Probing the transient rheology of accretionary prisms and megathrust earthquake hazard in the Indian Ocean basin

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The world's largest earthquakes occur in subduction zones, plate tectonic boundaries where one plate dives beneath another. These earthquakes have the potential to generate tsunamis that impact people and cities many thousands of miles from the location of the earthquake. The study of subduction zones and their earthquake and tsunami potential is often difficult, though, because much of the region where the geophysical and geological signals occur that are diagnostic of subduction zone behavior is underwater. This limitation in turn requires researchers to make simplifications of Earth behavior, such as whether the Earth behaves elastically like a rubber band or viscously like candle wax, that can negatively impact estimates of earthquake and tsunami potential. This project will use unique observations of a subduction zone in southern Pakistan and Iran, the Makran Subduction Zone, to better quantify how the rocks surrounding a subduction zone fault behave over time. This location provides a unique opportunity to better characterize subduction zones around the world because much of this subduction zone is exposed above the surface of the ocean. This means that the researchers will be able to quantify the behavior of the subduction zone with unprecedented detail using satellite monitoring tools. The Broader Impacts of the project include quantifying the future earthquake potential of the Makran Subduction Zone where an earthquake-generated tsunami would impact cities throughout the western Indian Ocean basin, including the megacities of Mumbai, pop. 23.9 million, and Karachi, pop. 16.9 million. The project also supports a graduate student and research in an EPSCoR state.To accomplish the goals above, the researchers will use interferometric synthetic aperture radar (InSAR) time series analysis to quantify post-seismic deformation caused by an Mw7.7 earthquake in within the Makran accretionary prism. Preliminary analysis of this deformation signal indicates that most of the accretionary prism itself undergoes viscoelastic relaxation at relatively shallow depths (20 km). By combining InSAR time series analysis with finite element modeling approaches, we will quantify the rheological structure and viscosities of the Makran accretionary prism, whether power-law viscoelastic behavior is required to explain the geodetic observations, and what deformation mechanisms are active to accommodate the post-seismic relaxation. Because InSAR observations miss the first 15 months of post-seismic deformation, a period when afterslip is expected to be a dominant deformation process, the researchers will explore new approaches in optical imagery time series analysis using Landsat-8 imagery, with the goal of quantifying early afterslip. Afterslip estimates will be incorporated into viscoelastic relaxation simulations to provide a more complete and unbiased model of the rheological structure of the Makran accretionary prism. Finally, this project will incorporate this rheological model into interseismic coupling models of the Makran Subduction Zone in an effort to better estimate the locking distribution and locking rate of the Makran megathrust. These results will additionally demonstrate the importance, or lack thereof, of including viscoelastic accretionary prisms in subduction zone locking models elsewhere in the world.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.