Fault slip and permeability in carbonates
In recent years our understanding of the nature of fault slip has been revolutionized by rock-mechanical experiments on natural materials at very high slip rates. These experiments have shown that the friction coefficient at slip rates that are appropriate for seismogenic fault slip (metres per second) is much lower than the value that had long been assumed, based on measurements at slower slip rates (microns per second). Quantifying this 'shear weakening' is crucial if we are to understand how a fault behaves during an earthquake.
The mechanism that causes shear weakening is not yet fully understood. Certain rocks, such as limestones and dolomites, heat up sufficiently during seismic slip to thermally-decompose, releasing CO2. If the gas cannot escape fast enough, it may act to lubricate the fault, causing shear weakening (see figure 1).
This project, funded through a NERC standard grant (NE/H021744/1), combines laboratory experiments, numerical simulations and field observations to quantify fault lubrication by thermal decomposition. The project currently funds two postdoctoral researchers (Igor Faoro and Eddie Dempsey) and we will employ a third postdoc to work on numerical modelling (see 'Opportunities' below). My role is to supervise the numerical modelling of gas flow through the evolving geometry of the fault zone during seismogenic slip. Permeability modelling will use a variety of simulation tools, including LBflow.
This work builds on earlier work, carried out with collaborators at Bristol University, on the development of permeability in carbonates.
A two-year postdoctoral position will be advertised here in the summer of 2011. The postdoc will use a variety of numerical modelling approaches to simulate the evolution of permeability in a fault zone during seismogenic slip; this plays a key role in determining the behaviour of the fault because gas liberated through thermal decomposition of limestone and dolomites during slip may lubricate the fault. The successful applicant will work under my supervision as part of the team detailed below.