Fault zone architecture, microstructures, deformation mechanisms and frictional behaviour of seismogenic, shallow-crustal, lithologically heterogeneous faults

Abstract

Earthquakes that rupture the Earth’s surface are typically the most damaging and highlight the need for us to better constrain the style of deformation and frictional behaviour of fault zones in the shallow crust. This thesis presents two studies of natural, seismogenic, shallow crustal fault zones: 1) the Gubbio fault zone, which has been exhumed from 2.5-3 km depth and deforms a mixture of limestone and phyllosilicate-rich marly limestone; and 2) the Masada fault zone, which deforms near-surface, poorly lithified lake sediments. Field studies were complemented by low- and high-velocity rotary shear experiments to constrain the frictional behaviours of the naturally observed fault gouges under representative conditions. In addition, microstructural analyses of both naturally- and experimentally-produced fault rocks were performed in order to constrain the deformation mechanisms operating during fault slip.
Our results show that the dominant deformation mechanisms operating within a fault zone, which are highly variable depending on environmental conditions such as depth, fault rock composition, fluid presence and composition, and strain-rate, will control: 1) fault zone architecture and therefore the distribution of seismicity; and 2) slip zone processes, which can subsequently affect the frictional behaviour of a fault, and also determine whether or not signatures of seismic slip are produced during rupture propagation. These are useful tools for geologists when trying to decipher the seismic history of natural faults. Frictional behaviour, in terms of the likelihood of rupture propagation through the shallow crust, is also found to vary significantly as a function of the aforementioned environmental conditions. A fuller knowledge of spatial, and possible temporal, variations in fault zone properties is therefore essential if more accurate earthquake forecasting models and assessments of their associated hazards are to be produced.