The Seismogenic Zone Experiment Revisited MARGINS Theoretical Institute The Seismogenic Zone Revisited Fault Friction and the Transition From Seismic to Aseismic Faulting Chris Marone 1 and Demian M. Saffer 2 1 Penn. State University 2 University of Wyoming Seismogenic zone Updip limit Characterizing the incoming material by non-riser drilling Riser drilling of the seaward limit of the seismogenic zone Quantify lateral changes In the physical, chemical, and hydrogeologic properties of the fault Image the seismogenic zone using earthquakes and artificial sources SEIZERRESIZE

The Seismogenic Zone Experiment Revisited : Scientific Objectives (Hyndman, DPG Report, Aug. 1999) What controls the earthquake cycle of elastic strain build-up and release? What controls the updip and downdip limits of the seismogenic zone in subduction thrusts? What controls the updip and downdip limits of great subduction earthquakes? Why does fault strength appear to be low? What causes tsunami earthquakes? Fault Friction and the Transition From Seismic to Aseismic Faulting Chris Marone 1 and Demian M. Saffer 2 1 Penn. State University 2 University of Wyoming Seismogenic zone Updip limit Characterizing the incoming material by non-riser drilling Riser drilling of the seaward limit of the seismogenic zone Quantify lateral changes in the physical, chemical, and hydrogeologic properties of the fault Image the seismogenic zone using earthquakes and artificial sources

Stability: Why is deformation stable in some cases and unstable in others? Strength: What controls fault strength? Rheology of the fault zone and surrounding materials: Slow earthquakes, postseismic slip, interseismic creep, fault healing, rupture dynamics. What processes, mechanisms, and constitutive law(s)? : Scientific Objectives (Hyndman, DPG Report, Aug. 1999) What controls the earthquake cycle of elastic strain build-up and release? What controls the updip and downdip limits of the seismogenic zone in subduction thrusts? What controls the updip and downdip limits of great subduction earthquakes? Why does fault strength appear to be low? What causes tsunami earthquakes? Key Issues in Fault Mechanics

Saffer, D. M., and C. Marone, Comparison of Smectite and Illite Frictional Properties: Application to the Updip Limit of the Seismogenic Zone Along Subduction Megathrusts, Submitted to EPSL, July Saffer, D. M., Frye, K. M., Marone, C, and Mair, K. Laboratory Results Indicating Complex and Potentially Unstable Frictional Behavior of Smectite Clay, GRL, 28, , Marone, C., Saffer, D., Frye K. M., and S.Mazzoni, Laboratory results indicating intrinsically stable frictional behavior of illite clay, AGU ABST, F Marone, C., Saffer, D., and K. M. Frye, Weak and Potentially Unstable Frictional Behavior of Smectite Clay, AGU ABST, F689, K. M. Frye, S. Mazzoni, K. Mair JOI –USSSP, ODP-Japan

Parkfield, CA Seismicity SW Nankai Subduction Zone % Depth Below Sea Floor (km) Marone & Scholz, 1988

SW Nankai Subduction Zone Parkfield, CA Seismicity 20% Key Questions about Fault Zone Friction Stability: Why is deformation stable in some cases and unstable in others? The seismogenic zone is defined by the transitions from stable to unstable frictional deformation Aseismic Seismogenic

Parkfield, CA Seismicity Seismogenic zone Brittle Friction Mechanics Stable versus Unstable Shear Aseismic N K F s f x x´ 1-D fault zone analog, Stiffness K B C Force Displacement Slope = - K Slip  s x´ x f

Parkfield, CA Seismicity Seismogenic zone Brittle Friction Mechanics Stable versus Unstable Shear Aseismic N K F s f x x´ 1-D fault zone analog, Stiffness K Frictional stability is determined by the combination of 1) fault zone frictional properties and 2) elastic properties of the surrounding material B C Force Displacement Slope = - K Slip  s x´ x f

Parkfield, CA Seismicity seismogenic zone Brittle Friction Mechanics Stable versus Unstable Shear aseismic N K F s f x x´ 1-D fault zone analog, Stiffness K Massless B C Force Displacement Slope = - K Slip  s x´ x f Stability transitions represent changes in frictional properties with depth Frictional stability is determined by the combination of 1) fault zone frictional properties and 2) elastic properties of the surrounding material

Laboratory Studies Slip  s  d L Slip Weakening Friction Law (v)  d ≠ N K F s f x x´ B C Force Displacement Slope = - K Slip  s x´ x f Quasistatic Stability Criterion K< K c ; Unstable, stick-slip K > K c ; Stable sliding  n  s -  d  L K c = Plausible Mechanisms for Instability

