Seismological Investigations of Earthquakes and Deep Earth Structure

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This project will use decades of records of seismic waves generated by thousands of earthquakes all over the world to learn about both earthquakes and deep Earth structure. Analyses of earthquakes will focus on the high-frequency components of the seismograms, which provide key constraints on the energy radiated by earthquakes and information about the speed and complexity of fault ruptures. These results should help improve forecasts of damaging ground motions produced by strong earthquakes. Planned analyses for Earth structure will examine sharp changes in seismic wave velocities that occur at depths of 50 to 400 miles below the surface that are related to changes in crystal structure or the presence of fluids, such as water or magma. A related project will examine scattering of seismic waves, which provides clues regarding small-scale variability in rock properties at depths extending from the surface to Earth's liquid core. These results will help in understanding the internal composition of the Earth and its relation to tectonic processes. The project will support undergraduate, graduate, and postgraduate students and involve them in research. Educational components include graduate student and postdoc support, as well as sponsoring an undergraduate summer intern in a long-standing and successful SIO program designed to increase student diversity. Systematic analyses of large datasets recorded by both permanent and temporary seismic stations all over the world provide rich opportunities to study problems concerning earthquakes and Earth structure. Planned research includes innovative investigations of upper-mantle discontinuity structure within and near the transition zone and the lithosphere-asthenosphere boundary, observing and modeling of seismic scattering to characterize small-scale compositional heterogeneity in the mantle, and spectral analyses of earthquake radiation to constrain earthquake rupture dimensions and dynamics. Some key questions this project plans to examine include: (1) What is the role of the low-velocity layer (LVL) above the 410-km discontinuity in mantle convection and water recycling? (2) What causes the mid-lithospheric discontinuity (MLD) observed under many continental cratons? (3) How variable is the sharpness and roughness of the 660-km discontinuity and what does that imply for mantle composition? (4) What is the strength of small-scale heterogeneity in the mid- to lower mantle and does it vary laterally? (5) What causes the large scatter in earthquake stress drop estimates, i.e., how much is due to modeling uncertainties versus real differences in earthquake dynamics? (6) Does earthquake stress drop vary systematically with tectonic region? (7) How do improved global high-frequency body-wave attenuation models compare to tomographic models and to surface-wave attenuation models?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.