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Sensitivity to Volcanic Field Boundary Melody G Runge et al. Journal of Applied Volcanology (2015) 4:22 DOI 10.1186/s13617-015-0040-z RESEARCH Open Access Sensitivity to volcanic field boundary Melody G. Runge1*, Mark S. Bebbington2, Shane J. Cronin1, Jan M. Lindsay1 and Mohammed Rashad Moufti3 Abstract Volcanic hazard analyses are desirable where there is potential for future volcanic activity to affect a proximal population. This is frequently the case for volcanic fields (regions of distributed volcanism) where low eruption rates, fertile soil, and attractive landscapes draw populations to live close by. Forecasting future activity in volcanic fields almost invariably uses spatial or spatio-temporal point processes with model selection and development based on exploratory analyses of previous eruption data. For identifiability reasons, spatio-temporal processes, and practically also spatial processes, the definition of a spatial region is required to which volcanism is confined. However, due to the complex and predominantly unknown sub-surface processes driving volcanic eruptions, definition of a region based solely on geological information is currently impossible. Thus, the current approach is to fit a shape to the known previous eruption sites. The class of boundary shape is an unavoidable subjective decision taken by the forecaster that is often overlooked during subsequent analysis of results. This study shows the substantial effect that this choice may have on even the simplest exploratory methods for hazard forecasting, illustrated using four commonly used exploratory statistical methods and two very different regions: the Auckland Volcanic Field, New Zealand, and Harrat Rahat, Kingdom of Saudi Arabia. For Harrat Rahat, sensitivity of results to boundary definition is substantial. For the Auckland Volcanic Field, the range of options resulted in similar shapes, nevertheless, some of the statistical tests still showed substantial variation in results. This work highlights the fact that when carrying out any hazard analysis on volcanic fields, it is vital to specify how the volcanic field boundary has been defined, assess the sensitivity of boundary choice, and to carry these assumptions and related uncertainties through to estimates of future activity and hazard analyses. Keywords: Probabilistic, Hazard forecasting, Harrat Rahat, Auckland volcanic field Background underpinning magmatic processes, evolution, and hazard Regions of distributed volcanism are often referred to as potential. As these subsurface processes are predomin- volcanic fields. As each eruption tends to occur in a dif- antly unknown, and highly complex, this determination ferent location, they can be further classified as mono- is accomplished via approximation of the known spatio- genetic volcanic fields and are frequently modelled as temporal evolution of the field, and then extrapolated spatial or spatio-temporal point processes (Connor and into the future to obtain hazard forecasts. This almost Hill 1995; Marzocchi and Bebbington 2012; Bebbington invariably uses spatial or spatio-temporal point processes 2013). Volcanic activity within these regions tends to be with model selection and development based on analysis predominantly basaltic, with low flux rates. Conse- of previous eruption data. The location of future erup- quently, volcanic fields tend to grow laterally rather than tions substantially influences the areas at risk from build up a single edifice in one location (Wood and eruption related hazards. The specific location may also Shoan 1981), and eruptions typically form cinder cones, affect the eruption style, for example, the presence of tuff cones and rings, maars, lava domes, shield volca- ground-water may lead to an explosive phreatomagmatic noes, and lava flows (Connor and Conway 2000). eruption (Ross et al. 2011). Thus, the probable location Determination of a distributed volcanic field’s spatio- is one of the most important pieces of information for temporal behaviour is key to understanding its volcanic hazard estimation. To obtain probable future locations, the first step is to assess the existence of patterns in previous eruption lo- * Correspondence: [email protected] 1University of Auckland, Private Bag 92019, Auckland 1142, New Zealand cations and timings. This is accomplished using various Full list of author information is available at the end of the article © 2015 Runge et al. 