Strong weakening of carbonate fault gouge at high slip velocity
This protocol is extracted from research article:
The role of aseismic slip in hydraulic fracturing–induced seismicity
Sci Adv, Aug 28, 2019; DOI: 10.1126/sciadv.aav7172

When modeling seismic slip, we recognized the propensity of calcite-rich gouges for flash heating–induced thermal decomposition (36, 37, 61), specifically decarbonation (CO2 release) and deposition of amorphous carbon (nano-lubrication), resulting in a dramatic drop of the steady-state friction (<0.1), with the slip velocity approaching seismic values. The rate-state frictional framework can accommodate the entire spectrum of frictional responses, from low to high slip velocity, when the steady-state friction expression is amended, e.g., asfss(V)=fw+(fLV(V)fw) (1+(V/Vw)m)1/m(4)where fw is the flash-heated (minimum) steady-state value of the friction coefficient and Vw is a characteristic weakening velocity value, such that the steady-state friction dependence on the velocity reduces to the end-members fLV(V) and fw when VVw and VVw, respectively. Exponent m ≥ 1 in Eq. 4 is a fitting parameter, with larger values corresponding to sharper transition between the low-velocity lnV and high-velocity ~1/V dependence of the friction on V [the limit of m = ∞ corresponds to the abrupt transition between the two asymptotic behaviors (62)].

We fit the friction versus slip rate data from the laboratory slip on a marble saw cut (37) with Eq. 4 to infer fw = 0.07, m = 6, and Vw,lab = 0.28 m/s (fig. S6, dashed line), while using the low-velocity friction fLV(V) (Eq. 3) characterized by fo = 0.6, Vo = 1 μm/s, a = 0.02, and a positive value for the velocity dependence coefficient (ab)lab = 0.0025 for calcite at room temperature (59) (as opposed to ab = −0.0025 in situ). The flash heating theory (38) predicts that Vw scales with the square of the difference between the weakening temperature Tw for the onset of thermal decomposition and ambient temperature T. This allows the in situ value of the weakening velocity to be inferred from the laboratory value, Vw = Vw,lab (TwT)2/(TwTlab)2 = 0.2 m/s, when Tlab = 20°C, T = 110°C (in situ temperature), and Tw = 600°C [temperature at the onset of decarbonation (37)] are used. The corresponding dependence of the steady-state friction of calcite on the slip velocity under the in situ conditions is shown in fig. S6.

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