SAN ANDREAS FAULT

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SAN ANDREAS FAULT

• From gulf of CA to Cape Mendecino CA 750 mi
  (1200 km)
• Right lateral strike slip fault.
• Slip rate 2-5 cm/yr
• Total displacement between NA plate and Pacific
  plate 200 miles in 28 to 24 million yr.

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• California was a subduction plate boundary
• from 200 million to 28 to 24 million years,
• evolved into a transform plate boundary

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Geologic History of California

• Subduction Zone Boundary
• Transform Zone Boundary

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Ocean transform turns into cont. transform
MOR migrates towards trench offshore
MOR cannot subduct, moves south
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San Andreas Fault System
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Seismic activity on sections of the SAF or other
faults can vary

• Creeping very slow gradual movement
• Moderate earthquakes (6-7 mag.) every 10 years
• Great earthquakes (gt 8.0) every 100 years

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1906 7.8 265 miles of rupture

• Parkfield to San Bernadino
• 1857 8.0 _at_ Fort Tejon
• 225 miles of rupture

Parkfield
From Abbott, 2004
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• Northern Section north of San Francisco, portion
  is offshore. Locked section.
• San Francisco region

• 1836 - Northern Hayward fault (M6.8)
• 1838 - Peninsula Segment of the San Andreas
  (M7)
• 1868 - Southern Hayward fault (M7)
• 1906 - San Andreas fault (M7.7-7.9)
• 1989 - Loma Prieta (M7.0)

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From Abbott, 2004
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(8.2)
On the basis of research conducted since the 1989
Loma Prieta earthquake, U.S. Geological Survey
(USGS) and other scientists conclude that there
is a 70 probability of at least one magnitude
6.7 or greater quake, capable of causing
widespread damage, striking the San Francisco Bay
region before 2030. Major quakes may occur in any
part of this rapidly growing region. This
emphasizes the urgency for all communities in the
Bay region to continue preparing for
earthquakes.
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San Francisco 1906

• Last great earthquake in California (8.2 on
  moment magnitude scale)
• 265 miles of ground rupture
• Epicenter was on the SAF
• Fire was the worst hazard
• 3000-5000 deaths
• 225,000 homeless from a population of about
  400,000
• 16-18, mag. 6.5 events between 1836 and 1906
• 28,000 buildings destroyed
• Repeat of the this event would cost 135 billion

Sacramento Street and approaching fire
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LOMA PRIETA EARTHQUKE 1989
 

• Occurred on the SAF
• Last large earthquake in the Bay area
• Liquefaction and fire
• Magnitude 7.1
• Epicenter in Santa Cruz mountains
• Mountains around Santa Cruz uplifted 1 ft
• 67 deaths (most on 880)

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Interstate 880
 
Marina district
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Seismicity along the SAF 1969 to early 1989
Locked section
Creeping section
 
Loma Prieta event
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Creeping section
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Parkfield?
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The 1987 Parkfield earthquake  Parkfield has
experienced strong (at least M6) earthquakes six
times between 1857 and 1966.  These quakes, have
an average repeat interval of 22 years (24, 20,
21, 12 and 32 years).  Excluding the larger and
more extensive 1857 earthquake, they have all
occurred on almost exactly the same part of the
fault.  Another similar earthquake was expected
to occur at Parkfield by around 1987, but...

• Parkfield, the most comprehensively instrumented
  section of a fault anywhere in the world,
• 12 creep meters (to measure slow aseismic slip on
  the fault)
• 2 electronic distance measurement instruments (to
  monitor displacement)
• 12 GPS stations (to monitor displacement)
• 8 dilatational strain meters (to assess strain
  build-up in rocks)
• 3 tensor strain meters (to assess strain build-up
  in rocks)
• 12 short-period seismometers
• 10 bore-hole seismometers
• 30 strong motion sensors (to measure the ground
  motion associated with a large earthquake)
• a 2.2 km deep borehole with various
  instrumentation
• a proposed 4 km deep borehole with various
  instrumentation

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Since 1857 6 mag. on avg. every 22 yr.
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Finally 2004 Magnitude 6.0 Drilling, sampling
and downhole measurements directly within the San
Andreas fault zone will substantially advance our
understanding of earthquakes.
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• Locked section From
• Parkfield to San Bernadino
• 1857 8.3 _at_ Fort Tejon
• 225 miles of rupture

Parkfield
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Parkfield
Faults in the LA and Santa Barbara areas
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"The earthquake hazard in Southern California is
larger than anywhere else in the United States
for two reasons. First, we have over 300 faults
but second, and equally important, we have 10
million people just in Los Angeles County alone,
and have a lot of building and a lot of
infrastructure being subjected to those
earthquakes. Lucy Jones, U.S.G.S.
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• Compressive stress
• Secondary faults--
• thrust and reverse faults
• Folds,

Big Bend left bend in SAF northeast of LA and
Santa Barbara Region
Los Angeles
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Los Angeles Region

• Many of the thrust faults in LA region are blind
  faults
• Many have the potential for a 6.0 to 7.0
  earthquake
• Northridge 1994
• 6.7 magnitude
• Took place on a blind thrust fault, not the SAF
• Surrounding mountains grew 1 ft
• Fault never broke the surface
• Most expensive natural disaster in US, so far.
• Same event downtown would kill 5000 and injure
  50,0000
• LA needs 15 more
• Northridge events or
• one 7.5 to release
• all the stain

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Known M gt 6 Earthquakes Since 1892
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Mojave Desert
San Fernando valley
http//earthobservatory.nasa.gov/Newsroom/NewImage
s/images.php3?img_id10861
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Mojave Desert
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Mojave
Garlock Fault
San Andreas Fault
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Recent Large Mojave Earthquakes

• Joshua Tree (April), Landers (June) and Big Bear,
  1992
• Landers 7.3 magnitude
• 3rd largest in CA in last century (1900-1999)
• 110 miles east of LA and 30 miles N. of Palm
  Springs
• 3 hrs later 6.7 Big Bear earthquake in San
  Bernardino Mts.
• Hecter Mine, 1999
• 7.1
• little destruction and no injuries because it
  occurred in an area with a sparse population and
  development.

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Hector Mine 1999 7.1 Mojave Desert
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Accelerated Stress Buildup on the Southern San
Andreas Fault and Surrounding Regions Caused by
Mojave Desert Earthquakes
Before Hector mine
San Bernardino segment