Structure of the North American Continent Using EarthScope USArray Data and Applied Wave Gradiometry Methods

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Wave gradiometry is a new technique utilizing the shape of seismic wave fields captured by EarthScope USArray transportable stations to determine fundamental wave propagation characteristics. Most techniques in seismology apply averaging methods, which smooth out wave field differences that may reflect real structure or complexity. Wave gradiometry methods are designed to take advantage of these wave field variations to resolve structure that cannot be derived strictly from averaging methods. The amazing thing about gradiomery is that if the spatial gradients of the wave field are known at a single seismic station, then the wave slowness, wave azimuth, change in geometrical spreading and radiation can also be determined at that point. The beauty of the EarthScope USArray, therefore, is that it provides an opportunity to quantify the spatial gradients of wave fields on a scale never before possible. This proposed work will involve further development and use of the wave gradiometry method in discovery-based research across the entire USArray, which will lead to new vital constraints for resolving 3-D structure within the lithosphere and upper-most mantle asthenosphere.We now have an important opportunity to take advantage of information within the wave fields, sampled by USArray, that have not been exploited to date. This observational approach, involving quantification of the displacement gradients within the wave fields, is the next important step in seismology that, when combined with theory, has the potential to lead to new important discoveries regarding 3-D structural complexity. We present here new adaptations to gradiometry method, including treatment of continuous displacement gradient fields within subarrays, and the use of multiple time series of spatial gradients obtained for each seismic station using reciprocity principle. Another important component of our research is the work with synthetic data (1) for calibration and to 'ground truth' the method and (2) for a better understanding of how wave field parameters estimates (slowness, wave azimuth, changes in geometric spreading, and radiation pattern) are linked with structural complexity, including the influence of lateral variations in anisotropy. Phase velocity maps generated to date show interesting jumps in velocity near some major terrane boundaries, indicating that some of these features extend deep into the mantle lithosphere. Furthermore, results from two events in Vanuatu show that changes in geometrical spreading and radiation are consistent with focusing and defocusing of energy as it propagates through low/high velocity zones. Use of such information from multiple different events, and from multiple different back azimuths, has the potential to be transformational in areas such as 3-D tomographic inversions. The Broader Impact of this work will consist of the training of a Ph.D. student, Yuanyuan Liu, the training of Undergraduate and High School students who will work in the lab during the funding period, and the generation of educational products involving wave field animations. The P.I. has a strong track record in working with both undergraduates and High School students. This substantial effort in mentoring will continue during the proposed period. During the funding period we will produce, and make available, wave field animations (animations of strain) as an educational product. These animations show the dilatations, shear, and rotations (see snap shot in Figure 1) associated with the wave fields that cross USArray. These will be useful for visualizing a component of the field that few researchers or students are used to thinking about, thereby providing the opportunity for unexpected insight and discovery. All codes will be shared, and displacement gradient tensor fields determined from the USArray seismograms will be archived using the NSF-supported Extreme Science and Engineering Discovery Environment (XSEDE) facility. An integrated model website will be set up where we will provide data sets that are used in the published models, published model results, benchmarking exercises and files, codes and tutorials for all gradiometry modeling, and wave field animations. Results are expected to be published in a timely way and they plan to make all data sets and codes, along with tutorials, available on the web site.