Bulletin of Volcanology (2019) 81: 76 https://doi.org/10.1007/s00445-019-1336-3 RESEARCH ARTICLE VOLCANS: an objective, structured and reproducible method for identifying sets of analogue volcanoes Pablo Tierz 1,2 & Susan C. Loughlin1 & Eliza S. Calder2 Received: 23 February 2019 /Accepted: 8 November 2019 /Published online: 6 December 2019 # The Author(s) 2019 Abstract The definition of a suite of analogue volcanoes, or volcanoes that are considered to share enough characteristics as to be considered exchangeable to a certain extent, is becoming a key component of volcanic hazard assessment. This is particularly the case for volcanoes where data are lacking or scarce. Moreover, volcano comparisons have often been based on similarities and differences inferred through expert judgement and not necessarily informed by volcano characteristics from global datasets. These similarities can be based on a range of features, from very simplified (e.g. statrovolcanoes) to very specific (e.g. detailed eruption chronologies), and may be strongly influenced by the personal experience of individuals or teams conducting the analogue analysis. In this work, we present VOLCANS (VOLCano ANalogues Search)—an objective, structured and reproducible method to identify sets of analogue volcanoes from global volcanological databases. Five overarching criteria (tectonic setting, rock geochemistry, volcano morphology, eruption size and eruption style), and a structured combination of them, are used to quantify overall multi-criteria volcano analogy. This innovative method is complementary to expert-derived sets of analogue volcanoes and provides the user with full flexibility to weigh the criteria and identify analogue volcanoes applicable to varied purposes. Some results are illustrated for three volcanoes with diverse features and significant recent and/or ongoing eruptions: Kılaueā (USA), Fuego (Guatemala) and Sinabung (Indonesia). The identified analogue volcanoes correspond well with a priori analogue volcanoes derived from expert knowledge. In some cases, single-criterion searches may not be able to isolate a reduced set of analogue volcanoes but any multi- criteria search can provide high degrees of granularity in the sets of analogue volcanoes obtained. Data quality and quantity can be important factors, especially for single-criterion searches and volcanoes with very scarce data (e.g. Sinabung). Nevertheless, the method gives stable results overall across multi-criteria searches of analogue volcanoes. Potential uses of VOLCANS range from quantitative volcanic hazard assessment to promoting fundamental understanding of volcanic processes. Keywords Global volcanism . Global database . Volcano morphology . Volcanic hazard assessment . Scarce data . Objective analogue volcano Introduction complex and operate on spatial and temporal scales ranging over many orders of magnitude (Sigurdsson et al. 2015). Volcanoes are powerful manifestations of our geodynamic Therefore, volcanic behaviour is truly challenging to under- planet. Volcanic physico-chemical processes are extraordinarily stand and even more challenging to forecast. Volcano scientists worldwide are asked, on a daily basis, to collect, process and Editorial responsibility: C.E.Gregg interpret varied types and sources of volcanological data (e.g. deposits of volcanic material, chemical compositions of volca- Electronic supplementary material The online version of this article nic minerals and rocks, seismic signals from volcano-tectonic (https://doi.org/10.1007/s00445-019-1336-3) contains supplementary material, which is available to authorized users. earthquakes, ground deformation, gas emissions) for its use in forecasting the future short- and long-term evolution of a par- * Pablo Tierz ticular volcanic system (e.g. Newhall and Hoblitt 2002; [email protected] Loughlin et al. 2015; Newhall et al. 2017). Some frequently active volcanoes may appear somehow predictable (e.g. 1 British Geological Survey, The Lyell Centre, Edinburgh, UK Stromboli, Italy), due to the fact that they tend to behave as they have behaved in the past (e.g. Blackburn et al. 1976;Rosi 2 School of Geosciences, University of Edinburgh, Edinburgh, UK 76 Page 2 of 22 Bull Volcanol (2019) 81: 76 et al. 2000; Taddeucci et al. 2015). Yet, this by no means ex- ensure homogeneity between sets of volcanoes (Bebbington cludes relatively rapid changes in behaviour towards less ex- 2014) and it sometimes corresponds well with broad classes of pected phenomena: e.g. paroxysmal activity at Stromboli (e.g. tectonic setting (Hone et al. 2007; Whelley et al. 2015). Bertagnini et al. 2003; Calvari et al. 2006; Aiuppa et al. 2010), Similarity in magma composition is typically described by of which the 3 July and 28 August 2019 