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What Kind of Movement Has Occurred 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 right-lateral strike-slip.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 northward.
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.
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