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Volcano #1 is a large submarine stratovolcano with a summit caldera in the south central part of the Tonga Arc. We collected and analyzed multichannel seismic profiles in conjunction with magnetic data from Volcano #1 to investigate the structure of the intracaldera fill and processes of caldera formation. The intracaldera fill, exhibiting stratified units with a maximum thickness of 2 km, consists of at least four seismic units and a thick wedge of landslide debris derived from the caldera wall. The structural caldera floor, deepening toward the northwestern rim, suggests asymmetric collapse in the initial stage, which, in turn, appears to have contributed to the creation of a caldera elongated to the northwest by enhancing gravitational instability along the northwestern caldera boundary. Occasional, but repeated, eruptions resulted in a thick accumulation of the intracaldera fill and further subsidence in the mode of piston collapse. Magnetization lows are well-defined along the structural rim of the caldera that is interpreted as the inner principal ring fault. The magnetization lows indicate sites of submarine hydrothermal vents that caused an alteration of magnetic minerals. Faults recognized on the outer slope of the volcano are interpreted to be involved in hydrothermal fluid circulation.

On Dec. 22, 2018, at approximately 20:55–57 local time, Anak Krakatau volcano, located in the Sunda Straits of Indonesia, experienced a major lateral collapse during a period of eruptive activity that began in June. The collapse discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano, which generated a tsunami with runups of up to 13 m on the adjacent coasts of Sumatra and Java. The tsunami caused at least 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 Anak Krakatau event provides an opportunity to study a major volcanically-generated tsunami that caused widespread loss of life and significant damage. Here, we present numerical simulations of the tsunami, with state-of the-art numerical models, based on a combined landslide-source and bathymetric dataset. We constrain the geometry and magnitude of the landslide source through analyses of pre- and post-event satellite images and aerial photography, which demonstrate that the primary landslide scar bisected the Anak Krakatau volcano, cutting behind the central vent and removing 50% of its subaerial extent. Estimated submarine collapse geometries result in a primary landslide volume range of 0.22–0.30 km 3 , which is used to initialize a tsunami generation and propagation model with two different landslide rheologies (granular and fluid). Observations of a single tsunami, with no subsequent waves, are consistent with our interpretation of landslide failure in a rapid, single phase of movement rather than a more piecemeal process, generating a tsunami which reached nearby coastlines within ~30 minutes. Both modelled rheologies successfully reproduce observed tsunami characteristics from post-event field survey results, tide gauge records, and eyewitness reports, suggesting our estimated landslide volume range is appropriate. This event highlights the significant hazard posed by relatively small-scale lateral volcanic collapses, which can occur en-masse , without any precursory signals, and are an efficient and unpredictable tsunami source. Our successful simulations demonstrate that current numerical models can accurately forecast tsunami hazards from these events. In cases such as Anak Krakatau’s, the absence of precursory warning signals together with the short travel time following tsunami initiation present a major challenge for mitigating tsunami coastal impact.

In 2018, Kīlauea Volcano experienced its largest lower East Rift Zone (LERZ) eruption and caldera collapse in at least 200 years. After collapse of the Pu‘u ‘Ō‘ō vent on 30 April, magma propagated downrift. Eruptive fissures opened in the LERZ on 3 May, eventually extending ∼6.8 kilometers. A 4 May earthquake [moment magnitude (M w) 6.9] produced ∼5 meters of fault slip. Lava erupted at rates exceeding 100 cubic meters per second, eventually covering 35.5 square kilometers. The summit magma system partially drained, producing minor explosions and near-daily collapses releasing energy equivalent to M w 4.7 to 5.4 earthquakes. Activity declined rapidly on 4 August. Summit collapse and lava flow volume estimates are roughly equivalent—about 0.8 cubic kilometers. Careful historical observation and monitoring of Kīlauea enabled successful forecasting of hazardous events.

In the past two decades, the central portion of Campi Flegrei caldera has experienced ground uplift up to 15 mm/month, with an increase of rate, magnitude and extent of the seismicity. In this work, we perform multi‐scale precise earthquake relocation of the 2014–2024 seismicity, mapping in detail activated fault zones. We relate the geometry, extent, and depth of these zones with up‐to‐date structural reconstructions of the caldera. The current seismicity is mainly driven by ground‐uplift‐induced stress concentration on pre‐existing, weaker fault zones, some of which identified for the first time. These structures are not only related to the inner caldera and dome resurgence but also to volcano‐tectonic events of the last 10 ka. The extent of imaged fault segments suggests they can accommodate ruptures up to a moment magnitude 5.1, significantly increasing seismic hazard in the area. Plain Language Summary During the past 2 yrs, there has been a marked increase in ground uplift, number and size of earthquakes at Campi Flegrei caldera. This increase in activity has raised concerns among the population and public authorities about the impact of seismic activity on buildings and infrastructure in the area and about the best actions to undertake during the seismic emergency to reduce the risk. Additionally, the possibility of a future volcanic eruption is being considered, although currently geochemical and geophysical monitoring shows no unequivocal signs of precursory phenomena. In this work, we map the last decade of seismicity with a new technique providing high‐precision earthquake locations that allow us to unveil the currently activated fault zones of the inner caldera. The results show a near‐elliptical distribution of seismicity at the global scale of the caldera, which at a smaller scale delineates complex seismogenic structures. The size of imaged faults suggests that earthquakes up to a moment magnitude 5.1 can occur, significantly increasing seismic hazard in the area. Key Points High‐precision location of 2014–2024 seismicity in Campi Flegrei depicts active fault zones with unprecedented detail Seismicity occurs along different volcano‐tectonic structures including the caldera inner ring fault zone and faults bounding the Solfatara crater A new structure has been recognized in the eastern sector of the caldera, hosting the largest magnitude event in the analyzed period

