An earthquake swarm rattling for nearly two weeks under the California-Mexico border region unleashed another nerve-jangling jolt and numerous aftershocks Tuesday.

The day's big quake struck at 2:41 p.m., centered in Mexico about 20 miles southeast of Calexico, a U.S. border city of more than 37,000 residents. There were no reports of any injuries or damage in Imperial County on the U.S. side, authorities said.

The U.S. Geological Survey reported the quake was magnitude 5.0 but USGS geophysicist Rafael Abreu said it occurred in the area of a seismic network operated by Mexico which put it at 5.3. Such differing magnitude reports and later adjustments of magnitude are common.

"It started off with a little jolt, then a rumble," said Sgt. Mike Misteriel of the Imperial County Sheriff's Department. The aftershocks producing a rolling sensation, he said.

It was the fourth moderate quake since the swarm began on Feb. 8, raising anxiety in southeastern California and across the border in the sprawling Baja California city of Mexicali.

Some Calexico residents who are used to the region's frequent quakes said they're rattled by the current swarm.

"A lot of my neighbors and co-workers, people in two-story homes, tell me they've been sleeping on the ground floor," said Cesar Aguilar, manager of a duty-free shop. "Me, my wife and kids are all sleeping together in the same room."

Customers have been stocking up on water, batteries, camping equipment and dry food at the local Wal-Mart, an assistant manager said.

"We ran out of flashlights," Manuel Martinez said. "We're calling different places to order some more."

"I was born and raised here, so I've got everything in my house in case of a big one. I probably got some stuff that are expired," he added.

When Tuesday's quake struck, merchants at a shopping center spilled into the parking lot, said Pablo Loo, manager of Calexico Market.

"They didn't want anything to drop on their heads," Loo said, adding that he stayed inside his shop because he's used to the quakes.

"It's Calexico, earthquakes happen all the time," Loo said. "What else can you do but ride with it?"

Enrique Alvarado said he leapt away from a plate-glass window in his office as soon as the shaking started.

"It was a little scary — you stand in the doorways," said Alvarado, who runs a vocational school. "I estimate it went on 25 or 30 seconds, but it feels like an eternity when you're in it."

Calexico Fire Chief Peter Mercado said some residents were overreacting to the swarm because they're influenced by radio reports from Mexicali, where some structures have been visibly damaged by earthquake activity.

"The panic there is being felt here, because people listen to the same newscast, the same radio station," Mercado said. "People are scared of going back inside their homes."

Following one moderate quake last week, up to 30 residents of a three-story retirement home went outside and huddled in the cold, refusing to return to their homes, Mercado said.

"We inspected the building and tried to reassure them that they're safer in the building than outside," he said.