What these mysterious vibrations say about future earthquakes is far from certain. But some think the deep tremors suggest underground stress may be building up faster than expected and may indicate an increased risk of a major quake.
Researchers at UC Berkeley monitored seismic activity on the fault's central section near Parkfield between July 2001 and February 2009 and recorded more than 2,000 tremors. The tremors lasted mere minutes to nearly half an hour.
Unlike earthquakes, tremors occur deeper below the surface and the shaking lasts longer.
Tremor episodes persist today. Though the frequency of tremors have declined since 2004, scientists are still concerned because they are still at a level that is twice as high as before the 2003 quake.