The inner workigs of Slow Earthquakes and Slow Slip Events: New insights based on field, microstructural and -chemical evidence along with targeted experiments

Prof Sandra Piazolo (SEE), Prof David Healy (SEE), Dr. Tim Chapman (University of New England, Australia), Dr. Lars Hansen (University of Minnesota)

In this exciting project, you will focus on the processes associated with the recently discovered Slow earthquakes and Slow Slip events. Until now, the processes that enable deformation at rates slower than “normal” earthquakes but faster than the commonly observed creep, remain elusive. You will tackle this problem by conducting targeted field work, perform in-depth microscopic analyses using state-of-the-art equipment and novel deformation experiments. Results promise to be of high relevance to society due to high population densities in areas close to Slow Slip Areas, and slow slip being one of the deformation modes of surging glaciers and catastrophic landslides.

Background: Geophysical and geodetic observations over the past two decades have shown that our view of how plates deform along plate boundaries needs to be revised. We now recognise the importance of Slow Earthquakes (SEs) in which slip occurs more slowly than in regular earthquakes, but significantly faster than can be attributable to normal plate motion. SEs are abundant and should leave a distinct imprint in the geological record, as they accommodate deformation within large, 50 km scale plate boundary regions e.g. subduction zones. Their observed characteristics cannot, at present, be reconciled with our current understanding of how rocks deform: extensively-studied ductile deformation processes are not compatible with the slip rates of SEs, while brittle processes would result in recognisable seismic signatures that are not observed. Interestingly, we now recognize similar behaviour in fast flowing glaciers and landslides. At present, there are number of theoretical suggestions for the processes responsible for Slow Earthquakes. However, to determine what processes are actually involved, we need to search for evidence of the relevant deformation processes in the geological record.

In summary, despite SEs potential importance in the evolution of subduction zones and societal hazard, research to date has not revealed any striking geological signature that allow for the determination of the underlying processes of SEs [KFS’21, PGPPS’24], significantly hampering our understanding of this important phenomena.

Project Research Hypothesis: We hypothesise that SEs have remained invisible because we currently do not know exactly what to look for. Luckily, we have decades of geophysical data to constrain what to look for.
(1) The process must be compatible with a wide range of slip rates (from 10-7 to 10-3 m/s) [H&R’13].
(2) The process occurs in a region where pore fluid pressure is within a few MPa of lithostatic pressure [F_etal’15]; SEs can be triggered by stresses of just a few kPa (i.e., the stress at the bottom of a filled bathtub), in contrast to the 4 MPa required for regular earthquakes [H&B’18].
(3) The process can occur spontaneously, and will accelerate, but must not reach seismic slip rates.
(4) The process must include a component of ductile flow [KFS’21].

We hypothesise that a combination of two processes could satisfy these constrains and be responsible for SEs: ductile fracture combined with granular flow. Ductile fractures are well-documented in metallurgy and ceramics and are associated with microvoids [A&G’79]. In geological systems, microvoids can be filled with fluids, buffering the fracture propagation to rates significantly lower than is typical for brittle fractures in purely elastic materials. This mode of failure is recognised by the presence of ductile deformation features adjacent to the fracture zone along with common fragmentation [S’02]. In geological shear and compression laboratory experiments such ductile fractures are known as “Slow fractures”, with failure occurring after significant ductile deformation in the presence of a fluid. We hypothesise that the combination of high strain, ductile flow and presence of fluids induce ductile fractures resulting in fast, near seismic slip in zones of fluid and fragment mixtures. Localised high fluid pressure enables minor stress perturbations to trigger rapid deformation within these zones. Seismic slip rates are never reached as crack propagation is buffered through fluid presence. We anticipate that flow in such a fluid-filled ductile fracture will leave distinct process-specific microstructures, with measurable chemical signatures caused by the fast, deformation-enhanced interaction of the fluid with the highly reactive fragments.
Objectives
In this project the student will work with leading scientist in Leeds and abroad to integrate latest techniques in characterizing deformation in metamorphic rocks and targeted experiments in order to understand the dynamics of the physiochemical processes at the core of Slow Earthquakes. As highlighted by [KFS’21] and [PGPPS’24], only a multi-scale and multi-disciplinary project can advance our understanding of SEs.
The project will address the following questions:
1) Distinguishing features: What features distinguish areas of inferred slow slip to areas known for very slow slip, i.e., creep, and very fast slip, i.e., earthquakes? What are the defining signatures both in the field and at the microscale?
2) Processes: What physiochemical processes are responsible for the different slow slip geophysical signatures? Is there a “goldilocks” combination of processes? Are different processes relevant at different scales?
3) Effects: How do the processes affect the long term behaviour of Slow Slip zones.

In order to answers the question posed above, it will be necessary to combine different techniques and approaches.

