'Puzzling' earthquake hits California

The Oct. 15 earthquake, of just 3.1 magnitude, was centered 7 miles northwest of Lathrop in the heart of the Sacramento-San Joaquin Delta, The Sacramento Bee reported.

"It was a surprise to us," Jack Boatwright, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif., said. "There's something down there that we don't know about."

Residents of Manteca, about 10 miles southeast of the epicenter, felt the 4:04 a.m. quake.

"My husband and I both woke up, because we heard the house creak and we didn't know what it was at first," Catie Marchini, 29, said. "It was kind of weird, because both my husband and I are pretty heavy sleepers."

"It was a lot different than other quakes I've felt before," she said. "It wasn't like a swaying motion. It was just one hard jolt in one direction."

Seismologists say earthquakes are rare within the delta region, which, like Sacramento, is considered seismically tame compared to the Bay Area.

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