Description: The 1888 Ritter Island volcanic sector collapse triggered a regionally damaging tsunami. Historic eyewitness accounts allow the reconstruction of the arrival time, phase and height of the tsunami wave at multiple locations around the coast of New Guinea and New Britain. 3D seismic interpretations and sedimentological analyses indicate that the catastrophic collapse of Ritter Island was preceded by a phase of deep-seated gradual spreading within the volcanic edifice and accompanied by a submarine explosive eruption, as the volcanic conduit was cut beneath sea level. However, the potential impact of the deep-seated deformation and the explosive eruption on tsunami genesis is unclear. For the first time, it is possible to parameterise the different components of the Ritter Island collapse with 3D seismic data, and thereby test their relative contributions to the tsunami. The modelled tsunami arrival times and heights are in good agreement with the historic eyewitness accounts. Our simulations reveal that the tsunami was primarily controlled by the displacement of the water column by the collapsing cone at the subaerial-submarine boundary and that the submerged fraction of the slide mass and its mobility had only a minor effect on tsunami genesis. This indicates that the total slide volume, when incorporating the deep-seated deforming mass, is not directly scalable for the resulting tsunami height. Furthermore, the simulations show that the tsunamigenic impact of the explosive eruption energy during the Ritter Island collapse was only minor. However, this relationship may be different for other volcanogenic tsunami events with smaller slide volumes or larger magnitude eruptions, and should not be neglected in tsunami simulations and hazard assessment.

Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347

Description: Several lines of evidence have previously been used to suggest that ice retreat after the last glacial maximum (LGM) resulted in regionally-increased levels of volcanic activity. It has been proposed that this increase in volcanism was globally significant, forming a substantial component of the post-glacial rise in atmospheric CO2, and thereby contributing to climatic warming. However, as yet there has been no detailed investigation of activity in glaciated volcanic arcs following the LGM. Arc volcanism accounts for 90% of present-day subaerial volcanic eruptions. It is therefore important to constrain the impact of deglaciation on arc volcanoes, to understand fully the nature and magnitude of global-scale relationships between volcanism and glaciation. The first part of this paper examines the post-glacial explosive eruption history of the Andean southern volcanic zone (SVZ), a typical arc system, with additional data from the Kamchatka and Cascade arcs. In all cases, eruption rates in the early post-glacial period do not exceed those at later times at a statistically significant level. In part, the recognition and quantification of what may be small (i.e. less than a factor of two) increases in eruption rate is hindered by the size of our datasets. These datasets are limited to eruptions larger than 0.1 km3, because deviations from power-law magnitude–frequency relationships indicate strong relative under-sampling at smaller eruption volumes. In the southern SVZ, where ice unloading was greatest, eruption frequency in the early post-glacial period is approximately twice that of the mid post-glacial period (although frequency increases again in the late post-glacial). A comparable pattern occurs in Kamchatka, but is not observed in the Cascade arc. The early post-glacial period also coincides with a small number of very large explosive eruptions from the most active volcanoes in the southern and central SVZ, consistent with enhanced ponding of magma during glaciation and release upon deglaciation. In comparison to non-arc settings, evidence of post-glacial increases in rates of arc volcanism is weak, and there is no need to invoke significantly increased melt production upon ice unloading, as occurred in areas such as Iceland. Non-arc volcanoes may therefore account for a relatively higher proportion of global volcanic emissions in the early post-glacial period than is suggested by the relative contributions of arc and non-arc settings at the present day. The second part of this paper critically examines global eruption records, in an effort to constrain global-scale changes in volcanic output since the LGM. Accurate interpretation of these records relies on correcting both temporal and spatial variability in eruption recording. In particular, very low recording rates, which also vary spatially by over two orders of magnitude, prevent precise, and possibly even accurate, quantitative analysis. For example, if we assume record completeness for the past century, the number of known eruptions (volcanic explosivity index ≥ 2) from some low-latitude regions, such as Indonesia, is approximately 1 in 20,000 (0.005%) for the period 5–20 ka. There is a need for more regional-scale studies of past volcanism in such regions, where current data are extremely sparse. We attempt to correct for recording biases, and suggest a maximum two-fold (but potentially much less) increase in global eruption rates, relative to the present day, between 13 and 7 ka. Although volcanism may have been an important source of CO2 in the early Holocene, it is unlikely to have been a dominant control on changes in atmospheric CO2 after the LGM.

