Karim Kelfoun

Digital photogrammetry as a tool in analogue modelling: applications to volcano instability

Abstract Three techniques of digital photogrammetry have been applied successfully to laboratory analogue models to study surface displacements caused by various volcano deformation types. Firstly, side-perspective videos are used to differentiate profile displacements between cryptodome intrusion models and models deforming by ductile inner-core viscous flow. Both models show similar morphologic features including a bulged flank and an asymmetric upper graben. However, differences in displacement trajectories of the bulge crest reflect upward intrusion push contrasting with essentially downward displacement vectors of weak core models. The other two techniques use vertical views correlated automatically either as time-sequence monoscopic views or as coeval stereoscopic pairs. This exploits to a maximum the method’s potential by imaging surface displacements over the whole model. Successive monoscopic photograms, because they suffer only moderate numerical processing for topographic effect removal, can detect very small displacements occurring early in deformation processes. As illustrated by analysis of intrusion models, the monoscopic method allows prediction of fault locations and main displacement locations. It can also anticipate the principal strain directions, and separate different deformation stages. On the other hand, the stereo-photogrammetry technique, although more complicated, provides topography and volume changes, as well as pictures of surface displacements in three dimensions. Results are presented for the spreading of volcano models on a ductile substratum and viscous cored cones. We have found digital photogrammetry to be a useful tool for analogue modelling, because it provides quantitative data on surface displacements, including movement invisible to the eye, as well as topographic changes. It is a good method for investigating and comparing different deformation mechanisms. It is especially useful for interpretation of displacement patterns obtained from monitoring of natural active volcanoes. In fact, results of the methods used in the laboratory can be directly compared with field data from geodetic or photogrammetric surveys, as at Mount St. Helens in 1980.

The influence of conduit geometry on the dynamics of caldera-forming eruptions

Abstract During caldera collapse events, new conduits may open and dramatically modify the style of the associated eruption. In order to investigate the effect of conduit geometry on the dynamics of caldera-forming eruptions, we performed numerical simulations of magma ascent in single-vent, cylindrical conduits and ring-fissure conduits. The results show that, for a given volume of conduit, the discharge rate is an order of magnitude smaller in ring-fissure conduits due to the higher friction. Furthermore, the transition from a sustained Plinian column to a collapsing fountain feeding pyroclastic flows occurs at a discharge rate an order of magnitude higher for ring-fissure conduits, due to the higher rate of air entrainment in a curtain jet than in a cylindrical jet. The production of pyroclastic flows from ring-fissure conduits therefore requires a much larger conduit volume than from a cylindrical conduit. We argue that this should be correlated with a much larger volume of lithics in the deposit. We confront these theoretical considerations with two geological examples. The Taupo ignimbrite has been emplaced by a high-discharge rate pyroclastic flow and contains a small volume of lithics. This is consistent with its emplacement during a single-vent eruptive phase. The Bishop Tuff has been emplaced by a lower discharge rate pyroclastic flow and yet contains much more lithics. This is consistent with an eruption through successive vents migrating along a ring fissure. The lithic content of an ignimbrite and the eruption discharge rate can therefore give some insights into the mechanism of caldera collapse and the role of ring fissures as magma conduits. A conclusion of our analysis is that perfectly annular ring-fissure conduits do not favour the formation of pyroclastic flows. The occurrence of pyroclastic flows in virtually all caldera-forming eruptions suggests that magma ascent is mainly localised in more restricted conduits. A corollary conclusion is that widespread ignimbrites, which record high-discharge rate eruptions, are not necessarily the result of ring fissure opening during caldera collapse.

On the ability of pyroclastic flows to scale topographic obstacles

We present a simple analysis of momentum dissipation in a pyroclastic flow with high initial velocity and show that the deceleration is inversely proportional to the volumetric flow rate. Consequently, for a given mass flow rate, dense flows slow down more rapidly than dilute flows. The analysis ignores density stratification and flow unsteadiness, and assumes that deceleration is due to turbulent dissipation alone, but the results have implications for a wide range of flow regimes. We apply our analysis to the distribution of the 1800-year-old deposits of the Taupo pyroclastic flow, the velocity of which is constrained by the height of topographic obstacles that it scaled. The volumetric flow rate required to maintain velocities high enough to overtop the rugged topography is consistent with a dilute flow, but strongly argues against the dense flow hypothesis. The same conclusion applies to other pyroclastic flow deposits distributed over rugged terrain.

VolcFlow capabilities and potential development for the simulation of lava flows

Abstract VolcFlow is a finite-difference Eulerian code based on the depth-averaged approach and developed for the simulation of isothermal geophysical flows. Its capability for reproducing lava flows is tested here for the first time. The field example chosen is the 2010 lava flow of Tungurahua volcano (Ecuador), the emplacement of which is tracked by projecting thermal images onto a georeferenced digital topography. Results show that, at least for this case study, the isothermal approach of VolcFlow is able to simulate the velocity of the lava through time, as well as the extent of the solidified lava. However, the good fit between the modelled and the natural flow may be explained by the short emplacement time (c. 20 h) of a thick lava (c. 5 m), which could limit the influence of cooling on the flow dynamics, thus favouring the use of an isothermal rheology.