The 15 January 2022, eruption at Hunga Tonga‐Hunga Ha'apai (HTHH) volcano started shortly after 4:00UTC. There had been noted unconfirmed precursory events. We analyzed seismometer data recorded in Fiji and Futuna, the closest stations operated during the eruption and located over 750 km away. We extracted Rayleigh waves and estimated their powers and source directions, assuming retrograde particle motions. We found a Rayleigh wave from the HTHH's direction about 15 min before the eruption onset. The arrival time difference of the Rayleigh wave between the two stations was consistent with that of the M5.8 earthquake during the eruption located beneath the HTHH. Referring to other seismic signals and satellite images, we concluded that the Rayleigh wave was the most significant eruption precursor with no apparent surface activity. Including our findings and results of previous studies, we propose a scenario of the beginning of the caldera‐forming eruption. Plain Language Summary Hunga Tonga‐Hunga Ha'apai (HTHH) volcano in Tonga had a caldera‐forming eruption on 15 January 2022. Disturbances associated with the eruption were recorded worldwide and by satellites. Many studies analyzed the data and reported that the eruption onset was shortly after 04:00UTC on January 15. However, some articles reported unconfirmed waves about 15 min before the eruption onset. This study is motivated by the following questions: (a) Were the unconfirmed waves actual? (b) Were they related to the eruption? (c) How did the huge eruption start? (d) How can we improve the monitoring of remote‐island and submarine volcanoes? Here, we analyzed data recorded at the closest seismic stations over 750 km away. We confirmed that a precursor event occurred ∼${\\sim} $ 15 min before the eruption and generated a significant seismic wave. This event might have been the trigger of the eruption. This study demonstrated that distant seismic stations and appropriate analysis methods will allow us to capture precursors leading to a catastrophic eruption. Key Points The volcano generated Rayleigh waves about 15 min before the giant eruption with no apparent surface activity These waves dominated in 0.03–0.1 Hz with amplitudes comparable to the amplitude of M4.9 Seismic stations 750 km from the volcano and appropriate data analyses allowed us to capture precursors of the catastrophic eruption

The Main Ethiopian Rift (MER) represents an area where volcanism and tectonics interact to create closely linked volcano-tectonic features. This linkage is paramount in the axial portion of the rift, where magmatic segments localize several large peralkaline eruptive centres. Many of them evolved into caldera collapse (the best preserved of which are younger than < 1 Ma ) generating large ignimbrites and registering the interaction between magmatism and tectonics along the MER. In this work we review the structure of the main collapsed calderas along the axial portion of the MER, to summarize the relationships between volcanism and tectonics proposed in the literature explaining their structural evolution. By doing this, we infer that tectonics had a strong influence in controlling the elongation of the majority of examined calderas. This control was induced by reactivation of inherited crustal fabrics or by stretching of the magma reservoirs under the MER regional stress field.

To investigate the magma plumbing system beneath the Akan Caldera (Hokkaido, Japan), we conducted broadband magnetotelluric surveys and imaged its three-dimensional resistivity structure to a depth of approximately 30 km. The caldera hosts post-caldera volcanoes, including Mt. Meakandake and Mt. Oakandake, which have recently shown ground inflation and heightened deep low-frequency seismicity. Our main finding is a prominent, westward-dipping conductive column beneath the caldera. This conductor is interpreted as the magma plumbing system of the Akan Caldera, comprising three distinct components, for which we estimated volumes based on the spatial extent of the conductive region: (1) the volatile-rich zone at depths of 2-5 km ( 10 km.sup.3, < 10 [Omega]m), (2) a mushy dacitic magma reservoir beneath the caldera center at depths of 3-15 km ( 500 km.sup.3, < 10 [Omega]m), and (3) a deeper basaltic magma reservoir to the west at depths of 15-30 km ( 4500 km.sup.3, < 70 [Omega]m). The active volcanoes, deformation sources, and earthquake hypocenters are located above or near the conductor edges (2), suggesting that magmatic fluids from the deeper western source (3) preferentially ascend along the column edges. A distinctive feature of the caldera is the flattened upper boundary of conductor (2), which is associated with sill-shaped ground inflation. This geometry may indicate the potential to store large volume of mushy magma, which could become eruptible if magma is directly supplied from greater depths. This study provides new insights into the magma plumbing system beneath the Akan Caldera and enhances our understanding of the mechanisms underlying its volcanic activity.

The 43 BCE eruption of Okmok Volcano has been proposed to have had a significant climate cooling impact in the Northern Hemisphere. In this study, we quantify the climate cooling potential of the Okmok II eruption by measuring sulfur concentration in melt inclusions (up to 1,606 ppm) and matrix glasses and estimate a total of 62 ± 16 Tg S released. The proportion reaching the stratosphere (2.5%–25%, i.e., 1.5–15.5 Tg S) was constrained by physical modeling of the caldera-collapse eruption. Using the NASA Goddard Institute for Space Studies E2.2 climate model we found a linear response between cooling and stratospheric sulfur load (0.05–0.08°C/Tg S). Thus, the 1–2°C of cooling derived from proxy records would require 16–32 Tg sulfur injection. This study underscores the importance of combining approaches to estimate stratospheric S load. For Okmok II, we find all methods are consistent with a range of 15–16 Tg S.