International Workshop on the mechanism of Phreatic Eruption

Managing the risk and perception of rapid onset phreatic eruptions: The
challenge of public preparedness through hazard maps, warnings,
signage and public education
Graham Leonard1, David Johnston2, Sally Potter1, Danielle Charlton3
1 GNS Science, 1 Fairway Drive, Lower Hutt 5010, New Zealand
2 Joint Centre for Disaster Research – Massey University – GNS Science, Wellington, New Zealand
3 The University of Auckland, New ZealandE-mail: g.leonard(at)gns.cri.nz

Rapid onset, often phreatic eruptions, are by their nature difficult to forecast. The eruption
mechanism usually relates to destabilization of a hydrothermal system which can occur withno or very little warning, especially when the system is long-established as in calderas or atthe vents of frequently active cones.
This creates a substantial challenge for public preparedness. In New Zealand this has led toincreased work on hazard models, especially for ballistics and pyroclastic density currents.
Social science has included 15 years of survey and observation data on warnings tocharacterize and quantify human response at Ruapehu and Tongariro volcanoes.
Geophysical monitoring through the GeoNet project has become iteratively more intensive
and diverse, covering 12 active and potentially active volcanic areas. Monitoring and eruption
forecasts through Volcanic Alert Bulletins are provided by the volcano monitoring group at
GNS Science. Volcanic Alert Levels are also used and they have been updated in the last 5years based on social science evidence, to include two unrest levels and hazard descriptors(Potter et al., 2014).
Risk awareness and self-protection advice is provided through signage, public education,and hazard maps. Automated warnings are used on Ruapehu where detection of an eruption
leads to lahar warnings for the Whakapapa ski area, which may be affected within minutes.
Evacuation drills are conducted annually, and a persistent minority of people do not move tosafety, or move quickly enough, from year to year. While the majority of risk in this case is
mitigated through warning, a residual risk is hard to eliminate.
Expectation management is, therefore, critical amongst scientists, decision makers,
emergency managers, national park officials, tourism providers and communities. It is
important to understand the level of overall risk, and how much reduction can be expectedfrom forecasts (and how much cannot be expected). The role of education and self-protective
actions must be widely understood. This includes finding natural shelter, shelter in buildings,

International Workshop on the mechanism of Phreatic Eruption

considering shelter structures, and using carried protection such as backpacks. There is also
a new national interagency volcanic hazard mapping framework in development for New
Zealand – with a link to safety maps for rapid onset eruptions (Figure 1).
The relationship with New Zealand’s economically-important tourism industry is critical andit is impossible to remove all risk while maintaining access. Therefore, substantial effort isbeing put in to calculate that risk and discuss acceptable or tolerable risk. Often the people
exposed to most risk are tourism operators themselves because of the large number of days
per year they are working or living in at-risk places. Surveys and behavioural science focusedon tourists and industry personnel have shown that the training and leadership of staff iscritical to optimal safety of both the staff and tourists.

Fig. 1 2019 Revised hazard map with key communication messages, partners and usercentred design. Text highlights that eruptions may occur with little or no warning.
Main References
Leonard, G.S., et al, 2014. Integrating multidisciplinary science, modelling and impact data
into evolving, syn-event volcanic hazard mapping and communication: A case study
from the 2012 Tongariro eruption crisis, New Zealand. JVGR, 286, pp.208-232.
Potter SH, Jolly GE, Neall VE, Johnston DM, Scott BJ. 2014. Communicating the status of
volcanic activity: Revising New Zealands volcanic alert level system. JApVolc. 3(13).