International Workshop on the mechanism of Phreatic Eruption

A search for precursors to phreatic eruptions
Arthur Jolly1, Ivan Lokmer2, Bruce Christenson1, Corentin Caudron3, Iseul Park1,4, Lauriane
Chardot5, Alec Yates6, Ben Kennedy4, Martha Savage6
1 GNS Science, Avalon, New Zealand, 2 University College, Dublin, Ireland, 3 Universite Grenoble Alpes, France, 4University of Canterbury, Christchurch, New Zealand, 5 Nanyang National University, Singapore, 6 Victoria
University-Wellington, New Zealand; Email: a.jolly(at)gns.cri.nz

New Zealand has hosted several volcanic eruptions over the past decade which have provided
an opportunity to examine the range of possible precursors to those events. A growing body ofretrospective research is now available for the 2006-2007 Ruapehu eruptions, the 2012 Tongariro
eruptions and the 2012-2016 White Island eruption sequence. This contribution will review themulti-parameter observations from seismic, deformation, gas geochemistry, and otherobservation parameters, to assess potentially useful contributions to volcano monitoring.
Precursors to the 2006-2007 Ruapehu eruptions included retrospective observation of anomalous
b-values, shear wave splitting delay times and the direction of anisotropy beginning about 3 years
before the September 2007 eruption (Keats et al., 2001). Just prior to the eruption, high frequency
and very-long-period (VLP) seismicity were observed (Jolly et al, 2010). Several minutes of highfrequency seismicity also preceded a separate eruption at Te Maari, located ~20 km north ofRuapehu, in August 2012 (Jolly et al., 2014) and were preceded by ~3 weeks of earthquake
swarms (Hurst, et al., 2014) and anomalous magmatic degassing (Christenson et al., 2013).
At White Island, 6 phreatic or phreato-magmatic eruptions were detected over the 2012-2016
period. This unrest was marked by higher-level tremor documented in the real-time seismic
amplitude measure (RSAM) which has proved valuable for hazards assessments of volcanic
unrest. For example, a retrospective tremor analysis showed that 4 of the 5 White Island eruptions
were related to the increased RSAM as part of a rigorous failure forecast (Chardot, 2015). In a
separate analysis, the ratios of the low and high frequency tremor and the relative seismic velocity
were shown to relate to eruption activity, possibly due to pressurization within the volcanohydrothermal system at Tongariro/Ruapehu (Caudron, et al., 2019) and White Island.
Numerous volcano-tectonic (VT), long-period (LP) and VLP earthquakes are recognized at White
Island. While the local VT seismicity is not a recognized precursor, the LP activity does appear tobe modulated by surface activity, including lake level changes and eruptions. VLP earthquakes
are particularly useful because they are repetitious, are locatable, and related directly with 2 ofthe 6 eruptions. White Island eruptions are also linked to anomalies in the seismic ambient noise
but the interpretation requires careful assessment, due to its modulation by external factors
including tectonic stresses, seasonal climate and the effects of volcanic tremor (Yates et al., 2019).

International Workshop on the mechanism of Phreatic Eruption

Together, these approaches offer great promise towards improved short to medium term hazards
assessments at New Zealand’s most active volcanoes.

Main References
Caudron, C.; Girona, T.; Taisne, B.; Suparjan; Gunawan, H.; Kasbani, K.

2019 Change in seismic

attenuation as a long-term precursor of gas-driven eruptions. Geology ; 47 (7): 632–636. doi:
https://doi.org/10.1130/G46107.1

Chardot, L.; Jolly, A.D.; Kennedy, B.; Fournier, N.; Sherburn, S. 2015 Using volcanic tremor for eruption
forecasting at White Island volcano (Whakaari), New Zealand. Journal of Volcanology and Geothermal
Research, 302: 11-23; doi: 10.1016/j.jvolgeores.2015.06.001

Christenson, B.; Britten, K.; Mazot, A.; and Fitzgerald, J. 2013 The 2012 eruption of Te Maari:Pre- andpost-eruption gas signatures from the magmatic-hydrothermal system, in Jolly, A.D. (ed) Te Maari day : aworkshop to discuss scientific advances from the 2012 Te Maari eruptions: GNS Science miscellaneous
series 66 28 p.

Hurst, A.W.; Jolly, A.D.; Sherburn, S. 2014 Precursory characteristics of the seismicity before the 6 August2012 eruption of Tongariro volcano, North Island, New Zealand. Journal of Volcanology and Geothermal
Research, 286: 294-302; doi: 10.1016/j.jvolgeores.2014.03.004.

Jolly, A.D.; Sherburn, S.; Jousset, P.; Kilgour, G.N. 2010 Eruption source processes derived from seismic
and acoustic observations of the 25 September 2007 Ruapehu eruption, North Island, New Zealand. Journalof Volcanology and Geothermal Research, 191(1/2): 33-45; doi: 10.1016/j.jvolgeores.2010.01.009

Jolly, A.D.; Jousset, P.; Lyons, J.J.; Carniel, R.; Fournier, N.; Fry, B.; Miller, C.A. 2014 Seismo-acoustic
evidence for an avalanche driven phreatic eruption through a beheaded hydrothermal system: an example
from the 2012 Tongariro eruption. Journal of Volcanology and Geothermal Research, 286: 331-347; doi:
10.1016/j.jvolgeores.2014.04.007

Keats BS; Johnson JH; Savage MK The Erua earthquake cluster and seismic anisotropy in the Ruapehu
region, New Zealand, Geophys. Res. Lett., Vol. 38, L16315, 6 pp., doi:10.1029/2011GL049014, 2011

Yates, A.S.; Savage, M.K.; Jolly, A.D.; Caudron, C.; Hamling, I.J.

2019. Volcanic, coseismic, and seasonalchanges detected at White Island (Whakaari) Volcano, New Zealand, using seismic ambient noise.
Geophysical Research Letters, 46(1): 99-108; doi: 10.1029/2018GL080580