Institute of Geophysics and Tectonics (IGT) PhD Projects
Dynamic Fault Mechanisms - Can They Be, And How Can They Be, Robustly Constrained From Measurements Of Seismic Transients
Supervisors: Dr Mark Hildyard, Dr Nicolas Houlié & Dr Tim Wright
Proposed models for earthquake rupture have progressed from simple circular crack models, to barrier versus asperity models, to slip weakening, and to mechanisms leading to self-organised criticality (e.g. Madariaga and Olsen, 2000). However, are these complex mechanisms constrained by the measurements we currently make of the seismic wavefield? To date, dynamic inversions of fault slip do not seem to confirm this. How do the waveforms change due to different models? Where and what needs to be recorded to truly constrain fault slip mechanisms? The project will use and develop numerical models to investigate this complex issue. Investigation through models allows all possible measurement positions and types to be considered in determining what best can uniquely determine fault rupture. Results will also be constrained by application to data-rich earthquakes, which have near- field seismic recordings (both broadband and high frequency), GPS and INSAR, coupled with a detailed knowledge of the localised geology and rheology. Examples of such data-rich datasets are available from the Alum Rock and Parkfield earthquakes. Testing proposed rupture mechanisms within this framework, the project will establish whether existing measurements are capable of truly constraining the rupture mechanism, or whether other measurements must be sought.
Figures 1 and 2 above show a model of the earthquake rupture process, and the resulting propagation of ground motions. The model represents a Magnitude 6 earthquake, and covers the near-field, with a volume of around 50 km x 50 km x 50 km. Not shown are the near-field waveforms which are also calculated. The above is a simplistic model of the earthquake process. Inversions of seismic recordings show that the final fault slip (and hence distribution of stress drop) is heterogeneous. Such inversions however have been found to be non-unique and it is of considerable current research to obtain robust reliable inversions. In addition, laboratory experiments have shown that earthquake mechanisms are much more complex than the approach in the above model. This project will investigate these more complex mechanisms, and their testability using current and future seismic data.