San Andreas Fault (California): Continental Faults
1. Zoom from the Canadian Rockies to Mid-California
2. If you out into the California desert, the Carrizo Plain, between San Luis
Obispo and Bakersfield, you will find a very strange thing. A large stream is
suddenly offset to the right about 130 m (400 ft). Even stranger is that
nearby there are some smaller gullies that are suddenly offset about 30 feet.
a. The subject of this lecture is the San Andreas Fault, so we know the
general answer to these questions – the Earth has actually moved
horizontally. It is obvious – at least to us now.
b. And as for why the different stream beds and gullies are offset by
different amounts? They are of different ages. The San Andreas keeps
on slipping, so the older stream beds are offset more than the younger
ones.
c. The San Andreas is the longest (?) strike-slip [define] fault in the
world, extending for more than 1300 km.
d. In some places it is not noticeable at all; in some places, like in the
Carrizo Plain, it is blatantly obvious (show photo).
e. Show map of California and San Andreas
i. LA is on Pacific Plate!
ii. There are many places you can go where you can stand with
each foot on a different plate!
3. But people didn’t always realize this. Geology had a real resistance to the idea
of rock moving horizontally.
a. Mapmakers and others knew early on that the continents had moved.
b. Franklin quote from 1747, to clergyman Jared Eliot: “Such changes in
the superficial parts of the globe seemed to me unlikely to happen, if
the earth were solid to the centre. I therefore imagined, that the
internal parts might be a fluid more dense, and of greater specific
gravity than any of the solids we are acquainted with, which therefore
might swim in or upon that fluid. Thus the surface of the globe would
be a shell, capable of being broken and disordered by the violent
movements of the fluid on which it rested.”
i. Good description of plate tectonics!!
c. But when Alfred Wegener actually proposed that the continents
“drifted,” he was ridiculed by North American and European
geologists, who simply could not imagine that solid rock continents
could plow through the solid rock of the earth.
d. For the San Andreas, the Spanish and English had been in California
since 1530, and were shaken by many earthquakes. But it wasn’t until
1895 that Berkeley geology professor Andrew Lawson proposed the
existence of a San Andreas fault in northern California (named after a
small elongated lake that occurs at one location along the fault).
i. And it was only after the devastating 1906 EQ that Lawson
realized that the fault continued down to southern CA.
ii. And still, it wasn’t until 1953, on the doorstep of the
discoveries of plate tectonics, that geologists Mason Hill and
Thomas Dibblee realized that an enormous amount of lateral
motion had occurred on the San Andreas.
iii. We now know there have been at least 550 km of offset across
the fault.
4. Of course – the San Andreas keeps sliding. Or rather, the Pacific Plate keeps
sliding northward relative to the North American plate. The rate is such that
the San Andreas Fault is not that old, geologically speaking. In fact it is only
about 40 million years old. So, what was along the west coast in California
before the San Andreas fault?
a. A clue comes from the existence of the Sierra Nevada mountains
(discussed later in Yosemite) – formed from Subduction Zone
volcanism!
b. Also, extinct volcanos, in places like Mono Craters and Mammoth
Lakes (well, mostly extinct!)
c. So, this was once a subduction zone. But what happened?
i. This clue is the Juan de Fuca plate – all that is left
ii. California rode over the plate and even the ocean trench
iii. We can see the body! Show seismic tomography of Farallon.
iv. Show cartoon of transition of plate boundary
v. In fact, the northward motion of Pacific plate is ripping off Baja
California
vi. (show Tanya’s demo)
vii. LA will be adjacent to SF in about 13 million years
5. But there have been no shortage of mysteries and problems – (often the case
with science – the more you know, the more you know you don’t know). [Did
I do the story of Socrates in HTEW?]
a. Q: Rates don’t match! NAmer/Pac plate motions at location of San
Andreas is about 4.5 cm/yr (2 in/yr), but motion measured along the
fault is only about half as much (2.5 cm/yr)
i. A: Distributed deformation over wide SAF Zone, including
other faults (Garlock, Hayward, Calavaras, etc.)
ii. Show PBO GPS map
b. Q: Fault should lock up and slip occasionally in large EQs: true some
places but not others. Why does the fault “creep” quietly in some
locations?
i. A: SAFOD drill core: serpentinite and talc!
ii. What’s more, the strange seismic tremor that has recently been
observed (explain) might be due to a mineral transformation of
existing minerals to flat minerals like talc.
6. Slip occurs differently in different locations:
a. San Francisco (1906; 1989; other faults; Tomales Bay)
b. Parkfield (prediction efforts)
c. SoCal (bends cause San Gabriel Mtns and Salton Sea (more about this
in talk on Dead Sea).
d. Show SCEC animation of predicted wave propagation along fault and
bad news for LA!
7. There are only a few long continuous faults on land.
a. Lots of subduction zone faults UNDER continents (Japan,
Oregon/Washington, etc), but not seen at surface
b. So, not too many to choose from for the rest of the Top 5:
i. Anatolian Fault (1999 Izmit; breaking westward; next spot is
Istanbul)
ii. Alpine fault (tearing New Zealand in half)
iii. Denali Fault
iv. Dead Sea Fault
v. (Special Mention: Europa)
Questions:
1. Why is it not quite accurate to say, as is sometimes
heard, that “Los Angeles will sink into the ocean?”
2. There have been volcanoes in places like New Mexico in
the not too distant past? How might this be related to
the uplift of the Colorado Plateau?