Ian Main FRSE
Professor, Seismology & Rock Physics

Overview of Research Interests

We live on a dynamic planet, whose upper layers respond in a brittle fashion to slow forcing from the underlying mantle. This takes the form of localised earthquake faulting on a large scale, and fracturing on a much smaller scale. Understanding the processes that lead to the mechanical, structural, hydraulic and chemical properties of rocks undergoing low-temperature deformation is a complex and fascinating business. The Earth is a complex material to start with, as a consequence of previous geological events. The acceleration from stable crack growth to unstable dynamic failure is extremely rapid, non-linear, and difficult to predict for individual events. There are strong feedbacks between the elements involved. Nevertheless, complex non-linear systems often 'self-organise' spontaneously to produce order and pattern in the population dynamics, for example the scale-invariant or fractal geometry of fault and earthquake populations. A quantitative understanding of such processes helps: to build our cities in a way that mitigates earthquake risk; to assess the predictability of volcanoes and earthquakes; to evaluate of the response of the Earth to anthropogenic activity such as hydrocarbon extraction; and to predict fluid flow and transport in oil reservoirs and groundwater aquifers.

I am interested in attacking these problems with a variety of analytical, numerical, laboratory, and field techniques, usually in collaboration with colleagues from a range of disciplines. The aim is to describe patterns of earthquakes and fractures, to work out the underlying mechanisms at work in their formation, and where possible to provide results in a form that can be used in a practical way.

1. Earthquake hazard. Earthquake hazard depends on the probability of occurrence of events, the nature and strength of the seismic source, and the propagation of the disturbance to the site of a particular building or facility. During my PhD I developed a method, based on statistical mechanics and information theory, to quantify earthquake hazard by synthesising short-term earthquake recurrence statistics with longer-term geological and tectonic information on deformation rates. This was one of the first attempts to pose a far-from-equilibrium problem such as earthquake occurrence in the framework of statistical mechanics. The results are consistent with current notions of earthquakes as self-organised critical phenomena. More recent work has concentrated on quantifying systematic and random uncertainties in time-independent seismic hazard analysis.

2. Earthquake precursors and predictability. The publically-perceived idea of earthquake "prediction" - i.e. "specifying in advance the precise location, size and time of occurrence of an individual event, above chance" (where every word counts!) - has long been a goal of Earth science. However, the extreme non-linearity of the underlying physics, the complexity of the Earth, and the lack of good data at large space and time scales make this problem extremely (perhaps inherently) intractable. My contribution to this field began with the development of analytical and numerical models, based on time-dependent fracture and damage mechanics, which predict the precise nature and form of seismic precursors observed under controlled conditions in laboratory rock samples. More recent work concentrates on examining the extent to which such behaviour scales in space and time, in particular the statistical evaluation of the degree of predictability in the population dynamics for natural events. At present the main applications are in assessing conditional probabilities for time-dependent seismic and volcanic hazard calculations, including using informatics-based web portals to test forecasting hypotheses in real time.

3. Scaling of faults and fractures. In order to underpin the models described above a direct investigation of the structure and scaling properties of fracture systems is necessary. In collaboration with others, I have shown that the observed fractal scaling of such patterns, where applicable, can be correlated with mechanical (e.g. the fracture toughness) and hydraulic (e.g. fluid permeability of fault gouge) properties of fault and fracture systems.

4. Earth Structure. Earthquakes may be used as natural sources to illuminate the structure of the Earth at different scales, using techniques similar to medical imaging by tomography, the main difference being the lack of control in the source location. I have been involved in determining earth structure in the form of subtle variations in seismic velocity and anelastic attenuation in the Aegean and the Pacific, in good agreement with tectonic models which have been suggested for these areas, and, in collaboration with the BGS Anisotropy project, using the scattered wavefield to track changes in the pore pressure field using time-lapse seismic imaging in oilfields.

5. Fluid-Rock Interactions. Fluid-rock interactions play a crucial role in many important geological process, from the formation of mineral veins to enhanced oil recovery. The rock physics group at Edinburgh has developed a novel experimental capability to investigate the geochemistry and geophysics of this interaction, demonstrating systematic changes in fluid permeability as a function of stress state and pore-fluid chemistry, even on relatively short timescales (hours to weeks). Computational simulations of fluid-rock interactions on a larger scale have been made in collaboration with the Edinburgh Parallel Computer Centre (EPCC), and are currently being applied to stress-sensitive fluid flow revealed by correlations in fluid pressure between oil wells. I have collaborated in European-funded projects to quantify fluid-rock interactions in boreholes drilled into the Aegion fault in the Gulf of Corinth, Greece and the experimental evaluation of salt transport in porous fractured media.

Technology Transfer: My work has been used directly in (a) earthquake hazard mitigation, (b) improved recovery of hydrocarbons and geothermal energy by more efficient reservoir management and better reservoir description, (c) prediction of groundwater flow and contaminant transport through fracture networks in aquifers, and (d) prediction of slope instability in Japan. Tangible applications include the development of a commercial, stress-sensitive oil reservoir simulator in collaboration with VIPS Ltd. and Heriot-Watt University, and the current effort to produce a commercial version of the Statistical Reservoir Analysis code to forecast and optimise oilfield production rates, funded by NERC.