In 2016, I was fortunate enough to be awarded a 34th IGC Early Career Travel Grant and the RSES Mervyn and Katalin Paterson Travel Fellowship. These awards allowed me travel for an extended period this year to attend conferences and undertake state-of-the-art laboratory experiments at the École Normale Supérieure (ENS) in Paris and the National Institute of Volcanology and Geophysics (INVG) in Rome.
In this article I will talk a little about my experiences at the ENS laboratories in Paris. During my stay I was able to use experimental techniques pioneered by the ENS lab to explore differences in fault processes between earthquakes resulting from increases in shear stress (such as classic mainshock-aftershock events) and those driven by changes in pore fluid pressure (e.g. during an injection driven swarm sequence). Working closely with Professor Alexandre Schubnel and PhD student Jérôme Albury, I was able to undertake six experiments during the four weeks of my visit.
The aim of my research is to use experiments undertaken at pressure and temperature conditions comparable to those found deep in the crust to learn about the strength and behaviour of faults. I am interested in understanding the processes that occur during the first seconds of fault slip as this is central to understanding whether a fault rupture grows to become a large, damaging earthquake, or whether strain is accommodated as a small, possibly non-seismic event. During the first seconds of slip, the extreme forces acting on fault contacts, or asperities, result in heat generation, formation of damage and changes in the physical properties of a fault surface. As slip proceeds, these processes can result in an evolution of fault strength through a process referred to as ‘dynamic weakening’.
A key aspect of my current research, and the reason for my trip to Paris, was to look at role that fluids play in affecting the behaviour of faults during the initial stages of slip. Fault zones are recognised as fluid conduits within the crust and large-scale injection experiments (such as in Basel, Switzerland) have shown a direct correlation between fluid pressure and rates of seismicity. This has important implications for the development of hydraulic fracture technologies such as enhanced gas recovery, geothermal energy extraction and geo-sequestration. However, little is currently known about how pore fluids modify fault strength and asperity behaviour during rupture and how this could facilitate or impede rupture propagation.
The lab at ENS has pioneered the use of two different types of sensors mounted on the sample as way of measuring deformation prior to and during the onset of slip. The first technique involves the detection and measurement of Acoustic Emissions (AEs). AEs are generated when structural changes, such as a brittle fracture, form a local source of elastic waves. Just like an earthquake only much, much smaller, the elastic waves generate tiny displacements on the sample surface that are detected using piezoelectric sensors. By looking at the rate and source of emissions we can gain insights into where the larger-scale macroscopic labquakes nucleate. The second type of measurement involves the use of strain gauges, which we glue adjacent to the slip surface. These sensors are important for recording the coseismic release of stored elastic energy. If we record multiple strain gauges synchronously we can use the time difference between the onset of energy release to determine the speed at which the labquake rupture propagates.
During my visit I performed six experiments on Fontainebleau sandstone, a pure quartz sandstone that is highly prized for use by experimental rock physicists owing to its purity, lack of preexisting deformation and beautiful pore structure. It just so happens that my piece of Fontainebleau sandstone was salvaged from the Palace of Versailles. It did seem rather surreal to be deforming part of one of France’s greatest icons!
Preparation for each experiment took the better part of a week. The samples had to be cored and ground, and the sixteen sensors carefully glued into position. However, all the fiddly preparation was worth it when the experiments worked and we were able to produce some exciting results. During the experiments we recorded thousands of little ‘fore-shock’ AEs in the lead up to the main macroscopic slip events.
Now, in a similar manner to the way seismologists pick events from different seismograms, we must pick the events from our 8 acoustic emission sensors. If we can correlate AE events between the different sensors we can calculate travel times and estimate the hypocenter locations. This work will take months to complete but we hope that it will provide us with new insights into how fluids alter the dynamics of rupture on a fault. Now that I am back in Australia, I am also using high-resolution electron microscopy imaging to look at physical changes that have occurred on the fault surfaces during slip in an attempt to correlate microstructural and behavioral evolution.
A month in Paris was a wonderful experience. Yes, I sampled many fine desserts from numerous patisseries, but I also had the opportunity to develop new skills and learn new experimental techniques in a hands-on way. No doubt, what I have learnt will shape my future endeavors and I would like to extend my sincere thanks to the Australian Geoscience Council, the Australian Academy of Science, the Research School of Earth Sciences and Mervyn and Katalin Paterson for the provision and the award of the travel grants that made this trip possible.