Periodic Reporting for period 2 - PRE-COLLAPSE (Slow sliding of volcanic flanks as PREcursor to catastrophic COLLAPSE)
Reporting period: 2022-08-01 to 2024-01-31
Volcanoes are among the most rapidly growing geological structures on Earth. Consequently, their edifices can be mechanically unstable and volcano flank collapses can form giant destructive landslides as in the case of the 1980 Mount St Helens collapse. At ocean island or coastal volcanoes these events pose an even larger threat as the sudden displacement of large amounts of material in water can cause tsunamis with extreme and ocean-wide effects. Slow seawards sliding of volcano flanks as it is observed at numerous volcanoes globally, can also be an expression of edifice instability. The aim of PRE-COLLAPSE is to understand if and how these two types of motion, slow sliding and catastrophic collapse, are related. To get there we will combine field observations, laboratory experiments, and numerical modelling. What is new is that all methods cross the shoreline so that they capture the entire volcano edifice from the volcano’s summit in the sky to its toe in the deep sea. The investigations will focus on four volcanoes: Etna (Italy), Anak-Krakatau (Indonesia), Ritter Island (Papua-Neuguinea), and Kilauea (Hawaii, USA). The outcome will help us identify flanks that are in transition to collapse.
A detailed review of global databases of volcanic edifices such as the Global Volcanism Program (GVP), and those of known mass movements quickly revealed the need for a separate database to compile information on volcanic islands. Out of the 2652 Holocene and Pleistocene volcanoes in the GVP database, we identified 370 edifices as volcanic islands. However, we were only able to find high-resolution bathymetry (<250m) of sufficient coverage of their submerged flanks for 47 of them. We therefore decided to evaluate how the use of a Digital Elevation Model (DEM) reliant on low-resolution, indirect satellite measurements where no shipborne data is available, will influence the quality of the geomorphometric parameters. The result was positive and most morphometric parameters show robust results.
We conducted a comprehensive review of historical marine volcano collapse events, focusing on the available event sequences during the collapses. Despite the majority of documented marine volcano collapses occurring during volcanic eruptions, the analysis revealed that collapses do not always follow eruptions, supporting the need to physically and quantitatively describe the processes leading to collapses.
In order to understand the generation of the slow sliding we created a static two-dimensional plain-strain finite-element model representing the volcano Anak Krakatau. The model geometry draws on pre- and post-collapse drone photogrammetry and satellite images. We find that a weak zone inside the volcano is needed to generate a slow sliding flank and to reproduce ground deformation observed by satellites prior to the collapse in December 2018. To understand mechanical properties of such a weak zone we conducted direct shear experiments were conducted on samples of the pre-collapse Anak Krakatau. The results show that all lithologies display velocity-weakening behavior independent of the applied normal load or fluid saturation. This is a prerequisite for a fault being unstable and capable of runaway acceleration.
We developed a 2D boundary element model for dike propagation while accounting for the topography of the volcanic edifice. We implemented a discretized, traction-free surface using dislocation elements within an existing 2D Boundary Element model designed for simulating the propagation of fluid-filled cracks. The numerical experiments show that high topography contrasts and asymmetric edifice geometries, as is often the case for marine volcanoes, have significant influences on calculated ground deformation.
A short research cruise for conducting marine research at Krakatau volcano was applied for and granted. The preparation phase required close exchange and collaboration with Indonesian scientists and authorities.
The main result of all work done in the reporting period is that there appears to be a strong link between flank collapses and eruptions. We find increasing evidence for that flank collapses precede large eruptions and not vice versa. This would exclude exceptionally large eruptions as triggers for flank collapse.
We conducted a comprehensive review of historical marine volcano collapse events, focusing on the available event sequences during the collapses. Despite the majority of documented marine volcano collapses occurring during volcanic eruptions, the analysis revealed that collapses do not always follow eruptions, supporting the need to physically and quantitatively describe the processes leading to collapses.
In order to understand the generation of the slow sliding we created a static two-dimensional plain-strain finite-element model representing the volcano Anak Krakatau. The model geometry draws on pre- and post-collapse drone photogrammetry and satellite images. We find that a weak zone inside the volcano is needed to generate a slow sliding flank and to reproduce ground deformation observed by satellites prior to the collapse in December 2018. To understand mechanical properties of such a weak zone we conducted direct shear experiments were conducted on samples of the pre-collapse Anak Krakatau. The results show that all lithologies display velocity-weakening behavior independent of the applied normal load or fluid saturation. This is a prerequisite for a fault being unstable and capable of runaway acceleration.
We developed a 2D boundary element model for dike propagation while accounting for the topography of the volcanic edifice. We implemented a discretized, traction-free surface using dislocation elements within an existing 2D Boundary Element model designed for simulating the propagation of fluid-filled cracks. The numerical experiments show that high topography contrasts and asymmetric edifice geometries, as is often the case for marine volcanoes, have significant influences on calculated ground deformation.
A short research cruise for conducting marine research at Krakatau volcano was applied for and granted. The preparation phase required close exchange and collaboration with Indonesian scientists and authorities.
The main result of all work done in the reporting period is that there appears to be a strong link between flank collapses and eruptions. We find increasing evidence for that flank collapses precede large eruptions and not vice versa. This would exclude exceptionally large eruptions as triggers for flank collapse.
In the reporting period we found evidence for a causal link between flank collapses and exceptionally large eruptions. In the upcoming project period we will further examine this link. New data collected during a research cruise in August 2023 will shed light on the event sequences of the 1883 events at Krakatau. We will use numerical and analytical dislocation modelling to further constrain for which cases topography needs to be included. At the same time we will develop further models that account for topography and the interaction between dike propagation and flank motion or collapse.
Our results so far demonstrate the need for a weak zone inside the volcano, possibly a fault, for reproducing observed ground deformation. Upcoming laboratory and numerical experiments will provide insights into the nature of the weak zone. Transient Finite Element Models will be applied to Anak Krakatau to constrain the conditions under which unstable sliding and eventual failure can occur. Direct shear experiments on samples from Kilauea (Hawaii) will be conducted and combined with numerical models to understand the volcano flank’s current stability and its potential to fail catastrophically.
Having demonstrated that global bathymetric charts can be used to deduce a large range of morphometric parameters, we are now able to systematically expand our global marine volcano database into less well studied areas. This large, comprehensive database will allow statistical analyses to be performed and to find correlation with the instability of volcanic islands.
Our results so far demonstrate the need for a weak zone inside the volcano, possibly a fault, for reproducing observed ground deformation. Upcoming laboratory and numerical experiments will provide insights into the nature of the weak zone. Transient Finite Element Models will be applied to Anak Krakatau to constrain the conditions under which unstable sliding and eventual failure can occur. Direct shear experiments on samples from Kilauea (Hawaii) will be conducted and combined with numerical models to understand the volcano flank’s current stability and its potential to fail catastrophically.
Having demonstrated that global bathymetric charts can be used to deduce a large range of morphometric parameters, we are now able to systematically expand our global marine volcano database into less well studied areas. This large, comprehensive database will allow statistical analyses to be performed and to find correlation with the instability of volcanic islands.