Could tremors be building for new California quake?

WASHINGTON, July 9 (Reuters) - Constant tremors along part of California's San Andreas fault could be building up to a more serious earthquake in an area due for a big one, researchers reported on Thursday.

A team at the University of California, Berkeley, has measured 2,198 tremors after the 6 magnitude quake in 2004 centered near Parkfield, California.

"The persistent changes in tremor suggest that stress is now accumulating more rapidly beneath this part of the San Andreas Fault, which ruptured in the ... magnitude 7.8 Fort Tejon earthquake of 1857," seismologist Robert Nadeau and colleagues wrote in their report, published in the journal Science.

That region, in southern California north of Los Angeles, experiences a quake every 85 to 142 years, they said -- making a quake theoretically 10 years overdue.

There have been some quakes nearby in the meantime, but the tremors keep occurring, Nadeau said.

"What's surprising is that the activity has not gone down to its old level," Nadeau said in a statement.

A series of small tremors was seen a few days before the Parkfield quake, so Nadeau hopes there may be a way to interpret them to get some kind of warning of future quakes.

Seismologists have taken many different tacks but remain unable to accurately predict when quakes will occur, although they usually know where.

"If earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes," Nadeau said.

The tremors are not volcanic but Nadeau is not sure what is causing them. Underground movement of fluids is one possibility, but Nadeau says it is possible that shattering bits of sharp rock in a deep region of hot soft rock may be responsible.

(Editing by Vicki Allen)