Heating and weakening of faults during earthquake slip J. R. Rice, JGR, 2006

1. Readings in Geophysics – Ge 169/277 Vito Rubino 6th March 2012 • Heating and weakening of faults during earthquake slip • J. R. Rice, JGR, 2006 • Thermal decomposition of carbonates • J. Sulem and V. Famin, JGR, 2009

2. Heating and weakening of faults during earthquake slip J. R. Rice, JGR, 2006

3. Heating and weakening of faults during earthquake slip • Earthquakes occur because fault strength weakens with increasing slip or slip rate • What physical processes determine how weakening occurs? • Relevant weakening processesin large crustal events are suggested to be thermal and to involve: • thermal pressurization of pore fluid • (at highly stressed frictional microcontacts • during rapid slip) • flash heating (at highly stressed • frictional microcontacts during rapid slip) • Macroscopic melting, or possibly gel formation in silica-rich lithologies, may become important at large slip. reduces effective normal stress and shear strength for a given friction coefficient reduces friction coefficient • Strength drop should often be nearly complete at large slip • Onset of melting should be precluted over much (and, for small enough slip, all) of the seismogenic zone Predictions implied by by these thermal mechanisms How to verify predictions Shear fracture energies Compare with seismic data of crustal events

4. Extreme localization of slip on mature faults (Punchbowl Fault) [Chester and Chester, 1998; Chester et al., 2003, 2004; Chester and Goldsby, 2003] 1 mm 5 mm

5. Field studies that support above morphology • Field observations of mature crustal faults suggest that slip in individual events occurs primarily within a thin shear zone < 1-5 mm, within a finely granulated, ultracataclastic fault core. • Evidence for such morphology emerged from studied of exhumed and now inactive faults

6. Thermal pressurization of pore fluid • This mechanisms assumes that fluids (water, typically) are present within the fault gouge which shears. • Shear strengtht during seismic slip can still be represented by the classical effective law: • Frictional heating would cause the fluid (if it was unconstrained rather then caged by the densely packed solid particles) to expand in volume much more than the solid cage • A pressure increase must be induced in the pore fluid during slip • Unless: • Shear-induced dilatancy of the gouge cage overwhelms the thermal expansion effects • Gouge is highly permeable Normal stress Pressure Strength t is reduced, ultimately, towards zero as shear heating continues to raise temperature so that p approaches sn. Since sn can typically be assumed to remain constant during slip

7. [Slide modified from J. Rice, Caltech GALCIT-SeismoLab seminar, 29 March 2007] Specific heat per unit volume of fault gouge Current mass of pore fluid Fluid mass flux First law of thermodynamics Fourier’s law Work of shear stress Darcy’s law Energy flux k - permeabilty hf - fluid viscosity (Segall and Rice, 1995, 2004) Assume constant friction coefficient It describes pore pressure evolution Hydraulic diffusivity Thermal diffusivity Pore pressure change per unit temperature change Increments dp in pore pressure can be related to increments dm in fluid mass and increments dT in temperature

8. Predictions of shear strength vs. slip f= 0.25 based on flash heating V = 1 m/s • Parameter L* includes the dependence of scaled friction strength on: • porothermoelastic properties of the solid and fluid • slip rate and friction Scaled friction strength Parameter L* has the unit of length diffusivity divided by slip rate Slip • Continued weakening over a very broad range of of slip d (scaled by L*) [Slide modified from J. Rice, Caltech GALCIT-SeismoLab seminar, 29 March 2007]

9. [Slide from J. Rice, Caltech GALCIT-SeismoLab seminar, 29 March 2007]

10. [Slide modified from J. Rice, Caltech GALCIT-SeismoLab seminar, 29 March 2007] • According to the predictions, G increases substantially with slip during an event • The average G in an event (as inferred from seismic slip inversions) also shows a clear tendency to increase with the average slip in the event (Abercombie and Rice, 2005; Rice et al., 2005; Tinti et al., 2005).

11. [Slide from J. Rice, Caltech GALCIT-SeismoLab seminar, 29 March 2007]

12. Other weakening mechanisms • Macroscopic melting (i.e., when a coherent melt layer has formed along the whole sliding surface) may be relevant for sufficiently large combination of slip and initial effective normal stress. • Gel formation. This mechanism has been identified with silica-rich lithologies, when: • Thermal pressurization caused by rapid emission of CO2 during the decarbonation. Melts, if sufficiently hot, have a low viscosity and may lubricate faults reducing dynamic friction (Sibson, 1975; Spray, 1993; Brodsky and Kanamori, 2001) Sliding produces fine wear debris in presence of water (Goldsby and Tullis, 2002; Di Toro et al., 2004) (J. Sulem and V. Famin, JGR, 2009)

13. Thermal decomposition of carbonates in fault zones: Slip-weakening and temperature-limiting effects J. Sulem and V. Famin, JGR, 2009

