Tremors are increasing near a "locked" stretch of the San Andreas fault in Central California near Parkfield. In a study released today, seismologists say this could mean the shaky state is on the verge of an enormous quake.
UC Berkeley seismologists Robert M. Nadeau and Aurélie Guilhem examined data gleaned from instruments buried deep in the earth around the Parkfield stretch of the San Andreas fault. What they discovered was that the area was constantly being rocked by small tremors from far underground, and that the number of these tremors had escalated greatly in the wake of two recent earthquakes.
In the map here, according to the researchers:
Parkfield is at the northern end of a locked segment of the San Andreas Fault (SAF) that, in 1857, ruptured south from Monarch Peak (MP) in the great 7.8 magnitude Ft. Tejon quake. As a result of nearby earthquakes in 2003 and 2004, tremors developed under Cholame and Monarch Peak. The black dots pinpoint 1250 well-located tremors. The square boxes are 30 kilometers (19 miles) on a side.
Color contours give regional shear-stress change at 20 km depth from the Parkfield earthquake (green segment) along the SAF. The thrust-type San Simeon earthquake rupture is represented by the gray rectangle and line with triangles labeled SS. The currently locked Cholame segment is about 63 km long (solid portion of the arrow) and is believed capable of rupturing on its own in a magnitude 7 earthquake. The gray lines within the Cholame box bound the west quadrant, where quasiperiodic episodes predominate.
According to a release from UC Berkeley:
The researchers conclude that the increased rate of tremors may indicate that stress is accumulating more rapidly than in the past along this segment of the San Andreas Fault, which is at risk of breaking like it did in 1857 to produce the great 7.8 magnitude Fort Tejon earthquake. Strong quakes have also occurred just to the northwest along the Parkfield segment of the San Andreas about every 20 to 30 years.
We've shown that earthquakes can stimulate tremors next to a locked zone, but we don't yet have evidence that this tells us anything about future quakes. But if earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes.
He noted that there were tremors before a recent Parkfield quake, and that he's hopeful we'll get a similar burst of tremors before future quakes. Those tremors could be an advanced warning system, after more research reveals what causes them and what their exact relationship is to quakes. The new research Nadeau has done, he says, strengthens the connection between elevated levels of tremors and earthquakes.
That means the San Andreas might be ready to snap. Or it could just mean that we have a lot more to learn about tremors.
via UC Berkeley
Photo of earthquake damage to Shinkan Dam via Ross Boulanger.