Description: Landslide deposits offshore many volcanic islands provide evidence of catastrophic lateral collapses. These deposits span a larger volume range than their continental equivalents, and can generate devastating tsunamis. All historical volcanic-island lateral collapses have occurred in arc settings, and have been characterised by rapid failure and efficient tsunami generation. The varied morphology of their deposits is influenced both by lithological properties and the nature of the substrate. Many deposits show evidence of extensive seafloor erosion and transformation into debris flows, and the propagation of frontally-confined sediment deformation beyond and beneath the primary deposit. Mobilised volumes can far exceed that of the initial failure, and accurate deposit interpretation requires internal geophysical imaging and sampling. Around intraplate ocean-island volcanoes, multi-unit turbidites suggest that lateral collapses may occur in discrete stages; although this would reduce their overall tsunamigenic potential, the volumes of individual stages of collapse remain very large. Numerical models of both landslide and tsunami processes in ocean-island settings are difficult to test, and the smaller collapses that typify island arcs are an important focus of research due to their higher global frequency, availability of direct failure and tsunami observations, and a need to better understand the signals of incipient collapse to develop approaches for tsunami hazard mitigation.

Description: Volcanic island flank collapses have the potential to trigger devastating tsunamis threatening coastal communities and infrastructure. The 1888 sector collapse of Ritter Island, Papua New Guinea (in the following called Ritter) is the most voluminous volcanic island flank collapse in historic times. The associated tsunami had run-up heights of more than 20 m on the neighboring islands and reached settlements 600 km away from its source. This event provides an opportunity to advance our understanding of volcanic landslide-tsunami hazards. Here, we present a detailed reconstruction of the 1888 Ritter sector collapse based on high-resolution 2D and 3D seismic and bathymetric data covering the failed volcanic edifice and the associated mass-movement deposits. The 3D seismic data reveal that the catastrophic collapse of Ritter occurred in two phases: (1) Ritter was first affected by deep-seated, gradual spreading over a long time period, which is manifest in pronounced compressional deformation within the volcanic edifice and the adjacent seafloor sediments. A scoria cone at the foot of Ritter acted as a buttress, influencing the displacement and deformation of the western flank of the volcano and causing shearing within the volcanic edifice. (2) During the final, catastrophic phase of the collapse, about 2.4 km³ of Ritter disintegrated almost entirely and travelled as a highly energetic mass flow, which incised the underlying sediment. The irregular topography west of Ritter is a product of both compressional deformation and erosion. A crater-like depression underlying the recent volcanic cone and eyewitness accounts suggest that an explosion may have accompanied the catastrophic collapse. Our findings demonstrate that volcanic sector collapses may transform from slow gravitational deformation to catastrophic collapse. Understanding the processes involved in such a transformation is crucial for assessing the hazard potential of other volcanoes with slowly deforming flanks such as Mt. Etna or Kilauea.

Description: Highlights • Ritter Island's sector collapse provides an exemplar of volcanic tsunami hazards. • Deposit heterogeneity reflects erosion, secondary failure and a triggered eruption. • The volume of the distal deposit alone far exceeds the tsunamigenic failure. • A single catastrophic collapse led to stratigraphically complex distal deposits. • Accurate assessment of tsunami potential requires internal imaging and sampling. Abstract The current understanding of tsunamis generated by volcanic-island landslides is reliant on numerical models benchmarked against reconstructions of past events. As the largest historical event with timed tsunami observations, the 1888 sector collapse of Ritter Island, Papua New Guinea provides an outstanding opportunity to better understand the linked process of landslide emplacement and tsunami generation. Here, we use a combination of geophysical imaging, bathymetric mapping, seafloor observations and sampling to demonstrate that the Ritter landslide deposits are spatially and stratigraphically heterogeneous, reflecting a complex evolution of mass-flow processes. The primary landslide mass was dominated by well-bedded scoriaceous deposits, which rapidly disintegrated to form an erosive volcaniclastic flow that incised the substrate over much of its pathway. The major proportion of this initial flow is inferred to have been deposited up to 80 km from Ritter. The initial flow was followed by secondary failure of seafloor sediment, over 40 km from Ritter. The most distal part of the 1888 deposit has parallel internal boundaries, suggesting that multiple discrete units were deposited by a series of mass-flow processes initiated by the primary collapse. The last of these flows was derived from a submarine eruption triggered by the collapse. This syn-collapse eruption deposit is compositionally distinct from pre- and post-collapse eruptive products, suggesting that the collapse immediately destabilised the underlying magma reservoir. Subsequent eruptions have been fed by a modified plumbing system, constructing a submarine volcanic cone within the collapse scar through at least six post-collapse eruptions. Our results show that the initial tsunami-generating landslide at Ritter generated a stratigraphically complex set of deposits with a total volume that is several times larger than the initial failure. Given the potential for such complexity, there is no simple relationship between the volume of the tsunamigenic phase of a volcanic-island landslide and the final deposit volume, and deposit area or run-out cannot be used to infer primary landslide magnitude. The tsunamigenic potential of prehistoric sector-collapse deposits cannot, therefore, be assessed simply from surface mapping, but requires internal geophysical imaging and direct sampling to reconstruct the event.

