Kelin%20Wang

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Observing an Earthquake Cycle Within a Decade
Kelin Wang Pacific Geoscience Centre, Geological
Survey of Canada
(Drawn by Roy Hyndman)
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Important points

1. Stress and strain evolve in earthquake cycles.
   Presently observed interseismic deformation is a
   snapshot of a changing field.
2. Earthquake cycle is a common process. There are
   fundamental similarities between earthquake
   cycles of different subduction zones.
3. Study of multiple subduction zones that are
   presently at different phases of earthquake
   cycles will help us understand the full cycle.
4. This will require us to distinguish between
   common/fundamental processes and site-specific
   processes.

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• Downdip limit of the seismogenic zone
• Frictional behavior of deeper part of the fault
• Mantle rheology

• Updip limit of the seismogenic zone
• Frictional behavior of shallow part of the fault

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Sumatra A few years after a great earthquake
Courtesy Kelly Grijalva and Roland Burgmann
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Alaska 40 years after a great earthquake
M 9.2, 1964
Freymueller et al. (2008)
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Chile 40 years after a great earthquake
GPS data Green Klotz et al. (2001) Red Wang
et al. (2007)
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Cascadia 300 years after a great earthquake
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Coast line
Coast line
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Locking
Rupture
ETS
Afterslip and transient slow slip short-lived,
fault friction Stress relaxation long-lived,
mantle rheology
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Viscoelastic stress relaxation model for Chile,
viscosity 2.5 ? 1019 Pa s
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1995 Antofagasta earthquake, N. Chile (Mw 8.0)
1993-95 Displacements (dominated by co-seismic)
1996-97 Velocities (2 years after earthquake)
Data from Klotz et al. (1999) and Khazaradze and
Klotz (2003)
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(No Transcript)
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Fluid pressure during a VLF episode
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Near-trench boreholes off Mutoto
VLF events
Davis et al. (2006)
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Average stress 15 MPa
?b? ? 0.04
Stress drop 4 MPa
??b? ? -0.01
??b? gt 0
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Evidence for a velocity-strengthening shallow
segment

• Lack of evidence for massive trench-breaking
  rupture
• Slip patterns from inversion of
  seismic/tsunami/geodetic data
• Inferences based on continental earthquakes
• Real-time monitoring at Hokkaido (2003) and
  Sumatra (2005)

Studying the shallow segment is as important as
studying the seismogenic zone
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Importance I Tsunamigenic seafloor deformation
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Importance I Tsunamigenic seafloor deformation
Earthquakes of same moment magnitude
Less strengthening of the shallow segment leads
to trench-breaking rupture. Trench-breaking
rupture causes less seafloor uplift.
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Importance II Deformation of the frontal
prism (Dynamic Coulomb wedge)
Inter-seismic lower basal stress
Co-seismic Basal fault strengthens greater
compression and pore fluid pressure within the
prism
Cumulative effects of numerous great earthquakes
control wedge taper
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Importance III Coseismic activation of megasplay
(Park et al., 2002)
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Co-seismic
Post-seismic
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Immediately following an earthquake
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Longer time after the earthquake
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(No Transcript)
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• Issues to be resolved by observations
• Does deformation at different stages of the
  earthquake cycle leave different signatures in
  rock samples?
• How far does coseismic rupture propagate updip?
  How common or rare is trench-breaking rupture?
• How do the frontal prism and splay faults
  respond to megathrust motion during, after, and
  between earthquakes?
• How does pore fluid pressure within the frontal
  prism and along the megathrust evolve in
  earthquake cycles?
• How does the coseismically strengthened shallow
  segment of megathrust relax after the earthquake?
  What is the time scale of the relaxation?
• How does the oceanic mantle respond to
  earthquake cycles? What viscosity model and
  value? Is it similar to the mantle wedge?

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Important points

1. Stress and strain evolve in earthquake cycles.
   Presently observed interseismic deformation is a
   snapshot of a changing field.
2. Earthquake cycle is a common process. There are
   fundamental similarities between earthquake
   cycles of different subduction zones.
3. Study of multiple subduction zones that are
   presently at different phases of earthquake
   cycles will help us understand the full cycle.
4. This will require us to distinguish between
   common/fundamental processes and site-specific
   processes.

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Suggestions for SEIZE

1. Study multiple subduction zones that are
   presently at different phases of earthquake
   cycles
2. Monitor strain, pore fluid pressure, etc.,
   correlate with land-based networks
3. Transects of shallow boreholes
4. Monitor locked and creeping segments

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Earthquake followed by locking
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Different along-strike rupture lengths and slip
magnitudes (surface velocities 35 years after an
earthquake mantle viscosity 2.5 x 1019 Pa s)
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Very-low-frequency earthquakes in Nankai
accretionary prism
?