An analysis of eight years of seismological data from deep in the central California crust shows that tremor rates along the area’s section of the San Andreas Fault have remained high for the past six years, signaling the possibility of a larger earthquake, according to a paper published today in Science.

“Tremors are a more sensitive indicator of stress change than are earthquakes,” the study authors write. And the tremor data seem to indicate that the San Andreas Fault “may have transitioned into a new state of stress and/or deformation,” bringing fears of something akin to the infamous 7.8-magnitute 1857 Fort Tejon earthquake, in which the fault ruptured significantly.

Contrary to popular belief, the tremors “are not relieving a lot of stress or making the fault less hazardous,” lead author Robert Nadeau, a seismologist at the University of California Berkeley, said in a statement. Instead, they seem to signal a change in the way stress is being distributed in sections of the fault that are “locked” (that is, they haven’t moved, indicating a build up of stress). 

The researchers studied readings taken from 76 monitoring stations around Parkfield, Calif. (which is about half way between Los Angeles and San Francisco). They found 2,198 tremors that had rumbled the depths (between 10 and 20 miles—15 and 30 kilometers—below the surface, rather than the 10-mile depth of full-fledged quakes) since 2001. 

Two nearby earthquakes—a 2003 6.5-magnitude in San Simeon and a 2004 6.0 in Parkfield—also helped Nadeau and co-author Aurelie Guilhem, a U.C. Berkeley graduate student, to paint a better picture of how these tremor rates changed before and after quakes. Both earthquakes saw a spike in tremors both before and after, and a few weeks before the Parkfield earthquake, tremor activity jumped 10-fold, the authors note.

“What’s surprising is that the activity has not gone down to its old level,” Nadeau said. 

That the area has escaped a major quake since 1857 doesn't bode well for the region, the authors note: "This segment is now fully locked,” and the fault was predicted to have ruptured again between 1942 and 1999, putting it already 10 years overdue for the big one.

Image of San Andreas Fault in Central California courtesy of Wikimedia Commons/USGS