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Axial Seamount is the most active underwater volcano in the northeast Pacific located ∼480 km off the coast of Oregon (45°56′N, 130°00′W). It is situated on the diverging midocean ridge boundary of the Juan de Fuca and Pacific plates, which has an intermediate spreading rate of 5-6 cm/yr (Wilson, 1988, Figure 1). It is also the location of the world's first underwater volcano observatory New Millennium Observatory and since 2014 has been the location of the National Science Foundation's Ocean Observatory Initiative Regional Cabled Array, which records real-time data from a network of monitoring instruments (Embley & Baker, 1999;Kelley et al., 2014). The volcano rises to a depth of about 1,400 m and has a distinct 3 × 8 km horseshoe-shaped caldera present at its summit (Figure 1). The caldera walls rise about 100 m above the caldera floor. The caldera floor is comprised of unsedimented basaltic lava flows and active hydrothermal vent systems are found along the caldera margin faults.

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Monitoring of active volcanoes is a challenging task due to unpredictable behavior of the magmatic system, lack of financial resources for monitoring, and a variety of other factors (Scarpa & Gasparini, 1996). These challenges are amplified when monitoring submarine volcanoes, which constitute the majority of volcanoes on earth (Crisp, 1984). For example, Axial Seamount is the best-instrumented submarine volcano in the world, but an eruption in 2011 was not identified until new lava flows were found 3 months later by the remotely operated vehicle Jason (W. W. Chadwick et al., 2012).The current array of volcano monitoring methods and techniques is vast and impressive, and it is clear that a monitoring network that uses many techniques is the most effective approach (Miller & Jolly, 2014;Scarpa & Gasparini, 1996). Seismic monitoring is a popular method to detect unrest via small earthquakes and tremors that are associated with magma movement (McNutt & Roman, 2015). However, magma can move aseismically, particularly if ascent is slow (Lu et al., 2000;Roman & Cashman, 2018). Gravity measurements are the most powerful technique that is currently available, as changes to the crustal gravity field are unambiguously associated with changes to the magmatic system (Battaglia et al., 2008;Carbone et al., 2019). However, high-quality gravimetric monitoring can cost hundreds of thousands of dollars for the instruments alone (Fernández et al., 2017). Current volcano monitoring programs could benefit from a method that is similar to gravimetry but operates at a much lower cost.