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Many of the world's volcanoes are hidden beneath the ocean's surface where eruptions are difficult to observe. However, seismo‐acoustic signals produced by these eruptions provide a useful means of identifying active submarine volcanism. A literature survey revealed reports of 119 seismo‐acoustically recorded submarine eruptions since 1939. Submarine eruptions have been recorded in all major tectonic settings, with a range of geochemistries, and at a variety of water depths, but the reports are dominated by eruptions in the Pacific and at only a few locations. Many of the reports offer little detail, with over half of the observations made from distances >500 km, and only about half were confirmed as eruptions by non‐seismo‐acoustic evidence. The reported seismo‐acoustic signals cover a wide variety of processes, including earthquakes, explosions, various types of tremor, signals related to lava extrusion, and landslides. Recorded signals can sometimes be difficult to classify or confidently associate with an eruption, although there has been progress in this regard. Real‐time monitoring of submarine eruptions has been on‐going for several decades on regional and global scales with growing interest and effort in local networks. Real‐time networks are complemented by short‐term instrument deployments that often give more detailed insights into the dynamics and processes of submarine eruptions. Thorough seismo‐acoustic monitoring and study has increased our understanding of submarine eruptions, especially of deep‐sea volcanoes and spreading centers. Despite this, there are still many outstanding questions that need to be addressed for submarine volcanoes to be as well understood and monitored as their terrestrial counterparts. Key Points: A literature survey found reports of 119 submarine volcanic eruptions recorded by seismic, hydroacoustic, or acoustic instruments since 1939This review summarizes the current state of submarine eruption seismo‐acoustics based on the literature survey resultsSeismo‐acoustic monitoring and study has increased our understanding of submarine eruptions but many scientific opportunities remain [ABSTRACT FROM AUTHOR]

In mid-December 2016, Bogoslof volcano, Alaska, began an 8.5-month-long eruption that produced at least 70 explosions and three lava domes. Bogoslof is an emergent submarine volcano with only the very top forming a small island, so the eruptive activity mostly occurred from a vent submerged beneath a bay or lagoon. The Bogoslof eruption was recorded on regional seismic and infrasound arrays as well as by a hydrophone that was deployed locally during the second half of the eruption. Since few emergent volcanic eruptions have seismo-acoustic recordings, these observations, taken in context with satellite and other observations, provide an opportunity to greatly improve our understanding of the seismo-acoustic signals produced by these eruptions. The instruments detected a range of activity including earthquakes, tremor, and mass flow events. Earthquakes occurred before, during, after, and unassociated with explosions and also often occurred in swarms. The 47 seismically-detected swarms and 27 additional hydroacoustically detected swarms can be further broken into four types: precursory, post-eruptive, general, and tremor-dominated. Most seismic swarms were less than 10 h long, had fewer than 50 earthquakes, and had average earthquake rates below 20/h. For the explosions, we calculate a tremor magnitude, frequency index, and average frequency. The tremor magnitudes determined from the hydrophone data show a roughly linear relation to explosion plume height, though this relation was not clear in the more limited seismic data. Lastly, we categorize the activity into five eruption phases based on seismo-acoustic character—precursory, opening, explosive I, pause, and explosive II. [ABSTRACT FROM AUTHOR]

Ahyi seamount, a shallow submarine volcano in the Northern Mariana Islands, began erupting on 23 April 2014. Hydroacoustic eruption signals were observed on the regional Mariana seismic network and on distant hydrophones, and National Oceanic and Atmospheric Administration (NOAA) scuba divers working in the area soon after the eruption began heard and felt underwater explosion sounds. The NOAA crew observed yellow‐orange bubble mats along the shore of neighboring Farallon de Pájaros Island, but no other surface manifestations of the eruption were reported by the crew or observed in satellite data. Here, we detail the eruption chronology and its morphologic impacts through analysis of seismic and hydroacoustic recordings and repeat bathymetric mapping. Throughout the 2‐week‐long eruption, Ahyi produced several thousand short, impulsive hydroacoustic signals that we interpret as underwater explosions as well as tremor near the beginning and end of the sequence. The initial tremor, which occurred for 2 hr, is interpreted as small phreatomagmatic explosions. This tremor was followed by a 90‐min pause before the characteristic impulsive signals began. Occasional tremor (lasting up to a few minutes) during the last 1.5 days of the eruption is interpreted as more sustained eruptive activity. Bathymetric changes show that a new crater, about 150 m deep, formed near the former summit and a large landslide chute formed on the southeastern flank. Comparing to other geophysically detected submarine eruptions, we find that the signals from the 2014 Ahyi eruption were more similar to those from other shallow or at‐surface submarine eruptions than those at deep (>500 m) eruptions. Plain Language Summary: Ahyi seamount, a shallow submarine volcano in the Commonwealth of the Northern Mariana Islands (CNMI), began erupting on 23 April 2014. The U.S. Geological Survey first noticed signs of the eruption during a routine data check on 24 April, while National Oceanic and Atmospheric Administration scuba divers working in the area heard and felt underwater explosion sounds. We analyze recordings of the eruption on the CNMI seismic network and on hydrophones located near Wake Island to detail how the eruption unfolded. The eruption started with about 2 hr of tremor from magma explosively interacting with water. After a 90‐min pause, short (up to a few seconds) explosions began and continued for 2 weeks. During the last 1.5 days of the eruption, longer tremor signals (up to a few minutes) from more sustained degassing eruptions occurred along with the short explosions. A comparison of bathymetric maps made before and after the eruption shows that the explosions formed a new crater 150 m deep near the summit and that a landslide chute formed on the southeastern flank. The seismic and hydroacoustic signals from the Ahyi eruption are more similar to those from eruptions at other shallow or at‐surface seamounts than to those from deep (>500 m) eruptions. Key Points: The submarine volcano Ahyi erupted for 2 weeks in April–May 2014 and was recorded by regional seismometers and distant hydrophonesThe eruption was characterized by several thousand explosions and occasional tremor at the beginning and end of the eruptive periodRepeat bathymetry reveals a new summit crater and a new, large landslide chute on the south flank [ABSTRACT FROM AUTHOR]