A 4.5 earthquake rocked The Geysers geothermal field Tuesday afternoon.

The geothermal field has several microquakes a day, usually below a magnitude 3, but one of the magnitude that occurred at 2:08 p.m. Tuesday is unusual.

"That's a pretty solid quake," said David Oppenheimer of the U.S. Geological Survey in Menlo Park.

The last one of that size occurred Oct. 20, and there have been 19 quakes of 4.5 in The Geysers since 1972, he said.

The Geysers is a steam field on the Sonoma-Lake county border that is seismically active, made up of hundreds of small faults, but not expected to produce a major quake.

The 4.5 quake "is thought to be the upper end. To get big quakes you need big faults. The hallmark of The Geysers is that the faults are distributed."

There were no immediate reports of damage.