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

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.