Subduction zone outer rises: a natural laboratory to reveal the development of fault networks and seismicity
Brittle deformation in the upper crust occurs through the progressive growth and linkage of a network of faults at plate boundaries. The evolution of these networks has major implications for the mechanics of crustal deformation and for seismic hazard. The connectivity of fault segments exercises strong control on whether multiple segments will rupture in a cascade (Manighetti et al., 2007), resulting in the largest magnitude earthquakes in these networks. Globally, this sort of rupture causes most earthquake fatalities (England & Jackson, 2011). However, how fault network connectivity and geometry relate to the frequency and size of multi-segment earthquakes is poorly understood. How fault networks develop is also poorly known, which limits our understanding of how these fault networks may change in the future.
Networks of extensional faults on the outer rise of subducting oceanic plates offer a unique opportunity to investigate how fault network evolution and growth relate to tectonic forcing (e.g., Nakamura et al., 2023). Stress at the outer rise increases as the plate approaches the subduction trench and bends, so we can use distance from the trench as a proxy for time (e.g., Boston et al., 2014). This means a ‘known’ stress history can be directly related to geometric observations of the fault network and spatio-temporal patterns of seismicity at outer rises. Key research questions will be answered by producing a new, high-resolution earthquake catalogue at several chosen locations using seismic waveform modelling, and combining this refined earthquake catalogue with other observational data. One major unanswered question this project will address is how fault network maturity and geometry affect the relative frequency of smaller single-segment earthquakes versus larger multi-segment earthquakes (e.g., the Mw7.9 Kaikoura, NZ earthquake, 2016 and the Mw7.2 Haiti earthquake, 2021).
- How do networks of normal faults grow and evolve as the downgoing plate approaches the trench and bending increases?
- How is this growth influenced by factors such as plate strength (linked to age) and strain rate (linked to convergence rate and slab angle)?
- What patterns of seismicity are associated with fault networks at different stages of maturity? What controls the balance between large, multi-segment earthquakes versus smaller events?
- Is there a link between rupture behaviour on the subduction megathrust and patterns of seismicity on the outer rise, as proposed in Sladen & Trevisan (2018)?
The first objective is to compile a comprehensive global database of outer rise bathymetry, seismic reflection data, and seismicity. You will then use this to map normal fault networks on the downgoing plate, and to estimate fault characteristics, such as length, total displacement, and strain histories. Waveform modelling will be used to relocate the earthquakes and reduce uncertainties in their position and focal mechanisms. The second phase will examine the spatio-temporal development of the fault networks, comparing the history of finite strain and network maturity with the inferred history of tectonic stress. Networks from different outer rises will be used to explore the influence of factors such as plate strength and strain rate. The final stage will focus on how the proportion of single-segment versus multi-segment earthquakes from the seismic data varies with fault network maturity. Here, you will also build simple probabilistic models to simulate seismicity records on varying types of fault network.
0-12 months: Literature review, compile database, map and investigate 1 or 2 chosen outer rises, 1st year report
12-24 months: Analyse seismicity distribution, extend mapping/development analysis globally, 1st manuscript
24-36 months: Seismicity modelling, preparation of 2nd manuscript, preparation of thesis
A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills. You will receive training in modern geophysical data analysis (including bathymetry, seismic reflection data, and earthquake catalogues), as well as the latest specialist computing packages. You will also have the opportunity to present results at national and international conferences and will be encouraged to publish in leading scientific journals. Through co-supervisor Craig, you will become a member of the Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), and will have the opportunity to attend COMET meetings and benefit from shared expertise in this area across several universities.
Applicants should have a good first degree in Geology, Geophysics, or a related subject. The ideal candidate would be enthusiastic to better understand earthquakes and faulting and have good numerical skills.