Global Search for D" Discontinuity Structure

University Of Utah, Salt Lake City UT

Investigators

Abstract

Approximately 1600 miles beneath the surface of our planet a sharp contrast in Earth properties, called the D" discontinuity, has been shown to exist in more than 100 studies. However, geoscientists still don’t have great knowledge of what this discontinuity physically represents. This is for two primary reasons: most seismic sensors used to detect this discontinuity are spatially restricted to those on land (only 30% of our planet is covered by land) and our current technology only allows us to detect this discontinuity from the deepest of earthquakes; extra noise in the seismic recordings have previously hindered our ability to use shallower earthquakes. Shallow earthquakes, however, are more numerous and occur in many more places on the Earth than deep earthquakes, so if shallow earthquakes could be utilized, they would greatly improve the coverage of the Earth in which we can search for the discontinuity. As such, this project seeks to develop new technology that allows geoscientists to sense the subtle seismic signals associated with this discontinuity using shallow earthquakes. In addition, the project will develop a new modeling capability that will allow a better understanding of what the materials making up this discontinuity are with greater accuracy. Being able to determine where and what the D” discontinuity represents is critical because it likely plays a large role in the ongoing processes inside the Earth. For example, depending on where and what the D" discontinuity is, could strongly affect the whole mantle convection process and control where we observe volcanic activity from hot spot volcanoes such as observed in Hawaii, Iceland, or Yellowstone. This research will be the focus of research for postdoctoral scholar at the University of Utah. This project will furthermore fund 2 years of undergraduate research experience for one student and will additionally support a research experience for one student as a part of the MS for Secondary School Teachers program at the University of Utah. All data collections and results from the extensive data analyses performed in this study will be shared openly on the University of Utah’s hive data repository and all software developed to carry out the array bootstrap technique described in this proposal will be made freely available through GitHub. Seismic array processing techniques are designed to enhance low amplitude seismic wave arrivals and are ideally suited for searching for laterally variable, low contrast seismic discontinuities. Yet only a limited number of studies have taken advantage of the vast amounts of seismic data available using these array processing methods. In addition, when using standard array processing approaches, one often observes potential arrivals that could be associated with known or unknown discontinuity structure but may also be due to correlated noise conditions on a subset of receivers. Here the investigators propose the development and application of a new array processing methodology. In particular, they group seismic stations into virtual subarrays, and compute velocity seismograms (vespagrams) for each subarray using a bootstrap resampling approach ultimately attaining multiple vespagrams for each subarray. For each subarray, the investigators automatically identify (1) seismic wave arrivals, (2) 95% confidence limits in travel-time and slowness, and (3) estimates on the likelihood that the arrival is not due to noise. They further demonstrate that they can image Dʺ discontinuity seismic arrivals using this methodology for shallow earthquakes which has the potential to increase data coverage of the lower mantle substantially. The investigators further develop a Bayesian inversion scheme to model the seismic velocity profile associated with their observations which provides further estimates of the errors, uncertainties and trade-offs associated with the observations. By combining the results from many subarrays, they further develop confidence constraints on discontinuity structures. With this approach the investigators hope to identify, catalog and map laterally variable yet consistent SH-wave seismic discontinuities in the bottommost 1,000 km of the mantle and to provide constraints on the depth range and seismic velocity structure at which these discontinuities exist based on error estimates and forward modeling. 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.