Numerical Simulation of Fluid Overpressure Driven Faulting and Seismicity In Low Porosity Rocks

Abstract

Pore fluid overpressures in active fault systems can drive fluid flow and cause fault weakening and seismicity. In return, deformation accommodated by different mode of failure (e.g. brittle vs. ductile) also affects fault zone permeability and, hence, fluid flow and pore fluid pressure distribution. The resulting non-linear, complex feedback between fluid flow, fluid pressure and fault deformation controls the length of the nucleation phase of an earthquake and the duration of the interseismic period.
In this thesis we: 1) model overpressured, supercritical CO 2 fluid flow in natural, exhumed faults in evaporite sequences, which represent an analogue of the seismic sources at hypocentre depth of recent seismic events in the Northern Apennines of Italy (e.g. M w 6.0 1997-98 Colfiorito and M w 6.5 2016 Norcia earthquakes). 2) perform parameter studies on pore pressure diffusion and
earthquake nucleation, with realistic models of ductile failure, varying the dimension of components of fault zone architecture and neighbouring lithology, outer fault core width and the height of pressurised layers abutting the fault core.
Our results show that: 1) the duration of the nucleation phase is significantly reduced, from a few years to a few months, when realistic models of fault zone architecture and pore pressure- and deformation-dependent permeability are considered. We implement a four-component model of fault zone architecture in simulations (damage zone, outer fault core, inner fault core and primary slip zone) in contrast to the one- or two-component models of fault zone architecture previously considered. 2) For a given tectonic loading rate, a thinner fault core results in a more effective fault weakening. The impact of fluid flow on the fault being more significant for faults with a thinner rather than thicker outer fault core.
In the absence of fluids, the base mechanical strength of the slipping portion of the fault did not vary with thickness. Similarly, an increasing the thickness of an
overpressured aquifer intersecting a fault in the damage zone produces a higher magnitude of pore pressure in the fault core, which weakens the principal slip zone. Understanding the controls exerted on the duration of the nucleation phase of earthquakes has important implications for premonitory signal detection, as
identifying extended nucleation phases of active faults would increase the likelihood of detection of early seismicity warnings.