From the trench to the seismogenic zone: Establishing links between low-T metamorphism, fluid pressure, and fault stability Demian Saffer, Penn State Univ. SEIZE-Subfac workshop Heredia, Costa Rica June, 2007

Outline 1. Overview: - Criteria for unstable slip - Roles of rock properties, fluid pressure 2. Low-T metamorphic processes - influence on frictional behavior - influence on fluid pressure - links to geochemical signals 3. Compaction- and dehydration-driven pore pressure: spatial distribution and links to the updip limit

SW Nankai Subduction Zone Parkfield, CA Seismicity 20% aseismic 5 5 seismogenic zone 10 10 15 aseismic The seismogenic zone is defined by the transitions from stable to unstable frictional deformation Does the proposed process actually occur where suspected? If so, does the proposed process have the hypothesized effect on sliding stability? Goal: Test qualitative interpretations and observations that provide only circumstantial evidence for processes that control the onset of unstable slip, through a combination of targeted experiments and modeling.

Does the process have the hypothesized effect on sliding stability? SW Nankai Subduction Zone Parkfield, CA Seismicity 20% aseismic 5 5 seismogenic zone 10 10 15 aseismic Does the process have the hypothesized effect on sliding stability? Does the process occur where proposed/suspected? Does the hypothesis successfully explain observations at other margins? Does the proposed process actually occur where suspected? If so, does the proposed process have the hypothesized effect on sliding stability?

Simple Spring-Slider System: Force Balance K F s f x x´ Ffriction = Fnormal * m Fspring = K * x after Scholz (2003)

Simple Spring-Slider System: Force Balance K F s f x x´ If frictional resistance decreases more rapidly than force from spring, forces are not balanced (block can accelerate): B C Force Displacement Slope = -K Slip m s x´ x f dFspring/dx = -K dFfriction/dx = (dm/dx)*Fnormal after Scholz (2003)

(a-b) < 0 (a-b) > 0 Slip weakening behavior: measured by velocity-stepping during shearing experiments (a) (a-b) = D µ/ D ln(V) SAFOD B1 s n (a-b) < 0 (MPa) 5 0.01 Coefficient of Friction 100 (a-b) > 0 SAFOD B21 Dc 10 30 100 Load Point Velocity (µm/s) Slip 200 µm p690, p692 Marone (2006)

Translated to Rate-and-State Friction Framework: Change in frictional resistance with slip given by: Dc sn’ (a-b) Unstable if: n‘ (a  b) Dc K <

Stability parameter: z = sn’ (a-b) Stability Criterion Stability parameter: z = sn’ (a-b) Rock properties: more negative (a-b) increases tendency for instability Pore pressure dissipation: Higher effective stress increases tendency for instability Thermal pressurization/melting can explain slip weakening, but not nucleation!

Hypotheses invoking low-T metamorphism and fluid pressure After Moore & Saffer (2001)

Conceptual model for Costa Rican Margin Von Huene et al. (2004)

Illite powder exhibits only velocity strengthening Does clay transformation cause a transition to unstable slip? Isolating effect of mineralogy Smectite powder exhibits both velocity weakening and velocity strengthening Illite powder exhibits only velocity strengthening

Does clay transformation cause a transition to unstable slip Does clay transformation cause a transition to unstable slip? Isolating effect of mineralogy 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 & Marone (2003)

25% illite in mixed-layer clays 61% illite in mixed-layer clays What about cementation and fabric that accompany low-T diagenesis & clay transformation? Triaxial Experiments on intact “wafers” of sediment: preserve cement, fabric, and porosity 25% illite in mixed-layer clays estimated T = ~75 C 61% illite in mixed-layer clays estimated T = ~140 C McKiernan, Saffer, & Lockner (2005)

Implications: Clay transformation and natural diagenetic processes in bulk sediment do not appear to cause a transition to negative (a-b)…based on experimental data to date. Therefore, consolidation, mineralization, and evolution of fault zones with high shear strain may be important in the onset of unstable slip (Moore et al., 2007; Ikari et al., 2007).

Role of tectonic loading, drainage, and fluid pressure on effective stress Vertical drainage of subducted section is a robust trend for several margins Behavior can be approximated by a simple model of consolidation Saffer (2007) Saffer, 2006

Lab data to constrain hydraulic diffusivity Saffer (2005)

Drainage of the subducted section updip extent of Costa Rica micro-seismicity 10 20 30 40 50 60 Distance from trench (km) updip extent of 1946 Nankai coseismic rupture Saffer (2007)

Correlation with fault behavior: Nankai Bangs et al. (2004)

distance from trench (km) 50 40 30 20 10 1 Cv = 1x10 m s 1 Cv = 1x10 -10 m 2 s -1 0.8 Cv = 1x10 -9 m 2 s -1 0.6 Underconsolidation Ratio (U) 0.4 Cv = 3x10 -9 m 2 s -1 Cv = 1x10 -8 m 2 s -1 0.2 décollement down-stepping Bangs et al. (2004)

Effects of metamorphism on fluid pressure Thermal Model Compaction Dehydration Bound. Conds Fluid Sources Fluid Pressure Permeabilities

Illitization model calibration: Nankai ODP Sites Huang (1993) and Pytte & Reynolds (1988) kinetic expressions 808 1174 1173 -1200 -1000 -800 -600 -400 -200 20 40 60 80 100 -1000 -800 -600 -400 -200 20 40 60 80 100 -700 -600 -500 -400 -300 -200 -100 20 40 60 80 100 100 200 200 200 400 400 300 Depth (mbsf) 600 600 400 800 800 500 1000 600 1000 1200 700 20 40 60 80 20 40 60 80 20 40 60 80 % Illite in I/S mixed Layer Clays

Application to Nankai Margin x 10-14 4 updip extent of 1946 coseismic rupture 3 compaction-driven sources Fluid Production (VH2O / Vsediment s-1) 2 Muroto Ashizuri 1 10 20 30 40 50 distance landward from trench (km)

Application to Nicoya Margin

Integrated 2-D Model for Nicoya Margin 0.2 0.4 0.6 0.8 1.0 20 40 60 80 distance from trench (km) 10 30 depth (km) l* along decollement Modeled Pore Pressure Interplate Earthquakes after Spinelli et al., 2004

Modeled heating and metamorphism: Also provides insight into source regions for geochemical signals This is a key constraint for hydrologic models used to estimate pore pressure at depth 100 10 [B] (mMol) 1 0.1 40 30 d11B (‰) 20 10

Implications: Simulated dissipation of fluid sources and pressure correlate broadly with updip limit Negative velocity-dependence is required for unstable slip, but causes remain unknown Consolidation and metamorphism intertwined as potential causes for both transition to negative (a-b) and increasing sn’.

Where to go from here? Causes for transition to velocity- weakening behavior: experiments on appropriate natural samples Permeability measurements for forward models of pore pressure Marriage of hydrologic models with geochemical data to better constrain hydraulic architecture, pressures at depth

Key Unknowns: Role of seamounts? (hydraulic architecture, distribution of incoming sediment, bsmt alteration) Role of fluids at greater depths (>> 150 C)?

Heterogeneity in the Seismogenic Zone ODP Drilling in Upper Aseismic Zone, with future Goals in the Seismogenic Zone Areas of concentrated slip or “asperities” Seismogenic Zone: Were slip occurs during the earthquake. Note that this slip may be patchy or concentrated in “asperities”. Deal with the mechanical definition of the seismogenic zone later. after Bilek et al., 2004; Lay and Bilek (2007)

Drilling will sample the transition Stable Slide Region Stick-slip Region ??? after Saffer & Marone (2003); Tobin (2007)