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

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

3. 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
Land observations

4. A few years after a great earthquake
Sumatra:
A few years after a great earthquake
Courtesy Kelly Grijalva and Roland Burgmann

5. Alaska: ~ 40 years after a great earthquake
M = 9.2, 1964
Freymueller et al. (2008)

6. Chile: ~ 40 years after a great earthquake
M = 9.5, 1960
GPS data:
Green: Klotz et al. (2001)
Red: Wang et al. (2007)

7. Cascadia: ~ 300 years after a great earthquake
Wells and Simpson (2001)

8. Coast line
Inter-seismic 2
(Cascadia)
Inter-seismic 1
(Alaska, Chile)
Post-seismic
(Sumatra)
Co-seismic
Coast line

9. Afterslip and transient slow slip: short-lived, fault friction
Rupture
Locking
ETS
Afterslip
Stress relaxation
Stress
relaxation
Afterslip and transient slow slip: short-lived, fault friction
Stress relaxation: long-lived, mantle rheology

10. Viscoelastic stress relaxation model for Chile, viscosity 2
Viscoelastic stress relaxation model for Chile, viscosity 2.5  1019 Pa s
(c)

11. 1995 Antofagasta earthquake, N. Chile (Mw = 8.0)
Displacements
(dominated by co-seismic)
Velocities
(2 years after earthquake)
Data from Klotz et al. (1999) and Khazaradze and Klotz (2003)

12. ? ? ? ? Inter-seismic 2 (Cascadia) Inter-seismic 1 (Alaska, Chile)
Co-seismic
Coast line
Post-seismic
(Sumatra) ? ? ? ?

13. coseismicslip (2 m contours)
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 ?

14. ? GPSA off Peru (Gagnon et al., 2005)
Updip segment is not slipping. Fully relaxed?
Coast line ?

15. Very-low-frequency earthquakes possibly in Nankai accretionary prism
Coast line
Very-low-frequency earthquakes possibly in Nankai accretionary prism
(Ito and Obara, 2006) ?

16. Near-trench boreholes off Mutoto
Fluid pressure during a VLF episode
Coast line ? ?
Near-trench boreholes off Mutoto
VLF events
Davis et al. (2006)

17. Average stress ~ 15 MPa Stress drop ~ 4 MPa Stress increase A few MPa
b  0.04
Stress drop
~ 4 MPa
Stress increase
A few MPa
b  -0.01
b > 0

18. 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

19. Tsunamigenic seafloor deformation
Importance I:
Tsunamigenic seafloor deformation

20. 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.

21. 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

22. Importance III: Coseismic activation of megasplay
Seismogenic Zone
(Park et al., 2002)

23. Stress increase; resisting slip
Co-seismic
Stress increase; resisting slip
Rupture;
Stress drop
Stress decrease
Locked;
Stress increase
Post-seismic

24. ? Immediately following an earthquake
Rate depends on friction properties ?
Rate depends on mantle rheology

25. shortening slowly slows down
Longer time after the earthquake
Slip quickly slows down
Fully locked?
Stress increases but
shortening slowly slows down

27. 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?
… …

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

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

30. Earthquake followed by locking

31. Different along-strike rupture lengths and slip magnitudes
(surface velocities 35 years after an earthquake;
mantle viscosity 2.5 x 1019 Pa s)

32. Very-low-frequency earthquakes in Nankai accretionary prism
Coast line
Very-low-frequency earthquakes in Nankai accretionary prism ?