A significant proportion of active volcanoes of the world sit offshore. Monitoring them is vital because of the impacts of eruptions on human activities, such as the disruption of marine traffic. However, offshore volcanoes are not studied as much as their onshore counterparts because technical challenges hinder continuous monitoring.
Recent technological developments have allowed monitoring of some submarine volcanoes. Hefner et al.  present new work on Axial Seamount, an active seafloor volcano off the Oregon coast. They demonstrate that vertical displacements from the 2015 eruption recorded by ocean-bottom pressure gauges are well modeled by a combination of a deflation of a shallow magma reservoir and a fault slip on the caldera wall. A series of these studies demonstrate the utility of offshore measurements to monitor activity.
It is not only vertical displacements that are useful in monitoring offshore volcanoes but also horizontal displacements. For example, a new system has been developed using a rigid buoy with a Global Navigation Satellite System antenna, attached to the seafloor. Future technological developments will allow us to monitor submarine and island volcanoes offshore more extensively with less cost.
Citation: Hefner, W. L., Nooner, S. L., Chadwick, W. W., & Bohnenstiehl, D. W. R. . Revised magmatic source models for the 2015 eruption at Axial Seamount including estimates of fault‐induced deformation. Journal of Geophysical Research: Solid Earth, 125, e2020JB019356. https://doi.org/10.1029/2020JB019356
—Yosuke Aoki, Associate Editor, JGR: Solid Earth