Structural and frictional heterogeneities along natural faults and implications for fault slip behaviour

Abstract:

For years, there have been lines of evidence that inform the weak vs. strong fault debate. Strong faults, or fault segments, are mainly characterized by a granular load-bearing microstructure with a frictional strength controlled by the Byerlee’s rule of friction. With increasing strain, localization along a principal slip zone promotes the passage from rate strengthening to rate weakening behavior. Along other faults, or fault segments, fluid-assisted reaction softening favours the development of interconnected networks of phyllosilicates that promote a significant reduction in friction and a marked rate-strengthening behaviour. The documentation of both strong and weak faults at different crustal depths, in different tectonic regimes and from a great variety of rock types, point to the heterogeneous structural nature of crustal scale faults. In particular, a single crustal scale fault, tens of kilometers long, can be characterized by weak fault patches in zones where crustal fluids exerted a chemical role, facilitating the replacement of strong with weak mineral phases, and strong fault patches, where fluid-assisted reaction softening was not efficient, and crustal deformation was achieved predominantly by fragmentation, grain-size reduction and localization.

Along frictional heterogeneous fault, creep along weak and rate-strengthening fault patches can promote earthquake nucleation within adjacent strong and locked, rate-weakening portions. Some new frontiers on this research topic regard: 1) when and how a seismic rupture nucleating within a strong patch might propagate within a weak velocity strengthening fault portion, and 2) if creep and slow slip can be accurately detected within the earthquake preparatory phase and therefore represent a reliable earthquake precursor.