Movement Along the Fault
Blocks on opposite
sides of the San Andreas fault move horizontally. If a person stood on one side
of the fault and looked across it, the block on the opposite side would appear
to have moved to the right. Geologists refer to this type fault displacement as
Movement of blocks along the San Andreas fault.
During the 1906 earthquake in the San Francisco region, roads, fences, and
rows of trees and bushes that crossed the fault were offset several yards, and
the road across the head of Tomales Bay was offset almost 21 feet, the maximum
offset recorded. In each case, the ground west of the fault moved relatively
Sudden offset that initiates a great earthquake occurs on only one section of
the fault at a time. Total offset accumulates through time in an uneven fashion,
primarily by movement on first one, and then another section of the fault. The
sections that produce great earthquakes remain "locked" and quiet over a hundred
or more years while strain builds up; then, in great lurches, the strain is
released, producing great earthquakes. Other stretches of the fault, however,
apparently accommodate movement more by constant creep than by sudden offsets
that generate great earthquakes. In historical times, these creeping sections
have not generated earthquakes of the magnitude seen on the "locked" sections.
Geologists believe that the total accumulated displacement from earthquakes
and creep is at least 350 miles along the San Andreas fault since it came into
being about 15-20 million years ago. Studies of a segment of the fault between
Tejon Pass and the Salton Sea revealed geologically similar terranes on opposite
sides of the fault now separated by 150 miles, and some crustal blocks may have
moved through more than 20 degrees of latitude.
Although it is difficult to imagine this great amount of shifting of the
Earth's crust, the rate represented by these ancient offsets is consistent with
the rate measured in historical time. Surveying shows a drift at the rate of as
much as 2 inches per year.