Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau

The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals. Here, we integrate physical, microtextural and geochemical characterisation of tephra deposits spanning the collapse period. We demonstrate that the first post-collapse eruptive phase (erupting juvenile clasts with a low microlite areal number density and relatively large microlites, reflecting a crystal-growth dominated regime) is best explained by instantaneous unloading of a relatively stagnant upper conduit. This was followed by the second post-collapse phase, on a timescale of hours, which tapped successively hotter and deeper magma batches, reflected in increasing plagioclase anorthite content and more mafic glass compositions, alongside higher calculated ascent velocities and decompression rates. The characteristics of the post-collapse products imply downward propagating destabilisation of the magma storage system as a response to collapse, rather than pre- collapse magma ascent triggering failure. Importantly, this suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short- and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.

Study of Bathymetry and Seabed Morphology Changes Around Anak Krakatau Waters of 2018 Post-Eruption

Anak Krakatau is an active volcanic mountain located in the Sunda Strait, Indonesia. On December 22, 2018, this mountain erupted and the flank collapses off into the water. This changes the bathymetry and morphology of the seabed around the waters of Anak Krakatau volcano. This study uses bathymetry data surrounding the Anak Krakatau waters from the Indonesian Navy Hydrographic and Oceanographic Center (PUSHIDROSAL) in 2016 and 2019. The data were obtained by survey using Echosounder Multibeam System MBES EM2040 and EM302. By means of surface analysis method (hillshade), the data were process to observe the bathymetry and seabed morphology changes around the Anak Krakatau volcano waters by the eruption. The results of bathymetry data processing show a depth change of about 25 m, with the lowest value in 2016 ranging from -250,328 m and in 2019 ranging from -226.12 m. The average value of the slope of the seabed in 2016 was 5.31° and 4.98° in 2019.

Potential Eruption and Current Activity of Anak Krakatau Volcano, Indonesia

Anak Krakatau Volcano is located in the Sunda Strait known for its paroxysmal eruption in 1883. During the January - November 2019 period, seismicity was dominated by types of quakes which indicated the occurrence of magma supply (VA and VB), near-surface volcanic activity (LF, Hybrid, Harmonic Tremors), and volcanic activity above the volcanic surface (eruptions, emission, and continuous tremors). In the period December 2019 - July 2020, there was an increase in the types of quakes near the surface (LF, Hybrid) and the types of quakes on the surface (emission and continuous tremors). Volcanic deformation monitors changes in tilt over the 2019-2020 period associated with pressure releases before, during and after the eruption. The results of GPS data modeling, the shallow pressure source is at a depth of 0.22 km below sea level. Volcanic activity until July 2020 was dominated by activity near and above the volcanic surface associated with the growth of lava domes. The volcanic system of Anak Krakatau is currently an open system, with the potential for eruptions. Strengthening the early warning system for the eruption of Anak Krakatau is important in mitigating efforts and understanding its eruption potential

The Tsunami Simulation Generated by ‘Anak Krakatau’ Volcano Flank Collapse using MIKE 21Hydrodynamics Flexible Mesh with Manning Number Variation

This study aims to calculate the tsunami investment and the estimated arrival time at several locations around the Sunda strait, caused by the December 2018 Krakatao's eruption. The propagation of the tsunami wave is simulated using MIKE 21 Hydrodynamics Flexible Mesh (HD FM). The spatial data consist of the bathymetry and topography of the Sunda Strait area and its surroundings, whilst assumptions are made on the tsunami source topology and its exact location. Several runs of the simulation are then conducted by varying the Manning Number, i.e. bed resistance values, at the tsunami source and throughout the simulation domain, which accordingly would influence the propagation speed, inundation, and arrival time. Smaller Manning's values, which correspond to increasing roughness, are applied at locations closer to the tsunami source. In this simulation, Manning's number ranges from 10 to 40 m1/3s-1. Surface elevation, still water depth, and u and v velocity components are generated from this simulation.

Spatial Analysis on Tsunami Predictions in Pandeglang Regency

Pandeglang Regency is an area that has the potentiel to be hit by tsunamis. The plate subduction paths of Indo-Australia and Anak Krakatau Volcano make Pandeglang Regency a region with a high tsunami potential. One step that can be taken to overcome and minimize losses is to do spatial planning to protect it against potential tsunami damage. This research aimed to evaluate the spatial area of Pandeglang Regency based on the identification of potential tsunami hazards. The concept of modelling the tsunami inundation height developed by Berryman and based on Head Regulation No.4 of 2012 of the Indonesian National Board for Disaster Management has been used to identify potential tsunami hazards. The modelling was carried out by calculating the potential distribution of tsunami wave heights in coastal areas. Three scenarios were used to estimate the distribution. The results showed that the first scenario predicted a maximum tsunami height of 7.5 meters above sea level with the furthest tsunami inundation reaching 1,700.12 meters. Second scenario predicted maximum height of 15 meters, with the furthest tsunami inundation reaching 3,384.62 meters. Meanwhile, the last scenario was able to predict a height of 20 meters and showed the furthest tsunami inundation reaching 5.155,11 meters. These results proved that in all scenarios, the widest inundation would occur in Panimbang Regency. This is due to the relatively small variations in roughness and slope of the surface. The same condition also occurs in the last two scenarios, in which Sumur District was the area most ffected. Therefore, the spatial plan of Pandeglang Regency needs to be evaluated and the function of residential area changed to reduce and prevent large losses.

VOLCANIC ERUPTION-INDUCED TSUNAMI AT ANAK KRAKATAU VOLCANO, SUNDA STRAIT, INDONESIA

On 22 December 2018, a volcanic eruption occurred at Anak Krakatau, Sunda Strait, Indonesia, which induced a tsunami. At the coastal area in the Sunda Strait, the destructive tsunami destroyed many structures and killed more than 400 people approximately 30 to 40 min after the eruption. In this event, it has been reported that many residents start to evacuate after seeing tsunami because alert of tsunami was not occurred. It is difficult to escape from a tsunami after seeing it waves, so early evacuation become important. Previously, many studies which handle Krakatau volcanic eruption induced tsunamis have been conducted. Pakoksung et al. (2019) conducted its simulation, but it was reported that the observed run-up heights and inundation depths were underestimated. Moreover, there were few studies which handle evacuation from non- seismic tsunami. The purpose of the study is to reveal the actual evacuation action from the tsunami induced by the 2018 volcanic eruption.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/ELOif7G4eNo