A powerful earthquake and aftershock hit quake-weary California this morning, destroying several mountain community mobile homes, triggering a roadway collapse and leaving many residents worried about whether more is to come.
The main temblor, which measured 6.0 on the Richter scale, struck at 7:42 a.m. (PDT) about 15 miles north of Bishop, according to U.S. Geological Survey (USGS) officials in Menlo Park. It was followed 9 minutes later by an aftershock registering 5.2.
This afternoon, an aftershock measuring 5.1 was recorded, bringing to six the number of sizable quakes to have rattled the state in the last two weeks.
Each increase of 1 on the Richter scale, a measure of ground motion, indicates a tenfold increase in quake intensity.
Radio stations in southern California, where an earthquake of 8.0 magnitude or stronger is expected to take many lives sometime in the next 30 years, were soon deluged with fearful callers speculating that the spurt of quakes foretold greater damage and wondering whether the full moon might have been a factor.
Authorities in the Sierra Nevada mountain towns nearest the epicenter, about 220 miles north of Los Angeles, reported no serious injuries. Earthquake scientists said that the quakes appeared to have no connection to a magnitude-5.9 quake July 8 that caused about $5 million damage in the Palm Springs area or to a magnitude-5.3 quake that originated off the Pacific coast July 13.
Lucile Jones, a geophysicist with the USGS in Pasadena, said studies have shown no significant correlation between major quakes and the phases of the moon. However, scientists have noticed a decline in the number of small quakes, of 2.0 or 3.0 magnitude, in southern California since late last year and may try to determine whether such quiet periods mean that large quakes are on the way.
"It was a rolling quake. It seemed like it lasted for eternity but it actually was only about 30 seconds," said Ray Schaaf, a U.S. Forest Service employe in Bishop. He said he also felt Sunday's 5.5-magnitude temblor, thought to have been a foreshock of today's. "This one was enough to make my pulse quicken," he said.
Schaaf said reports from the little town of Chalfant Valley, perhaps two miles from the epicenter, indicated that about 50 mobile homes -- almost one-third of all dwellings in the town -- lurched off their braces or foundations. One fire official said that about 20 homes appeared to be beyond repair.
Emergency food, water and shelter was rushed to the town, Schaaf said. Rangers also checked for campers who may have been cut off by one of several quake-triggered rock slides.
A 150-yard-long section of Pleasant Valley Road, about 10 miles southwest of the quake epicenter, dropped 30 feet, overturning a pickup truck, Schaaf said. He said he thought that the road probably was already severely undermined by water runoff and that the quake accelerated the process. Several dozen campers cut off by the road collapse were taken out on a narrow emergency road.
Kate Hutton, a seismologist at the California Institute of Technology, said the quakes struck near an earthquake fault known as the Sierra Nevada system, but in an area where extremely complex geological formations hamper the identification of a clear fault line. Several USGS measuring devices that are in the area will be studied to try to pinpoint the depth and location of the latest shocks.
The last severe quake to hit that area, a 6.3-magnitude temblor in 1980, was part of what scientists called the Mammoth swarm. This series of quakes, over a relatively short time near Mammoth Lakes, a popular ski resort area, prompted several studies. Some suggested that molten rock close to the surface had been expanding and helped trigger the tremors. The area has a history of volcanic activity, including an eruption thought to have happened in 1500, but the molten rock mass linked to the 1980 tremors recently appears to have been contracting, Jones said. Today's main quake was felt in San Francisco, 230 miles west, and Salt Lake City, 420 miles northeast.