The inner workings of the earthquake cycle: New insight from integrating geophysical observations and microstructures

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 multi-disciplinary 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 forecasting.

Earthquakes are one of the main hazards that humanity faces, and improving our ability to anticipate how fault zones behave through time is of major importance. However, we have 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 is novel in its cross-disciplinary nature integrating earthquake cycle analysis on real rocks including information on the time-averaged fault activity using 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 of this integration as experts in the respective fields are within the same school. In addition, strong personal links to supervisors at other universities (Dr Jess Hawthorne, University of Oxford and Prof Ken McCaffrey, Durham University) strengthen 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 in 2016 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 (Walters et al., 2018). This regions hosts normal faults that are at varying stages of maturity, which have been shown to demonstrate slip rate variability (Figures 1 & 3, Cowie et al., 2017). 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. 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
You will integrate the latest techniques in characterising fault zone structures in order to understand the dynamics of physical processes preserved from the earthquake cycle, focused on the following questions:
• Processes: What physiochemical processes are involved in fast fault slip, creep, and postseismic afterslip?
• 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? Based on the latter, can we forecast 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, which can be tailored to your interests and experience.
1. 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.
2. Develop models of process dynamics derived from field and sample analysis. Link structures observed at all scales through careful sample selection.
3. Integrate key parameters derived from microstructural analyses into fault slip modelling/physical calculations and compare the results with observed fault behaviours (Hawthorne et al. 2016)
4. Conduct well-constrained experiments of fault slip in the laboratory collaborating with project partner Prof Shengwen Qi at the Chinese Academy of Sciences, followed by subsequent in-depth analysis of experimental samples (e.g. Piazolo et al. 2015).
5. 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.