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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]

In December 2016, the unmonitored Bogoslof volcano unexpectedly began erupting, producing at least 70 explosions over the next 8.5 months. Roughly half of the explosions were preceded by observable short-term precursory activity, defined here as seismic activity ending within 30 min of the explosion onset or an earthquake swarm ending within a few hours before the explosion onset. Five different types of precursors were noted during the Bogoslof eruption: non-swarm earthquakes, earthquake swarms, rapid rate events, tremor, and chromatic tremor. Most of the precursors that were detectable by the nearby seismic networks (> 45 km away) occurred during the first 3 months of the eruption, with nearly all precursory earthquake swarms occurring during this period. A hydrophone deployed ~ 7 km northeast of the summit for the last 3 months of the eruption detected weaker activity that was not recorded by the more distant seismic networks. We describe the observed precursors and discuss their inter-relations. Overall, the precursors seem indicative of processes occurring throughout the volcanic system and suggest possible changes in the system during the eruption. We then compare the short-term precursors with explosion parameters, such as plume height, duration, and occurrence of lightning, to look for relations that may provide more insight into the eruption dynamics and subsurface processes and help to further improve forecasting. We find no obvious relations between any of the precursor or explosion parameters, suggesting that their use in forecasting eruption characteristics at Bogoslof is limited. [ABSTRACT FROM AUTHOR]

Abstract: The 2016–2017 eruption of Bogoslof volcano, a submarine stratovolcano in the Bering Sea, produced 70 discrete explosive eruptions over 8 months. With no local monitoring data, activity was seismically recorded on nearby islands 50–100 km away, limiting the detection and resolution of seismic observations. We construct a matched filter catalog of 3,199 events from 49 earthquake families, many of which occurred with hydroacoustic T waves of varying strength. We then use a 2‐D finite difference model to show that hydroacoustic amplitudes should decrease with increased source depth beneath the edifice and leverage each family's seismically recorded T wave amplitude as a proxy for source depth, which we compare to regional infrasound data. This unique combination of using P and S waves to detect events, T waves as a proxy for depth, and infrasound for precise timing of emissions allows us to interpret the dynamics and evolution of the Bogoslof eruption. [ABSTRACT FROM AUTHOR]

The Failure Forecast Method (FFM) was introduced as an empirical model for forecasting catastrophic material failures related to natural hazards, such as landslides and volcanic eruptions, with mixed success. During the 2018 eruption of Kilauea volcano, Hawaii, the draining of the summit magma reservoir into the Lower East Rift Zone resulted in the formation of a new caldera at the summit. I tested the applicability of the FFM to caldera collapse by analyzing the cyclical earthquake swarms and ground deformation that occurred between 62 sudden major caldera collapse events. The progression of both the cumulative moment release of the cyclical earthquakes and the GNSS displacement show a major change in mid-June. In late May through early June, the progression of the parameters is consistent with strain localization or creep progression related to the development or activation of the ring fault system. From late June until the end of the eruption, parameter progression is roughly steady with initial accelerating increases in cumulative moment and displacement that shift to approximately linear progression. Analysis of repeating earthquake families in the cyclical swarms showed that the behavior of the repeaters was consistent with that of the cyclical swarms as a whole and suggested that each family undergoes its own progression of activation to termination. While the FFM analysis identified the system change in mid-June, it did not demonstrate an ability to forecast collapse events or the end of the eruption. • Examined the 2018 Kilauea caldera collapse with the failure forecast method. • Analyzed cumulative moment and ground displacement preceding collapse events. • Shift from ring fault development to stick-slip motion identified in mid-June. • Cumulative moment behavior suggests earthquakes linked to stress state of system. • Ground displacement from GNSS stations have character similar to creep curves. [ABSTRACT FROM AUTHOR]

