March 3, 2017

Featuring

William Wilcock
University of Washington

Seafloor cabled observatories are expensive to deploy but provide infrastructure to support real-time long-term observations of episodic processes that may be challenging to make by other means. NSF’s Ocean Observatories Initiative cabled observatory was installed in the Northeast Pacific Ocean in 2014. It supports an expandable network of ~130 sensors divided between sites on the Cascadia margin off central Oregon and Axial Seamount, a hotspot volcano that lies at the center of the Juan de Fuca mid-ocean ridge. Axial Seamount erupted in 1998 and 2011, and the cabled instrumentation deployed on its summit is designed to observe the volcanic and hydrothermal systems. Within just over 5 months of the observatory coming on line, the volcano erupted on April 24, 2015. A small seismic network at the summit has detected over 200,000 local earthquakes most of which occurred in the build-up to the eruption and during the seismic crisis that accompanied the eruption. Prior to the eruption, the earthquakes are very strongly correlated with ocean tides. During the eruption, the caldera floor subsided 2.4 m, a dike propagated along the seamount’s North Rift, and lava erupted within the caldera and at several locations along the North Rift. The seismic network recorded numerous seafloor explosions on the lava flows that appear to document the emplacement of lava on the seafloor. The earthquake locations delineate an outward-dipping caldera ring fault that slipped one way to accommodate inflation and reversed motion during the eruption. The volcano has now resurged about 1 m but seismicity levels remain very low. For the next eruption, the observatory would benefit from the installation of a mooring or resident autonomous underwater vehicle (AUV) to monitor hydrothermal plumes and in the case of an AUV, visit eruption sites. Because eruptions at Axial appear to be predictable, there will also be opportunities to enhance observations temporarily with autonomous instruments. The Axial observatory demonstrates the reliability of cabled observatories for geophysical observations. Other applications of cabled observatories of interest to the speaker are the expansion of cabled observatories on the Cascadia subduction zone to support earthquake and tsunami early warning and hazards research, and the long-standing idea of incorporating scientific sensors into the optical repeaters that are distributed uniformly along transoceanic commercial telecommunication cables.