An earthquake with a preliminary magnitude of 3.9 sent a sharp, short jolt through the San Francisco Bay area Thursday night.
The US Geological Survey says the quake was centered near Piedmont, a densely populated suburb 12 miles east of San Francisco. The quake struck around 6:40 p.m.
Police in Oakland and Piedmont say there are no reports of damage or injuries.
Chevron officials were monitoring a small fire at their East Bay refinery just before the temblor struck. They didnÂ´t realize the jolt was just a small quake and hit the sirens, signaling residents to take shelter. But, the fire was out and all was clear by 7:00 p.m.
An aftershock with a preliminary magnitude of 3 shook the same area about an hour later.
A USGS seismologist says the quake was probably on the Hayward fault.