Ricerc@Sapienza

In this multimethodological study, microstructural observations of fault rocks are combined with
micromechanical property analyses (contact resonance atomic force microscopy (CR-AFM)) and with rotary
friction experiments (Slow- to High-Velocity rotary-shear friction Apparatus apparatus) to find evidence of
seismic to aseismic slip and understand the nanoscale rheology of clay-bearing, carbonate-hosted faults.
Fluidized structures, truncated clasts, pores and vesicles, and phyllosilicate nanosized spherules and tubes
suggest fast deformation events occurred during seismic slip, whereas clay-assisted pressure-solution
processes, clumped clasts, foliation surfaces, and mantled clasts indicate slow deformation events occurred
during postseismic/interseismic periods. CR-AFM measurements show that the occurrence of ~5 wt % of clay
within the carbonate-hosted gouges can significantly reduce the fault core stiffness at nanoscale. In addition,
during high-velocity friction experiments simulating seismic slip conditions, the presence of ultrathin
phyllosilicate-bearing (?3 wt %) layers within calcite gouges, as those observed in the natural fault, show
faster dynamic weakening than that of pure calcite gouges. The weak behavior of such layers could facilitate
the upward propagation of seismic slip during earthquakes, thus possibly enhancing surface faulting.
Microstructural observations and experimental evidence fit some well-known geophysical and geodetic
observations on the short- to long-term mechanical behavior of faults such as postseismic/interseismic
aseismic creep, interseismic fault locking, and seismic slip propagation up to the Earth’s surface.