Kelin Wang Pacific Geoscience Centre, Geological Survey of Canada Observing an Earthquake Cycle...
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Transcript of Kelin Wang Pacific Geoscience Centre, Geological Survey of Canada Observing an Earthquake Cycle...
Kelin WangPacific Geoscience Centre, Geological Survey of Canada
Observing an Earthquake Cycle
Within a Decade
(Drawn by Roy Hyndman)
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.
Important points
Land observations
• 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
Offshore observations
Sumatra: A few years after a great earthquake
Courtesy Kelly Grijalva and Roland Burgmann
Alaska: ~ 40 years after a great earthquake
M = 9.2, 1964
Freymueller et al. (2008)
GPS data:
Green: Klotz et al. (2001)Red: Wang et al. (2007)
Chile: ~ 40 years after a great earthquake
M = 9.5, 1960
Cascadia: ~ 300 years after a great earthquake
Wells and Simpson (2001)
Inter-seismic 2 (Cascadia)
Inter-seismic 1(Alaska, Chile)
Co-seismic
Coast line
Coast line
Post-seismic (Sumatra)
Rupture
Stress relaxation
Stress relaxation
Afterslip
Locking
Afterslip and transient slow slip: short-lived, fault frictionStress relaxation: long-lived, mantle rheology
ETS
Viscoelastic stress relaxation model for Chile, viscosity 2.5 1019 Pa s
(c)
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)
Inter-seismic 2 (Cascadia)
Inter-seismic 1(Alaska, Chile)
Co-seismic
Coast line
Coast line
Post-seismic (Sumatra)
?
?
?
?
?
Coast line
Coast line
?
?coseismicslip (2 m contours)
Hsu et al. (2006)
GPS stations
Coseismic (contours) and 1-yr postseismic (color) slip of 2005 Nias-Simeulue earthquake
Coast line
Coast line
?
GPSA off Peru (Gagnon et al., 2005)
Updip segment is not slipping. Fully relaxed?
Coast line
Coast lineVery-low-frequency earthquakes possibly
in Nankai accretionary prism
(Ito and Obara, 2006)
?
Coast line
Coast line
?
?
Fluid pressure during a VLF episode
VLF events
Near-trench boreholes off Mutoto
Davis et al. (2006)
b 0.04
b -0.01
Average stress
~ 15 MPa
Stress drop~ 4 MPa
b > 0Stress
increaseA few MPa
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)
• Soft frontal prism sediment• Presence of stable-sliding minerals • Dilatancy of granular gouge material upon fast shearing• Inability to localize deformation during fast shearing
Mechanism of the velocity-strengthening behavior:
Studying the shallow segment is as important as studying the seismogenic zone
Importance I:
Tsunamigenic seafloor
deformation
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.
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
(Park et al., 2002)
Importance III: Coseismic activation of megasplay
Seismogenic Zone
Co-seismic
Post-seismic
Stress increase; resisting slip
Rupture;Stress drop
Stress decrease
Locked;Stress increase
Immediately following an earthquake
? Rate depends on friction properties
Rate depends on mantle rheology
Longer time after the earthquake
Slip quickly slows down
Stress increases butshortening slowly slows down
Fully locked?
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?
• … …
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.
Important points
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
Suggestions for SEIZE
Earthquake followed by
locking
Different along-strike rupture lengths and slip magnitudes(surface velocities 35 years after an earthquake;
mantle viscosity 2.5 x 1019 Pa s)
Coast line
Coast lineVery-low-frequency earthquakes in
Nankai accretionary prism
?