Assessing Eruption Forecasts Based on Volcanic Ground Deformation Events Caused by Deeper Pressure Sources

Abstract

Ground deformation is often observed to precede eruptions. For assessments of volcanic activity and hazards, it is therefore important to clarify the relationship between the occurrence of ground deformation and any forthcoming eruptions. Here, we report volcanic ground deformation events that were detected at 9 volcanoes in Japan by applying the stacking method to strain measurements along baselines of GEONET, the GNSS network managed by the Geospatial Information Authority of Japan. We compiled the durations and amounts of strain change during 38 ground deformation events caused by deep pressure sources beneath the volcanoes. Sixteen of these 38 events were accompanied or followed by eruptions, whereas no deformation event was detected before or during seven eruption events. We compared the durations of ground deformation events to the elapse times between the beginning of deformation and eruption. In all ground deformation events preceding eruptions, the first eruption occurred during the ground deformation event. Eruptive activity ceased during the ground deformation event in 10 of the 16 eruptions preceded by deformation, and within the three months following the ground deformation event in 14 of the 16 events. The durations of ground deformation events not accompanied by eruptions (“failed eruptions”) were shorter than those of events accompanied by eruptions (“succeeded eruptions”). “Missed eruptions” were those not accompanied by deformation events. Succeeded eruptions accounted for 8% of the deformation events shorter than 180 days, and 60% of those longer than 180 days. The threat score is an evaluation of forecast accuracy on a scale from 0 to 1, with higher values indicating more accurate forecasts. Considering deformation events as eruption forecasts, we obtained a threat score of 0.36 for 44 events (including succeeded, failed, and missed eruptions, but excluding one difficult-to-classify event at Miyakejima), and a miss rate of 0.30. Importantly, the missed eruptions included two fatal eruptions. It is therefore difficult to use only this method to assess forthcoming eruptions. To improve the accuracy of eruption forecasts, it might be more efficient to evaluate ground deformation caused by shallow pressure sources.