The Seismogenic Zone Experiment Revisited

1. Fault Friction and the Transition From Seismic to Aseismic FaultingChris Marone1 and Demian M. Saffer21Penn. State University2University of Wyoming RESIZE Riser drilling of the seaward limit of the seismogenic zone Characterizing the incoming material by non-riser drilling Seismogenic zone Updip limit Quantify lateral changes In the physical, chemical, and hydrogeologic properties of the fault Image the seismogenic zone using earthquakes and artificial sources The Seismogenic Zone Experiment Revisited SEIZER MARGINS Theoretical Institute The Seismogenic Zone Revisited

2. Fault Friction and the Transition From Seismic to Aseismic FaultingChris Marone1 and Demian M. Saffer21Penn. State University2University of Wyoming • : 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? Riser drilling of the seaward limit of the seismogenic zone Characterizing the incoming material by non-riser drilling Seismogenic zone Updip limit Quantify lateral changes in the physical, chemical, and hydrogeologic properties of the fault Image the seismogenic zone using earthquakes and artificial sources The Seismogenic Zone Experiment Revisited

3. : 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 • 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)?

4. JOI –USSSP, ODP-Japan K. M. Frye, S. Mazzoni, K. Mair 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. 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, 2297-2300,2001. Marone, C., Saffer, D., Frye K. M., and S.Mazzoni, Laboratory results indicating intrinsically stable frictional behavior of illite clay, AGU ABST, F 2001. Marone, C., Saffer, D., and K. M. Frye, Weak and Potentially Unstable Frictional Behavior of Smectite Clay, AGU ABST, F689, 1999.

5. SW Nankai Subduction Zone Parkfield, CA Seismicity 20% 0 0 5 5 Depth Below Sea Floor (km) 10 10 15 Marone & Scholz, 1988

6. Aseismic Seismogenic Aseismic The seismogenic zone is defined by the transitions from stable to unstable frictional deformation • Key Questions about Fault Zone Friction • Stability: Why is deformation stable in some cases and unstable in others? SW Nankai Subduction Zone Parkfield, CA Seismicity 20% 0 0 5 5 10 10 15

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

8. N x x´ 1-D fault zone analog, Stiffness K K F s f • Brittle Friction Mechanics • Stable versus Unstable Shear Parkfield, CA Seismicity Aseismic Seismogenic zone Aseismic Frictional stability is determined by the combination of1) fault zone frictional properties and 2) elastic properties of the surrounding material Slope = -K B m s f Force x´ x C Slip Displacement

9. N x x´ 1-D fault zone analog, Stiffness KMassless K F s f • Brittle Friction Mechanics • Stable versus Unstable Shear Parkfield, CA Seismicity aseismic seismogenic zone aseismic Stability transitions represent changes in frictional properties with depth Frictional stability is determined by the combination of1) fault zone frictional properties and 2) elastic properties of the surrounding material Slope = -K B m s f Force x´ x C Slip Displacement

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

11. Rate and State Dependent Friction Law Vo V1 = e Vo Slip rate N m x x´ K F Velocity Weakening a b s f b-a >0 Dc Slip Slope = -K Quasistatic Stability Criterion B m s n (  ) Kc = Dc f K < Kc; Unstable, stick-slip K > Kc; Stable sliding Force x´ x C Slip Displacement Laboratory Studies Plausible Mechanisms for Instability

12. Stick-Slip Instability Requires Some Form of Weakening: Velocity Weakening, Slip Weakening, Thermal/hydraulic Weakening Rate and State Dependent Friction Law m Slip Weakening Friction Law s Vo V1 = e Vo Slip rate m m d Velocity Weakening a b m (v) ≠ d L b-a >0 Dc Slip Slip Stability Criterion Stability Criterion nms-md Kc = n (  ) Kc = L Dc K < Kc; Unstable, stick-slip K > Kc; Stable sliding K < Kc; Unstable, stick-slip K > Kc; Stable sliding

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

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

15. 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 6 m No Evidence of Buried Slip No Shallow Postseismic Afterslip Wald, 1996 1 m Wald, 1996

16. 1979, M6.6 a  b Earthquake Stress Drop Seismicity ( - ) ( + ) ( - ) ( + ) seismogenic zone Observations: Shallow Region is Poorly Consolidated Sediment. Shallow Region:Coseismic Slip Deficit Negative Stress Drop 1 m Wald, 1996

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

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

19. n (a  b) Kc= Dc • Frictional Strength Does Not Dictate Deformation Stability

20. What controls the updip seismic limit and rupture extent for subduction zone earthquakes? • Hypotheses for velocity weakening • 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

21. Laboratory Friction Experiments Displacement Clay Gouge Layer • 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 2001. • Direct comparison of frictional properties: • 1) Illite-shale • 2) Pure smectite • 3) Smectite-quartz mixtures • 4) Natural gouge: Nankai, San Gregorio Fault Transducer Aligned smectite grains R B 1 mm

22. 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% SiO2 • 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

23. Results: Stress-Strain Characteristics Failure Envelope Absolute Frictional Strength

24. Results: Velocity stepping. Measuring the velocity dependence of friction

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

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

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

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

29. 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)

30. What controls the updip seismic limit and rupture extent for subduction zone earthquakes? • Hypotheses for velocity weakening • 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

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

32. a 500 m m 5 0 0 m m

33. a 500 m m Fracture and Consolidation (Rate Strengthening Processes) Adhesive Friction at Contact Junctions (Potentially Rate Weakening) 5 0 0 m m 1 mm

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

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

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

37. Seismicity • What Causes the Updip Transition from Stable to Unstable Frictional Regimes? • 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

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

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

40. 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 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

41. What is the nature of the fault zone at depth? Materials, fluid conditions, fault structure? Future Work • 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

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

43. 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 Key Observations, Outstanding Questions 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?