The dynamics of the earthquake cycle: New insight from field work, experiments and novel microstructural investigations
The dynamics of the earthquake cycle: New insight from field work, experiments and novel microstructural investigations
Prof Sandra Piazolo (SEE), Dr Laura Gregory (SEE), Ali Ghanbarzadeh (School of Mechanical Engineering); collaborator: Prof Ken McCaffrey (Durham University)
Contact email: s.piazolo@leeds.ac.uk
Slip behaviour at and around faults has been shown to be highly dynamic with variability in behaviour occurring both spatially and temporally. This exciting project explores the underlying physical processes that lie at the core of dynamic slip behaviour by probing the rock record of fault slip. In this novel project, you will integrate knowledge obtained from high resolution laser scanning (LiDAR), quantitative microstructural work, and Quaternary fault studies to gain an in-depth understanding of the physical processes acting on a fault and/or fault zone. Results will be far reaching in fundamental science with direct implications for applied science in terms of earthquake hazard evaluation and forcasting.
Earthquakes are one of the main hazards that humanity faces, therefore improving our ability to anticipate how fault zones behave through time is of major importance. However, we have still very little understanding of why some faults appear to accommodate different slip modes and others do not, how different slip processes are represented in the rock record, and why faults cycle between different modes. This project in novel in its cross-disciplinary nature. The project builds on existing earthquake cycle analysis in terms fault history determined by isotopic age dating on fault rocks, patterns of fault surface roughness, and their link to microstructures from natural and experimental fault rocks. In Leeds we have the rare opportunity for this integration as experts in the respective fields are within the same University. In addition, the highly specialized experimental and analytical equipment exists in house, making this challenging project possible. This project is further strengthened by the strong personal and scientific links between the Leeds team and the project collaborator (Prof Ken McCaffrey, Durham).
The earthquake cycle typically consists of three periods of fault slip: (1) coseismic, when earthquake rupture occurs at speeds of 1-2 km/sec and results in a huge release of energy during an earthquake (Heaton, 1990; Rice, 2006); (2) postseismic – during which the fault relaxes and is thought to experience an exponentially decaying rate of stable afterslip (Perfettini and Ampuero, 2008; Ingleby and Wright, 2017); and (3) an interseismic period when the fault is locked and accumulates elastic strain prior to the next earthquake (Marone, 1998; Barbot et al., 2012). It is known that in principle, each of these periods will imprint a specific signature onto the fault rocks and their surroundings while at the same time, the evolution of the rocks themselves will influence the character of the next period of the earthquake cycle. How this dynamic interplay actually works remains unknown. This lack of knowledge hampers our inability to assess earthquake hazard for individual faults with confidence. This project aims to unravel this dynamic interplay through field and state of the art chemical and physical sample analysis, novel experiments and high end microstructural analysis.
Figure 1:(a) Red lines show examples of two different slip histories that have the same long-term slip rate but highly variable recurrence intervals. (b) Demonstrates how cosmogenic nuclides can be used to model variable fault slip, with sampling strategy indicated in the inset of (a), see Cowie et al., (2017)
The mechanical behaviour of a fault zone is governed by physical and chemical processes occurring on grain-size (micro) scales. Important processes in the development and evolution of faults include stress induced fracturing and frictional sliding, grain communition, crystal plasticity and pressure solution. These processes occur at a variety of timescales where fracturing is instantaneous but crystal plasticity may be continuously active over thousands to millions of years. So far much of our knowledge of these processes has been gained indirectly by fitting theoretical mechanical behaviour to observed slip behaviour. However, microstructures in fault rocks allow direct derivation of the processes responsible for behaviour during slip at a range of rates (e.g. Verbene et al. 2015, DelPiane, Piazolo et al. 2017). Even though it is clear that the microstructures in fault rocks must hold key answers to the underlying questions of fault slip behaviour, the use of microstructures has been hampered by analytical problems as many structures are extremely fine grained and micron to nanometre scale analysis is necessary. In addition, scaling the effects of microscale processes as observed in laboratory experiments to large scale fault rock behaviour of natural faults is not straightforward.
Recent advances in analytical techniques (e.g. Piazolo et al. 2015, Piazolo et al. 2016, DelPiane, Piazolo et al. 2017) allow us for the first time to investigate microstructures of fault rocks in unprecedented detail, promising to gain the much needed fundamental knowledge of the mechanical and chemical processes governing the rheological behaviour of faults through time and space (e.g. Figure 2). To gain such knowledge it is imperative that analyses be conducted in a framework of well constrained case studies, utilizing the natural laboratory of real faults. In order to do so, we must combine the real-world record of the main types of fault behaviour, made on geological scales in time and space, with in-depth microstructural analysis.
Different to laboratory experiments, it is possible to investigate a fault zone over different scales in field-based studies with the background knowledge of a particular fault’s behaviour through time (Goodall et al., 2021). Such work will augment experimental work focussing on the physics of one time-step in the earthquake cycle.
