An earthquake of modest intensity, measured at a 4.7 magnitude, struck deep beneath a mountain range in the Southern California desert on Monday, rattling populated areas as far away as San Diego and Los Angeles, but no damage or injuries were reported.
Automated sensors initially reported the quake as a flurry of three tremors occurring in rapid succession at magnitudes of 5.1 or higher, but the seismic strength was quickly downgraded to between 4.6 and 4.7 before scientists determined that only a single quake of that size had actually struck.
Susan Hough, a seismologist for the U.S. Geological Survey in Pasadena, said the agency's real-time quake-reporting system was "confused" by a 2.3 magnitude foreshock that preceded the main jolt by 16 seconds, and a 3.2 aftershock less than a minute later.
The main quake, which hit shortly before 10 a.m. local time about 22 miles south of the resort town of Palm Springs, ended up being followed by about 100 very small aftershocks, she said.
The principal tremor occurred about 6 to 8 miles beneath the San Jacinto Mountains, a granite range towering over California's southern desert, on a branch of an active fault line named for the mountain, Hough said.
Quakes of similar size typically generate too little ground motion to cause any property damage.
"If you were sitting right on top of a 4.7, it might have knocked some things off the wall," she told Reuters, but centered in the mountains as it was, "we'd expect little or no damage." None was reported, she said.
Still, light rattling or rumbling was reported felt about 65 miles to the south in San Diego and about the 100 miles to the northwest in Los Angeles, where some high-rise buildings swayed slightly.
Hough said the state of California overall averages about eight magnitude 5.0 quakes a year over the long term.