Mapping and modelling phreatic ballistic fields at tourism hotspots : a methodological assessment at Tongariro and Whakaari (White Island) Volcanoes, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Ballistic projectiles ejected during explosive volcanic eruptions pose a significant hazard to people, infrastructure, buildings and the environment due to their high impact and sometimes heat energy, accounting for 40% of deaths within 5 km of volcanoes. Phreatic, steam-driven explosive eruptions, which often produce ballistics, are the most common eruption type on earth and can occur without warning. Active volcanoes with geothermal features such as bubbling crater lakes and fumaroles attract tourists, yet are most at risk of erupting phreatically. Recent mass casualties following hydrothermal or phreatic eruptions at Ontake Volcano, Japan in 2014, and a near miss at Tongariro, New Zealand in 2012, have highlighted the hazards posed by ballistics to tourists from relatively small but unheralded explosive eruptions. Ballistic hazard assessments are essential for informing risk management for these unexpected, ballistic-producing eruptions.
A ballistic hazard assessment seeks to determine the probability of eruptions which may eject ballistics, and to identify the areas or elements that ballistics may impact. A key part of this assessment is quantifying the area impacted and the intensity of the hazard within it, in metrics such as impact energy or number of blocks per area. However, current ballistic hazard assessments are limited by mapping methods which sample only a small proportion of a ballistic deposit, and guidance for field-appropriate methods is lacking. This thesis aims to advance ballistic hazard assessments by: (1) improving methods for mapping ballistic deposits through a comparative study; (2) using unique empirical data to improve input parameters for applying state-of-the-art 3D numerical modelling and; (3) creating a simple guide for future ballistic assessments that considers available resources, field time and state of the volcano. Two case-study tourist volcanic centres in New Zealand are the basis of these investigations, due to pressing need for their ballistic hazard to be assessed.
Current ballistic mapping approaches often inadequately describe the size distribution of erupted clasts, particularly small clast sizes, due to time and resource constraints of fieldwork and an under appreciation of their hazard. This constraint then limits the ability of numerical modelling to adequately describe the eruption dynamics and hazards. To investigate this, the most common mapping methods from the literature were applied to a complex phreatic block field at Red Crater, Tongariro and the results were compared to determine their efficiency, limitations and usefulness to risk assessments. Both a handheld photography or a newly-developed aerial imagery method was found to accurately determine the spatial density of ballistics, and, when combined with ground-truthed field data, accurately and efficiently determined size and spatial distribution of blocks as small as 5 cm in diameter.
Using lessons learnt from this methodological comparison, and considering limited field time due to high risk of further eruptions, the handheld mapping method was applied at Whakaari/White Island. This provided a detailed spatial density and size distribution dataset to allow inverse, 3D multiparticle modelling to clarify the dynamics of an unwitnessed eruption. The modelled scenarios using both field and seismo-acoustic data indicate 3 highly directional eruptive bursts ejected blocks at speeds of 45 - 65 m/s at low angles of 30° from horizontal. 30% of the modelled mass did not exit the crater, and 8-21% of the tourist pathway was hit with blocks with sufficient kinetic injury energy to cause casualty. The addition of seismo-acoustic information also allowed the development of a 4D ballistic hazard scenario to analyse the ballistic hazard through time. The demonstration of a field to 4D ballistic hazard assessment approach and the development of a guide for future ballistic hazard assessments has supplied tools for increasing ballistic hazard assessment applicability to risk managers at volcano-tourism centres.