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.
Highlights
Regions of distributed volcanism are often referred to as volcanic fields
The size of the buffer zone was not investigated here and while the 5 km buffer zone imposed for Harrat Rahat is a relatively small increase in total area, this value has a substantial effect on the area designated to the Auckland Volcanic Field (Fig. 2)
Forecasts of long-term volcanic activity should be accompanied by an estimate of their precision and uncertainties imposed by the assumptions made
Summary
Regions of distributed volcanism are often referred to as volcanic fields. As each eruption tends to occur in a different location, they can be further classified as monogenetic volcanic fields and are frequently modelled as spatial or spatio-temporal point processes (Connor and Hill 1995; Marzocchi and Bebbington 2012; Bebbington 2013). The addition of a buffer zone is non-trivial, especially where specific spatial densities are important as excessively increasing the volcanic field area will result in an underestimate of average ventdensity and spatial intensity This is illustrated here using the city of Al-Madinah, located immediately to the north-west of Harrat Rahat (Fig. 1a) which is spread over an area of ~30 km (Fig. 6a), with a population of 1.5 million. OF the five boundary shapes tested, the convex hull results were most sensitive to the addition of a buffer zone, varying up to ~160 % (Table 3), predominantly due to the change in proportion of the area of interest falling within a susceptible region (Fig. 6b) This is closely followed by the rectangular volcanic field boundary with a variation of ~140 % for the same reason (Fig. 6d)
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