The September 27, 2014 phreatic eruption of Mount Ontake near Nagano, Japan, left 57 climbers dead, and another 6 missing and presumed dead. The conventional wisdom (as still reported on Wikipedia) is that phreatic eruptions have no discernable precursors—no warning signs—and so experts cannot give warnings and advisories.
Now, however, in an article recently published in Earth, Planets and Space, Aitaro Kato, Toshiko Terakawa, Yoshiko Yamanaka, Yuta Maeda, Shinichiro Horikawa, Kenjiro Matsuhiro, and Takashi Okuda, from Nagoya University and the University of Tokyo, have back-reviewed the seisemic data from the year leading up to the eruption—and especially the two weeks immediately prior—and have discovered a pattern of increased activity. More than that, they have hypothesized how the pre-eruption mechanism-of-action would lead to the activity that they discovered, and they suggest studying additional observational techniques to help better predict future phreatic eruptions.
But, first—what is a phreatic eruption? As distinct from the more stereotypical magmatic eruption (which is what it sounds like), phreatic eruptions center on superheated steam rather than magma. Sometimes also called “steam cannon” eruptions, these events throw superheated steam along with ash and rock into the air. They can also lead to pyroclastic mud flows, as well. The 1980 eruption of Mount Saint Helens in Washington State might be the best known example of a phreatic eruption.
In their article, “Preparatory and precursory processes leading up to the 2014 phreatic eruption of Mount Ontake, Japan,” Kato et al., built a catalogue of seismic activity beneath the volcano going back a year before the eruption. They then used those data to construct a filter for searching the larger set of seismic data to look for patterns, which they found. From those patterns—specifically increased activity along a trench running from the north-north-west to the south-south-east—they were able to build a model of how this eruption evolved.
They hypothesize that over time, water began to infiltrate small cracks in the base of the volcano; and that the magma chamber would heat this water, building pressure, which then weakened the surrounding rock, leading eventually to the full eruption. They conclude the article with a prescription:
To our knowledge, the present study was the first to describe the rapid propagation of a vertical conduit during the accelerating stage prior to a phreatic eruption. It is important to capture short-term precursors as early as possible and to raise the alarm for climbers to evacuate from potential eruption sites. Further understanding of the excitation mechanism of phreatic eruptions will require continuous monitoring of seismicity and crustal movement in the vicinity of the summit because the geophysical signals related to the pre-eruption propagation of a vertical crack are too localized to be observed in the far field.
Read the entire article here.