 V 1 = e V o a b DcDc Slip rate Rate and State Dependent Friction Law Velocity Weakening b-a >0 Slip VoVo Quasistatic Stability Criterion K < K c ; Unstable, stick-slip K > K c ; Stable sliding  n (    ) DcDc K c = B C Force Displacement Slope = - K Slip  s x´ x f N K F s f x Plausible Mechanisms for Instability Laboratory Studies

Stick-Slip Instability Requires Some Form of Weakening: Velocity Weakening, Slip Weakening, Thermal/hydraulic Weakening Slip  s  d L Slip Weakening Friction Law (v)  d ≠  V 1 = e V o a b DcDc Slip rate Rate and State Dependent Friction Law Velocity Weakening b-a >0 Slip VoVo Stability Criterion K < K c ; Unstable, stick-slip K > K c ; Stable sliding  n  s -  d  L K c = Stability Criterion K < K c ; Unstable, stick-slip K > K c ; Stable sliding  n (    ) DcDc K c =

Frictional Instability Requires K < K c   n (a  b) DcDc Kc =Kc = (a-b) > 0 Always Stable, No Earthquake Nucleation, Dynamic Rupture Arrested (a-b) < 0 Conditionally Unstable, Earthquakes May Nucleate if K < K c, Dynamic Rupture Will Propagate Uninhibited Friction Laws and Their Application to Seismic Faulting a  b ( + ) (  ) Seismicity seismogenic zone Earthquake Stress Drop ( + ) (  )

Seismic Moment Released Continuously as the Event Ruptures to the Surface? Or Negative Stress Drop in the Upper Region with Resulting Postseismic Afterslip

Observations: Shallow Region is Poorly Consolidated Sediment. Shallow Region: Coseismic Slip Deficit Negative Dynamic Stress Drop Strong Correlation Between Region of Negative Stress Drop and Postseismic Afterslip 1979, M6.7 1 m Wald, m No Evidence of Buried Slip No Shallow Postseismic Afterslip

Observations: Shallow Region is Poorly Consolidated Sediment. Shallow Region: Coseismic Slip Deficit Negative Stress Drop 1979, M6.6 1 m Wald, 1996 a  b ( + ) (  ) Seismicity seismogenic zone Earthquake Stress Drop ( + ) (  )

Prism material is weak and therefore aseismic? Prism material is aseismic and therefore weak? Strength of the Subduction Fault Zone   n (a  b) DcDc Kc =Kc = Fault Strength and Frictional Stability Are Independent Unstable Behavior Requires That the Local Stiffness, K, be less than Kc

Strong Material, Stable (aseismic) Deformation Weak Material, Unstable (seismic) Deformation Laboratory Measurements of Frictional Strength (Granular Gouge)

Frictional Strength Does Not Dictate Deformation Stability   n (a  b) DcDc Kc =Kc =

What controls the updip seismic limit and rupture extent for subduction zone earthquakes? Hypotheses for velocity weakening 1)Clay mineral transformation from smectite to illite structure Illite is strong and may exhibit velocity weakening at elevated temperature Smectite is weak and exhibits velocity strengthening under some conditions 2) Consolidation/lithification state of fault gouge and accretionary prism materials Poorly consolidated granular gouge exhibits velocity strengthening Lithified materials and highly localized shear exhibit velocity weakening

Saffer, D. M., and C. Marone, Comparison of Smectite and Illite Frictional Properties: Application to the Updip Limit of the Seismogenic Zone Along Subduction Megathrusts, Submitted to EPSL, July 2002 Marone, C., Saffer, D., Frye K. M., and S.Mazzoni, Laboratory results indicating intrinsically stable frictional behavior of illite clay, AGU ABST, F Direct comparison of frictional properties: 1) Illite-shale 2) Pure smectite 3) Smectite-quartz mixtures 4) Natural gouge: Nankai, San Gregorio Fault Clay Gouge Layer Displacement Transducer Aligned smectite grains 1 mm B R Laboratory Friction Experiments

Materials Clay Mineralogy Illite-shale: (Rochester shale) Total clay 68%, quartz 28%, plag 4% Clay: 87% illite, 13% kaolinite/dickite Smectite clay: (GSA Resources, Mg-smectite) 100% clay (pure montmorillonite with trace amounts of zeolite and volcanic glass) (XRD analyses from M. Underwood) Quartz powder: (US Silica, F-110) 99% SiO 2 Shale crushed, ground, sieved < 500 microns Uniform layers produced in a leveling jig Initial layer thickness measured on the bench and under applied normal load

Results: Stress-Strain Characteristics Failure Envelope Absolute Frictional Strength

Results: Velocity stepping. Measuring the velocity dependence of friction

Results: Velocity stepping Measuring the velocity dependence of friction Illite-shale exhibits steady-state velocity strengthening: (a-b) > 0 Frictional Instability Requires K < K c    n (a  b) DcDc Kc =Kc =

