The San Andreas Fault Shows Signs of Awakening

California is used to minor earthquakes. Sitting atop a myriad of fault lines, both minor and major, earthquakes are away of life. Most of them are un-noticed as they are too small to feel. But over the last ten days, California has had a series of quakes of magnitude 3.0 or above on the Richter scale.

The LA Times records three such incidents. Another has been reported by the USGS today.

California sits in an area that is geologically very active. Plate margins grind against each other causing the tremors. The San Andreas Fault is a huge gash in the landscape that runs from Cape Mendocino down to the Mexican border. It’s a transform, or slide fault. The leading edge of the North American Plate and the Pacific plate slide past each other. They move in small fits and starts but on occasion get stuck. The pressure builds and builds, until whatever prominence of rock holding them in position gives way, and the plates move causing an earthquake.

The San Andreas Fault last had a major rupture in 1680. The average lapse between major slips is 150 years, meaning that the pressure has now been building for 334 years.

Other faults branch off the San Andreas Fault and it’s some of these that geologists believe cause the constant minor quakes that Californians live with.

To the west of California, off shore in the Pacific Ocean lies the Cascadia fault. This is a subduction zone, an area where one plate is forced down and under the one it collides with. The Juan de Fuca plate is sliding under the North American plate. Any earthquake produced here would be a megathrust quake, similar to the one that destroyed parts of Indonesia on December 26th 2004.

The Cascadia fault line runs from Northern Vancouver Island to Cape Mendocino California. The last time this one unzipped was in 1700. The resulting earthquake is well documented and the huge tsunami it generated affected not only the United States, but Japan, where it was until recently known as The Orphan Tsunami.

The USGS reported in December that a large quake would devastate cities such as Los Angeles. San Francisco has been hit by several large earthquakes that have been generated by the San Andreas or one of the faults that branch off it. 1906, 1925, 1933, 1957, 1964, 1971, 1987, and 1989 all saw earthquakes that resulted in the loss of life.

The San Andreas Fault may be awakening, foreshocks usually come in groups, are of a higher magnitude than the area usually experiences, and sometimes rumble on for weeks or even months before a major earthquake strikes.

In major urban areas such as Los Angeles or San Francisco, the destruction and loss of life would be substantial, and it’s to be hoped that this series of shocks is not the San Andreas Fault clearing its throat before the main event.

Sources:

http://www.sci.sdsu.edu/salton/San%20AndreasFaultSyst.html

http://geology.com/articles/san-andreas-fault.shtml

http://www.pnsn.org/outreach/earthquakesources/csz