1. Conduct field work in inferred slow slip areas: Investigate the large and small scale features, collect samples for in-depth analysis.
2. Analyse collected samples (e.g. nanoscale electron microscopy, microtomography) and geochemical techniques. One field area is New Caledonia (samples already existing, [T_etal’18; C&C’21]), while a second field area is to be determined (Option e.g. Santa Catalina (USA), HP-LT areas of E Australia, Corsica, France, Syros, Greece). In each case, PT fluid conditions will have to be determined to assess the conditions under which deformation happened. As fluids are known to play a major part, we will be using chemistry as a tracer of fluid movement, hence analyses will be focussed on a combination of physical and chemical characterisation techniques. Specifically, you will quantify the signatures of inter- and intragrain deformation and any associated chemical or isotopic changes by combining state of the art EBSD and chemical analyses using a unique Focussed Ion Beam-Scanning Electron Microscope with time-of-flight detector at the University of Leeds. The spatial resolution, data analysis procedures, and elemental and orientation precision of these highly advanced techniques allow for an investigation, in unprecedented detail, of the link between chemistry and deformation microstructures
3. Develop models of process dynamics derived from field and sample analysis.
4. Conduct well-constrained experiments of slow slip in the Geosolutions geomechanics laboratory at the School of Earth and Environment, as well as at the University of Minnesota (USA), followed by subsequent in-depth analysis of experimental samples. Experiment will use the isotope labelling technique that allows an examination of the chemical signature [S_etal’17] and real-time accoustic emission and porosity data to track dynamics.
5. Develop and test hypotheses linking the observations from the rock record into slow slip behaviours from the earthquake and geodetic records.
6. There is the potential to also employ numerical modelling to test the viability of the proposed processes to generate the geophysical signatures observed.
We expect the balance between these approaches to vary depending on the specific interests of the student. Scope exists to develop novel methods of integrating what you may observe in the rock record with physical models of slip; a challenging but important endeavour.

Potential for high impact outcome
Active tectonics and earthquake hazard is a pressing issue facing many countries. We are in a unique position at Leeds together with our international collaborators to bring together a range of observational, experimental and field approaches to answer important unresolved questions about the inner workings of Slow Earthquakes and Slow Slip Events and their potential link to large, devastating Earthquakes. The research topic has immediate relevance to improving our understanding of the link between slow slip 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.
The project sits in an emerging research field with important fundamental research to be done but also important societal implications especially since this intriguing deformation behaviour is recognized also in association with glacial flow and landslides. Consequently, we anticipate the project generating several papers being suitable for submission to high impact journals.

Training
You will be part of an active group of researchers and students at SEE that focus on earthquake dynamics including experts in active faulting and microstructural investigation of rocks and minerals. In addition you will have the benefit of the wide geochemical expertise in-house through Dr. Ivan Sarov and Dr. Dan Morgan. Specifically, the student will work under the supervision of Prof Sandra Piazolo and Prof. David Healy and within the Tectonics as well as the Rocks, Fluids and Melts Research clusters of 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 deformation experiments, and (iii) laboratory analysis including state-of-the-art microstructural and chemical analysis (from outcrop to nanometer scale). As fluid presence and pressure is thought to be important, you will also be trained in thermodynamic modelling to assess reaction induced fluid pressure variations [CMV’21]. 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, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile
The student should have a strong interest in tectonics problems in metamorphic rocks with interests in both physical and chemical processes and their interaction. The student should have 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 mechanics, chemical and microstructural analysis utilizing a combination of technique (field analysis, in-depth microstructural analysis, experiments and/or numerical modelling) is a prerequisite.

References
[A&G’79] Gandhi, & Ashby, 1979, Acta Metallurgica, 27, 1565, [C&C] Chapman & Clarke 2021, JMG, 39, 343; [CZT19] Carter, Zimmerman,Teyssier, 2019, JSG, 127, 103871; [CMV’21] Chapman, Milan, Vry, GRL, e2021GL096415; [F+’15] Frank et al. 2015, EPSL, 413, 135; [H&R’13] Hawthorne & Rubin, 2013. JGR, 118, 3785; [H&B’18] Hawthorne & Bartlow, 2018, JGR, 123, 4243; [HZK’12] Hansen, Zimmerman, Kohlstedt 2012, Nature, 492, 415; [KFS’21] Kirkpatrick, Fagereng, Shelley 2021, Nature Reviews, 2, 285: [M_etal’02] Mei, Bai, Hiraga, Kohlstedt, 2002, EPSL, 201, 491; [PGPPS’24 ] Platt,Grujic, Phillips, Piazolo, Schmidt, 2024, Geospheres; [S’02] Schulson, 2002, ActaMat., 50, 3415; [S_etal’17] Spruzeniece, Piazolo, Daczko, Kilburn, Putnis, 2017, JMG, 35, 281; [T_etal’18] Taetz, John, Broecker, Spandler, Stracke, 2018, EPSL, 482, 33.