Selva et al. Journal of Applied Volcanology (2019) 8:5 https://doi.org/10.1186/s13617-019-0086-4 REVIEW Open Access Multiple natural hazards at volcanic islands: a review for the Ischia volcano (Italy) J. Selva1* , V. Acocella2, M. Bisson3, S. Caliro4, A. Costa1, M. Della Seta5, P. De Martino4, S. de Vita4, C. Federico6, G. Giordano2, S. Martino5 and C. Cardaci7 Abstract Volcanic islands pose several major types of natural hazards, often interconnected and concentrated in relatively small areas. The quantification of these hazards must be framed from a multi-hazard perspective whilst building on existing single-hazard analyses. Ischia is a densely inhabited volcanic island with a long eruptive history lasting more than 150 ka (last in 1302 AD) characterized by the significant asymmetric resurgence of a caldera block. Here, we review the state-of-art of the natural hazards of Ischia, aiming at building a solid base for future holistic multi- hazard quantifications. We frame our analysis in three steps: i) review of geological, historical and current activity; ii) review of available hazard models and analyses; iii) development of an interpretative framework for the interdependent hazards. The results highlight that volcanic activity has been quite intense and many volcano- related hazardous phenomena have affected the island including in very recent times, both for eruptive (phreatic or magmatic eruptions) and non-eruptive (earthquakes, landslides, and tsunamis) phenomena. The effects of some of them (e.g. tsunamis, tephra) are also relevant beyond the island territory. Quantitative hazard assessments are almost absent and should be developed in the future considering the evident interconnections between hazards. To this end, we propose a conceptual interpretative multi-hazard framework that highlights the fundamental role played by the resurgent block in controlling and connecting the different hazards, in terms of both spatial distribution of the sources and temporal clustering. Keywords: Ischia island, Volcanic hazards, Multi-hazard, Conceptual model, Resurgence Introduction the different “states of the volcano” (quiescence, unrest, The management of long-term volcanic risks is particu- eruption) and the definition of robust methods to fore- larly crucial in volcanic islands, which are characterized cast the transition between these states. This requires by multiple hazards concentrated in a relatively small the development of multi-disciplinary studies targeted to area, often associated with a large seasonality of expos- comprehensive multi-hazard quantifications (e.g., Mar- ure due to tourism. Volcanic islands are indeed particu- zocchi et al. 2012; Selva 2013; Mignan et al. 2014; Liu et larly fragile environments due to their special geological al. 2015) that, for volcanic islands, are fundamentally un- formation and their societal development concentrated derrepresented in the literature. In addition, for these in relatively small areas, in which cascading events also topics, the emerging scientific knowledge is often rather connect emerged and submarine environments (e.g., limited and uncertain and, also when well constrained, Carey et al. 1996; Marti et al. 2010; Giachetti et al. 2012; difficult to communicate to decision makers due to its Casalbore et al. 2018; Hunt et al. 2018; Rosi et al. 2019). intrinsic complexity. The scientific challenges are mainly the quantification In this paper, we discuss the experience gained by one and the characterization of the interactions among the working group in charge of reviewing the state of know- multiple hazardous phenomena that may occur during ledge about volcanic hazards for Ischia, Italy, established within the agreement between DPC (Italian Department * Correspondence: [email protected] of Civil Protection) and INGV (Istituto Nazionale di 1Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Bologna, Geofisica e Vulcanologia; INGV-DPC-T4C 2016–2017). Italy Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Selva et al. Journal of Applied Volcanology (2019) 8:5 Page 2 of 43 Ischia volcano belongs to the Neapolitan volcanic sys- SSHAC 1997), so the full spectrum of scientific opinions tem. It is located to the SW of the Campi Flegrei cal- (epistemic uncertainty) retrieved from literature and dis- dera, and of the volcanic islands of Procida and Vivara, tilled through a transparent and documented interaction all together forming the Phlegraean volcanic district. Is- within the review has been treated. Noteworthy, STEPs chia volcano has been the site of a large number of erup- 2 and 3 are strongly oriented toward a multi-hazard and tions in historical times, the most recent of which multi-risk perspective, to meet the requirement of build- occurred in 1302 AD (Vezzoli 1988; Iacono 1996), and ing a holistic multi-hazard framework on which to base of other interdependent hazardous phenomena con- future hazard quantification. nected to its magmatic system. It is also characterized by STEP 1 consists of the review of the available geo- an active hydrothermal system (Chiodini et al. 2004;Di logical, historical and instrumental data, as well as their Napoli et al. 2011 and references therein) that, along interpretations. The main goals are the definition of a with its favourable position within the Neapolitan gulf reference period and the characterization of the volcanic and its beaches, makes it one of Italy’s major touristic states (rest/unrest/eruption), allowing us to define a ref- hubs. The island is densely populated, with more than erence catalogue of eruptive and non-eruptive events. 