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The Dec. 22, 2018 lateral collapse of the Anak Krakatau (AK) volcano in the Sunda Straits of Indonesia discharged volcaniclastic material into the 250 m deep caldera southwest of the volcano and generated a large tsunami, causing runups of up to 85 m in the near-field, and 13.5 m in the far-field, on the nearby coasts of Sumatra and Java. The tsunami caused 437 fatalities, the greatest number from a volcanically-induced tsunami since the catastrophic explosive caldera-forming eruption of Krakatau in 1883 and the sector collapse of Ritter Island in 1888. For the first time in over 100 years, the 2018 AK 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 collapse and tsunami generation, propagation, and coastal impact, with state-of the-art numerical models, using both a new parametrization of the collapse and a near-field bathymetric dataset based on our 2019 field surveys and satellite images. These subaerial and submarine data sets are used to constrain the geometry and magnitude of the landslide mechanism, which show that the primary landslide scar bisected the AK edifice, cutting behind the central vent and removing 50% of its subaerial volume. The primary landslide volume is estimated to range from 0.175–0.313 km3, based on uncertainties in the shape of the submerged part of the failure plane. This is supported by an independent estimate of the primary landslide deposit volume of 0.214 ± 0.036 km3. Given uncertainties in the failure volume, we define a range of potential failure surfaces that span these values in 4 collapse scenarios of volume ranging from 0.175 to 0.313 km3. These AK collapses are modeled, assuming either a granular or viscous fluid rheology, together with their corresponding tsunami generation and propagation. 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 min. For both modeled rheologies, the 0.224 km3 collapse (second and preferred scenario) most successfully reproduces the near- and far-field tsunami flow depth and runup observed in all post-event field survey results, tide gauge records, and eyewitness reports to date, 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, stressing the need to develop and install early warning systems for such events.

Adaptive designed clinical trials allow the possibility to monitor the progress of an ongoing trial (in a blinded or unblinded manner) and make appropriate design changes as the trial progresses. Such possible design changes should generally be prespecified and not simply reactive fixes to trials that are going wrong. Wide-scope trials comparing many treatments for a single condition (platform trials) and many conditions targeted by a single treatment (umbrella trials) are also more commonly appearing. There are many technical, statistical, as well as operational challenges to planning and running such studies. Identifying and solving problems of bias are particularly paramount, and data monitoring committees are being used more and more to help with this problem. Notwithstanding the obstacles, there are also benefits in efficiency to be gained.

A wide variety of volcanic processes can produce tsunamis at coastal, island, and submarine volcanoes and also within lakes on or near inland volcanoes. These processes include submarine volcanic eruptions, ocean-entering volcanic flows including pyroclastic flows, landslides including volcano lateral collapses, and sudden ground movements at volcanoes. Of these the largest and most dangerous tsunamis are those produced by large explosive eruptions that generate ocean-entering pyroclastic flows, and large lateral collapse landslides. Although more frequent, other processes such as submarine eruptions typically produce only locally hazardous tsunamis. Volcano tsunamis typically have shorter wave periods than earthquake-generated tsunamis and so undergo more rapid dispersion during propagation: as a result they are mainly local to regional hazards, although the largest volcano tsunamis maybe destructive at transoceanic distances. Modeling of tsunami generation by volcanic processes is particularly difficult because of their variety and complexity, especially in the case of those generated by large landslides and by the impact of pyroclastic flows. Large uncertainties therefore exist regarding the scale and frequency of occurrence of the resulting hazards.

Adaptive designed clinical trials allow the possibility to monitor the progress of an ongoing trial (in a blinded or unblinded manner) and make appropriate design changes as the trial progresses. Such possible design changes should generally be prespecified and not simply reactive fixes to trials that are going wrong. There are many technical statistical as well as operational challenges to planning and running such studies. Identifying and solving problems of bias are particularly paramount and data monitoring committees are being used more and more to try to help with this problem. There are also benefits in efficiency to be gained.

New multibeam bathymetry data are presented for the South Sandwich intra-oceanic arc which occupies the small Sandwich plate in the South Atlantic, and is widely considered to be a simple end-member in the range of intra-oceanic arc types. The images show for the first time the distribution of submarine volcanic, tectonic and erosional–depositional features along the whole length of the 540 km long volcanic arc, allowing systematic investigation of along-arc variations. The data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well-defined volcanic front, but with three parallel cross-cutting seamount chains extending 38–60 km from near the volcanic front into the rear-arc. There is no evidence for intra-arc rifting or extinct volcanic lines. Topographic evidence for faulting is generally absent, except near the northern and southern plate boundaries. Most of the volcanic arc appears to be built on ocean crust formed at the associated back-arc spreading centre, as previously proposed from magnetic data, but the southern part of the arc appears to be underlain by older arc or continental crust whose west-facing rifted margin facing the back-arc basin is defined by the new bathymetry. The new survey shows nine main volcanic edifices along the volcanic front and ca. 20 main seamounts. The main volcanoes form largely glaciated islands with summits 3.0–3.5 km above base levels which are 2500–3000 m deep in the north and shallower at 2000–2500 m deep in the south. Some of the component seamounts are interpreted to have been active since the last glacial maximum, and so are approximately contemporaneous with the volcanic front volcanism. Seven calderas, all either submarine or ice-filled, have been identified: Adventure volcano, a newly discovered submarine volcanic front caldera volcano is described for the first time. All but one of the calderas are situated on summits of large volcanoes in the southern part of the arc, and most are associated with current or historic volcanic or hydrothermal activity. Shallow shelves around the islands are generally 1–10 km wide. Submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces. Seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows.

New multibeam (swath) bathymetric sonar data acquired using an EM120 system on the RRS James Clark Ross, supplemented by sub-bottom profiling, reveals the underwater morphology of a not, vert, similar 12,000 km2 area in the northern part of the mainly submarine South Sandwich volcanic arc. The new data extend between 55° 45′S and 57° 20′S and include Protector Shoal and the areas around Zavodovski, Visokoi and the Candlemas islands groups. Each of these areas is a discrete volcanic center. The entirely submarine Protector Shoal area, close to the northern limit of the arc, forms a 55 km long east–west-trending seamount chain that is at least partly of silicic composition. The seamounts are comparable to small subaerial stratovolcanoes in size, with volumes up to 83 km3, indicating that they are the product of multiple eruptions over extended periods. Zavodovski, Visokoi and the Candlemas island group are the summits of three 3–3.5 km high volcanic edifices. The bathymetric data show evidence for relationships between constructional volcanic features, including migrating volcanic centers, structurally controlled constructional ridges, satellite lava flows and domes, and mass wasting of the edifices. Mass wasting takes place mainly by strong erosion at sea level, and dispersal of this material along chutes, probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields. Large scale mass wasting structures include movement of unconsolidated debris in slides, slumps and debris avalanches. Volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical, being steep with abundant recent volcanism on their western flanks, and gently sloping with extinct, eroded volcanic sequences to their east. This is consistent with the calculated rate of subduction erosion of the fore-arc.