Signature of earthquake ruptures in fault zone rocks

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Signature of earthquake ruptures in fault zone
rocks
Ory Dor PhD Thesis Defense May 8th, 2007
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Dissertation outlines

• Structural asymmetry across faults of the S. San
  Andreas Fault system (Dor, Rockwell, Ben-Zion,
  PAGEOPH 2006).

• Structural asymmetry across faults of the S. San
  Andreas Fault system (Dor, Rockwell, Ben-Zion,
  PAGEOPH 2006).

2.Geological and geomorphological asymmetry
across the 1943-44 rupture sections of the North
Anatolian fault (Dor, Yildirim, Rockwell,
Ben-Zion, Emre, Sisk, Duman, GJI 2007).
2.Geological and geomorphological asymmetry
across the 1943-44 rupture sections of the North
Anatolian fault (Dor, Yildirim, Rockwell,
Ben-Zion, Emre, Sisk, Duman, GJI 2007).
3.Pulverized rocks in the Mojave section of the
SAF (Dor, Ben-Zion, Rockwell, Brune, EPSL 2006).
3.Pulverized rocks in the Mojave section of the
SAF (Dor, Ben-Zion, Rockwell, Brune, EPSL 2006).
4.Constraints on the depth and mechanism of
Dynamic damage generation during SAF earthquakes
(Dor, Ben-Zion, Chester, Brune, Rockwell).
4.Constraints on the depth and mechanism of
Dynamic damage generation during SAF earthquakes
(Dor, Ben-Zion, Chester, Brune, Rockwell).
5.Damage architecture in the hanging-wall of the
Sierra Madre fault an expression for off-fault
Coulomb failure during earthquakes (Dor, Sammis,
Ben-Zion).
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Asymmetric radiation field and rupture directivity
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Asymmetric radiation field and rupture directivity
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Mechanism for preferred rupture direction
bimaterial interface

• Model predictions
• Asymmetric damage structure
• Damage is generated at a shallow depth (top 3
  km)
• Correlation with the velocity structure

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Studied fault sections

• 3 sites on the SAF
• 3 sites along the San Jacinto fault
• 2 sites along the Punchbowl fault

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Trench across the SAF, Palmdale
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LARSE seismic experiment
Fuis et al. 2003
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San Jacinto fault, Anza
Hog Lake
Anza
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Road cut exposure of the San Jacinto Fault at
Anza area
Road cut exposure of the San Jacinto Fault at
Anza area
NE
SW
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SJF - Low velocity trapping structure south of
Anza
NE
SJF
SW
Lewis et al. 2005
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Velocity structure, Anza area
Scott et al. (1994)
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• Conclusions so far
• Sections of the San Andreas, San Jacinto and
  Punchbowl faults have an asymmetric damage
  structure.
• The sense of asymmetry is compatible with
  northwestward preferred rupture direction.
• The more damaged sides of the SAF and SJF are on
  the crustal blocks with faster seismic
  velocities, in agreement with predictions of the
  bimaterial theory.
• Implications!!!

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The North Anatolian fault, Turkey
in collaboration with the GENERAL DIRECTORATE
OF MINERAL RESEARCH AND EXPLORATION OF TURKEY
(MTA), Ankara
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Erosion patterns related to rock damage
Next adjustments of river valleys to the trace
of the fault
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Adjustments of river valleys to the trace of the
fault results

• Fault zone anomalies on the scale of Few km
• Gravity (Stierman, 1984)
• Elevated geodetic signals (e.g., Fialko et al.,
  2002 Hamiel and Fialko, 2007)
• Seismic anisotropy with fault-parallel cracks
  (e.g., Peng and Ben-Zion, 2004)
• Elevated seismic scattering (e.g., Ravenaugh,
  2000)

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Morphometric analysis, 1944 rupture section
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Strahler stream order (Horton, 1945
Strahler, 1957)
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Morphometric analysis, 1944 rupture section
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Aspect analysis
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Aspect analysis
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• Morphometric parameters compared here
• Drainage density
• Stream frequency
• Ruggedness number
• Hypsometric integral

