Microstructural evidence for seismic and aseismic slips along clay‐bearing, carbonate faults

• Published in: Journal of geophysical research. Solid earth Vol. 122; no. 5; pp. 3895 - 3915
• Main Authors: Smeraglia, Luca, Bettucci, Andrea, Billi, Andrea, Carminati, Eugenio, Cavallo, Andrea, Di Toro, Giulio, Natali, Marco, Passeri, Daniele, Rossi, Marco, Spagnuolo, Elena
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-05-2017
• Subjects: Atomic force microscopy; Bearing; Calcite; carbonate fault; Carbonates; Clay; Deformation; deformation mechanisms; Distance; Earth; Earth surface; earthquake; Earthquakes; Experiments; Fault lines; Faults; Friction; Geological faults; Geophysics; Locking; Mechanical properties; Microscopy; Microstructure; Pores; Porosity; Pressure; Propagation; Resonance; Rheology; Rock; Rocks; rotary experiments; Seismic activity; seismic cycle; Seismic engineering; Shear; Simulation; Solifluction; Spherules; Stiffness; Structures; Tubes; Velocity; Vesicles
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB014042

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.
Key Points
Fault microstructures record fast (seismic) to slow (aseismic) deformation stages consistent with some geophysical observations
A few % of clays in natural fault gouge may strongly affect frictional behavior
Shear experiments suggest that clay‐bearing layers have shorter weakening distance than carbonate‐pure gouges during seismic slip