Structural Geology, Tectonic Geomorphology, and Recent Slip History of the South-Central San Andreas Fault

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Structural Geology, Tectonic Geomorphology, and
Recent Slip History of the South-Central San
Andreas Fault

• J Ramón Arrowsmith
• School of Earth and Space Exploration, Arizona
  State University
• http//activetectonics.la.asu.edu
  ramon.arrowsmith_at_asu.edu

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Fault zone is comprised of heterogeneous non
coplanar fault surfaces bounding oblate blocks
whose geometry and activity varies in time and
space
3D mixed mode stepover
Cross-section view mode III step (bend)
Map view mode II step (bend)
Questions GeometricFault surface and block
shapes and sizes TimeHow long are they active?
What is slip history? Block motion
history? DevelopmentLinkage and evolution of
roughness

• Strong influences on
• Stress and displacement fields around the fault
  surfaces
• Further development and linkage
• Fluid flow
• Rupture dynamics
• Fault zone strength

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• Plan Present observations of fault zone
  structure and motions from south-central San
  Andreas fault system
• Large scale geologic mapping characterizes
  106-107 yr time scale
• High resolution topography from Airborne Laser
  Swath Mapping illuminates tectonic geomorphology
  (104-106 yr time scale)
• Detailed earthquake geology investigation of 2004
  earthquake and earlier slip events suggests
  structural model
• Result The upper few km of the SAF zone is
  comprised of relatively independently moving
  moderate aspect ratio blocks elongate parallel to
  the SAF. Their relative motions are controlled by
  fault activity which may vary on the earthquake
  recurrence timescale

Carrizo Plain
Geologic mapping
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Large scale geological mapping (16,000)
Thayer, 2006
Rock units
Structures
Simplified Middle Mountain Geologic Map
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F
H
J
Next slides Serial SAF-normal cross
sections Controlled by geologic contact
geometries at surface
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Incorporation of blocks into shear zone
L-shaped fault (Dickinson, 1966)
Slide blocks (Thayer, 2006)
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Example of fault block form Hull shaped block
51 aspect ratio Preliminary apatite Fission
track ages on granite blocks such as these are
40-50 Ma (Fayon and Arrowsmith, unpublished)
F
RMS QEII
Length 293 m Beam 32 m Height 50 m
25x
Length 640 m Beam 140 m Height 150 m
F
-wikipedia
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Variation in slip location in 1966 and 2004
earthquakes suggests process of
formation SE-directed rupture (1966) in red on
NE and NW-directed rupture (2004) in blue bound
100 m wide and 800 m long zone
Thayer, 2006
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A
Parkfield
?
B
Elevation (m)
?
Distance (m)
Middle Mountain
Northern Cholame
2500 m long twisting block (101 aspect ratio)
A
A new tool Airborne laser swath mapping
measures the landscape at unprecedented
resolution (0.5 m DEMs)
B
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B4 0.5 m DEM view NW over Carr Hill
Millers Field paleoseismic site (Toké et al.,
2006)
SW dip along SAF
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Paleoseismological trench excavation
Toké, 2005 and Toké, et al., 2006
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Millers Sag Paleoseismic Investigation
Toké, et al., 2006
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Toké, et al., 2006
Photo at right
Survey of 369 earthquake fractures in Millers
field 2 days after the 2004 earthquake
Spectacular pattern of shearing and association
with pre-existing tectonic landforms
Simple shear zone model
q
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Crack coordinate system
NW
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Shear zone coordinate system
NW
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6.6 cm offset over 70 m aperture on 10/1/04
(Lienkaemper, et al., 2005)
Creepmeter from nearby (Bilham, 2005)
Survey
Significant afterslip (2x) since these
observations no doubt caused further crack growth
and linkage
2 cm offset across cracks of shear zone on
10/1/04
Up dip decrease in slip
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These observations and inferences suggest a model
in which the upper few km of the SAF zone is
comprised of relatively independently moving
oblate blocks elongate parallel to the SAF.
Their relative motions are controlled by fault
activity which may vary on the earthquake
recurrence timescale.
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Acknowledgments

• Maurits Thayer, Nathan Toké, Annia Fayon
• Mark Zoback, Basil Tikoff, Steve Hickman, Ken
  Hudnut, Bill Ellsworth, George Hilley, Chris
  Crosby, Jeri Young, Michael Rymer, Andy Snyder,
  Parkfield landowners, Ken Jones, ASU Active
  Tectonics group, GEON colleagues
• B4 ASLM data
• NSF
• USGS
• SCEC