Most earthquakes occur within plate-tectonic convergence zones. Earthquakes which occur at depths larger than ~50 km (30 miles) result from rupture events in oceanic plates that are sinking into the Earth?s mantle. These deep earthquakes have similar characteristics than shallow earthquakes, yet they are different. Observations suggest that deep earthquakes involve frictional sliding along planar faults, as the shallow ones. Yet it is still unclear how faults can break at the extreme pressure prevailing in the seismogenic mantle, reaching up to 250,000 times the atmospheric pressure. The mechanisms responsible for deep earthquakes have thus been debated for decades. Here, the researchers investigate these mechanisms using new analyses of seismic waves. Seismic waves travel away from the earthquakes that generated them, through the Earth?s mantle and crust. When they reach the surface, they can be used to investigate events in the deep Earth. Here, the team analyze the wave spectra, analogous to how astronomers analyze the electromagnetic spectra of stars. It tests the hypothesis that deep earthquakes rupture small fault planes with high rupture speeds. The researchers document the differences in shear-stress states of faults in the crust (at shallow depths) and in the mantle (at large depths). This project leverages national investments in seismological infrastructure, such as the IRIS Global Seismic Network (GSN) and the Earthscope USArray. It provides support for an early career female scientist and a female graduate student. It also provides training in seismology, geophysics, scientific computing, and big-data analytics for several undergraduate students at University of Michigan - Ann Arbor.
Here, the analysis of the earthquake rupture process and seismic wave propagation are tightly linked, as both the earthquake source complexity and the heterogeneous properties of rock along the wave paths shape seismic wave spectra. The team follows a new two-step spectral-ratio approach aimed at optimally separating the source and propagation effects in teleseismic P-wave and S-wave spectra. Using the four-decade long IRIS database, the researchers select pairs of nearby earthquakes to determine rupture durations and stress drops of several hundred large earthquakes in the crust and mantle. From pairs of GSN and USArray ground-motion recordings, they estimate how seismic wave attenuation and scattering affect wave spectra. By processing the spectra consistently in both steps, they can compare the source characteristics and chart with robust statistics how stress drop varies with earthquake depth, magnitude, and tectonic plate boundary. Furthermore, they quantify wave propagation along several well-travelled paths to add new constraints on variations of wave scattering and attenuation in the Earth.
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.
