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An earthquake with a preliminary magnitude of 4.1 struck the San Francisco Bay Area early Tuesday, according to the U.S. Geological Survey.

The temblor, which was centered one mile north of Fremont, hit about 2:41 a.m., USGS reported.

It was later downgraded to a 4.0, according to a USGS incident report.

There were no immediate reports of damages or injuries.

The initial quake was followed by at least a dozen smaller ones and triggered a 15-to 20-minute delay on the area’s rail system before the morning commute, the Los Angeles Times reported.  The largest of the smaller temblors was a magnitude 2.7.

All of the earthquakes were centered in Fremont, which is located roughly 38 miles southeast of San Francisco and about 17 miles north of San Jose, according to Google Maps.