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Fig. 2: A &B Example of calcite fault rock investigated using EBSD (Electron Backscatter Diffraction analysis) at the nanometer scale (spatial resolution of 10 nm) to investigate the changes in orientation within the fault rock. This allows us to identify different deformation mechanisms. (see DelPiane, Piazolo et al. Geology 2017) |
| (c) TEM images of gouge particles produced after 120 mm of shear displacement. Micrometre-sized calcite crystal surrounded by a sub-micrometre matrix. Note the difference in dislocation density (dislocations are seen as grey “wiggles” between the core and the rim of the crystals. (see DelPiane, Piazolo et al. Geology 2017) |  |
In this project you will collect a set of fault breccia samples “caught in the act” from active fault zones in several potential field areas. One area of focus will be from the Apennines in central Italy, which recently experienced a devastating earthquake sequence that began with an Mw 6.2 resulting in nearly 300 deaths and relocation of tens of thousands of people. This regions hosts normal faults that are at varying stages of maturity, which have been shown to demonstrate slip rate variability (Figures 1 & 3). You will also have the opportunity to investigate normal faults in southwest Turkey, and other potential regions depending on your interest and progress. Target faults in Central Italy and Turkey have been researched in terms of their seismic activity and their average Quaternary slip behaviour, and the location of ideal field sites is already known. You will have the unique opportunity to combine knowledge gained through microscopic studies with mesoscale features on these faults, using terrestrial laser scanning datasets detailing the metre-scale fault surface, in collaboration with Prof Ken McCaffrey, from Durham University (e.g. Figure 3c). These faults provide the opportunity to investigate structures from the outcrop to the nanoscale, allowing for a process-oriented analysis of fault rock structure.
Objectives
In this project, the student will work with leading scientists at Leeds (Sandra Piazolo, Laura Gregory and Ali Ghanbarzadeh), and Durham (Ken McCaffrey) to integrate the latest techniques in characterising fault zone structures in order to understand the dynamics of the physical processes preserved from the earthquake cycle. The project will address the following questions:
- Processes: What physiochemical processes are involved in fast fault slip, creep, and postseismic afterslip? How do these processes evolve as a fault grows and develops?
- Recognition: How can various earthquake cycle behaviour be identified in natural rocks? What is the link between micro- and meso- scale features of damage on a fault, if any?
- Effect: What is the mechanical effect of the different processes identified in (1)? What is an appropriate mathematical representation of such dynamic behaviour? This mathematical description will form the foundation to improve the timescale and spatial behaviour of fault zones past, present and future?
In order to answers the question posed above, it will be necessary to combine different techniques and approaches.
- Investigate small scale features in samples close to or on the fault using the latest field based (e.g. fault zone laser scanning) and analytical (e.g. nanoscale electron microscopy, microtomography) techniques. The field area in central Italy with faults of known Quaternary fault slip rates (e.g. Cowie et al. 2017) will be the initial focus but we anticipate expansion of the field area to southwestern Turkey and/or the western USA.
- Conduct experiments using novel experimental designs (e.g. modified Universal Material Tester-UMT) allowing for direct shear experiments with high frequency cycles that can directly measure tangential loads and frictional behaviour in-situ while varying the normal force. Here, starting material with different material properties (e.g. porosity, chemical composition) will be subject to direct shear. Chemical fingerprinting will allow recognition of the sequence of “events”.
- Analysis of experimental samples before and after experiments using high-end microscopy including an unique Focussed Ion Beam Scanning Electron Microscope with time of flight spectrometer and Electron backscatter diffraction detectors). Analysis will be from the nanometer-scale, to the micron and to mm scale. Analytical results from experiments will be compared to natural rocks from geological faults provided by the supervisor and/or collected by the student.
- Develop and test hypotheses linking the observations from the rock record into fault slip behaviours, relying on what we already know from the earthquake and Quaternary records on the faults you have studied.
We expect the balance between these approaches to vary depending on the specific interests of the student. There is the potential to develop novel methods of integrating what you may observe in the rock record with physical models of fault slip; a challenging but important endeavour.
Potential for high impact outcome
Active tectonics and the resulting earthquake hazard is a pressing issue facing many countries. We are in a unique position at Leeds to bring together a range of observational, modelling and field approaches to answer important unresolved questions about the earthquake cycles. The research topic has immediate relevance to improving our understanding of the link between faulting and timescale and nature of seismic hazard. There will be ample opportunities to deliver the results of the project at international conferences in addition to UK meetings. Through in-country collaborators, there will be the opportunity to communicate the earthquake hazard to local authorities and civil protection planners.
The project sits in an emerging research field with important fundamental research to be done but also important societal implications. Research outcomes will be a step-change in the science underpinning prediction and assessment of fault rock behaviour with large impact. Consequently, we anticipate the project will generate several international publications in high impact journals.