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. Runge et al. Journal of Applied Volcanology (2015) 4:22 Page 2 of 18 exploratory statistics including lineament identification ensures a conservative ‘safe’ solution, but potentially un- (Wadge and Cross 1988; Lutz and Gutmann 1995) and derestimates average vent-density and consequently cluster analyses (Clark and Evans 1954; Hopkins and spatial intensity. It also increases the coverage area over Skellam 1954; Ripley 1979). For these, a surface region which supplementary data is required for a hazard ana- representing the volcanic field must be defined, both for lysis, thus either increasing costs, or lowering the data area calculations and Monte Carlo point placement resolution. steps. A region must also be defined for the evaluation In this work, various approaches for the definition of of spatio-temporal models (Connor and Hill 1995; volcanic field boundaries are evaluated for spatial inten- Cronin et al. 2001), and for assessment of all spatial and sity estimates at two quite different volcanic fields: spatio-temporal models during verification and valid- Harrat Rahat, Kingdom of Saudi Arabia, and Auckland ation steps. Volcanic Field (AVF), New Zealand. This paper begins The spatial boundaries of a volcanic field are conven- with an introduction to these two volcanic fields selected tionally defined as the extent of the existing lava flows, as case studies. Five boundary options are then outlined scoria cones, or other volcanic edifices. These are usually which comprise of three commonly applied fitting well constrained via geological mapping. This provides a methods (an ellipse, a rectangle, and a convex hill), present-day view of the location of past volcanic material alongside two vent-density based suggestions (isotropic from a resource-mapping perspective. However, for as- and anisotropic kernel smoothing methods). Five geo- sessment of future volcanic activity, this definition is not logical considerations are then discussed for the two vol- sufficient as it does not account for the locations of new canic fields that then form the underlying assumptions eruption vents and may confound the locations of previ- and related uncertainties for each fitted boundary. These ous volcanic eruptions with factors controlling lava dis- considerations combined with the five boundary options tribution. Depending on rheology and underlying result in ten potential boundary options for the volcanic topography, lava may extend tens of kilometres from a field case studies. These provide the basis for the sensi- source vent and accumulate in depressions giving false tivity analyses for each field and the affect of boundary weight to these regions far from potential eruption definition is assessed using exploratory methods locations. (summary statistics) and spatial intensity estimates. Volcanic fields form above mantle regions where par- tial melting produces magma, which may be related to Harrat Rahat low rates of crustal extension (Takada 1994). In these Harrat Rahat is one of at least nineteen volcanic fields sites, magma supply is insufficient to maintain a single within the western Arabian Plate (Brown 1970). The harrat conduit, such that each new magma batch has to find its (‘volcanic-field’) has a total erupted volume of ~2000 km3 own path to the surface (Fedotov 1981). Due to the var- (Blank and Sadek 1983) from at least 968 identified vents iety of geological conditions under which mantle mate- (Runge et al. 2014, Fig. 1a) and is predominantly basaltic rials melt to form magmas (e.g., within subduction with some more evolved compositions. The cause of vol- zones, in areas of weak crustal thinning, or hot-spot up- canism across the Arabian Shield is still very much under welling), boundary parameters remain unknown, thus a discussion (Moufti et al. 2013; Rolandone et al. 2013), and well-informed geological approach to boundary defin- although there are other volcanic fields, and a major zone ition is currently impossible. For forecasting purposes of crustal rifting (the Red Sea rift) proximal to Harrat however, the volcanic field boundary has a significant in- Rahat, little is known about their respective influences on fluence on estimates of event recurrence rate and spatial magma generation within the harrat. intensity. Where a hard boundary is required (e.g., for Erupted products include extensive lava flows (Murcia clustering statistics, or spatio-temporal models), it is et al. 2014), as well as scoria and spatter cones, shield assumed that there is zero probability of an eruption volcanoes, tuff rings, and lava domes (Camp and Roobol occurring outside of this region. Often, even when a soft 1989). Eruptions in Harrat Rahat have been dated be- boundary is feasible, a hard boundary
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