Marcel Hürlimann

Vertical and lateral collapses on Tenerife (Canary Islands) and other volcanic ocean islands

Recent studies demonstrate that Tenerife has undergone large lateral collapses. This has led to the suggestion that the Las Canadas caldera, one of the best exposed calderas in the world, is the result of lateral collapse. We have tested this idea using the available structural, stratigraphic, volcanological, and geochronological data. We conclude that the Las Canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of Tenerife. Our results indicate, however, that vertical collapses may have played a major role in triggering lateral collapses. We propose that the association of vertical and lateral collapse events, such as inferred for Tenerife, can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands.

Characterisation of a volcanic residual soil and its implications for large landslide phenomena: application to Tenerife, Canary Islands

Large landslides are common processes during the evolution of volcanoes and individual events can exceed several cubic kilometres in volume. Volcanic slope failures are a significant risk for the neighbouring population due to their huge volumes and great runout distances. Around the Canary archipelago, a total of seventeen deposits of large landslides have been found, and on Tenerife, seven large landslides have affected the subaerial and submarine morphology during the last ∼6 Ma. However, the causes of such mass movements are still poorly understood. This work analyses the events around the Canary Islands and focuses on the ones that occurred on Tenerife in order to obtain new insights into the mechanisms of large volcanic landslides. The study is divided into a first part that includes site investigations examining the general features favouring large-scale failures at volcanoes. The second part describes the laboratory tests used to analyse a residual soil that may be the potential slip surface of the slides on Tenerife. The site investigation revealed that regional tectonics and the climate have a significant influence on the spatial distribution of the landslides. Moreover, morphological and geological features such as deep fluvial canyons, a high coastal cliff and persistent dike intrusion may favour the initiation of slope failure. A typical residual soil sample from the lateral scarp of the La Orotava amphitheatre on Tenerife was studied by carrying out standard laboratory tests. The microstructure was analysed using environmental scanning electron microscopy and a particular bonding was found. This bonding was also detected by the geotechnical tests. Consolidation tests and direct shear tests revealed that the mechanical behaviour of the residual soil changes greatly if the bonding of the soil is broken. The bonded structure generally fails when the effective normal stress surpasses the yield strength of the bonding. In the case of large volcanic landslides with thicknesses up to several hundred meters, the high overburden easily exceeds this yield strength and generates a broken bonding. Therefore, volcanic residual soils, such as the one analysed in this study, are perfect candidates for the potential failure surfaces of large volcanic landslides. Referring to the La Orotava events, we assume that residual soil layers and morphological, geological and climatic features reduced the slope stability to critical conditions, whereas a strong earthquake associated with a caldera collapse episode may have finally triggered the landslide. The results obtained indicate that the residual soils play an important role in affecting the stability of volcano slopes and their destabilising influence significantly favours large-scale sliding. We suggest that the results obtained from this study can be applied to other locations since volcanic residual soils are common in volcanic areas.

Mechanical relationship between catastrophic volcanic landslides and caldera collapses

Large-scale sector collapses of volcanic edifices and collapse calderas represent some of the most catastrophic geological events taking place at the earths surface. Examples of these two processes occurring simultaneously suggest that a mechanical relationship between landslides and collapse calderas may exist. We demonstrate that a caldera collapse can trigger large-scale landslides in volcanic terrains. Moderate seismic shocks caused by seismogenic slip on the ring fault on which the caldera subsidence takes place act as the driving force necessary to destabilize the volcano flank. This process is favored on steep volcanoes and where flank strength is reduced by agents such as hydrothermal alteration, pore fluid pressure increase or the presence of weak soils.

Large landslides triggered by caldera collapse events in Tenerife, Canary Islands

Abstract Landsliding is a significant process on volcanic edifices, with individual events exceeding several cubic kilometres in volume. The causes of such mass movements and their relationship with volcanic activity are still poorly understood. Landslide events are an important factor in the evolution of volcanic islands such as Tenerife, where vertical and lateral collapses have occurred repeatedly. Subaerial and submarine processes related to landslide events strongly influence the morphology of the island. On Tenerife there are three very big valleys, Guimar, La Orotava and Icod, that have been created by large landslide events with ages ranging from Upper Pliocene to Middle Pleistocene. The landslides affect the northern flanks of the island and the slopes of a large central volcanic edifice, the Las Canadas volcano, which is truncated by the Las Canadas caldera, a multicyclic collapse depression, formed between 1.02 and 0.17 Ma. We have focused our studies on the potential for caldera collapse events to trigger large scale landslides. The available geological and morphological information has been incorporated into numerical models, which simulate the destabilising effects of a caldera collapse episode. The results of the numerical modelling indicate that processes associated with caldera collapse events can overcome the stabilising forces on the volcano flank and trigger landslides. We propose that caldera collapse events may have triggered large landslides on the slopes of the Las Canadas volcano.

Results and experiences gathered at the Rebaixader debris-flow monitoring site, Central Pyrenees, Spain

The wired and wireless monitoring system installed in the Rebaixader catchment detected six debris flows and 11 debris floods between 2009 and 2012. Apart from results directly related to the processes, many experiences associated with monitoring were collected. Debris flows and debris floods showed clear differences in both the recorded data and field observations. The distinction was especially visible in the stage measurements and the ground vibration registered by the most downstream geophone. At this geophone, a positive relation between the maximum ground vibration and the volume was also observed. The triggering of most events was associated with short, high-intensity rainstorms in summer, but some were also generated in spring, when the melting of snow cover and frozen soil played an additional role. A positive correlation between the volume and both the amount and the intensity of the triggering rainfall was observed. Regarding technical aspects, a switch between a “no-event” mode with a low sample rate and an “event” mode with a fast sampling was particularly useful at the station that register the passing of a flow. In addition, the stations, which most recently were installed at Rebaixader, apply wireless devices because wireless techniques include multiple advantages against standard wired systems. Although recorded data or even video images provide detailed information on the debris-flow behavior, we strongly recommend periodic field surveys along the entire torrent to verify and improve the interpretation obtained from the monitoring system.