60,000 inhabitants distributed in less than 50 km2.In This catalogue represents a fundamental input to fore- touristic seasons this population increases substantially. casting tools (e.g., Newhall and Hoblitt 2002; Aspinall et This makes the volcanic and related risks at Ischia very al. 2003; Marzocchi et al. 2008; Newhall and Pallister high, also for relatively small events, as demonstrated by 2015; Hincks et al. 2014), hazard quantifications in rest, the recent Mw = 3.9 earthquake occurred in August unrest, and eruption periods (e.g., Bonadonna et al. 2017 (Gruppo di Lavoro INGV sul terremoto dell’isola 2005; Costa et al. 2009; Selva et al. 2010, 2014, 2018; di Ischia 2017) that caused 2 fatalities, several tens of in- Jenkins et al. 2012; Biass and Bonadonna 2012; Tierz et jured, and substantial damages mainly to the village of al. 2017), as well as to the definition of potential strat- Casamicciola Terme, located in the northern part of the egies for risk management (e.g., Marzocchi and Woo island. 2007; Winson et al. 2014; Woo 2015; Papale 2017; Pallis- Although the potential volcanic and related risks at Is- ter et al. 2019). chia are very high, the scientific literature on their quan- STEP 2 consists of the systematic review of the state- tification is sparse and relatively poor. In particular, a of-art of hazard quantifications for all the potential haz- critical review of the basic knowledge regarding the vol- ards, adopting a multi-hazard perspective.. We extended canic system and the related natural hazards is still lack- the review to all the phenomena defined in IAEA (2012, ing, even in the presence of a relatively rich literature on 2016), including eruptive (new vent, ballistics, tephra fall the geology of the island (Rittmann and Gottini 1980; and atmospheric phenomena, lava flows and pyroclastic Vezzoli 1988; de Vita et al. 2010; Sbrana and Toccaceli density currents) and non-eruptive (hydrothermal phe- 2011; Sbrana et al. 2018 and references therein), as well nomena, gases, deformations, seismicity, gravitational in- as a relatively large number of studies regarding the phe- stability, and tsunamis). We adopted four reviewing nomena that it may produce (especially on historical milestones (past observations including the largest earthquakes, landslides and tsunamis, e.g., Cubellis and known and the most recent events, occurrence in rest/ Lungo 2018; de Vita et al. 2006; Della Seta et al. 2012; unrest/eruption, intensities and probabilistic hazard Paparo and Tinti 2017). This situation does not even curves, triggering/cascading events) and on a rough allow establishing solid ground for hazard quantifica- discretization of probabilistic and spatial information. tions in the future. Our goal is to provide a homogeneous foundation for fu- To fill this gap and prepare the ground for future haz- ture probabilistic single and multi-hazard quantifica- ard quantifications, we propose a standardized multi- tions, bounding on the already observed natural hazard review scheme organized in 3 steps. STEP 1 is variability and on the links with the volcanic states and dedicated to the establishment of the state-of-art on the with the other hazards. volcanic system. STEP 2 is focused on the review of the STEP 3 develops a conceptual interpretative frame- available quantification of all the hazardous phenomena work providing a comprehensive and rational interpret- that a volcanic system may be related to. STEP 3 is dedi- ation of the system based on the available information cated to the development of a reference interpretative provided by STEPs 1 and 2. This type of models, often conceptual framework, focusing on specific topics of the basis of hazard quantifications (e.g., IAEA 2012, interest that emerge during STEPs 1 and 2. All the three 2016), reflects the opinion that the authors formed dur- steps provide concrete results useful for future develop- ing the review project, and has a more speculative char- ments. In all STEPs, we accounted for i) the hazard/risk acter than the other STEPs. As such, no alternative separation principle (e.g., Jordan 2014; Papale 2017); ii) models were considered, even if theoretically possible.