YES-27-D-05: Characterising Earthquakes in Oceanic Lithosphere: New Seismological Constraints on Plate Stresses and Deformation

Project Description:

This project will characterise earthquakes occurring at shallow depths in oceanic lithosphere, determining their location, depth, source mechanism, and rupture characteristics, down to magnitudes not previously possible, and use the resulting dataset to explore the tectonic and geodynamic implications of the distribution of seismicity.  Oceanic lithosphere makes up approximately 2/3 of the Earths surface, and how this lithosphere supports stresses plays a dominant role in our understanding of plate tectonics.  However, with geodesy blocked by the overlying ocean, and with near-field observation reliant on logistically-expensive ocean-bottom seismometers, global-scale seismology remains the only tool we can effectively deploy to study earthquakes in oceanic lithosphere on a global scale.

In this project, you will build on a range of recent advances in observational earthquake seismology for studying earthquakes using remote, far-field data.  You will develop approaches to apply these techniques to shallow earthquakes in oceanic lithosphere, determining accurate depths and locations, and constraints on the earthquake source process, for increasingly small earthquakes.

Key to this will be the integration of array-based signal processing with waveform modelling to allow not only accurate location but also the determination of source mechanisms.  With array-based techniques allowing us to work with earthquakes down to M4.7, compared with ~M5.5 for single-station data, this will allow a vast increase in the number of earthquakes we can characterise. Following a phase of technical development, you will then apply these approaches to characterise seismicity in oceanic lithosphere in a range of key tectonic settings:

• In the outer rise region, starting with the outer rise of Central America. The outer rise of Central America has recently ended a period of seismic quiescence, with a number of outer rise earthquakes now having occurred along this previously-understudied margin.  As an outer rise in a young, relatively hot downgoing plate, this offers a chance to study how stress is supported in such settings.
• Following initial work on Central America, there is scope to expand work on the outer rises into a more global study. Of particular interest is understanding the controls on the orientation of deeper compressional seismicity beneath the outer rise, and how this related to the pre-existing fabric present in oceanic lithosphere following its formation at the mid ocean ridge.
• On Mid Ocean Ridges, where the depth extent of brittle deformation, and how related to spreading rate, thermal structure, magmatic contribution, and style of rifting remains uncertain, with only limited information coming from sparse local seismic surveys using ocean-bottom seismometers.
• Along oceanic transform faults, where understanding the along-variability in depth is key to understanding the extent and segmentation of large earthquakes, as well as in understanding how oceanic lithosphere cools away from the mid ocean ridge.

In later years there is flexibility within the scope of the project to tailor the goals to suit a students interests and skills, either by studying different tectonic environments, pushing the frontiers of source seismology using novel machine learning-based approaches to the imagine-based processing of stacked seismograms, or through the incorporation of elements of numerical geodynamic modelling, probing the physics controlling earthquake distributions in oceanic lithosphere.

 

 

References:

• Blackwell, T.J. Craig and S. Rost (2024). Automatic relocation of intermediate-depth earthquakes using adaptive array analysis, Geophysical Journal International, v239, pp821-840, doi:10.1093/gji/ggae289.
• Sandiford and T.J. Craig (2023). Plate bending earthquakes and the strength distribution of the lithosphere, Geophysical Journal International, v235, doi:10.1093/gji/ggad230.
• J. Craig, A. Copley, and T. A. Middleton (2014). Constraining fault friction in oceanic lithosphere using the dip angle of newly-formed faults at the outer rise, Earth and Planetary Science Letters, v392, p94-99.
• J. Craig, A. Copley, and J. Jackson (2014). A reassessment of outer-rise seismicity and its implications for the mechanics of oceanic lithosphere, Geophysical Journal International, doi: 10.1093/gji/ggu013.

Aims and Objectives: 

This studentship will study earthquakes occurring in oceanic lithosphere, building on observations from earthquake seismology, and integrating these with our geological and geodynamic understanding. An initial technical phase will build on recent advances in waveform processing to develop new approaches to characterising and locating earthquakes occurring at shallow depths in oceanic lithosphere.  After the initial technical phase, target tectonic settings can be tailored to suit the interests and strengths of the student, but could include:

• The distribution and style of faulting in the outer rise regions, considering both the depth range of given types of faulting, and the how the orientation of seismicity relates to the stress state in the outer rise.
• The along-strike variation in the depths of earthquakes along major oceanic transform faults.
• The style and depth extent of seismicity associated with Mid Ocean Ridges, and how this varies as a function of spreading rate, magmatic contribution, and style of tectonic rifting.

This project will use freely-available global seismological dataset, principally waveform data from the Federation of Digital Seismograph Networks, equipping students with a fully transferable and portable skills set, not limited by laboratory facilities.

 

Applicant Background:

This project would suit candidates with a background in quantitative geology, geophysics, or physics with an interest in solid-Earth processes.  Prior experience with observational seismology and/or prior skills in coding are desirable, but not required.  We encourage applicants from all backgrounds.

Training:

The student will be based the Institute for Geophysics and Tectonics at the University of Leeds.  The graduate student population in the Institute comes from many countries around the world and are well supported by a comprehensive programme of training and an inclusive supervision network.  The student will receive extensive training in observational earthquake seismology, with potential additional training in numerical geodynamics and in tectonic interpretation. Within Leeds, they will have the opportunity to interact with the internationally-excellent research group in Tectonics and Volcanology, hosted within the Institute for Geophysics and Tectonics.  The School also hosts numerous staff from the NERC-funded Centre for the Observation and Modelling of Earthquakes and Tectonics (www.comet.nerc.ac.uk), with whom the student will be able to interact.