Merchandise is scattered on the floor of a Albertson's grocery store Saturday, July 6, 2019 following an earthquake in Ridgecrest, Calif. The Friday night quake, preceded by Thursday's temblor, was the largest Southern California quake in at least 20 years. ( AP Photo/Marcio Jose Sanchez)

Los Angeles - Southern California was rattled Friday by a 7.1-magnitude earthquake, the US Geological Survey (USGS) said, the strongest quake to hit the region in two decades.

The tremor, which surpassed a 6.4-magnitude quake on Thursday, struck at 8.19 pm (0319 GMT) near Ridgecrest around 180 kilometres north-east of Los Angeles.

The extent of the damage of Friday's quake in Ridgecrest - a desert city of about 30,000 people - was initially unclear.

David Witt, the Kern County fire chief, said that no major building collapses were known of but authorities were searching the area.

"There's so many calls for help that we have a backlog of calls," Witt told reporters.

On Thursday the city experienced several structural fires and some homes were destroyed, but no major injuries were reported.

Dozens of aftershocks measuring over 3.0-magnitude hit near Ridgecrest following Friday's quake, including two of over 5.0-magnitude.

"This was definitely stronger shaking and unfortunately it looks to be pretty high levels of shaking in Ridgecrest," Lucy Jones, a seismologist for the USGS, told reporters on Friday.

"That's why we keep on saying 'one in 20 chance,' well this is the 20," Jones added, referring to comments made earlier on the chances of having an aftershock larger than Thursday's earthquake.

There is a five per cent chance that a larger earthquake will follow Friday's quake, which would likely occur within a day, Jones said.