1. Subduction Zone Geodynamics:
Walking the Maze of Coupling and Decoupling
Kelin Wang
Pacific Geoscience Centre, Geological Survey of Canada

2. Subduction Zone Geodynamics:
Walking the Maze of Coupling and Decoupling
Coupling or decoupling
Velocity continuous or discontinuous (long-term)
Seismic or aseismic
Locked or creeping (short-term)
Strong or weak interface

3. Low seismic attenuation Low Vp/Vs Serpentinization Stagnant
Cold Forearc
Hot Arc, Back Arc
70 ~ 80 km
~ 100 km
Low seismic attenuation
Low Vp/Vs
Serpentinization
Stagnant
High attenuation
High Vp/Vs
Melting
Vigorous wedge flow

4. End-member warm-slab and cold-slab subduction zones
N Cascadia
NE Japan
Blue:
Basaltic crust
Purple:
Serpentine stability
Basalt to eclogite ~ km depth
Feeble arc volcanism
Serpentinized mantle wedge corner
Intraslab earthquakes to ~90 km depth
Basalt to eclogite ~ km
Active arc volcanism
High-velocity wedge corner
Earthquakes to hundreds of km
Kirby et al., 1996; van Keken et al., 2002; Wada and Wang, 2009; Syracuse et al., 2010

5. Survival depth of basaltic crust (blue diamonds) and
Warm
Cold
Survival depth of basaltic crust (blue diamonds)
and
depth range of intraslab earthquakes (purple)
Model-predicted peak dehydration depth (blue)
and
serpentine stability in subducting slab (purple)
Wada and Wang, 2009

6. 70 ~ 80 km
~ 100 km ?
Questions:
What controls the abrupt transition from decoupling to coupling?
What is the role of petrology, fluids, and rheology?

7. velocity-weakening, “seismically coupled”)
Seismogenic zone
(stick-slip,
velocity-weakening, “seismically coupled”)
For coseismic deformation (a few minutes), this is all elastic.

8. End-member warm-slab and cold-slab subduction zones
Blue:
Basaltic crust
Purple:
Serpentine stability
N Cascadia
NE Japan
Temperature plays a role but perhaps not via a single critical value
Continental Moho seems to be a limit, but there are counter examples: Many events in NE Japan, 2004 Sumatra (Klingelhoefer et al., 2010)

9. ? Seismogenic zone Questions:
What determines the downdip limit of seismic rupture?
What is the frictional behavior of the slab – mantle wedge interface?

10. Locked zone Interseismic Coseismic
coastline
coastline
Interseismic deformation changes with time and therefore is not a mirror image of coseismic deformation.
Locked zone (future rupture zone) cannot be determined by inverting interseismic geodetic data using an elastic model.

11. Locked zone
For interseismic deformation (decades to centuries), this is viscoelastic.

12. Locking Rupture Afterslip Stress relaxation Stress relaxation
Three primary processes after an earthquake:
afterslip, viscoelastic stress relaxation, and fault locking.

13. A few years after a great earthquake:
Sumatra:
A few years after a great earthquake:
All sites move seaward
Courtesy Kelly Grijalva and Roland Burgmann

14. Alaska and Chile: ~ 40 years after a great earthquake:
Opposing motion of coastal and inland sites
M =
M =
Freymueller et al. (2009)
Wang et al. (2007)

15. All sites move landward
Cascadia: ~ 300 years after a great earthquake:
All sites move landward
Wells and Simpson (2001)

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

17. ? Locked zone Questions:
What do interseismic deformation observations tell us about fault friction, rock rheology, and state of locking?
What can we learn by observing subduction zones presently at different stages of the earthquake cycle?

