Enigmatic tremors erupt across West.

Like millions of other Californians, residents of Mammoth Lakes felt the earth shake on June 28, the day a magnitude 7.5 earthquake hit the southern Mojave Desert near the town of Landers. But Mammoth Lakes is a bit different from most other places rocked by that Sunday shock. Situated on the eastern edge of the Sierra Nevada, this town lies more than 400 kilometers away from the epicenter, too distant for slumbering people to feel the early morning Landers jolt.

Instead, Mammoth Lakes was having its own little earthquakes, which started within minutes of the major quake far to the south. At that same time, similar bursts of perceptible jolts and microquakes started at a dozen other sites in the western United States, including Mt. Shasta, southern Nevada, southern Utah, and even a little later at Yellowstone National Park, which lies a full 1,200 kilometers from the Landers epicenter.

"This is very unusual. I think bizarre is a good word to describe it," says geophysicist Paul Reasenberg, one of the many researchers at the US. Geological Survey (USGS) in Menlo Park, Calif., who are trying to make sense of the far-flung microquakes. Reasenberg says that before June 28, he wouldn't have believed it possible for a quake in southern California to trigger jolts at the other end of the state, let alone several states distant.

"This is one of those rare moments in science when your observational systems bring you something that you've never seen before," he says.

The phenomenon of distantly triggered quakes went unnoticed until now because large earthquakes had not occurred within a network of sensitive seismometers capable of detecting the microquakes.

There have been hints of this kind of activity in the past. In 1906, 11 hours after the great San Francisco earthquake, a shock with an estimated magnitude of 6.2 occurred in the imperial Valley, at the opposite end of the state.

Prior to the Landers event, investigators were wary about drawing connections between the San Francisco and Imperial Valley quakes. But recent events make a link seem more plausible. The post-Landers swarms led USGS seismologist William L. Ellsworth to take a close look at the seismicity records for 1906. In addition to the Imperial Valley shock, Ellsworth found nine earthquakes that occurred in California and Nevada within two days of the San Francisco quake. He calls these "candidates" for triggered earthquakes, cautioning that they may have been unrelated to the great quake.

Similarly, seismologists cannot be certain that the Landers quake triggered the seismic swarm at Yellowstone because this volcanic area has similar swarms quite often and the activity started almost two hours after the quake in southern California. In contrast, the swarms at Mammoth Lakes and several other areas were clearly triggered by the Landers shock because they started so soon after the major earthquake, Reasenberg says.

At present, geophysicists cannot explain how a major shock can trigger seismic unrest so far away. It is well known that tremors on one fault can raise the stress on nearby faults; indeed, the Landers earthquake triggered the magnitude 6.6 Big Bear earthquake on a separate fault. But such stress changes weaken with distance from the epicenter. Calculations made by Robert W. Simpson of the USGS indicate that at Mammoth Lakes, the static stress changes from the Landers quake would be weaker than the stresses induced daily by the tidal forces of the sun and moon.

Because static stresses appear too puny to spark distant quakes, most researchers are focusing on another type of stress change, a temporary one caused by seismic waves traveling through the Earth.

According to one theory, the seismic waves traveling under Mammoth Lakes may have shaken up a chamber of molten rock known to exist at a very shallow depth under this volcanic crater. Like shaking a soda bottle, the seismic waves would cause dissolved gases in the magma to come out of solution, forming bubbles in the magma. The expanding magma would then push on the crust, setting off small earthquakes, says USGS seismologist David Hill.

Hill believes the soda bottle model cannot explain all the seismic bursts, however, because many have occurred far from magma chambers. Another theory holds that the seismic waves somehow reduced the friction on faults, allowing ones already stressed to cause microquakes.

In any case, the mystery bursts have given geophysicists in northern California something to think about while their colleagues in the south study the nearby Landers quake. "I've been in this profession for close to 20 years," says Reasenberg, "and this has been the most exciting week I can remember."