Investigating the Lithosphere-Asthenosphere System by Integrating New Constraints on Seismic Attenuation with Existing Geophysical Observations

Seismic waves propagating though a truly elastic material would, in principle, keep on propagating forever. In the Earth, various mechanisms make the seismic energy 'leak' out of the waves, such that it is not available for further propagation; leading eventually to the dissipation of the energy. This is what is known as seismic attenuation. Although seismic attenuation occurs in a matter of seconds, some of the same mechanisms that control this phenomenon have profound effects on the way the plates respond to tectonic stresses over millions-of-years timescales. For this reason, understanding where seismic attenuation is strong and weak can help address questions such as: How do failed rifts (scars left on the plates by episodes of extension) affect the strength of the plates? And, what controls the location of mountain ranges and earthquakes that occur away from the plate boundaries? Additionally, the effect of small amounts of melt in mantle rocks may or may not significantly affect attenuation depending on how that melt is distributed within the rock matrix. Therefore, understanding attenuation in what are likely melt-bearing regions can help us understand where and how melt is retained in the upper mantle.

Seismic attenuation is relatively difficult to study both in the field and in the lab, and thus constraints on attenuation structure are much scarcer than those available for other properties of the deep subsurface, primarily seismic propagation velocity. This project will apply a recently developed robust methodology for imaging the attenuation structure beneath the continents to the high-quality seismic data sets that have resulted from the EarthScope initiative. Several focus sites were selected that have been covered by FlexArray seismic deployments and that represent prime examples of the features that this project is aiming to investigate. Furthermore, combining the new results that will be generated with other geophysical information will provide stronger answers to the questions the project poses than would have been possible with any one technique alone. The attenuation measurements will be combined with previous constraints on seismic velocity and conductivity (where available) jointly analyzed using software tools developed by another EarthScope-funded project to discern whether the observed anomalies are best explained by factors such as enhanced temperature, presence of small amounts of melt, enhanced water content, or others. Finally, the data analysis software developed as part of this project will be enhanced by building intuitive user interfaces and self-contained software packages that will be made available to the scientific community. Facilitating the dissemination and use of these tools will lower the barriers to making this kind of measurement and doing this kind of research. Broader Impacts of the work are many. The MATLAB code used to process attenuation as part of this proposal will be publicly released to the community. By pairing the code with an intuitive GUI, scientists new to the field will more easily be able to measure the attenuation of teleseismic body waves. This proposal will fund one postdoctoral researcher and a Hispanic early career PI.

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.