Lindsay, Jan M

Uncertainties in modelling volcanic hazards are often amplified in geographically large systems and in volcanoes which have a diverse eruption history that comprises variable eruption compositions and styles from different vent locations. The large ~ 700 km2 Okataina Volcanic Centre (OVC) is a large silicic caldera complex in a geodynamic region of New Zealand which has displayed a range of eruption styles and compositions over its current phase of activity (26 ka - present), including one basaltic maar-forming eruption, one basaltic Plinian eruption, and nine rhyolitic Plinian eruptions. All three of these eruption styles have occurred within the past 3.5 ky, and any of these styles could occur in the event of a future eruption. The location of a future eruption is also unknown. Future vents could potentially open in one of three different possible areas of the OVC: the Tarawera linear vent zone (LVZ) (5 eruptions over the past 26 ky), the Haroharo LVZ (5 eruptions over the past 26 ky), or outside of these LVZs (1 eruption over the past 26 ky). A future rhyolitic or basaltic Plinian eruption from the OVC is likely to generate widespread tephra fall in loads that will cause significant disruption and socio- economic impacts throughout the surrounding region. Past OVC tephra studies have focused on evaluating hazard from a rhyolitic Plinian eruption at select vent locations in the OVC's Tarawera LVZ. Here, we expand upon these past studies by evaluating tephra hazard for all possible OVC eruption vent areas and for both rhyolitic and basaltic Plinian eruption styles, and exploring how these parameters influence tephra hazard forecasts. Probabilistic volcanic hazard model BET_VH and advection-diffusion tephra hazard model TEPHRA2 were used to assess the hazard of accumulating ≥ 10 kg m-2 of tephra from both basaltic Plinian and rhyolitic Plinian eruption styles, occurring from within the Tarawera LVZ, the Haroharo LVZ, and other potential vent areas within the caldera. We present the results of these analyses as a first-order tephra hazard assessment for the entire OVC. Our results highlight the importance of considering all the potential vent locations of a volcanic system, in order to capture the full eruption catalogue in analyses (e.g., 11 eruptions over 26 ky for the OVC, versus only 5 eruptions over 26 ky for the Tarawera LVZ), as well as the full potential distribution of tephra hazard. Although the Tarawera LVZ has been prominently discussed in studies of OVC hazard because of is recent activity (1886 and ~1315 AD), we find that, in the event of future eruption, the likelihood of a vent opening within the Haroharo LVZ (last eruption 5.6 ka) is equivalent (< 1% difference) to that for the Tarawera LVZ (31.8% compared to 32.5%). We also find that an eruption from within the Haroharo LVZ presents a relatively higher hazard to several localities, such as the town of Kawerau, where the average absolute probability of accumulating ≥ 10 kg m-2 of tephra is 1.3 times greater than for an eruption from within the Tarawera LVZ. While the absolute probabilities of accumulating ≥ 10 kg m-2 of tephra in the next one year from a basaltic Plinian eruption are on average 7.2 times lower than for a rhyolitic Plinian eruption throughout the surrounding region, our results suggest that the hazard posed by a basaltic Plinian eruption does contribute to the overall OVC tephra hazard, raising absolute probabilities for the entire OVC by an order of 0.14, which may have implications when considering sensitive decision-making thresholds.

We report on the first ‘‘real-time’’ application of the BET_UNREST (Bayesian Event Tree for Volcanic Unrest) probabilistic model, during a VUELCO Simulation Exercise carried out on the island of Dominica, Lesser Antilles, in May 2015. Dominica has a concentration of nine potentially active volcanic centers and frequent volcanic earthquake swarms at shallow depths, intense geothermal activity, and recent phreatic explosions (1997) indicate the region is still active. The exercise scenario was developed in secret by a team of scientists from The University of the West Indies (Trinidad and Tobago) and University of Auckland (New Zealand). The simulated unrest activity was provided to the exercise’s Scientific Team in three ‘‘phases’’ through exercise injects comprising processed monitoring data. We applied the newly created BET_UNREST model through its software implementation PyBetUnrest, to estimate the probabilities of having (i) unrest of (ii) magmatic, hydrothermal or tectonic origin, which may or may not lead to (iii) an eruption. The probabilities obtained for each simulated phase raised controversy and intense deliberations among the members of the Scientific Team. The results were often considered to be ‘‘too high’’ and were not included in any of the reports presented to ODM (Office for Disaster Management) revealing interesting crisis communication challenges. We concluded that the PyBetUnrest application itself was successful and brought the tool one step closer to a full implementation. However, as with any newly proposed method, it needs more testing, and in order to be able to use it in the future, we make a series of recommendations for future applications.