14. Thermal decomposition of carbonates in fault zones: Slip-weakening and temperature-limiting effects Coseismic temperature increase to less ≈ 800 °C 1) Endothermic reaction of calcite decomposition Limits Decarbonation has two critical consequences on seismic slip Numerical simulations of seismic slip at depths of 5-8 km Slip-weakening effect of thermal pressurization 2) Rapid emission of CO2 by decarbonation Increases Chemical decomposition of calcite Decarbonation CaCO3 CaO + CO2

15. Why studying thermal decomposition of carbonates? Field observations (Pizzino et al., 2004;Lewicki and Brantley, 2000; Lewicki et al., 2003; Famin et al., 2008; Italiano et al., 2008; Hirono et al., 2006, 2007) Additional mechanism to be investigated among possible fault weakening processes. Production of CO2 attested important Experimental observations (Han et al., 2007)

16. Evidence of CO2 release in several active crustal faults • Corinth rift, Greece. Chemical analyses of water springs near the seismogenicHeliki and Aegion faults revealed an anomalously high content of dissolved CO2 compared with the regional values (Pizzino et al., 2004) • San Andreas Fault. The surface trace of the SAF displays a positive anomaly of CO2 fluxes (Lewicki and Brantley, 2000) and this CO2 comes from a shallow source, not from the mantle (Lewicki et al., 2003). • Nojima Fault, Japan. Kobe earthquake, 1995. A microinfrared analysis of exhumed pseudotachylites (i.e. friction induced melts produced by seismic slip) from the Nojima fault revealed a carbon supersaturation in the melts, and the quantity of CO2 released by friction melting during the 1995 Kobe earthquake was evaluated to 1.8 to 3.4 103 tons (Famin et al., 2008). • Central Apennines. Italiano et al., 2008 reported enhanced fluxes of crustal CO2 (i.e. not mantellic) during the 1997-1998 seismic crisis of major faults, and proposed that coseismicdecarbonation was responsible for the CO2 emission. • Chelungpu Fault, Taiwan. ChiChi earthquake. The fault core was depleted in carbon relative to the damage zone, and the deplation was attributed to a decarbonation induced by frictional heat (Hirono et al., 2006, 2007) • High velocity friction experiments on Carrara marble have shown that thermal decomposition of calcite due to frictional heating induces a pronounced fault weakening (Han et al., 2007).

17. Evidence of CO2 release in several active crustal faults Field observations Experimental observations

18. Deforming shear band with heat and fluid fluxes Governing equations Mass Balance Energy Balance

19. Evolution of temperature, pore pressure and shear stress Temperature increase drastically slowed due to the energy consumed in the endothermic chemical reaction Decomposition of carbonate rock begins at ~ 700 °C 2 1 fault strengthening fault weakening Two competing effects act on the evolution of the pore pressure: CO2 production induces an additional fluid mass and thus a pressurization of the pore fluid Increase of porosity due to the solid decomposition induces an increase of the permeability of the medium which limits the pressurization Also evolution of T and p with slip, if considering constant slip rate V = 1 m/s (slip is: d= V t). Shear stress decreases rapidly during initial pressurization and then increases again Mineral decomposition of the rock is a mechanism of fault weakening in a first stage and of fault strengthening in a second stage.

20. Correction to J. Sulem & V. Famin, JGR, 2009 Account for mass of CaO produced in the chemical reaction of calcite decomposition (decarbonation) in the mass balance of the solid phase

21. Influence of some parameters • Shear band thickness • Friction coefficient • Slip velocity • Initial permeability • Initial state of stress and initial temperature

22. How much can these models explain ? Rice, JGR, 2006 • In absence of a strong weakening mechanism, temperature rise would lead to widespread melting, yet evidence of glass (pseudotachylyte) that would be left from rapid recooling is not pervasive on most exhumed faults. • Relevant weakening processes in large crustal events are therefore likely to be thermal.Two possible weakening processes are: • 1) thermal pressurization of pore fluid • 2) flash heating at highly stressed frictional microcontacts • For sufficiently large slip, a macroscopic melt layer may form too at high enough normal stress • In silica-rich lithologies, weakening by gel formation may occur instead. Sulem & Famin, JGR, 2006 • Decarbonation is a source of CO2 that significantly increases the slip-weakening effect of thermal pressurization • An important result of this model is that the endothermic reaction of calcite decomposition limits the coseismic temperature increase to less than ~ 800°C within a carbonate shear band under rapid slip • Large earthquakes are more prone to temperature limitation • This model provides another explanation to the anomalous absence of positive heat flow on active crustal faults, such as San Andreas: a large part of the heat produced by friction would be consumed by endothermic reactions • Friction melting hampered by endothermic calcite decomposition in carbonate fault rocks (and probably in other faults containing a sufficiently high proportion of volatile-rich secondary minerals) • This is consistent with relative scarcity of pseudotachylytes in mature faults such as the Punchbowl fault, and their occurrence in less evolved faults such as Nojima.