Volcano-structural study and long-term volcanic hazard assessment on el Hierro island (Canary islands)
- Autores: Laura Becerril Carretero
- Directores de la Tesis: Joan Martí Molist (dir. tes.), Inés Galindo Jiménez (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2014
- Idioma: español
- Tribunal Calificador de la Tesis: María José Blanco (presid.), Carlos Galé Bornao (secret.), Antonio J. Brumnich (voc.)
- Materias:
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- Resumen
SUMMARY The Canary Islands, one of the world¿s largest oceanic volcanic regions, are home to over 2 million inhabitants and were visited by more than 12.5 million of tourists during 2013 (ISTAC, 2013: http://www.gobiernodecanarias.org/istac/). This makes the archipelago particularly vulnerable to the hazardous volcanic processes that, on the other hand, have formed the islands. In this context, studies focused on the reduction of the impact of future eruptions and on the improvement of resilience of the Canary socio-economic system should be a priority. Volcanic hazard and risk analyses pose the most useful tools for achieving that goal. Despite the great importance of these types of studies, methodologies for developing a complete volcanic hazard assessment have only been applied to Tenerife, Gran Canaria and Lanzarote Islands. Consequently, it is highly advisable to carry out similar analyses in the remainder of the islands of the archipelago taking the opportunity for improving or testing enhancements of the applied methodologies.
This PhD Thesis is focused on the youngest, smallest and the most recent affected by an eruption island of the Canary archipelago, the El Hierro Island. The beginning of this PhD preceded the 2011-2012 submarine eruption, at the south part of El Hierro. Nonetheless, this eruption made the objectives previously proposed of this PhD clear and raised the need to perform, as well as a comprehensive volcano-structural study, a systematic volcanic hazard evaluation on the island. Thus, the objective of this Thesis is to develop and broaden our knowledge of the structure of El Hierro volcanic edifice and, based on it and on the analyses of previous eruptions, to estimate the long-term volcanic hazard of the island. The latter was carried out analysing, through the application of state-of-the-art probabilistic methods, (1) the gathered volcano-structural data, (2) the information about past volcanic activity behaviour, (3) the spatial probability of future eruptions and their associated hazards, (4) and the estimations of temporal occurrence of eruptions.
The development of this study required the design of a purpose-built database for facilitating subsequent hazard and risk analyses. To this goal, a volcanological database for El Hierro (HADA) was constructed for compiling all the necessary information to carry out the subsequent volcanic hazard analysis. Additionally, in the framework of this study, the database for the assessment and management of the volcanic risk (VERDI) was also designed to become the basis for future volcanic risk studies. Both databases were constructed on the basis of a Geographic Information System (GIS).
A detailed volcano-structural analysis of the island was carried out, in which vents, eruptive fissures, dykes and faults were identified measured and analysed. In addition, a new volcano-structural map was developed for El Hierro volcanic edifice at 1:10,000-scale.
The main results show a radial strike distribution of the on-land and submarine structures suggesting the existence of a rather uniform stress field during the constructive episodes, that result from a combination of loading, gravitational spreading, and magma-induced upwelling. By contrast, in the shallower parts of the edifice the NE-SW, N-S and WNW-ESEstriking structures are seen, reflecting local stress fields related to the formation of megalandslides which mask the general and regional radial patterns. This study suggest that the rifts zones described on El Hierro are shallow structures that commonly capture and divert ascending magma towards different parts of the island but do not condition magma ascent at depth.
In addition, the length-thickness ratios of feeder dykes were used to estimate the depth to the source magma chamber. Accurately measured volcanic fissures/feeder-dykes indicate a source depth of 8.4 14.5 km, which coincides with the main cloud of the earthquake foci surrounding the magma chamber associated with the 2011¿2012 eruption.
On the other hand, the fragile structure of the island is characterised by the existence of normal faults that in general are parallel to the trend of the rifts. In the NE rift they form a graben structure that has an exposed length of nearly 6 km and a width of approximately 1.5 km. The graben is bounded by two major normal faults trending ENE-WSW and inside it, three subtle antithetic faults were recognised. The graben¿s main faults dip an average of 65° towards SE and have escarpments up to 30 m high. Related to the scarp of El Golfo landslide, some other faults and fractures were identified following the trend of the scarp related to the decompression process favoured by the landslide.
Cortical extension produced by dyke intrusion has also been performed in order to evaluate the spreading rates on the Island, being very low in the most recent rocks, 0.04%, and greater close to the axis of the rifts, 25.15%. This value is significant since it indicates a substantial growth by intrusions, and it could be considered as a conditioning factor for large landslides on the island.
Additionally to the volcano-structural analysis, several sequential steps were followed to ensure quality and representative results of the long-term volcanic hazard assessment.
