up a long list of potential precursors, none has so far proved reliable. Earthquakes appear too variable to provide the scientists with a simple warning sign.
One of these proposed precursors is an increasing concentration of radon, a radioactive gas, in wells along a fault. Radon is a short-lived gas that is produced by the decay of uranium in rock. Soviet researchers in Tashkent assert that a number of shocks have been preceded by increases in radon concentration.
In the last two years, California scientists have seen radon increases before two moderate quakes. But they also have seen an equivalent increase without a quake and a quake without an increase.
"These radon variations undoubtedly mean something, but we just aren't sure what," Dr. Rayleigh says.
The most enigmatic and potentially significant of the measurements made recently is the new pattern of stresses along the fault line. That the earth is doing unusual things in this region has been known for several years, since the discovery of the "Palmdale bulge." This is an area where the arid hills and canyons have been uplifted by an average of 18 inches.
Local deformations like that at Palmdale are easier to explain than the changes recorded over a much larger area in the past year. The first indication of these changes came from researchers at the Jet Propulsion Laboratory who are using radiotelescopes to measure distance on Earth with extreme accuracy. Late last year they reported an increase of nine inches in the distance between their stations in the Owens Valley and Pasadena, a path that crosses the San Andreas. This was quickly confirmed by USGS scientists who have set up a network of laser surveying stations crisscrossing the fault.
These measurements, Dr. Rayleigh reports, show that the forces which lock the fault, keeping it from slipping in an earthquake, have decreased substantially at several points. How significant this change is remains uncertain because the scientists do not know the total forces acting on the fault.
Still, on a statistical basis, southern California is considered overdue for a large earthquake. A temblor of magnitude 7 or greater, the size of the 1906 San Francisco earthquake, has occurred an average of every 70 years.