Plume Structure and Mantle Layering Beneath the South Pacific: Modeling Teleseismic Waveforms from Traditional and Floating Sensors

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Besides formidable hazards to humans, earthquakes are sources of energy that help us image the Earth’s interior. Earth is a dynamic planet, its interior always in convective motion, and to understand it as a system, seismologists mine the information contained in the measurement of earthquake waves recorded by seismometers across the globe. An area of specific interest is Earth’s mantle below the Pacific, which is strewn with volcanoes, most of them underwater, but many present as oceanic islands such as Hawaii, Samoa, and Tahiti. The source region of these volcanoes is insufficiently known; in particular it is not clear at what depths in the mantle they originate. Part of the reason for the lack of clarity is that seismic observations across the oceans are very hard to obtain. A new type of instrument, MERMAID, is a free-floating “hydrophone”, essentially an underwater microphone, drifting with the deep ocean currents, that picks up earthquake waves, almost like traditional seismometers. In this project, the researchers will use recordings from about 65 of these new instruments to update Earth models based on our current state of knowledge about earthquakes and Earth’s interior. The results will be new and better images of the Earth’s interior, which can be linked back to the processes operating at depth and expressed as volcanism at the surface. These scientific modeling activities will contribute to the research education of undergraduate geosciences majors, a geophysics graduate student and a post-doctoral researcher, the researchers engage in community and capacity building by archiving, curating, and sharing the data with the public via national data centers.MERMAID was originally designed to capture compressional first-arriving “P”-wave arrivals. Preliminary research on new techniques for using this data has shown detail in the entire wave train that can be used for waveform modeling, allowing the researchers to go beyond simple travel-time picking. The researchers will further develop this new technique, and apply it to distant, or “teleseismic” events identified in the entire dataset returned by all the MERMAID instruments active in the global oceans today, and to integrate them into mantle wavespeed and impedance contrast models for Polynesia. The waveform modeling technique for tomography focuses on smooth wavespeed variations and relies on modeling the ``oceanic last mile'' of the teleseismic mantle wavefield via spectral-element mapping using a transfer-function approach. The impedance contrast modeling technique focuses on sharp contrasts in wave speed and density and uses precursors to surface-reflected seismic phases to produce images of deep reflectors of both mantle transition-zone and mid-mantle discontinuities. It is a wave-equation imaging method that involves reverse-time migration employing full-waveform tomographic models for wavefield extrapolation. The modeling takes into account all three components of the seismogram and is accompanied by detailed sensitivity and hypothesis tests to give confidence in interpretation of the physical nature of mantle discontinuities in relation to the mantle plumes that will hopefully be revealed from wavespeed structure mapping.This project is supported by the Geophysics Program and the Geoinformatics Program in the Division of Earth Sciences.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.