Toward Global-Scale Submarine Fiber Sensing: Early Results from Multispan DAS at the OOI Regional Cabled Array

A recently developed multispan distributed acoustic sensing (multispan-DAS) technique from Nokia Bell Labs enables strain measurements along submarine fiber-optic cables across multiple repeater-separated spans. By leveraging the high-loss loopback couplers within optical repeaters, this technique overcomes the long-standing limitation of conventional DAS to the first span of a repeated cable, typically < 100 km offshore. Dense, continuous arrays of seafloor strain sensors can now extend to hundreds or thousands of kilometers. This technique has been used to successfully record the 2025 M8.8 Kamchatka earthquake and tsunami at teleseismic range with a spatial resolution of ~100 m across 4400 km of a repeated submarine cable.In November 2025, the multispan-DAS system from Nokia Bell Labs was deployed for three months on both repeated submarine cables of the Ocean Observatories Initiative Regional Cabled Array (OOI RCA) offshore Oregon. The deployment traverses the Cascadia subduction zone forearc and extends approximately 500 km offshore to Axial Seamount. During this period, the first span of the southern cable was simultaneously interrogated using a multiplexed conventional DAS unit, while data continued to stream from co-located cabled seismometers, hydrophones, and other oceanographic instruments on the OOI RCA.The multispan-DAS system recorded a regional earthquake beyond the first repeater of both cables during testing as well as the ambient seafloor seismic wavefield, demonstrating sensitivity to a broad range of seismic, oceanographic, and acoustic signals. These observations provide a unique opportunity to directly compare multispan-DAS measurements with conventional DAS and established seafloor instrumentation across a large spatial extent. The resulting dataset will be publicly released following documentation and quality control. We will present preliminary results characterizing the noise floor, sensitivity, and signal fidelity of multispan-DAS relative to co-located sensors, and examine the consistency of observed seismic and oceanographic signals across measurement modalities. These results will highlight the potential of multispan-DAS for applications including routine earthquake monitoring, earthquake early warning, and broader seafloor observation, and represent an important step toward establishing this technique as a new tool for the seismological and oceanographic communities.