CAUGHT IN THE ACT: EVIDENCE FOR SEISMIC ARREST ON NATURAL FRICTION-MELT-BEARING FAULTS?

During seismic slip in the earth, frictional work is converted primarily into heat, and may cause melting of the material surrounding the fault plane at depth. The generation of melt will either lubricate the fault by decreasing the dynamic stress relative to the static stress, or it will act as a viscous brake that may impede or arrest the propagating earthquake rupture. Footwall rocks beneath the Whipple detachment fault in eastern California provide an opportunity to examine and potentially quantify this process, as they preserve brittle and ductile features characteristic of both steady-state and transient conditions. We compare natural measurements of static shear stress (τ_s), from recrystallized grain size paleopiezometry on quartz-rich, brittle-to-ductile shear zones, to natural measurements of the strain-integrated dynamic stress (τ_f) on adjacent pseudotachylite-bearing faults with measurable displacements. We find that minimum estimates of τ_f (56 ± 7 MPa) are within error of estimates of τ_s (60 ± 8 MPa), suggesting that the formation of melt did not cause lubrication and may have caused seismic arrest. The dynamic shear stress measurements are significantly higher than other published natural observations of pseudotachylites, but may be explained by the high clast content present in these fault zones, which would have significantly increased the melt viscosity.