18. ETS
Seismogenic zone

19. Northern Cascadia ETS event of May 2008
Comparison with a worst-case scenario of megathrust rupture
GPS displacements and slip distribution on subduction interface determined by inverting the GPS data.
Tremor located by Kao (white) and Wech (gray)

20. 1944 1946 1944 Non-volcanic tremor 1944 1946 1944 Including afterslip
(Ichinose et al., 2003)
(Baba and Cummins, 2005)
1944
1946
1944
(Sagiya and Thatcher, 1999)
Including afterslip
(Kikuchi and Yamanaka, 2001)

21. Survival depth of basaltic oceanic crust (blue) and
Warm
Cold
Nankai
Mexico
Alaska
Costa Rica
Survival depth of basaltic oceanic crust (blue)
and
depth range of intraslab earthquakes (purple)
Model-predicted peak dehydration depth (blue)
and
serpentine stability in subducting slab (purple)
Wada and Wang, 2009

22. End-member warm-slab and cold-slab subduction zones
N Cascadia
NE Japan
Blue:
Basaltic crust
Purple:
Serpentine stability
ETS at mantle wedge corner
No ETS has been reported
In addition:
Other types of slow slip events: long- and short-duration slow slip without tremor, very-low-frequency earthquakes in ETS zone … …
Vp/Vs anomaly associated with ETS (fluid?)
Mike Brudzinski will provide other details this afternoon

23. ETS Seismogenic zone Questions:
What is the relation between the earthquake cycle, ETS, and other slow slip phenomena?
What are the thermally controlled petrologic and hydrologic conditions of ETS?

24. Seismogenic zone

25. Average stress ~ 15 MPa b  0.04 Stress drop a few MPa b  -0.01
coastline
Interseismic
Average stress
~ 15 MPa
b  0.04
Stress drop
a few MPa
coastline
Coseismic
b > 0
Stress
increase
a few MPa
b  -0.01

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

27. Understanding how the prism is made … …
Classical
Coulomb Wedge
Dynamic
Coulomb Wedge
Updip zone Seismogenic zone

28. Relation between seismogenic zone and prism structure … …
Nankai
Moore et al., 2007
Costa Rica
(based on von Huene et al. (2004)
Ranero et al., 2007

29. Prism stress from NanTroSeize boreholes
Byrne et al., 2009

30. How the leading edge behaves in earthquake cycles … …
Inter-seismic 2
(Cascadia) ?
Inter-seismic 1
(Alaska, Chile)
Post-seismic
(Sumatra)
Co-seismic
Coast line

31. 2005 Nias-Simeulue earthquake: 1-yr postseismic slip (color)
Updip segment off Peru:
Not slipping. Fully relaxed?
Hsu et al. (2006)
Gagnon et al. (2005)

32. CORK fluid pressure transients associated with Nankai VLF
Very-low-frequency earthquakes possibly in Nankai accretionary prism
(Ito and Obara, 2006)
Fluid transients have also been observed at prism toe, Costa Rica, using flowmeters and also interpreted to indicate transient fault slip (Brown et al., 2005; Labonte et al., 2009).
(Davis et al., 2006)

33. ? Seismogenic zone Questions:
What stops coseismic rupture at accretionary and erosional margins?
How does the updip segment move during the interseismic period?
How do stress and fluid in the wedge evolve throughout earthquake cycles at accretionary and erosional margins?
What can we learn by observing subduction zones presently at different stages of the earthquake cycle?

34. Seismogenic zone

35. very rough smooth rough Bilek (2007) Two aspects of the megathrust:
Fault zone material and its frictional behaviour
Fault zone morphology and its scale variability

36. Frictional contact Uneven fault zone
Slip can break velocity-strengthening barrier, allowing large displacement in earthquakes.
– localized shear
Large displacement requires modification of fault geometry, involving complex deformation of the fault-zone volume.
– distributed cataclastic shear

37. very rough smooth rough Bilek (2007) Two aspects of the megathrust:
Fault zone material and its frictional behaviour
Fault zone morphology and its scale variability

38. Smoothly coupled fault;
Rate-state friction;
Giant earthquakes possible
Roughly coupled fault;
Friction and complex deformation;
Earthquakes and creep
Very roughly coupled fault;
Complex fault zone deformation;
Creep and small earthquakes

39. Built on Ruff (1985)

40. Evolution throughout earthquake cycles
Thermal state
Evolution throughout earthquake cycles
Comparison between subduction zones
Seismogenic zone