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It was a small earthquake, measuring just 3.1 on the Richter scale, but its location in the heart of the Sacramento-San Joaquin Delta has experts buzzing.

The Oct. 15 quake was centered 7 miles northwest of Lathrop, on Union Island. No faults are known to exist in that area, where earthquakes are rare.

The temblor could offer new insights on safety issues in the Delta, where concerns about flood protection and water quality during a major quake have been growing. It is also a reminder that many mysteries lurk below ground – even in California, a nucleus of earthquake research.

“It was a surprise to us,” said Jack Boatwright, a geophysicist at the U.S. Geological Survey in Menlo Park who is studying the quake. “There’s something down there that we don’t know about.”

Catie Marchini felt the quake when it struck at 4:04 a.m. She and her husband live on the western edge of Manteca, about 10 miles southeast of the epicenter.

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

Marchini grew up on a farm that sits on the quake’s epicenter. Her parents still live there, in a house on Roberts Island, just across the river from Union Island.