explosions were a simple geochemical divisions (e.g. mafic/felsic or basalt/an- powerful reminder. Other frequently active volcanoes may desite/dacite/rhyolite, Hone et al. 2007;Mastinetal.2009; show more complex behaviours, such as switching between Ogburn et al. 2015), but more elaborate schema may include open-conduit (i.e. magma is filling the conduit below the crater rock affinity (e.g. tholeiitic/calc-alkaline/alkaline, Sobradelo and feeds the eruption) and closed-conduit regimes (i.e. the et al. 2010). In terms of (explosive) eruption size and style, conduit is plugged by a semi- or totally-crystallysed rock). Volcanic Explosivity Index (VEI, Newhall and Self 1982)isa Some volcanoes may erupt frequently enough that sufficient customary choice (Bebbington 2014; Ogburn et al. 2015)but data can be collected to postulate qualitative or quantitative a great variety of more-or-less specific metrics have been forecasting models of volcanic activity (e.g. Volcán de used. These include the following: repose period, open/ Colima, Mexico; Luhr and Carmichael 1990; De la Cruz- closed conduit, mass eruption rates, grain size distribution, Reyna 1993; Luhr et al. 2002). Volcanoes that erupt infrequent- lava-dome-growth durations and extrusion rates and genera- ly and/or have very few data available concerning past behav- tion of specific types of pyroclastic density currents (PDCs) iour are more numerous: Loughlin et al. (2015) estimated that (Marzocchi et al. 2004;Mastinetal.2009; Sandri et al. 2012, about 70% of the world’s Holocene (last ~ 12 kyr) volcanoes 2014;Sheldrake2014; Ogburn et al. 2015). are very poorly studied. These volcanoes may show no eruptive Classically, analogue volcanoes are identified by classi- signs for centuries and, therefore, go unnoticed by local popu- fying volcanoes into isolated compartments or categories lations but they may, in a matter of weeks to months, grow (e.g. andesitic stratovolcanoes that generate dome-collapse restless and produce energetic eruptions. For instance, Mount PDCs). At high levels of detail, the number of categories Pinatubo (Philippines), erupted in 1991 after approximately can be very large and volcanoes become seemingly unique 500 years of repose, in what was the second largest eruption (Cashman and Biggs 2014). Consequently, the identified of the 20th century (Newhall and Punongbayan 1996). These sets of analogue volcanoes may be suited for a given spe- poorly-studied, potentially dangerous volcanoes are one major cific problem but wider applicability of the methods and reason why the volcanological community has sought to iden- results is prevented. In addition, the common practice in tify analogue volcanoes, which are volcanoes believed to share choosing analogue volcanoes is almost purely based on the some commonalities in terms of their specific “type, magma personal experience or knowledge of the hazard analyst or composition, repose period, or any other characteristics of in- team. The selection of analogue volcanoes under such cir- terest” (Newhall et al. 2017). cumstances may be significantly biased by specific erup- Volcano observatories and crisis-response teams world- tion behaviour patterns known from a handful of volcanoes wide, such as the Volcano Disaster Assistance Program of rather than from global databases. the United States Geological Survey (https://volcanoes.usgs. In this study, we develop an objective, structured and re- gov/projects/VDAP/about.html), have been using analogue producible method for identifying sets of analogue volcanoes volcanoes to inform their volcanic hazard assessments over based on information from three global volcanological data- the last decades, which has helped protect communities bases. We refer to this tool as VOLCANS (VOLCano living around active volcanoes in many countries around the ANalogues Search). Different criteria (tectonic setting, rock world (Sandri et al. 2012;Ogburnetal.2015; Newhall and geochemistry, volcano morphology, etc.) are used to quantify Pallister 2015; Newhall et al. 2017). More broadly, analogue either single-criterion or multi-criteria volcano analogy, which volcanoes have been identified for the purpose of classifica- is interpreted as the inverse of a distance metric defined be- tion (Hone et al. 2007) or, in general, to improve hazard as- tween any two volcanic systems in the databases. This dis- sessments at different temporal and spatial scales (Marzocchi tance metric is a proxy for dissimilarity and is calculated using et al. 2004;Mastinetal.2009; Sandri et al. 2012;Whelley normalised numerical variables, which provides a quantifica- et al. 2015). Table 1 contains a summary of previous studies tion of the volcano analogy between zero and one. This gen- on analogue volcanoes, the