Conclusion: recommendations and findings of the RED SEED working group

Abstract RED SEED stands for Risk Evaluation, Detection and Simulation during Effusive Eruption Disasters, and combines stakeholders from the remote sensing, modelling and response communities with experience in tracking volcanic effusive events. The group first met during a three day-long workshop held in Clermont Ferrand (France) between 28 and 30 May 2013. During each day, presentations were given reviewing the state of the art in terms of (a) volcano hot spot detection and parameterization, (b) operational satellite-based hot spot detection systems, (c) lava flow modelling and (d) response protocols during effusive crises. At the end of each presentation set, the four groups retreated to discuss and report on requirements for a truly integrated and operational response that satisfactorily combines remote sensors, modellers and responders during an effusive crisis. The results of collating the final reports, and follow-up discussions that have been on-going since the workshop, are given here. We can reduce our discussions to four main findings. (1) Hot spot detection tools are operational and capable of providing effusive eruption onset notice within 15 min. (2) Spectral radiance metrics can also be provided with high degrees of confidence. However, if we are to achieve a truly global system, more local receiving stations need to be installed with hot spot detection and data processing modules running on-site and in real time. (3) Models are operational, but need real-time input of reliable time-averaged discharge rate data and regular updates of digital elevation models if they are to be effective; the latter can be provided by the radar/photogrammetry community. (4) Information needs to be provided in an agreed and standard format following an ensemble approach and using models that have been validated and recognized as trustworthy by the responding authorities. All of this requires a sophisticated and centralized data collection, distribution and reporting hub that is based on a philosophy of joint ownership and mutual trust. While the next chapter carries out an exercise to explore the viability of the last point, the detailed recommendations behind these findings are detailed here.

Modern Multispectral Sensors Help Track Explosive Eruptions

Due to its massive air traffic impact, the 2010 eruption of Eyjafjallajokull was felt by millions of people and cost airlines more than U.S.

Tsunami hazard related to a flank collapse of Anak Krakatau Volcano, Sunda Strait, Indonesia

1.7 billion. The event has, thus, become widely cited in renewed efforts to improve real-time tracking of volcanic plumes, as witnessed by special sections published last year in Journal of Geophysical Research, (117, issues D20 and B9).

Sustained blasts during large volcanic eruptions

Abstract Numerical modelling of a rapid, partial destabilization of Anak Krakatau Volcano (Indonesia) was performed in order to investigate the tsunami triggered by this event. Anak Krakatau, which is largely built on the steep NE wall of the 1883 Krakatau eruption caldera, is active on its SW side (towards the 1883 caldera), which makes the edifice quite unstable. A hypothetical 0.280 km3 flank collapse directed southwestwards would trigger an initial wave 43 m in height that would reach the islands of Sertung, Panjang and Rakata in less than 1 min, with amplitudes from 15 to 30 m. These waves would be potentially dangerous for the many small tourist boats circulating in, and around, the Krakatau Archipelago. The waves would then propagate in a radial manner from the impact region and across the Sunda Strait, at an average speed of 80–110 km h−1. The tsunami would reach the cities located on the western coast of Java (e.g. Merak, Anyer and Carita.) 35–45 min after the onset of collapse, with a maximum amplitude from 1.5 (Merak and Panimbang) to 3.4 m (Labuhan). As many industrial and tourist infrastructures are located close to the sea and at altitudes of less than 10 m, these waves present a non-negligible risk. Owing to numerous reflections inside the Krakatau Archipelago, the waves would even affect Bandar Lampung (Sumatra, c. 900 000 inhabitants) after more than 1 h, with a maximum amplitude of 0.3 m. The waves produced would be far smaller than those occurring during the 1883 Krakatau eruption (c. 15 m) and a rapid detection of the collapse by the volcano observatory, together with an efficient alert system on the coast, would possibly prevent this hypothetical event from being deadly.

Explosive eruption, flank collapse and megatsunami at Tenerife ca. 170 ka

We carried out numerical simulations to investigate magma ascent in wide conduits during large explosive eruptions. Wide conduits allow high discharge rates, low frictional pressure losses, and shallow levels of explosive fragmentation of the magma within the conduit. In contrast with the commonly modeled lower rate eruptions during which magma fragments inside the conduit at depth and feeds a vertical eruptive jet, we find that for sufficiently high discharge rates (>10 10 kg ṁ s −1 ) the fragmentation level may rise up to the surface. Gas-rich, unfragmented magma reaches the surface at high pressure and feeds a sustained volcanic blast. Geologic evidence for very high discharge rate eruptions, wide conduits, and shock waves in large pyroclastic flows supports the occurrence of this type of explosive eruption.

Post-eruptive flooding of Santorini caldera and implications for tsunami generation

Giant mass failures of oceanic shield volcanoes that generate tsunamis potentially represent a high-magnitude but low-frequency hazard, and it is actually difficult to infer the mechanisms and dynamics controlling them. Here we document tsunami deposits at high elevation (up to 132 m) on the north-western slopes of Tenerife, Canary Islands, as a new evidence of megatsunami generated by volcano flank failure. Analyses of the tsunami deposits demonstrate that two main tsunamis impacted the coasts of Tenerife 170 kyr ago. The first tsunami was generated during the submarine stage of a retrogressive failure of the northern flank of the island, whereas the second one followed the debris avalanche of the subaerial edifice and incorporated pumices from an on-going ignimbrite-forming eruption. Coupling between a massive retrogressive flank failure and a large explosive eruption represents a new type of volcano-tectonic event on oceanic shield volcanoes and a new hazard scenario.