Collaborative Research: Vertical signatures of lithospheric deformation in the western US

Continuous space geodetic observations using GPS and similar constellations allow estimating relative motions of the Earth’s crust, easily down to sub-mm/yr levels. We now have long, 10-20-year GPS records of vertical ground surface motions in the United States with precision levels of 1-2 mm/yr. These precise vertical motions contain important signatures of deformation deep in the Earth.

For example, recorded vertical motions close to the subduction zone boundary in the Pacific North-West where the Juan de Fuca plate subducts underneath North America imply that there is a signature of the associated mantle convection processes at depth as well as the process of stick-slip behavior at the plate interface associated with the megathrust earthquake cycle underneath Cascadia. This project will use a range of numerical models and GPS data to capture both the seismic cycle and long-term mantle timescales and work toward disentangling the two.

These efforts will help to better understand how the crust deforms and plate boundaries evolve in general, and how the data from Cascadia can be best integrated for a physics-based estimate of seismic hazard in the region. The project engages summer students of UT Austin Jackson School's GeoFORCE program, and with Indiana University’s College of Arts and Sciences Undergraduate Research Experience, which are designed to increase the number of students pursuing STEM degrees.

Space geodetic constraints on present-day vertical motions of the crust within the western US are critical for discriminating between competing hypotheses of deformation processes working at various temporal and spatial scales. It has been suggested that small-scale convection signals from the asthenosphere may mask some of the expected signatures of the visco-elastic seismic cycle in Cascadia, for example.

This proposed work will use numerical modeling and inversions to disentangle deep-seated and shallow contributions. This research will compare visco-elastic cycle deformation models with visco-plastic mantle flow predictions, and build viscosity models of the mantle in the western US and geodynamic models of long-term deformation with elasto-visco-plastic deformation of the crust and viscous flow in the mantle.

The researchers will explore both permanent deformation in the crust as well as transient viscous flow in the mantle and compare with the geodetic velocity field, and also examine deformation at the time scale of the earthquake cycle through kinematic interseismic earthquake cycle models which account for mantle flow due to past earthquakes and interseismic coupling across faults. They will design techniques to integrate results from geodynamic models and kinematic cycle models to obtain a consistent estimate of vertical surface velocities.

Such efforts have implications for understanding the tectonic evolution of the United States as well as implications for seismic hazard assessment in the Pacific Northwest.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.