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Morphometric analysis results
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Gully networks in bad-land topography,
1943 rupture section
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Conclusions for the North Anatolian
Fault Structural asymmetry is compatible with
eastward and westward preferred rupture direction
for the 1943 and 1944 rupture sections,
respectively, similar to the rupture directions
in these two recent earthquakes.
If this behavior is dictated by fault properties,
future tomographic studies are expected to find
faster seismic velocities on the more damaged
sides.
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Back to the SAF Pulverized rocks
Brune (2001) rock powder in places near the
SAF, with no SAF-parallel shear and with original
fabrics preserved.
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Back to the SAF Pulverized rocks

• Brune (2001) rock powder in places near the
  SAF, with no SAF-parallel shear and with original
  fabrics preserved.
• Wilson et al. (2005) Tejon Pass
• Grain size at the sub-micron scale
• No SAF-parallel shear
• No significant weathering mechanical process
• Peaked distribution (not fractal)

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Systematic mapping of crystalline pulverized
rocks between Cajon Pass and Tejon Pass
The Mojave section of the SAF
Dor et al., EPSL 2006
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• Pulverized results and conclusions
• 93 of the total outcrop area is pulverized ?
  this is a systematic damage product of the SAF,
  forming a 100 wide tabular zone parallel to the
  fault slip zone.

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Pulverized results and conclusions (cont.) 2.
70 of the total pulverized outcrop area is on
the NE side of the fault similar to the sense
of asymmetry of the smaller scale damage products.

• 3. Pulverization occurred at shallow depth. Why?
• Max. of 2-4 km exhumation in the Punchbowl area
  (very generous considering this is the mountain
  side of the SAF).
• Successive sedimentation since Miocene little
  incision (Quail Lake M ? Hungry Valley P?
  Quaternary fans)
• Deeply exhumed terrains do not imply deep
  pulverization.

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Pulverized results and conclusions (cont.) 5.
The structural asymmetry, shallow inferred
generation depth and the abundance of tensional
features are all compatible with predictions of
the bimaterial theory.
But how shallow and can we provide better
constraints on the depth and mechanism?
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Plio-Pleistocene Juniper Hills formation
Map after Barrows et al.(1985)
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Factor of Increase in Perimeter Length (FIPL)
5.77
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500 mm
FIPL 6.94
FIPL (SEM correction) 9.41 Combined
correction 9.97 (44 increase)
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Average FIPL for all 104 grains 8.14 STDV 5.34
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Average FIPL for all 104 grains 8.14 STDV 5.34
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Average FIPL for all 104 grains 8.14 STDV 5.34
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FIPL results for fault-normal traverse
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FIPL results for fault-normal traverse
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Andersonian, homogeneous stress
Process zone model crack tip stress
concentration (e.g. Vermilye and Schulz, 1998)
Wear along frictional, wavy fault local stress
concentrations. (e.g. Chester and Chester, 2000)
Wilson et al. (2003)
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Fracture orientation measurements
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long
intermediate
short
FIP
10 m
Data 40 Mean dir 042
Data 95 Mean dir 054
Data 84 Mean dir -
Data 428 Mean dir 027
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Sample 8E, 10 SW of the San Andreas fault
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Predicted maximum compressive stress due to
stress cycling during slip events on a frictional
wavy fault surface
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Main conclusion 1. The measurements delineate a
100 m wide damage zone on the SW side of the
fault, implying that dynamic generation of damage
occurs near the surface of the Earth. This length
scale is similar to that of the pulverized rocks
layer.

• Tentative conclusions (i.e. more work is needed)
• Damage is not the result of absolute tensile
  stresses (although grain-scale failure may be
  tensile).
• The orientation of microfractures near the fault
  is compatible with local principle stress
  orientation associated with slip on a wavy
  frictional fault surface with high loading angle.
  
• The variation in microfractures orientation is
  compatible with dynamic loading and unloading of
  the normal stress (a property of rupture along a
  bimaterial interface).

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• Summary
• This body of work is largely compatible with
  previous knowledge about the structure of major
  continental strike slip faults (e.g. Chester et
  al. 2004) and adds to it the following aspects
• Structural asymmetry, a new type of observation,
  can be used to infer about preferred rupture
  direction with implications for earthquake
  physics (the bimaterial theory) and seismic
  hazard.
• Pulverized rocks are recognized as a major
  structural component of fault zones. They contain
  valuable information about rupture processes.
• Dynamic generation of damage can occur near the
  surface of the Earth ( introduction of a new
  image analysis method).

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Thanks
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This talk is dedicated to Rotem, who didnt see
me much lately
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Questions?