FAULT LUBRICATION AND EARTHQUAKE PROPAGATION IN CARBONATE ROCKS

Abstract

SummaryFriction experiments performed on dolomite/calcite at conditions typical during the propagation of large earthquakes (slip velocities v > 1 m/s and displacements d > 1 m) show that the frictional strength of experimental faults decays exponentially from peak values, in the Byerlees’ range (μ p ≈ 0.8), to extremely low steady-state values (μ ss ≈ 0.1), attained over a weakening distance Dw. The integration of CO2-emission data, recorded during the experiments, with microstructural observations shows that nanoparticles were produced in the slip zone due to cataclastic and thermally activated chemical/thermal processes (e.g. decarbonation reactions). Steady-state shear strength, soon after the onset of CO2 emissions during the transient stage, is reduced below best fit exponential law. During the transient stage of dynamic weakening, flash heating temperature rises, greater than the value necessary to activate the thermal decomposition of dolomite (T=550°C), have been locally reached at the highly stressed frictional microcontacts. Pressurized fluids (CO2), temporarily trapped within the slip zone, may also have contributed to dynamic weakening in accordance with the effective normal stress principle. Flash heating and thermal pressurization processes will be inhibited at the end of the transient stage as nanoparticles are produced and fluids can escape the slip zone. Steady-state dynamic weakening may be controlled by velocity-dependent frictional properties of nanoparticles. Seismic source parameters (e.g. slip weakening distance Dw and breakdown work Wb), calculated from experimental data, match those obtained by modelling of seismological data from earthquakes nucleated in the same carbonate rocks in Italy during the 1997 M=6 Colfiorito earthquakes. Wb scales with Dw according to a best fit power law, in a similar fashion to that inferred from existing seismological data sets.