Quantitatively assessing long-term volcanic risk can be challenging due to the many variables associated with volcanic hazard and vulnerability. This study presents a structured first-order approach for considering variables in hazard and vulnerability anal- yses, such as eruption style and cyclic fragility, in order to quantitatively estimate risk. Probabilistic volcanic hazard data derived from advection–diffusion–sedimentation tephra fall model TEPHRA2 and probabilistic volcanic hazard analysis tool BET_VH (Bayesian Event Tree for Volcanic Hazards) are combined with fragility functions and seasonal vulnerability coefficients for agricultural production to calculate volcanic risk indices which represent the likelihood of damage or loss to farm production over a given time frame. The resulting dataset allows for approximations of quantitative risk over a con- tinuous range of ash thickness thresholds, at multiple levels of uncertainty, and in the context of fluctuating hazard and vulnerability environments (e.g., seasonal wind patterns and crop phases). We illustrate this approach through a case study which evaluates the risk of incurring 90% damage to agricultural production at dairy and fruit farms in the Bay of Plenty region of New Zealand (BoP) due to ashfall from a Plinian eruption phase at the large local caldera volcano, the Okataina Volcanic Centre (OVC). Consideration of sea- sonal wind profiles, seasonal fluctuations in fruit and dairy farm vulnerability, multiple possible OVC eruption styles, different possible OVC vent locations, and a continuous distribution of ash thickness and damage thresholds enables a multi-dimensional analysis that aims to reflect the natural complexity and interdependencies associated with volcanic risk. A risk uncertainty matrix is introduced as a conceptual scheme to help guide eval- uation and communication of the results of such quantitative risk analyses by showing how different types of uncertainty can yield ‘‘maximum’’, ‘‘average’’, or ‘‘minimum’’ estimates of risk. Results of this case study indicate that BoP fruit farms are at higher risk of experiencing damage and production loss from OVC ashfall than dairy farms, and farms to the east of the OVC are typically at higher risk than farms to the north of the OVC. Forecasts based on the annual maximum estimate of risk for fruit farms show a regional average of 2.3% probability (greater than 1 in 50 likelihood) of experiencing 90% damage from a basaltic or rhyolitic Plinian eruption from anywhere within the OVC over a period of 100 years. Seasonal-level analyses revealed that the risk of experiencing losses due to OVC ashfall at fruit farms is cyclic and fluctuates with time of year and harvest season, with the highest risk experienced during peak harvest season (15 October–14 April) when crop vulnerability is high and westerly winds dominate in the BoP.

By using BET_VH, we propose a quantitative probabilistic hazard assessment for base surge impact in Auckland, New Zealand. Base surges resulting from phreatomagmatic eruptions are among the most dangerous phenomena likely to be associated with the initial phase of a future eruption in the Auckland Volcanic Field. The assessment is done both in the long-term and in a specific short-term case study, i.e. the simulated pre-eruptive unrest episode during Exercise Ruaumoko, a national civil defence exercise. The most important factors to account for are the uncertainties in the vent location (expected for a volcanic field) and in the run-out distance of base surges. Here, we propose a statistical model of base surge run-out distance based on deposits from past eruptions in Auckland and in analogous volcanoes. We then combine our hazard assessment with an analysis of the costs and benefits of evacuating people (on a 1km x 1km cell grid). In addition to stressing the practical importance of a cost-benefit analysis in creating a bridge between volcanologists and decision makers, our study highlights some important points. First, in the Exercise Ruaumoko application, the evacuation call seems to be required as soon as the unrest phase is clear; additionally, the evacuation area is much larger than what is recommended in the current Contingency Plan. Secondly, the evacuation area changes in size with time, due to a reduction in the uncertainty in the vent location and increase in the probability of eruption. It is the tradeoff between these two factors that dictates which cells must be evacuated, and when, thus determining the ultimate size and shape of the area to be evacuated.

The Auckland Volcanic Field (AVF) is a young basaltic field that lies beneath the urban area of Auckland, New Zealand’s largest city. Over the past 250,000 years the AVF has produced at least 49 basaltic centers; the last eruption was only 600 years ago. In recognition of the high risk associated with a possible future eruption in Auckland, the New Zealand government ran Exercise Ruaumoko in March 2008, a test of New Zealand’s nation-wide preparedness for responding to a major disaster resulting from a volcanic eruption in Auckland City. The exercise scenario was developed in secret, and covered the period of precursory activity up until the eruption. During Exercise Ruaumoko we adapted a recently developed statistical code for eruption forecasting, namely BET_EF (Bayesian Event Tree for Eruption Forecasting), to independently track the unrest evolution and to forecast the most likely onset time, location and style of the initial phase of the simulated eruption. The code was set up before the start of the exercise by entering reliable information on the past history of the AVF as well as the monitoring signals expected in the event of magmatic unrest and an impending eruption. The average probabilities calculated by BET_EF during Exercise Ruaumoko corresponded well to the probabilities subjectively (and independently) estimated by the advising scientists (differences of few percentage units), and provided a sound forecast of the timing (before the event, the eruption probability reached 90%) and location of the eruption. This application of BET_EF to a volcanic field that has experienced no historical activity and for which otherwise limited prior information is available shows its versatility and potential usefulness as a tool to aid decision-making for a wide range of volcano types. Our near real-time application of BET_EF during Exercise Ruaumoko highlighted its potential to clarify and possibly optimize decision-making procedures in a future AVF eruption crisis, and as a rational starting point for discussions in a scientific advisory group. It also stimulated valuable scientific discussion around how a future AVF eruption might progress, and highlighted areas of future volcanological research that would reduce epistemic uncertainties through the development of better input models.