Constitutive Modelling Rate and State Friction Law Elastic Interaction, Testing Apparatus Results: Velocity stepping Measuring the velocity dependence of friction

Constitutive Modelling Rate and State Friction Law Elastic Interaction, Testing Apparatus Results: Velocity stepping Measuring the velocity dependence of friction

Comparison of Smectite and Illite Frictional Properties Smectite exhibits both velocity weakening and velocity strengthening Illite exhibits only velocity strengthening

Normal stress dependence of the friction rate parameter for smectite and illite-shale Smectite exhibits velocity weakening at low normal stress and velocity strengthening at higher normal stress (for v < 20 micron/s) Illite exhibits velocity strengthening for all normal stresses and velocities studied (Saffer, Frye, Marone, and Mair, GRL 2001) (Saffer and Marone, 2002)

What controls the updip seismic limit and rupture extent for subduction zone earthquakes? Hypotheses for velocity weakening 1) Clay mineral transformation from smectite to illite structure Illite is strong and may exhibit velocity weakening at elevated temperature Smectite is weak and exhibits velocity strengthening under some conditions 2) Consolidation/lithification state of fault gouge and accretionary prism materials Poorly consolidated granular gouge exhibits velocity strengthening Lithified materials and highly localized shear exhibit velocity weakening

a  500m Consolidation, Comminution, and Fabric Development in Granular Gouge

500  m a  500m

500  m 1 mm a  500m Fracture and Consolidation (Rate Strengthening Processes) Adhesive Friction at Contact Junctions (Potentially Rate Weakening)

Frye and Marone, JGR 2002 Water Weakening at Adhesive Contact Junctions Highly Consolidated Gouge Hydrolytic Weakening causes enhanced rate of strengthening, but base level frictional strength is unchanged

Frictional Character Dominated by Adhesion at Contact Junctions Highly Consolidated Gouge Frye and Marone, JGR 2002

Marone, Raleigh, and Scholz, JGR, 1990 Effect of Consolidation/Lithification on Frictional Properties Highly Consolidated Granular Gouge Exhibits Velocity Weakening Frictional Behavior

What Causes the Updip Transition from Stable to Unstable Frictional Regimes? 1) Clay mineral transformation from smectite to illite structure Illite is strong and may exhibit velocity weakening at elevated temperature Smectite is weak and exhibits velocity strengthening under some conditions 2) Consolidation/lithification state of fault gouge and accretionary prism materials Poorly consolidated granular gouge exhibits velocity strengthening Lithified materials and highly localized shear exhibit velocity weakening Seismicity

a  b (  ) ( + ) Seismicity Field Observations Effect of Clay Mineralogy Smectite Illite Summary of laboratory data related to the updip seismic limit These data, collected at room temperature, indicate that Illite-rich shales and mudstones are unlikely to host earthquake nucleation

Quartz Gouge, Effect of Shear Strain and Consolidation a  b (  ) ( + ) Seismicity Field Observations These data, collected at room temperature, are consistent with an upper stability transition and shallow aseismic fault behavior

Summary of laboratory data related to the updip seismic limit Fluids: We performed experiments dry and found dilatant porosity changes. Pore pressure and the presence of fluids in our experiments would tend to increase (a-b) and further stabilize frictional shear. Fault Stability: At present our data imply that Illite-rich shales and mudstones are unlikely to host earthquake nucleation We have compared the frictional behavior of smectite-clay and illite- shale under identical conditions. Illite Intrinsically-stable velocity strengthening frictional behavior for all normal stresses and velocities studied Smectite: for v < 20 mm/s: Velocity weakening at low normal stress and velocity strengthening for normal stresses above 50 MPa for v > 20 mm/s: Velocity velocity strengthening

Extend experiments to higher temperature Include controlled pore- pressure Investigate the effects of gouge consolidation Study natural samples Study the smectite-illite transformation in-situ What is the nature of the fault zone at depth? Materials, fluid conditions, fault structure? Future Work

(a-b) > 0 Always Stable, No Earthquake Nucleation, Dynamic Rupture Arrested (a-b) < 0 Conditionally Unstable, Earthquakes May Nucleate if K < K c, Dynamic Rupture Will Propagate Uninhibited Summary of laboratory and field observations related to the updip stability transition

Key Observations, Outstanding Questions Aseismic slip Slow earthquakes, Creep events, Tsunamogenic earthquakes Slow precursors to “normal” earthquakes Earthquakes with a distinct nucleation phase Afterslip and transient postseismic deformation Normal (fast) earthquakes Seismic and Aseismic Faulting: End Members of a Continuous Spectrum of Behaviors What causes this range of behaviors? One (earthquake) mechanism, or several? How best do we describe the rheology of brittle fault zones?