Ocean Bottom Distributed Acoustic Sensing for Oceanic Seismicity Detection and Seismic Ocean Thermometry.

A T‐wave is a seismo‐acoustic wave that can travel a long distance in the ocean with little attenuation, making it valuable for monitoring remote tectonic activity and changes in ocean temperature using seismic ocean thermometry (SOT). However, current high‐quality T‐wave stations are sparsely distributed, limiting the detectability of oceanic seismicity and the spatial resolution of global SOT. The use of ocean bottom distributed acoustic sensing (OBDAS), through the conversion of telecommunication cables into dense seismic arrays, is a cost‐effective and scalable means to complement existing seismic stations. Here, we systematically investigate the performance of OBDAS for oceanic seismicity detection and SOT using a 4‐day Ocean Observatories Initiative community experiment offshore Oregon. We first present T‐wave observations from distant and regional earthquakes and develop a curvelet denoising scheme to enhance T‐wave signals on OBDAS. After denoising, we show that OBDAS can detect and locate more and smaller T‐wave events than regional OBS network. During the 4‐day experiment, we detect 92 oceanic earthquakes, most of which are missing from existing catalogs. Leveraging the sensor density and cable directionality, we demonstrate the feasibility of source azimuth estimation for regional Blanco earthquakes. We also evaluate the SOT performance of OBDAS using pseudo‐repeating earthquake T‐waves. Our results show that OBDAS can utilize repeating earthquakes as small as M3.5 for SOT, outperforming ocean bottom seismometers. However, ocean ambient natural and instrumental noise strongly affects the performance of OBDAS for oceanic seismicity detection and SOT, requiring further investigation. Plain Language Summary: Oceanic earthquakes can produce loud sounds in the ocean. These sounds usually arrive at a seismic station as the tertiary wave, a so‐called T‐wave, following the arrival of the primary P‐wave and secondary S‐wave. T‐waves can propagate thousands of kilometers in the ocean's SOund Fixing And Ranging (SOFAR) channel with little energy loss. Thus, they are useful for monitoring earthquakes and ocean temperature changes. However, currently available instruments for measuring these waves are limited. Recently, a new type of technique, Distributed Acoustic Sensing, provides an opportunity to expand the seismic‐recording capability in the ocean. Ocean bottom distributed acoustic sensing (OBDAS) can effectively turn submarine telecommunication cables into dense seismic sensors that complement conventional seismometers. In this study, we explore the OBDAS potential for T‐wave detection. With a 4‐day OBDAS community experiment offshore Oregon, we demonstrate that OBDAS does a better job than a conventional seismic network for detecting T‐waves when a specifically designed denoising scheme is applied. In addition, OBDAS has the potential to measure ocean temperature changes using more repeating earthquakes of smaller magnitudes, outperforming conventional sensors. However, the accuracy of the OBDAS system can be strongly affected by various types of noise, which requires further research. Key Points: We develop a curvelet denoising scheme for ocean bottom distributed acoustic sensing to enhance T‐wave signalsThe denoised distributed acoustic sensing data detects three times more T‐wave events than cabled ocean bottom seismometersThe improved detection capability makes more small repeating earthquakes useable for seismic ocean thermometry [ABSTRACT FROM AUTHOR]