La Soufrière volcano, St Vincent, eruption 2020–21: assessing unrest and eruptive states from limited volcano-seismic data

Abstract Following a month of unrest in June–July 2019 and 3 months of effusive activity from late December 2020 to March 2021, La Soufrière volcano, St Vincent, transitioned to a 2-week period of explosions on 9 April 2021. During initial unrest, there was one working seismograph station on the volcano, providing the only information for tracking fluctuations in seismicity. Subsequently, full capability for locating volcanic earthquakes was achieved on 27 January 2021. Because events prior to this did not have reliable locations, unconventional data tracking approaches were adopted for assessing unrest evolution. Holistic inferences, suggested by combining these novel analyses with decades of knowledge of Lesser Antilles volcanoes, provided punctual evidential support for assessing the possibility that unrest could culminate in explosive activity. However, this case history is not offered as a paradigm for minimalist seismic monitoring at active volcanoes, notwithstanding that information gains were possible. This said, evaluations of seismic moment release, using event duration magnitudes as proxies for moment magnitudes, allowed the magma volume intruded or activated in the volcano to be estimated: 68 ± 14 × 10 6 m 3 dense rock equivalent (DRE) magma; this is a value remarkably similar to a geological estimate of total erupted volume (71 ± 14 × 10 6 m 3 DRE), with matching uncertainties.