These steps were the following: (1) the compilation of geological and volcanological information, (2) the characterisation of past eruptions, (3) the spatial and temporal probabilistic studies, (4) the simulation of the most probable eruptive scenarios, and (5) the development of the volcanic hazard maps.
The characterisation of the volcanic activity behaviour on the island through the study of the variety of forms, eruptive styles, deposits, petrology and size of the eruptions allowed settling that mafic monogenetic eruptions represent the most likely eruption types in the near future on the island. Nevertheless, submarine eruptions will be also greatly expected processes, if it is considered the entire volcanic edifice. The on-land eruptions would generate: small-size cones with total volumes that can at least range from 0.0016 to 0.15 km3, being most of VEI values in the range 0¿2; lava flows which did not reach the sea have total lengths of 8 km, but they can be even longer when they arrive at the sea having mean thickness of 3 m; proximal scoria, fallout and, occasionally, PDCs, these latters in cases in which eruptions are related to hydromagmatic episodes. The most common eruptive styles are Hawaiian and Strombolian to violent Strombolian.
Due to the volcano-oceanic nature of the island, hydrovolcanic eruptions (submarine or littoral eruptions affecting coastal areas or phreatomagmatic explosions), are also expected. These eruptions would produce tuff rings and tuff cones, rhythmic laminated sequences of coarse juvenile ash and lapilli rich beds with accidental lithic fragments or even the formation of maars or pseudo-craters. The study of the only one explosive felsic eruption on the island, named Malpaso Member, confirmed that the eruption deposit was originated from a base-surge-type explosive eruption with a subsequent radial emplacement of dilute PDC currents in which magma/water interaction controlled the dynamics of the eruption covering an area of more than 15 km2. It corresponds to the final episodes in the construction of the El Golfo-Las Playas Edifice. The presence of this deposit opens up the possibility that eruptions other than monogenetic magmatic and/or hydrovolcanic mafic ones may also occur. Despite they represent higher explosively eruptions, they have much lower probability of occurrence than the mafic magmatic ones.
The subsequent step on the long-term volcanic hazard evaluation consisted of evaluating the likelihood of a future eruption, i.e., to identify the spatial distribution of vent open, in order to estimate which areas are most likely to host future vents. This study was based on the probabilistic analysis of volcano-structural data of the island, to finally develop the susceptibility map of El Hierro. The most likely area to host new eruptions in El Hierro is in the south-western part of the west rift. High probability locations are also found at the southern end of the south rift and in the northeast rift, and along the submarine prolongation of the rifts axes. This map can be a support tool for decision makers in land planning, emergency measures and civil defence actions.
Another step forward on the long-term volcanic hazard assessment is to estimate the temporal probability of any possible volcanic event. To address this issue, on one hand four new age data were obtained to time constrain the recent Quaternary volcanism of El Hierro and to estimate its recurrence rate. Three of them were obtained by the 40Ar/39Ar method, whereas one was obtained using the 14C technique, this latter representing the most recent dated eruption so far on the island at the northeast rift, giving an age of 2280±30 14C BP. The maximum eruptive recurrence rate taking into account all available ages onshore for the last 33 ka is 9.7 x10-4 events per year, for the emerged part of the island, which means that the recurrence period of the eruptions is approximately 1000 years.
On the other hand a probabilistic statistical method based on Bayesian event tree for longterm probability of each possible scenario was used, through the HASSET tool that took into account the previous dating information. Thus, the long-term probability of a basaltic eruption with magmatic unrest in the submarine area, occurring in the next 20 years is 0.11±0.04. But if a submarine mafic magmatic eruption of VEI 2 generating short lava flows is considered the probability is 0.04±0.02.
By studying the past eruptive behaviour and once the spatial and temporal probabilities were estimated, the most probable scenarios were computing as a means of evaluating the potential extent of the main expected volcanic processes and associated hazards. For this purpose, VORIS 2.0.1 was used to model lava flows, PDCs and ash fallout. Combination of the most probable scenarios related to basaltic eruptions of VEI 2 that generate lava flows, fallout and PDCs in case of hydrovolcanic events, provided the first qualitative integrated volcanic hazard map of El Hierro. The resulting map shows that medium and high volcanic hazard zones coincide with some of the main inhabited areas. However, this map is reliant on the current information, which may be subject to further revision thus implying possible changes in the hazard assessment. In addition, the spatiotemporal probability map of vent opening or quantitative hazard map of El Hierro was developed through the combination of the spatial probability map and the estimated recurrence period.
As a concluding remark, this PhD dissertation offers a methodological procedure that facilitates undertaking volcanic hazard assessment in a systematic way, which can be easily applied to other volcanic areas, with similar characteristics, around the world.
Furthermore, this study can be a valuable document for enabling local authorities to apply more rational land use and, in the particular case of the Canary Islands Civil Protection Organisation, to design more adequate emergency plans to face future volcanic crises.
