Recording earthquakes for tomographic imaging of the mantle beneath the South Pacific by autonomous MERMAID floats

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Authors

Joel D. Simon , Frederik J. Simons 

Abstract

We present the first 16 months of data returned from a mobile array of 16 freely-floating diving instruments, named MERMAID for Mobile Earthquake Recording in Marine Areas by Independent Divers, launched in French Polynesia in late 2018. Our 16 are a subset of the 50 MERMAIDs deployed over a number of subsequent cruises in this vast and understudied oceanic province as part of the collaborative South Pacific Plume Imaging and Modeling (SPPIM) project, under the aegis of the international EarthScope-Oceans consortium. Our objective is the hydroacoustic recording, from within the oceanic water column, of the seismic wavefield generated by earthquakes worldwide, and the nearly real-time transmission by satellite of these data, collected directly above and on the periphery of the South Pacific Superswell. This region, characterized by anomalously elevated oceanic crust and myriad seamounts, is believed to be the surface expression of a deeply-rooted mantle plume. Tomographically imaging Earth's mantle under the South Pacific with data from these novel instruments requires a careful examination of the earthquake-to-MERMAID travel-times of the high-frequency P-wave detections within the windows selected for reporting by the discrimination algorithms on board. We discuss a workflow suitable for a fast-growing mobile sensor database to pick the relevant arrivals, match them to known earthquakes in the global earthquake catalogs, calculate their travel-time residuals with respect to global seismic reference models, characterize their quality, and estimate their uncertainty. We detail seismicity rates as recorded by MERMAID over 16 months, break these statistics down by magnitude to quantify the completeness of our catalog, and discuss magnitude-versus-distance relations of detectability for our network. The projected lifespan of an individual MERMAID is five years, allowing us to estimate the final size of the data set that will be available for future study. To prove their utility for seismic tomography we compare the MERMAID data quality against “traditional" land seismometers and their low-cost Raspberry Shake counterparts, using waveforms recovered from instrumented island stations in the geographic neighborhood of our floats. Finally, we provide the first analyses of travel-time anomalies for the new ray paths sampling the mantle under the South Pacific over the first 16 months of operation of our array.

DOI

https://doi.org/10.31223/X59C7N

Subjects

Geophysics and Seismology

Keywords

MERMAID, South Pacific Ocean, Seismic-phase identification, Seismic velocities

Dates

Published: 2020-12-02 22:17

Last Updated: 2021-02-23 01:43

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License

CC BY Attribution 4.0 International

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