Tremors deep within the San Andreas Fault suggest California should not become complacent about future earthquakes, a leading seismologist said.

"The San Andreas fault is changing down deep and it's changing down deep in places where large earthquakes have happened in the past," said Robert Nadeau, a research seismologist at the University of California, Berkeley.

Seismic activity in the central part of the fault has increased in the years since the magnitude 6.5 San Simeon quake in 2003 and the magnitude 6.0 Parkfield quake in 2004, Nadeau and his team said in a study published Friday in the journal Science.

Unusually strong tremors preceded the Parkfield quake three weeks before it struck, leading scientists to believe such tremors could provide an early warning single to a big quake, said Greg Beroza, a seismologist at Stanford University.

Earthquakes usually generate clear seismic waves with sharp onsets, while tremors vibrate quietly and can continue for days. Tremors usually occur in a deeper, softer part of the Earth's crust, rather than in the upper part typically thought to generate earthquakes, the Los Angeles Times reported Friday.