How do scientists forecast eruptions?
Scientists use a wide variety of techniques to monitor volcanoes, including seismographic detection of the earthquakes and tremor that almost always precede eruptions, precise measurements of ground deformation that often accompanies the rise of magma, changes in volcanic gas emissions, and changes in gravity and magnetic fields. Although not diagnostic individually, these techniques, when used in combination at well-monitored volcanoes, have resulted in successful predictions. At Pinatubo volcano (Philippines) in 1991, a successful forecast saved thousands of lives. The USGS Volcano Hazards Prgram notes that the key to an accurate short-term eruption forecast is being able to recognize when such monitored data show consistent changes from normal background levels of activity.
Monitoring-based forecasts are becoming much more reliable, but they remain imperfect. If scientists are fortunate, precursors to an eruption follow the same course as they followed before previous eruptions. Patterns often change, though, and wholly new behavior is observed. The best forecasts will be based on an integration of geologic history, realtime monitoring, and a deep understanding of the internal plumbing processes of the specific volcano. Even with the best of monitoring and interpretations, reliable forecasts are rarely possible more than a few days in advance of an eruption.
Some forecasts of volcanic eruptions are based on eruption recurrence intervals, but these are notoriously unreliable for two reasons: 1) few volcanoes are sufficiently well studied to provide an accurate eruptive history over the many hundreds, or tens of thousands, of years necessary to establish a reliable recurrence interval; and 2) few volcanoes maintain the same behavior for long (more often than not, as soon as a repetitive pattern becomes apparent, the volcano changes behavior).
Volcano observatories make forecasts with great caution as they can have huge impacts on the affected populations, in some cases forcing people to leave behind homes, farms, and livestock. Inaccurate forecasts can lead to unnecessary obligation of scarce resources and/or undermine residents' confidence in future forecasts.
Reliable forecasts, however, can be made by volcano observatory staff, who have the experience to interpret their monitoring that detects eruption precursors. Most nations with volcanoes have tasked an established observatory, run by the government or by a university, to provide eruption forecasts to the public. All of these observatories are members of the World Organization of Volcano Observatories (WOVO).
Global Volcanism Program, 2013. Volcanoes of the World, v. 4.7.6. Venzke, E (ed.). Smithsonian Institution. Downloaded 16 Feb 2019. https://doi.org/10.5479/si.GVP.VOTW4-2013