The tremor was centered 2.5 miles (four kilometers) south of Julian, which is around 20 miles from the US border with Mexico, and was eight miles deep, according to data from the US Geological Survey.

The initial quake, which hit just after 10:00 am (1700 GMT), was followed by a series of smaller aftershocks.

There were no immediate reports of serious damage, and scientists said structural problems were unlikely.

Because the quake occurred on land, there was no chance of a tsunami.

Paul Nelson, the owner of the Eagle Mining Company, which offers tours down a decommissioned gold mine in Julian, said the quake had created quite a shake.

"I thought the windows were going to blow out," he told AFP.

"It was kinda scary. It did knock some stuff down in the gift shop."

One man in Carlsbad told broadcaster KTLA there had been a long continuous rolling movement.

"It was pretty wild," he said.

"It was pretty intense for a good four to seven seconds.

"I was sitting in my kitchen, and all the chandeliers started to roll around, and all the lights are moving all over the place."

People in the Mexican border city of Tijuana took to social media to say they had felt the quake.

Tremors were also felt north of Los Angeles, while local train services were suspended.

The US West Coast is the confluence of tectonic plates, where several fault lines generate frequent tremors.

Southern California has been struck by a number of major earthquakes, including one in 1994 that hit Northridge, in the Los Angeles area, killing dozens of people and injuring thousands more.

It also caused billions of dollars of damage to homes and infrastructure.

The San Francisco earthquake of 1906, which also caused a tsunami, is thought to have killed upwards of 3,000 people, some of whom died in fires that erupted after the powerful tremor.