Training
You will be part of an active group of researchers and students at SEE that focus on earthquake dynamics and advance microstructural analysis including experts in active faulting and microstructural investigation of rocks and minerals. In addition, the student will be able to join the Institute for Functional Surfaces at School of Mechanical Engineering, a world class research institute focussed on surface chemistry and deformation. The student will work under the supervision of Prof Sandra Piazolo and Dr. Laura Gregory within the Tectonics and Geodynamics group in the Institute of Geophysics & Tectonics at the School of Earth & Environment at Leeds. The Institute also hosts the Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET http://comet.nerc.ac.uk/), which provides a large group of researchers engaged in active tectonics research with whom the student can interact. This project provides a high level of specialist scientific training in: (i) geological field skills, (ii) laboratory analysis including state-of-the-art microstructural and –chemical analysis (from outcrop to nanometer scale) (iv) data processing and interpretation of laser scanner data and (vi) deformation experiment with and without fluids using an unique tribology rig available. Microscale analysis will be using the most advanced analytical suite currently available in the UK housed in the new Bragg Centre (https://www.leeds.ac.uk/info/130565/bragg_centre_for_materials_research) a £96million investment in engineering and physical sciences at the University of Leeds. The successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range from scientific computing through to managing your degree and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).
Student profile
The student should have a strong interest in active tectonics challenges, a desire to undertake laboratory and fieldwork overseas, and a strong background in a quantitative science (earth sciences, geophysics, geology, physics, natural sciences). Willingness and excitement for taking up the challenge to work at the boundary of earth science, mechanics and microstructural analysis utilizing a combination of technique (field analysis, in-depth microstructural analysis, and experiments) is a prerequisite. This is a multidisciplinary project but we welcome students with enthusiasm for any relevant field as the project is flexible based on your interests.
References
Barbot, S., N Lapusta, and JP Avouac (2012). Under the hood of the earthquake machine: toward predictive modelling of the seismic cycle. Sciece 336, pp 707-710, doi: 10.1126/science.1218796
Cowie, PA, Phillips, RJ, Roberts, GP, McCaffrey, K, Zijerveld, LJJ, Gregory, LC, Faure Walker, J, Wedmore, LNJ, Dunai, TJ, Binnie, SA, Freeman, SPHT, Wilcken, K, Shanks, RP, Huismans, RS, Papanikolaou, I, Michetti, AM, and Wilkinson, M (2017). Orogen-scale uplift I nthe central Italian Apennines drives episodic behaviour of earthquake faults. Scientific Reports 7:44858, doi: 10.1038/srep44858.
Davidesko, G, Sagy, A, Hatzor, YH (2014). Evolution of slip surface roughness through shear. Geophysical Research Letters 41, 1492–1498, doi:10.1002/2013GL058913.
Delle Piane, C., Piazolo, S., Timms, N. E., Luzin, V., et al. (in press). Sub-seismic slip in nano calcite fault gouge generates amorphous carbon and crystallographic texture at low temperature. Geology 46, no. 2 (2017): 163-166.
Dunham, EM, D Belanger, L Cong, and JE Kozdon (2011). Earthquake ruptures with strongly rate-weakening friction and off-fault plasticity, part 2: Nonplanar faults.” Bulletin of the Seismological Society of America 101 (5), pp 2296–2307, doi: 10.1785/0120100076
Ingleby, T and Wright, TJ (2017). Omori-like decay of postseismic velocities following continental earthquakes. Geophysical Research Letters 44, 3119–3130, doi:10.1002/2017GL072865.
Marone, C (1998). The effect of loading rate on static friction and the rate of fault healing during the earthquake cycle. Nature 391, pp 69-72.
Perfettini, H and Ampuero, JP (2008). Dynamics of a velocity strengthening fault region: Implications for slow earthquakes and postseismic slip. Journal of Geophysical Research 113, B09411, doi:10.1029/2007JB005398.
Piazolo S; La Fontaine A; Trimby P; Harley S; Yang L; Armstrong R; Cairney JM (2016) Deformation-induced trace element redistribution in zircon revealed using atom probe tomography, Nature Communications, 7, . doi: 10.1038/ncomms10490
Piazolo S; Montagnat M; Grennerat F; Moulinec H; Wheeler J (2015) Effect of local stress heterogeneities on dislocation fields: Examples from transient creep in polycrystalline ice, Acta Materialia, 90, pp.303-309. doi: 10.1016/j.actamat.2015.02.046
Rice, JR (2006). Heating and weakening of faults during earthquake slip. Journal of Geophysical Research 111; B05311, doi:10.1029/2005JB004006.
Weldon, R, Scharer, K, Fumal, T, and Biasi, G (2004). Wrightwood and the earthquake cycle: what a long recurrence record tells us about how faults work. GSA Today 14 (9), pp 4-10, doi: 10.1130/1052-5173(2004)014<4:WATECW>2.0CO;2.