The Department of Earth Sciences is seeking to fill two PhD positions of in total 11 within the EU TREAD doctoral network: 'Data and Processes in Seismic Hazard'. The two positions at Utrecht University offer a four-year PhD track with help of additional non-EU funding, in contrast to the other 3-year tracks in the network. A personalized training program will be set up, reflecting your training needs and career objectives. About 20% of your time will be dedicated to this training component, which includes training on the job in assisting in the BSc and MSc teaching programs of our department. The two projects are:

• Project #5: Flow to friction transition and back in carbonate rocks. Main supervisor: Dr. André Niemeijer (Experimental Rock Deformation group / High Presssure & Temperature Lab)

Objectives: The rheology of carbonates during the seismic cycle, especially in the presence of pressurized fluids and at the viscous-plastic to elasto-frictional transition, remains poorly understood.

In the project, we will perform experiments on both intact carbonate rocks as well as fault gouges under conditions where the transition from crystal-plastic flow to frictional behavior might be activated. Detailed microstructural analyses down to the nanoscale (UU & UNIPD) of the experimental products and comparison with natural fault rocks from the deep roots of fault zones exposed in the Apuan Alps (Italy) and Western Alps (Switzerland) (UNIPD) will allow us to:

1. test whether the deformation mechanisms activated in the experiments occur in natural faults,
2. test and update existing calcite paleo-piezometers to estimate the state of stress at earthquake nucleation depths and beyond,
3. define the conditions under which the transition from volume-conservative crystal-plastic deformation to volume-dependent frictional deformation occurs (i.e., viscous-plastic to elasto-frictional transition).

Additionally, existing flow laws for creep in fine-grained calcite aggregates that have been used to predict shear strength during seismic sliding will be tested and updated, also for their utilization in other fellow projects on the modelling of the seismic cycle proposed in TREAD.

Expected Results:

1. Identification of the dominant deformation mechanisms across the transition from friction to flow behavior in experimental and natural carbonate fault rocks;
2. Updated and tested microphysical models (laws) for the full range of velocities encountered in the seismic cycle; (3) Critical assessment of existing paleopiezometers for wet calcite rocks.

• Project #7: How tectonics affects seismic hazard parameters in complex continental settings. Main supervisor: Dr. Ylona van Dinther (Tectonics group)

Objectives: Recent 2D tectonic earthquake sequence modelling of the Northern Apennines reveals that realistic tectonic loading and deep structures and rheology have a major impact on earthquake sequences in the upper continental crust. Specifically, the stress field and the type, distribution and rate of earthquakes in Northern Apennines are significantly affected by slab pull and lower crustal rheology, although these are not taken into account in earthquake sequence modelling or seismic hazard assessment. To understand these key features this doctoral candidate will:

1. extend 2D/3D visco-elasto-plastic, seismo-thermo-mechanical models, simulating earthquake sequences following millions of years tectonic, topography and fault evolution, down to milliseconds of earthquakes from strike slip to complex continental settings. To computationally efficiently simulate wave-mediated stress transfer in 3D, faults stress states will be coupled to the dynamic rupture model following recent achievements.
2. apply these new state-of-the-art models to spontaneously simulate and understand seismic hazard parameters (i.e., Mmax and b-value) as a function of important tectonic and rheological parameters (e.g., loading by mantle and lower crust, carbonate rheology, fluid flow).
3. tightly constrain a scenario in the Betics by observations from field studies, geodesy, seismology and fault geometries, and microphysical friction laws, using instantaneous modelling to assess its seismic hazard and compare those outcomes to more traditional  PSHA approaches to converge towards a more physics-inspired PSHA methods.

Expected Results:

1. 2D/3D coupled visco-elasto-plastic tectonic earthquake sequence models for complex continental settings;
2. Improved understanding of how key tectonic and rheological parameters affect seismic hazard parameters in complex continental settings;
3. Data-constrained physics-based scenario for seismic hazard assessment in the Betics.