A moderate earthquake that rattled a swathe of Southern California forced several dozen people in one community out of their homes after firefighters discovered foundation problems that made the buildings unsafe, authorities said yesterday.

Fire crews red-tagged 20 apartment units in a building in the Orange County city of Fullerton after finding a major foundation crack.

Structural woes including broken chimneys and leaning were uncovered in half a dozen single-family houses, which were also deemed as unsafe to occupy until building inspectors clear the structures. The damage displaced 83 residents.

Despite the evacuations, Saturday's magnitude-5.1 quake centred about 40km south of downtown Los Angeles mostly frayed nerves. The quake was preceded by two smaller foreshocks, and more than 100 aftershocks followed, including a magnitude-4.1.

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No injuries were reported.