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

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

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

End-member warm-slab and cold-slab subduction zones N Cascadia NE Japan Blue: Basaltic crust Purple: Serpentine stability Basalt to eclogite ~ 40-50 km depth Feeble arc volcanism Serpentinized mantle wedge corner Intraslab earthquakes to ~90 km depth Basalt to eclogite ~ 100-140 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

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

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

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

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)

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

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.

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

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

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

Alaska and Chile: ~ 40 years after a great earthquake: Opposing motion of coastal and inland sites M = 9.5 1960 M = 9.2 1964 Freymueller et al. (2009) Wang et al. (2007)

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

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

? 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?

ETS Seismogenic zone

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)

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)

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

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

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?

Seismogenic zone

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

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

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

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

Prism stress from NanTroSeize boreholes Byrne et al., 2009

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

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)

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)

? 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?

Seismogenic zone

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

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

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

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

Built on Ruff (1985)

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