FAULT ZONE STRUCTURE AND STRENGTH DURING COSEISMIC AND SLOW SLIP IN LABORATORY EXPERIMENTS

The most convincing field evidences for seismic slip on faults are thermal signatures, such as severe mineral alteration and pseudotachylites, both rare relative to typical fault structures. Though earthquake slip may be accompanied by significant heating, this is not a requirement, because coseismic shear resistance may be low in many cases. Generally, earthquakes occur because faults weaken with increasing slip and/or slip rate suggesting that slip is accompanied by shear localization. Laboratory tests demonstrate the association between slip and slip-rate weakening and localization. Whether extremely localized (sub-mm-thick) slip zones on faults are evidence for seismic slip is an unresolved question.

In an effort to determine associations between the mode of sliding, slip velocity and structure, it is instructive to compare microstructures of lab-scale faults driven to weaken at near-seismic slip rates with those for which slip was aseismic or slow. Relevant high-speed rock friction experiments include those in which thermal and non-thermal weakening, respectively, occur due to 1) flash heating or melting of short-lived, highly stressed, microscopic asperities, or 2) thixotropy of finely comminuted, wet, amorphous wear debris ('silica gel'). For both mechanisms, the friction coefficient extrapolates to 0.3 or less at a seismic rate of 1 m/s. Microstructures expected from 1) are highly deformed microscopic contacts (without melting), quenched melt films at contacts, and asperities amorphized in the solid state. Microstructures observed for 2) are microns-thin, fault-coating amorphous films, which shear viscously during, but become brittle after, rapid slip. Efforts to uniquely link such microstructures in nature to either of these dynamic weakening mechanisms will be complicated, since similar features arise due to plastic contact deformation and solid state amorphization during slow sliding in lab tests. Furthermore, natural coseismic textures may evolve, particularly amorphous materials, which may crystallize or dissolve at elevated temperatures and with fluids present. Thus, whether flash heating or gel microstructures are preserved or their altered character is recognizable, and whether they are unique signatures of coseismic slip, are questions requiring further investigation.