The temblor struck 11 kilometers (seven miles) southeast of the town of Parkfield at 10:15 am (1715 GMT), the US Geological Survey said.

The USGS upgraded the magnitude of the quake after shifting it between 5.9 and 6.0 in the immediate aftermath of the quake that struck along the notorious San Andreas fault.

The quake was felt as far afield as San Francisco, which lies 350 kilometers (220 miles) to the northwest of Parkfield, and was followed by a wave of smaller quakes measuring up to 5.0 on the Richter scale.

"It's a strong quake," said Dale Grant, a geophysicist with the National Earthquake Information Center in Golden, Colorado. "It has been felt all the way up to Monterrey and Salinas," which lie south of San Francisco, he said.

Police in the town of Paso Robles, which lies just 52 kilometers (33 miles) southwest of Parkfield, said there were no reports of damage or injuries.

The shaker was followed four minutes later by a second temblor measuring 5.0 on the Richter scale, which was centered five miles from Parkfield, the quake monitors said.

More than 40 aftershocks measuring as much as 4.7 and 3.6 on the Richter scale shook the region that hugs the golden state's notorious San Andreas fault over the following 30 minutes, the USGS said.

Residents of the quake-prone region, which is shaken by a temblor measuring at least six on the Richter scale once every 22 years on average, said they felt the rumble coming deep from the ground.

"We don't see anything new in terms of damage, but we certainly felt it, said Paso Robles Mayor Frank Mecham. "A lot of folks gathered in the square after the quake, and a lot were shook up after our last one."

The quake came nine months after a temblor measuring 6.5 on the Richter scale hit the same area of central California, sending one building crashing to the ground and killing two people in Paso Robles.

That shaker was centered around of the coastal town of San Simeon on December 22, 2003 and wrought the most severe damage in California since a 6.7 quake hit Los Angeles in 1994.

A scientist who successfully predicted that temblor, had also forecast that southern California would be hit by a major earthquake measuring 6.4 or stronger on the Richter scale by September 5.

But University of California at Los Angeles seismologist Vladimir Keilis-Borok went back on his forecast on September 4, saying the prediction had been based on false data.

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