LUDLOW, Calif. (Reuters) - A powerful earthquake measuring 7.0 on the open-ended Richter scale and centered in the California desert, rocked a large portion of the southwestern United States early Saturday, derailing a passenger train and knocking out power in parts of the area.

The earthquake struck at 2:46 a.m. PDT (5:46 a.m. EDT), jolting millions of people awake in southern California, but no serious injuries were reported.

Earthquakes of this magnitude can cause widespread damage. The actual damage, however, was limited by the fact that the epicenter of the earthquake was located just north of the California desert community of Joshua Tree, a sparsely populated region of scrub in the Mojave Desert.

``This is an earthquake larger than the Northridge quake. The good news is that not many people live there,'' said Lucy Jones, a seismologist with the U.S. Geological Survey.

The 1994 Northridge quake, with a magnitude of 6.7, killed 57 people in the Los Angeles area and caused $20 billion in damage.

Saturday's earthquake, which came one day before the 10th anniversary of a major quake in the San Francisco area, was felt in Las Vegas casinos, where gambling halted briefly, as well as in Tucson, Arizona, and all the way south to San Diego, near the Mexican border.

Power And Water Affected

In Los Angeles County, the quake caused power outages, rock slides and water main ruptures.

A California Institute of Technology spokeswoman said the quake was centered 32 miles north of Joshua Tree.

In the Mojave desert, 21 cars of the 24-car Southwest Chief, traveling from Chicago to Los Angeles, left the rails near the small mining town of Ludlow, about 110 miles northeast of Los Angeles, a spokeswoman for Burlington Northern Santa Fe, which owns the tracks, said.

The Amtrak train, traveling at 60 mph, was carrying 155 passengers and crew members when it derailed about eight miles west of Ludlow.

The carriages stayed upright and only four minor injuries were reported, including a woman who was thrown from her sleeper berth and injured her shoulder, officials said.

Sleeping passengers were jolted awake as the train left the buckled tracks at 3:10 a.m. PDT (6:10 a.m. EDT), about 25 minutes after the quake struck.

``It felt like the train hit a pile of rocks. I heard a lot of stuff falling,'' said Ruth Cohen, a tourist from Israel who had been on a trip to the Grand Canyon in Arizona and was on her way to relatives near Los Angeles.

Slowdown Limited Injuries

Injuries were limited, because the Southwest Chief was following a freight train and therefore traveling well below its maximum speed of 90 mph, Burlington Northern spokeswoman Lena Kent said.

After being fed breakfast aboard the stopped train, the tired and upset passengers, clutching bags and babies, were put aboard buses to take them to their destination.

A spokesman for the San Bernardino County Fire Department said buildings at a U.S. Marine Corps base at Twentynine Palms, near Joshua Tree, suffered structural damage and a supermarket in Joshua Tree was severely damaged.

In the hardscrabble mining town of Ludlow, about 15 miles from the quake's epicenter, a number of mobile homes were knocked off their foundations. Broken glass and smashed dishes littered the floor of the town's only coffee shop.

Ludlow resident Jackie Adams said the quake knocked over furniture and sent dishes flying around her mobile home.

``Every drawer in that house was opened. All my heirlooms were smashed,'' she said. Her boyfriend, David Rockwell, said, ''It looked like somebody took everything and threw it.''

A spokesman for the Southern California Edison Co (AMEX:SCEq - news). said as many as 180,000 customers suffered power outages because of the quake, although only 2,500 were without power by midafternoon.

Dike Holds Spilled Naphtha

At the Port of Los Angeles, 2,000 gallons of naphtha, a petroleum product, spilled when the quake ruptured a storage tank. A fire department official said the spill, at the Ultramar Oil Co., was contained by a surrounding dike.

Witnesses described the quake, which lasted between 45 seconds and a minute, as having a rolling motion that increased in intensity as it continued.

Seismologist Jones said the temblor had been named the Hector quake, after an old mining town nears its epicenter.

By noon, three aftershocks with a magnitude greater than 5.0 had struck, and there were 15 with a magnitude over 4.0. The main temblor was preceded by a series of smaller quakes Friday night, the largest registering 3.8.

The quake was a fresh temblor and not a long-delayed aftershock to the 7.2-magnitude Landers quake that killed at least one person in the area in 1992, the Geological Survey's Jones said.

She said there was a 5 percent chance that the 7.0-magnitude quake was a precursor to an even larger one but the probability would decrease daily.