Transformation of Ground Vibration Signal for Debris-Flow Monitoring and Detection in Alarm Systems

Debris flows are fast mass movements formed by a mix of water and solid materials, which occur in steep torrents, and are a source of high risks for human settlements. Geophones are widely used to detect the ground vibration induced by passing debris flows. However, the recording of geophone signals usually requires storing a huge amount of data, which leads to problems in storage capacity and power consumption. This paper presents a method to transform and simplify the signals measured by geophones. The key input parameter is the ground velocity threshold, which removes the seismic noise that is not related to debris flows. A signal conditioner was developed to implement the transformation and the ground velocity threshold was set by electrical resistors. The signal conditioner was installed at various European monitoring sites to test the method. Results show that data amount and power consumption can be greatly reduced without losing much information on the main features of the debris flows. However, the outcome stresses the importance of choosing a ground vibration threshold, which must be accurately calibrated. The transformation is also suitable to detect other rapid mass movements and to distinguish among different processes, which points to a possible implementation in alarm systems.

A 2D finite volume model for bebris flow and its application to events occurred in the Eastern Pyrenees

Abstract FLATModel is a 2D finite volume code that contains several original approaches to improve debris-flow simulation. Firstly, FLATModel incorporates a ‘stop-and-go’ technique in each cell to allow continuous collapses and remobilizations of the debris-flow mass. Secondly, flow velocity and consequently yield stress is directly associated with the type of rheology to improve boundary accuracy. Thirdly, a simple approach for entrainment is also included in the model to analyse the effect of basal erosion of debris flows. FLATMODEL was tested at several events that occurred in the Eastern Pyrenees and simulation results indicated that the model can represent rather well the different characteristics observed in the field.

Processing the ground vibration signal produced by debris flows

Ground vibration sensors have been increasingly used and tested, during the last few years, as devices to monitor debris flows and they have also been proposed as one of the more reliable devices for the design of debris flow warning systems. The need to process the output of ground vibration sensors, to diminish the amount of data to be recorded, is usually due to the reduced storing capabilities and the limited power supply, normally provided by solar panels, available in the high mountain environment. There are different methods that can be found in literature to process the ground vibration signal produced by debris flows. In this paper we will discuss the two most commonly employed: the method of impulses and the method of amplitude. These two methods of data processing are analyzed describing their origin and their use, presenting examples of applications and their main advantages and shortcomings. The two methods are then applied to process the ground vibration raw data produced by a debris flow occurred in the Rebaixader Torrent (Spanish Pyrenees) in 2012. The results of this work will provide means for decision to researchers and technicians who find themselves facing the task of designing a debris flow monitoring installation or a debris flow warning equipment based on the use of ground vibration detectors. We present the processing of the seismic signal produced by a debris flow.Two methods of seismic data processing, amplitude and impulses, are compared.We process the geophone signal of a debris flow to reveal its main features.The processing of the debris flow seismic signal reveals to be useful for warning.A geophone network allows an early detection of debris flow for warning purposes.

Estimate of the debris-flow entrainment using field and topographical data

Abstract The entrainment of material is a common process in debris-flow behaviour and can strongly increase its total volume. However, due to the complex nature of the process, the exact mechanisms of entrainment have not yet been solved. We analysed geomorphological and topographical data collected in 110 reaches of 17 granular debris flows occurred in the Pyrenees and the European Alps. Four governing factors (sediment availability, channel-bed slope, channel cross section shape and upstream-contributing area) were selected and defined for all the 110 reaches. One dataset of the resulting database was used to develop two models to estimate the erosion rates based on the governing factors: a formula derived from multiple linear regression (MLR) analysis and a decision tree (DT) obtained from J48 algorithm. The models obtained using these learning techniques were validated in another independent dataset. In this validation set, the DT model revealed better results. The models were also implemented in a torrent (test set), where the total debris-flow volume was known and two empirical methods (available in literature) were applied. This test revealed that both MLR and DT predict more accurately the final volume of the event than the empirical equations for volume prediction. Finally, a general DT was proposed, which includes three governing factors: sediment availability, channel-bed slope and channel cross section shape. This DT may be applied to other regions after adapting it regarding site-specific characteristics.

Experiences of Debris-Flow Monitoring and Warning at Catchment Scale in the Pyrenees

Debris-flow monitoring improves the understandings of debris flows and also provides fundamental information for an efficient early warning and alarm system (EWAS), which commonly focuses on ground vibration and flow depth. Preliminary results from the Senet station (Central Pyrenees, Spain) show that monitoring is a complex task, especially if different torrential processes should be distinguished using merely geophone data. Video images are not useful for an EWAS, but strongly improves the analysis, because they allow to identify the flow type and to characterise the ground vibrations corresponding to each type. In addition, an EWAS can also be based on triggering rainfall patterns, which have to be defined by a one-parameter o multi-parameter condition. Our first experiences on the possible implementation of an EWAS at Senet test site are promising, but indicate the complexity regarding the critical rainfall conditions, the definition of reliable thresholds to avoid false alarms and the difficulties related to technical shortcomings.