From the trench to the seismogenic zone: Establishing links between low-T metamorphism, fluid pressure, and fault stabilityDemian Saffer, Penn State Univ.SEIZE-Subfac workshopHeredia, Costa RicaJune, 2007

Outline1. 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 ZoneThe seismogenic zone is defined by the transitions from stable to unstable frictional deformationaseismicaseismic5105101500Parkfield, CA Seismicity20%seismogeniczoneGoal: 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.

SW Nankai Subduction Zoneaseismicaseismic5105101500Parkfield, CA Seismicity20%seismogeniczoneDoes 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?

Simple Spring-Slider System: Force BalanceFspring = K * x Ffriction = Fnormal * mafter Scholz (2003)

Simple Spring-Slider System: Force BalanceIf frictional resistance decreases more rapidly than force from spring, forces are not balanced (block can accelerate):dFspring/dx = -KdFfriction/dx = (dm/dx)*Fnormal

after Scholz (2003)

Coefficient of FrictionSlipLoad PointVelocity (m/s)100p690, p6923010sn(MPa)5100200 m(a-b) = D/Dln(V)0.01SAFOD B1SAFOD B21(a)Slip weakening behavior:measured by velocity-stepping during shearing experiments(a-b) < 0(a-b) > 0Marone (2006)Dc

Translated to Rate-and-State Friction Framework:Change in frictional resistance with slip given by:Unstable if:

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 pressureAfter Moore & Saffer (2001)

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

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

Does clay transformation cause a transition to unstable slip? Isolating effect of mineralogy(Saffer, Frye, Marone, and Mair, GRL 2001)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 studiedSaffer & Marone (2003)

What about cementation and fabric that accompany low-T diagenesis & clay transformation?25% illite in mixed-layer claysestimated T = ~75 C61% illite in mixed-layer claysestimated T = ~140 CTriaxial Experiments on intact wafers of sediment: preserve cement, fabric, and porosityMcKiernan, 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 stressSaffer, 2006 Vertical drainage of subducted section is a robust trend for several margins

Behavior can be approximated by a simple model of consolidation

Saffer (2007)

Lab data to constrain hydraulic diffusivitySaffer (2005)

060Distance from trench (km)1020304050updip extent of 1946 Nankaicoseismic ruptureupdip extent of Costa Rica micro-seismicitySaffer (2007)Drainage of the subducted section

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

00.20.40.60.8101020304050distance from trench (km)Underconsolidation Ratio (U)dcollement down-steppingBangs et al. (2004)

Effects of metamorphism on fluid pressure Fluid SourcesFluid Pressure

Illitization model calibration: Nankai ODP SitesHuang (1993) and Pytte & Reynolds (1988) kinetic expressions700600500400300200100060040020008001000120010008006004002000020406080020406080020406080% Illite in I/S mixed Layer ClaysDepth (mbsf)80811741173

distance landward from trench (km)Fluid Production (VH2O / Vsediment s-1)x 10-14012341020304050compaction-driven sourcesAshizuriMurotoupdip extent of 1946 coseismic ruptureApplication to Nankai Margin

Application to Nicoya Margin

after Spinelli et al., 2004Integrated 2-D Model for Nicoya Margin

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 depth0.1110100[B] (mMol)10203040d11B ()

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)?

ODP Drilling in Upper Aseismic Zone, with future Goals in the Seismogenic ZoneAreas of concentrated slip or asperitiesafter Bilek et al., 2004; Lay and Bilek (2007)Heterogeneity in the Seismogenic Zone

Drilling will sample the transitionafter Saffer & Marone (2003); Tobin (2007)

Does the proposed process actually occur where suspected?If so, does the proposed process have the hypothesized effect on sliding stability?Does the proposed process actually occur where suspected?If so, does the proposed process have the hypothesized effect on sliding stability?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.