By Steve Gorman
LOS ANGELES (Reuters) - A magnitude 4.7 earthquake struck deep beneath a mountain range in Southern California on Monday, jangling the nerves of tennis stars competing in the nearby desert town of Indian Wells and rattling urban areas as far away as Los Angeles, but no damage or injuries were reported.
Automated sensors initially reported the quake as a flurry of three tremors in rapid succession at magnitudes of 5.1 or higher, but 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 occurred.
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.
But the quake proved a distraction for some of the tennis players competing at the BNP Paribas Open in the nearby desert community of Indian Wells.
"We just had an earthquake!!!," Russian pro Maria Kirilenko exclaimed in a Twitter message from the elite ATP Masters 1000 event. "Suddenly everything start moving, I was holding the walls in my room not to fall! OMG"
Two-time champion Rafael Nadal of Spain later told reporters he was startled by the tremor, the first he had ever experienced.
"I was very scared," Nadal said, smiling. "I was on the massage table preparing for my warm-up. I think the massage table moves even more."
"I finish the earthquake, and my legs were like this," he grinned, wobbling his legs.
Light rattling or rumbling was reported felt about 60 miles to the south in San Diego and about 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 earthquakes a year over the long term.
(Additional reporting by Mark Lamport-Stokes, Dan Whitcomb, Cynthia Johnston and Nichola Groom; Editing by Richard Chang and Eric Walsh)