California prepares for aftershocks, but big jolt not likely

Two major earthquakes that hit Southern California last week should be a warning to people in the United States to prepare for natural disasters, the state's governor said as officials expressed relief that the damage wasn't worse.

No fatalities or major injuries were reported after the magnitude 7.1 quake, which jolted an area from Sacramento to Mexico on Saturday (NZT) and prompted the evacuation of the Navy's largest single landholding, Naval Air Weapons Station China Lake in the Mojave Desert.

The quake was centred 18 kilometres from Ridgecrest, the same area of the desert where a magnitude 6.4 shock hit on Friday. It left behind cracked and burned buildings, broken roads, obstructed railroad tracks and leaking water and gas lines.

Governor Gavin Newsom said on Sunday that governments must strengthen alert systems and building codes and that residents should ensure they know how to protect themselves during an earthquake.

"It is a wake-up call for the rest of the state and other parts of the nation, frankly," Newsom said at a news conference on the state's efforts to help the region hit by earthquakes on Friday and Saturday.

Saturday's earthquake was the largest in Southern California in nearly 20 years.

The quake probably ruptured along about 40km of fault line and was part of a continuing sequence, said Lucy Jones, a seismologist at the California Institute of Technology and a former science adviser at the US Geological Survey.

The seismic activity is unlikely to affect fault lines outside the area, Jones said, noting that the gigantic San Andreas Fault is far away.

On Monday, the US Geological Survey predicted just a 1 per cent chance of another magnitude 7 or higher earthquake in the next week, and a rising possibility of no magnitude 6 quakes.

Officials have voiced concerns about the possibility of large aftershocks in the days and even months to come, though the chances have dwindled.

The damage wasn't worse largely because of how remote the area is, but Newsom cautioned after touring Ridgecrest that "it's deceiving, earthquake damage. You don't notice it at first."

The Democratic governor estimated the damage at more than US$100 million (NZ$150m) and said US President Donald Trump called him to offer federal support.

Only 28,000 people live in the Ridgecrest area, which is sandwiched between more populated areas of Southern California and Clark County, where Las Vegas is located.

Seismologists warned that the area could see up to 30,000 aftershocks over the next six months, though many of those will be too small for people to notice.

April Hamlin said she was "already on edge" when the second quake rattled her Ridgecrest home. She and her three kids initially thought it was another aftershock.

"But it just kept on intensifying," Hamlin said. "The TV went over, hanging by the cord. We heard it break. We heard glass breakage in the other rooms, but all we could do was stay where we were until it stopped."

With the possibility of aftershocks and temperatures expected to reach 38 degrees Celsius over the next several days, officials were taking precautions.

The California National Guard was sending 200 troops, logistical support and aircraft, Major General David Baldwin said. The Pentagon had been notified, and the entire California Military Department was on alert, he said.

The California Office of Emergency Services brought in cots, water and meals and set up cooling centres in the region, Director Mark Ghilarducci said.

Highway officials shut down a 48km section of State Route 178 between Ridgecrest and the town of Trona, southwest of Death Valley, because of a rockslide and severe cracking. It temporarily cut off Trona, a town of about 2,000 people.

Crews worked through the night to patch the roadway, but it remained rough and uneven, California Department of Transportation spokeswoman Christine Knadler said.

In Ridgecrest, fire and police officials said they were initially swamped by calls for medical and ambulance service. But Police Chief Jed McLaughlin said there was "nothing but minor injuries such as cuts and bruises, by the grace of God."

Two building fires - one involving a mobile home - were quickly doused, McLaughlin said, and natural gas lines where leaks were reported were shut off.

In Trona, considered the gateway to Death Valley, fire officials said up to 50 structures were damaged.