The strongest earthquake in 20 years shook a large swath of Southern California and parts of Nevada on Thursday, rattling nerves on the July 4th holiday and causing injuries and damage in a town near the epicenter, followed by a swarm of ongoing aftershocks.

The 6.4 magnitude quake struck at 10:33 a.m. in the Mojave Desert, about 150 miles (240 kilometers) northeast of Los Angeles, near the town of Ridgecrest, California.

Multiple injuries and two house fires were reported in the town of 28,000. Emergency crews were also dealing with small vegetation fires, gas leaks and reports of cracked roads, said Kern County Fire Chief David Witt.

He said 15 patients were evacuated from the Ridgecrest Regional Hospital as a precaution and out of concern for aftershocks.

Kern County District Supervisor Mick Gleason told CNN there were some structural issues with the hospital and some patients had to be moved from one ward to another and that others were taken to a neighboring building.

Gleason did not say what the structural issues were.

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