Direct impact from volcanic ballistic projectiles, fragments of solid rock or molten lava, are one of the most common causes of fatalities and injuries on volcanoes and have caused substantial damage and destruction of property and infrastructure. Despite this, ballistic hazard, impact and risk research trails behind other volcanic hazards. There is a good understanding of how ballistics are transported, how far they travel and their size, though little is understood of how they are distributed within a ballistic field, the intensity of ballistic hazard within the field, and how the spatial distribution changes over time. Consequently, when ballistic hazard has been included in hazard and risk assessments and management decisions, it is managed by placing a precautionary zone around the volcano, often based on the maximum travel distance.

In addition, it is well known that an impact by a ballistic can cause injury or death, yet this is not the only aspect of the hazard footprint from an individual ballistic. Other aspects such as impact ejecta (surface debris and/or shrapnel from the ballistic) also contribute to the hazard footprint size and little is known about their ability to cause death or injury and how this changes over the hazard footprint. It is critical for hazard and risk managers to know the potential size of the hazard footprint that a person could be affected by and the hazard intensity that may be experienced to calculate risk effectively. Previously only direct impact and impact angle have been considered in risk calculations. This thesis aims to improve our understanding of ballistic hazard so that a more risk-based approach to hazard and risk assessment and management can be applied. This is achieved through review of ballistic hazard characteristics, hazard and risk assessments, maps, management and communication literature to get an overview of the topic and determine knowledge gaps; and field and experimental work to investigate the ballistic hazard footprint and hazard intensity.

To assess how ballistic distribution and intensity change over a ballistic field, the ballistic hazard footprint at Yasur Volcano, Vanuatu was mapped from drone-captured orthophotos taken in two field campaigns two months apart. Mapping revealed that the spatial density of ballistics changed over small areas. Spatial density and ballistic size decreased with distance from the crater, while an increased spatial density was also noted to the S – SSE of the vent area indicating explosion directionality. When compared with eruption footage captured over a three-day period, it was found that explosion directionality slightly differed between the two data sets (mapping and video analysis) taken over different timescales suggesting that directionality may evolve over time.

The size of the hazard footprint from an individual ballistic was found to be influenced by impact energy of the ballistic, ballistic diameter, crater diameter, ejecta travel distance, ejecta impact energy, ballistic density, substrate hardness, impact angle and slope. Pneumatic cannon experiments were used to investigate the contribution of impact ejecta to the hazard footprint. The kinetic energy and travel distance of impact ejecta produced from varying ballistic densities impacting different surfaces were analysed and findings showed that ejecta have the potential to cause injury or fatality on impact (based on hazard intensity values found in the literature). However, this was greatly dependent on the density of the impacting ballistic, the surface hardness, ballistic impact energy and where along the ejecta trajectory it impacted. Initial kinetic energy values were not retained over the entire ejecta trajectory, indicating that hazard intensity varies over the individual ballistic hazard footprint.

To make the most effective risk management decisions it is important to understand both the hazard footprint and the hazard intensity. Ballistic hazard and risk assessments should be conducted over as much of the volcano as possible and assess the spatial density (hazard intensity) as well as the extent. Additionally, assessments should include all temporal hazard changes that may occur in the timeframe relevant to the assessment to get the greatest understanding of the spatial and temporal aspects of the hazard. When calculating hazard intensity, vulnerability and risk to people from ballistics on volcanoes, the kinetic energy of the impact ejecta should be included in the hazard footprint in addition to ballistic energy and impact angle. Improving current understanding of how ballistics are distributed in space and time, and how hazard intensity varies over the ballistic hazard footprint will vastly improve our ability to assess ballistic hazard and risk.