Description: We have successfully constructed and tested a new, portable, Hybrid Lister‐Outrigger (HyLO) probe designed to measure geothermal gradients in submarine environments. The lightweight, low‐cost probe is 1‐3 m long, contains 4‐12 semiconductor temperature sensors that have a temperature resolution of 0.002 oC, a sample rate of 〈2 seconds, and a maximum working depth of ~2100 meters below sea level (mbsl). Probe endurance is continuous via ship‐power to water depths of ~700 mbsl, or up to ~1 week on batteries in depths 〉500 mbsl. Data are saved on solid‐state disks, transferred directly to the ship during deployment via a data cable, or transmitted via Bluetooth when the probe is at the sea surface. The probe contains an accelerometer to measure tilt, and internal pressure, temperature, and humidity gauges. Key advantages of this probe include (1) near‐real time temperature measurements and data transfer; (2) a low‐cost, transportable, and lightweight design; (3) easy and rapid two‐point attachment to a gravity corer, (4) short (3‐5 minute) thermal response times; (5) high temporal/spatial resolution and (6) longer deployment endurance compared to traditional methods. We successfully tested the probe both in lakes and during sea trials in May 2019 offshore Montserrat during the R/V Meteor Cruise 154/2. Probe‐measured thermal gradients were consistent with seafloor ocean‐drilling temperature measurements. Ongoing probe improvements include the addition of real‐time bottom‐camera feeds and long‐term (6‐12 month) deployment for monitoring. Key Points - We have designed, developed, and tested a low‐cost, portable hybrid Lister‐type probe to measure shallow thermal gradients - The probe consists of lightweight, quickly interchangeable/expendable components deployable to 2100 meters depth - The probe provides high vertical and temporal temperature resolution and rapid data transmission, reducing down‐time

Description: Highlights • We report two previously unknown sector collapse deposits on the flank of Sarkar. • Deposits originated from the same island and traveled on the same slope. • The associated failure led to very different mass transport deposits. • Difference of deposits is attributed to geological characteristics of slide planes. Abstract Volcanic island sector collapses have produced some of the most voluminous mass movements on Earth and have the potential to trigger devastating tsunamis. In the marine environment, landslide deposits offshore the flanks of volcanic islands often consist of a mixture of volcanic material and incorporated seafloor sediments. The interaction of the initial volcanic failure and the substrate can be highly complex and have an impact on both the total landslide deposit volume and its emplacement velocity, which are important parameters during tsunami generation and need to be correctly assessed in numerical landslide-tsunami simulations. Here, we present a 2D seismic analysis of two previously unknown, overlapping volcanic landslide deposits north-west of the island of Sakar (Papua New Guinea) in the Bismarck Sea. The deposits are separated by a package of well-stratified sediment. Despite both originating from the same source, with the same broad movement direction, and having similar deposit volumes (~15.5–26 km3), the interaction of these landslides with the seafloor is markedly different. High-resolution seismic reflection data show that the lower, older deposit comprises a proximal, chaotic, volcanic debris avalanche component and a distal, frontally confined component of deformed pre-existing well-bedded seafloor sediment. We infer that deformation of the seafloor sediment unit was caused by interaction of the initial volcanic debris avalanche with the substrate. The deformed sediment unit shows various compressional structures, including thrusting and folding, over a downslope distance of more than 20 km, generating 〉27% of shortening over a 5 km distance at the deposit's toe. The volume of the deformed sediments is almost the same as the driving debris avalanche deposit. In contrast, the upper, younger landslide deposit does not show evidence for substrate incorporation or deformation. Instead, the landslide is a structurally simpler deposit, formed by a debris avalanche that spread freely along the contemporaneous seafloor (i.e., the top boundary of the intervening sediment unit that now separates this younger landslide from the older deposit). Our observations show that the physical characteristics of the substrate on which a landslide is emplaced control the amount of seafloor incorporation, the potential for secondary seafloor failure, and the total landslide runout far more than the nature of the original slide material or other characteristics of the source region. Our results indicate the importance of accounting for substrate interaction when evaluating submarine landslide deposits, which is often only evident from internal imaging rather than surface morphological features. If substrate incorporation or deformation is extensive, then treating landslide deposits as a single entity substantially overestimates the volume of the initial failure, which is much more important for tsunami generation than secondary sediment failure.