LOS ANGELES (AP) — 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 rep...