Photonic seismology in Monterey Bay: Dark fiber DAS illuminates offshore faults and coastal ocean dynamics

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Nathaniel Lindsey, Craig T. Dawe, Jonathan Ajo-Franklin


Emerging fiber-optic sensing technology coupled to existing subsea telecommunications cables can provide access to unprecedented seafloor observations of both ocean and solid earth phenomena. During March 2018, we conducted a Distributed Acoustic Sensing (DAS) measurement campaign along a buried fiber-optic cable typically used for data transfer to and from a scientific cabled observatory offshore Monterey Bay called the Monterey Accelerated Research System (MARS) node. During a 4-day period of MARS node maintenance the MARS cable was repurposed as an evenly-spaced ~10,000-component, 20-kilometer-long DAS array. Full wavefield observation of a M3.4 earthquake that occurred 45-km inland near Gilroy, CA illuminated multiple recently-mapped and previously unmapped submarine fault zones, which were observed to slow the propagating wavefront and act as point scatterers reradiating body-wave energy as Scholte waves. In the shallow water of the MARS cable (h<100m), dominant noise (f~0.1-0.3 Hz) was found to match the predicted seafloor pressure field induced by shoaling ocean surface waves, otherwise known as the primary ocean microseism. DAS amplitudes track sea state dynamics during a storm cycle in the Northern Pacific, correlating with features of local bay buoy and onshore broadband seismometer data streams. We also observed secondary microseisms (f~0.5-2 Hz). Decomposing the incoming and outgoing wavefield components of the primary microseism noise we validated the Lougnet-Higgins-Hasselmann theory that bi-directional ocean wind-waves setup by the coast reflection undergo nonlinear wave mixing to cause the secondary microseisms, even when the outgoing energy is only 1% of the incoming energy. We observe additional noise patterns at higher and lower frequencies that are consistent with previous point sensor observations of post-low-tide tidal bores (f~1-5 Hz), storm-induced sediment transport (f~0.8-10 Hz), infragravity waves (f~0.01-0.05 Hz), and breaking internal waves (f~0.001 Hz). The number of geophysical interactions observed over this brief four-day dark fiber recording evidences the introduction of an important new technique for seafloor science.



Earth Sciences, Geophysics and Seismology, Physical Sciences and Mathematics


Seismology, marine geophysics, DAS, Ambient noise, Distributed acoustic sensing, microseism, fiber-optic, array seismology, earthquake wavefield, infragravity waves, internal waves, Monterey Bay


Published: 2019-07-09 05:28

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