Through the Ocean to the Mantle: Seismic Study of the Pacific Mantle with Long-Lived Autonomous Floating Seismic Sensors

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Seismometers record seismic waves generated by earthquakes. Changes in seismic wave speeds are caused by variations in the temperature and composition of Earth. These changes in wave speeds are the primary observation used to understand the structure and evolution of Earth, from the scale of mantle convection and the way heat is transferred from the core to the surface to the interaction between deep Earth and surface processes such as tectonic plate motion and crustal deformation. Geographical data coverage by seismometers that record the seismic waves is not uniform, however, and this remains the fundamental limiting factor in seismic studies. Because two thirds of Earth's surface is covered by oceans, and it is difficult to deploy seismometers in the oceans, instrument coverage in these areas is sparse. Thus, all three-dimensional models of Earth are marked by blank spots in areas where little or no information can be obtained. An autonomous robotic earthquake sensor has been developed and deployed in the Pacific Ocean near French Polynesia. This project will analyze the data from this fleet of mobile sensors to image small-scale structures in Earth's mantle and provide a better understanding of the thermal history of Earth. An Adopt-a-Float smartphone app will be developed during this project and used in K-12 outreach projects. The project supports the training of students in marine geology and geophysics and mantle tomography, and in software design, education and outreach, statistics and applied mathematics.The resolving power of global seismic-tomographic wave speed models in the mid- to deep mantle remains limited due the lack of seismic data from the oceans, hampering a more detailed exploration and interpretative understanding of Earth's uncharted interior. Deploying and recovering ocean-bottom devices is technically challenging and expensive. Low-cost, freely-floating, autonomous hydrophone arrays are enhancing the resolving power of P-wave tomography. With an international consortium, floats have been deployed in French Polynesia to record teleseismic wave arrivals over the next five years. In this first two-year phase of the project, the data recovered in near-real time will be collected, quality-controlled, archived, documented and publicly disseminated. The first-arrival travel-time anomalies will be analyzed with respect to one-dimensional velocity reference models with an eye towards creating a curated high-quality data set suitable for regional tomographic inversions. The seismological analysis of travel-time anomalies from the oceanic array will be compared with new results obtained from mantle transition-zone receiver functions constructed from records collected at nearby land stations. The aperture of the array is a substantial improvement over the handful of stations in the global networks, hence the results of both lines of study will represent concrete improvements on understanding of the mantle. Mantle tomography is limited by data availability; sampling is poorest in the southern hemisphere, which is home to a number of mantle plumes that drive the heat engine that cools deep Earth. The connections of hot spots to mantle structure at great depth remain uncertain. The new data analyzed during this project will be used to frame and ultimately answer major questions on present-day mantle dynamics and on the thermal evolution of 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.