Transcript The San Andreas Fault System Lecture Notes Page

The San Andreas Fault System
“The Big One”
Introduction
• The California landscape is
cut by many faults capable
of making earthquakes, but
one of these, the San
Andreas Fault, more
important than all the others.
It is the principal boundary
between the Pacific Plate
and the North American
Plate, and its displacement
rate is the highest of all
California faults – more than
one inch per year. Two of
the greatest earthquakes in
the history of California, the
1857 Fort Tejon Earthquake
and the 1906 San Francisco
Earthquake, resulted from
slippage along this fault.
Tectonic Evolution of the San
Andreas Fault Zone
• The San Andreas Fault that we see today however, is merely a snapshot
in time, as the fault has migrated eastward. To understand this, let’s
start about 30 million years ago. Three tectonic plates are involved, the
North American, Pacific and Farallon Plates. Dividing the Farallon and
Pacific oceanic plates was the East Pacific Rise (divergent boundary),
and the Farallon was being subducted beneath the North American
continental plate.
•By 20 million years ago, the
westward leading edge of the
North American Plate began to
overrun over the East Pacific
Rise.
•As this continued, the once
convergent boundary instead became
a transform boundary.
M – Mendocino Triple Junction
R – Rivera Triple Junction
• The proto-San Andreas Fault probably ran
near or slightly off the coast of California.
For perhaps 10-12 million years, the plate
boundary, although gradually lengthening,
remained at or near the edge of the continent.
• About 5 million years ago, the long sliver of
coast that we now know as Baja California
was split off from the mainland of Mexico by
an extension of the East Pacific Rise and
started drifting northwestward with the
Pacific Plate.
• The San Andreas Fault at this
time “stepped inland” to roughly
its position today. The spreading
center that now runs up the Gulf
of California to the Salton Sea is
broken into numerous short
segments by long transform
faults.
•The “stepping” of the San Andreas
Fault also caused the fault to take on
a rather distinctive “big bend” in the
vicinity of the Los Angeles Basin,
causing the uplift of the Transverse
Ranges.
Divergent Boundary
Divergent Boundary
Transform Boundary
In cross-section, this change over from a convergent, subduction
boundary to a transform environment is shown above.
Another way of looking at this change over time
diagrammatically. The proto-San Andreas Fault
begins as a fault paralleling the coastline. As the
East Pacific Rise extended inland, the San Andreas
likewise stepped inland.
• Depending on which way
a strike-slip fault like the
San Andreas “bends”, it
may be a “restraining
bend”, as it is in southern
California, or a “releasing
bend” as it is more
commonly in areas more
to the north.
Restraining Bend
Restraining bends will often cause the
generation of hills or mountains due to
compression. The mountains surrounding the
Los Angeles Basin in the LANDSAT image to
the right were generated along a restraining
bend on the San Andreas fault.
A releasing bend will generally
create a “pull apart” or sag pond
due to extensional forces acting
in the area.
Releasing Bend
Lost Lake, a spring fed sag pond
generated by a releasing bend along
the southern San Andreas Fault near
Cajon Pass.