San Andreas Fault Tour: Parkfield to the Salton Sea, California
FULL GALLERY: All San Andreas Fault photos in gallery format
The southern San Andreas Fault from Parkfield to the Salton Sea in southern California is considered by seismologists as a prime threat for a major earthquake (exceeding magnitude 7) in the near future. The fault marks the boundary of two massive blocks of the earth's crust called tectonic plates: the Pacific plate to the west and the North American plate to the east. The Pacific plate moves north relative to the North American plate at a constant speed of 1.5 inches per year. In most places along the San Andreas, the two plates stay locked together for a hundred years or more - the elastic strain slowly and steadily building in the rock along the fault zone. Eventually, the accumulated tectonic strain overcomes the frictional strength of the rock, and the two sides slip suddenly and violently past each other in a matter of seconds: an earthquake.
Illustration of the right-lateral strike-slip fault motion during earthquakes on the San Andreas Fault
Science tells us some key facts about the southern San Andreas' threat. First, the two tectonic plates that create the fault zone are continuously moving relative to each other, slowly accumulating strain in the parts of the fault that are locked together. Second, trenching studies along the fault zone have revealed that the average recurrence interval for major earthquakes in these regions is on the order of 100 years. And finally, much of this "locked" section of the fault has not ruptured in more than 160 years - most recently in the magnitude 7.9 Fort Tejon earthquake of 1857. The far southern segments of the fault (which move slower, due to the plate motion being accommodated via multiple faults), have been dormant for much longer, over 300 years. It is inevitable: all of that accumulated strain has to be released at some point. It could be tomorrow, it could be in 30 years - but scientists agree it is certain to come.