International Workshop on the mechanism of Phreatic Eruption

From monitoring to forecasting– Toward statistical evaluation of non-magmatic unrest –Takeshi Hashimoto1
1 Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University
E-mail: hasimoto(at)sci.hokudai.ac.jp

Phreatic eruptions are often referred to be exceptionally difficult to forecast by traditional
geophysical techniques (de Moore et al., 2016). Although non-magmatic unrest phenomena
can be hazardous in themselves, they are "too often overshadowed by eruption forecasting"(Rouwet et al., 2014). In many cases, volcanoes hosting a shallow hydrothermal system havebackground activities such as steaming ground, micro-seismicity, sporadic grounddeformation, even during inter-eruptive periods, sometimes exhibiting states of non-eruptive
and non-magmatic unrest. It is readily envisaged that such non-magmatic unrest eventsreflect a certain change in the shallow hydrothermal system. However, it is currently difficult
to link them to the assessment of eruption imminence, or to the prediction of eruption intensitywith a physical basis. On the other hand, given that an impermeable seal layer in a shallowhydrothermal system, or conduit obstruction due to deposition of alteration minerals can beimportant factors in the processes leading to a phreatic/hydrothermal eruption (e.g.,Christenson et al., 2010; Tanaka et al., 2017, 2018; Stix and de Moor, 2018), monitoring suchprocesses is a clue to evaluating the build-up phase of the eruption.
Here, some sort of statistical evaluation that is based on the monitoring records for the unrest
events from multiple volcanoes of a similar type may be practically useful, instead ofdeterministic forecasting that generally requires the detailed information on the source
parameter and the physical properties of the surrounding structure for a specific volcanos
environment. Even if we do not have perfect knowledges on the detailed source geometry orlocation, ground deformation and the magnetic field changes, for example, can be indirect
proxies of such processes, since they are considered to reflect pressure and/or temperaturechanges within a volcano. Hashimoto et al. (2019) provided one such example, where theycollected information on reported non-magmatic unrest events mainly from Japanese
volcanoes that have been monitored for extended time periods. They compiled the source
depth, intensity, and the rate, based on the single magnetic dipole model or the Mogi model.
Clear positive correlations were found between the source depth and its intensity and rate
as a linear trend on the log-log scatter plots. Upward deviation from the linear trend on theInternational Workshop on the mechanism of Phreatic Eruption

scatter plot was found to be a potentially useful criterion, whereas the source depth by itself
had no definitive relationship with the subsequent occurrence of eruptions. I consider thatsuch an event collection can be the background data to predetermine the criteria of theVolcano Unrest Index (Potter et al., 2015), especially for "wet" volcanoes, as well as it canbe optionally used to augment the VUIs functio n with a probabilistic evaluation.

Fig. 1 Conceptual scatter plot demonstrating the statistical evaluation of non-magmaticunrest events (Hashimoto et al., 2019). The symbols × and ○ represent “erupted” and“unerupted” unrests, respectively. Both kinds occur in the intermediate zone between thetwo dashed lines. Very small events at great depths, which belong in the shaded
triangular zone on the lower-right, are not observable (undetected unrests). Very strong
events close to the ground surface, which belong in the shaded triangle on the upper left,
are physically impossible.
Main References
Hashimoto T, Utsugi M, Ohkura T, et al (2019) On the source characteristics ofdemagnetization and ground deformation associated with non-magmatic activity. Bull.
Volcanol. Soc. J., 64: doi: 10.18940/kazan.64.2_103
Potter S, Scott BJ, Jolly GE et al (2015) Introducing the volcanic unrest index (VUI): a tool toquantify and communicate the intensity of volcanic unrest. Bull. Volcanol., 77: doi:
10.1007/s00445-015-0957-4