The 2018 eruption of Kīlauea volcano, Hawai'i, resulted in a major collapse of the summit caldera along with an effusive eruption in the lower East Rift Zone. The caldera collapse comprised 62 highly similar collapse cycles of strong ground deformation and earthquake swarms that ended with a magnitude 5 collapse event and one partial cycle that did not end with a collapse event. We analyzed geodetic and seismic data to better understand how the caldera collapse progressed over 3 months of activity, focusing on the cyclical activity. We identified 3 main phases of collapse: initial ring-fault activation and small explosions (Phase 1), an eastward shift in activity and freeing of the central piston (Phase 2), and a recoupling of the piston to the reservoir followed by relatively steady behavior until the eruption's end (Phase 3). Additionally, we observed geodetic evidence of tangential motion from the localization of the main ring fault (Phase 2) and the formation of a major peripheral ring fault on the eastern side of the collapse caldera during Phase 3. Both geodetic and seismic parameters suggest that the collapse may have had an eastward-component of motion after the ring fault system had formed. The cyclical seismic and geodetic parameters show no obvious signs that the collapse was coming to an end, with the only notable change being a significant increase in the ratio of cyclical displacement to co-collapse displacement observed during the last complete cycle on GNSS stations outside the caldera region. • Analyzed cyclic deformation and earthquake swarms from stress loading and faulting. • Evidence for asymmetric failure of the magma reservoir roof observed. • Identified 3 phases of caldera collapse, including decoupling/recoupling of roof. • Observed tangential motion of central piston and movement of peripheral block. • Caldera collapse mostly a passive result of magma reservoir drainage. [ABSTRACT FROM AUTHOR]

We analyze seismic tremor recorded during eruptive activity over the course of the 2016–2017 eruption of Bogoslof volcano, Alaska. Only regional recordings of the tremor wavefield exist for Bogoslof, making it a challenge to place the recordings in context with other eruptions that are normally captured by local seismic data. We apply a technique of time-frequency polarization analysis to three-component seismic data to reveal the wavefield composition of Bogoslof eruption tremor. We find that at regional distances, the tremor is dominated by P-waves in the band from 1.5 to 10 Hz. Using this information, along with an enriched Bogoslof earthquake catalog, we obtain estimates of average reduced displacement (DR) for eruption tremor during 25 of the 70 Bogoslof events. DR reaches as high as approximately 40 cm2 for two of the major events, similar to other VEI~3 eruptions in Alaska. Overall, average reduced displacement displays a weak correlation to plume height during the first half of the 9-month-long eruption sequence, with a few notable exceptions. The two events with the highest DR values also generated measurable eruption tremor at very-long-periods (VLP) between 0.05 and 0.15 Hz. [ABSTRACT FROM AUTHOR]

Basaltic shield volcanoes of the western Galápagos islands are among the most rapidly deforming volcanoes worldwide, but little was known of the internal structure and brittle deformation processes accompanying inflation and deflation cycles. A 15-station broadband seismic array was deployed on and surrounding Sierra Negra volcano, Galápagos from July 2009 through June 2011 to characterize seismic strain patterns during an inter-eruption inflation period and to evaluate single and layered magma chamber models for ocean island volcanoes. We compare precise earthquake locations determined from a 3D velocity model and from a double difference cluster method. Using first-motion of P-arrivals, we determine focal mechanisms for 8 of the largest earthquakes ( M L ≤ 1.5 ) located within the array. Most of the 2382 earthquakes detected by the array occurred beneath the broad (∼9 km-wide) Sierra Negra caldera, at depths from surface to about 8 km below sea level. Although outside our array, frequent and larger magnitude ( M L ≤ 3.4 ) earthquakes occurred at Alcedo and Fernandina volcano, and in a spatial cluster beneath the shallow marine platform between Fernandina and Sierra Negra volcanoes. The time–space relations and focal mechanism solutions from a 4-day long period of intense seismicity June 4–9, 2010 along the southeastern flank of Sierra Negra suggests that the upward-migrating earthquake swarm occurred during a small volume intrusion at depths 5–8 km subsurface, but there was no detectable signal in InSAR data to further constrain geometry and volume. Focal mechanisms of earthquakes beneath the steep intra-caldera faults and along the ring fault system are reverse and strike-slip. These new seismicity data integrated with tomographic, geodetic, and petrological models indicate a stratified magmatic plumbing system: a shallow sill beneath the large caldera that is supplied by magma from a large volume deeper feeding system. The large amplitude inter-eruption inflation of the shallow sill beneath the Sierra Negra caldera is accompanied by only very small magnitude earthquakes, although historical records indicate that larger magnitude earthquakes (Mw <6) occur during eruptions, trapdoor faulting episodes without eruptions, and large volume flank intrusions. [ABSTRACT FROM AUTHOR]