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

• Thermal decomposition of carbonates J. Sulem and V. Famin, JGR, 2009

Readings in Geophysics – Ge 169/277

Vito Rubino 6th March 2012

Heating and weakening of faults during earthquake slip

J. R. Rice, JGR, 2006

• Earthquakes occur because fault strength weakens with increasing slip or slip rate

• What physical processes determine how weakening occurs?

• Relevant weakening processes in large crustal events are suggested to be thermal and to involve:

1) thermal pressurization of pore fluid (at highly stressed frictional microcontacts during rapid slip) 2) 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.

Heating and weakening of faults during earthquake slip

reduces effective normal stress and shear strength for a given friction coefficient

€

τ = f σ n − p( )

reduces friction coefficient

Predictions• 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

How to verify predictions

Shear fracture energiesimplied by by these thermal mechanisms

with seismic data of crustal events

Compare

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

5 mm

1 mm

• 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

Fault Location Papers

North Branch San GabrielSan Andreas systemSouthern California

Chester et al., 1993

Punchbowl Chester and Chester, 1998; Chester 2004

Median Tectonic Line Japan Wibberly and Shimamoto, 2003

Hanaore Japan Noda and Shimamoto, 2005

Nojima Japan Lockner et al., 2000

Field studies that support above morphology

• This mechanisms assumes that fluids (water, typically) are present within the fault gouge which shears.

• Shear strength t 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

Thermal pressurization of pore fluid

€

τ = f σ n − p( )

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

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

Fourier’s law

Darcy’s law

Energyflux

Current mass of pore fluid

Fluid massflux

Work of shear stress

Specific heatper unit volume of fault gouge First law of thermodynamics

Pore pressure change per unit temperature change

(Segall and Rice, 1995, 2004)

Hydraulic diffusivity

Thermal diffusivity

It describes pore pressure evolution

k - permeabiltyhf - fluid viscosity

Increments dp in pore pressure can be related to increments dm in fluid mass and increments dT in temperature

Assume constant friction coefficient

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

f = 0.25 based on flash heatingV = 1 m/s

Parameter L* has the unit of length diffusivity divided by slip rate

Parameter L* includes the dependence of scaled friction strength on:• porothermoelastic properties of the solid and fluid • slip rate and friction

Scaled friction strength

Predictions of shear strength vs. slip

• Continued weakening over a very broad range of of slip (d scaled by L*)

Slip

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

[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).

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

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)

Thermal decomposition of carbonates in fault zones: Slip-weakening and temperature-limiting effects

J. Sulem and V. Famin, JGR, 2009

Numerical simulations of seismic slip at depths of 5-8 km

Decarbonation has two critical consequences on seismic slip

1) Endothermic reaction of calcite decomposition

2) Rapid emission of CO2

by decarbonation

Coseismic temperature increase to less ≈ 800 °C

Slip-weakening effect of thermal pressurization

Limits

Increases

Decarbonation Chemical decomposition of calcite CaCO3 CaO + CO2

Thermal decomposition of carbonates in fault zones: Slip-weakening and temperature-limiting effects

Production of CO2

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 etal., 2008; Hirono et al., 2006, 2007)

Experimental observations(Han et al., 2007)

attested

Additional mechanism to be investigated among possible fault weakening processes.important

• Corinth rift, Greece. Chemical analyses of water springs near the seismogenic Heliki 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 coseismic decarbonation 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).

Evidence of CO2 release in several active crustal faults

Fault Location Reference

Heliki and Aegion Corinth rift, Greece Pizzino et al., 2004

San Andreas CaliforniaLewicki and Brantley,

2000; Lewicki et al., 2003)

Nojima Japan Famin et al., 2008

Central Appenienes Italy Italiano et al., 2008

Chelungpu Fault Taiwan Hirono et al., 2006, 2007

High velocity friction experiments Carrara marble Han et al., 2007

Evidence of CO2 release in several active crustal faults

Field observations

Experimental observations

Deforming shear band with heat and fluid fluxes

Governing equations

Mass BalanceEnergy Balance

Evolution of temperature, pore pressure and shear stress

Also evolution of T and p with slip,if considering constant slip rate V = 1 m/s (slip is: d = V t).

Decomposition of carbonate rock begins at ~ 700 °C

Temperature increase drastically slowed due to the energy consumed in the endothermic chemical reaction

Two competing effects act on the evolution of the pore pressure:1) CO2 production induces an additional fluid mass

and thus a pressurization of the pore fluid2) Increase of porosity due to the solid

decomposition induces an increase of the permeability of the medium which limits the pressurization

1 2

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.

fault weakening

faultstrengthening

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

Influence of some parameters

• Shear band thickness

• Friction coefficient

• Slip velocity

• Initial permeability

• Initial state of stress and initial temperature

How much can these models explain ?

• 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.

• 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.

Rice, JGR, 2006

Sulem & Famin, JGR, 2006