Seismicity and outgassing dynamics of Nyiragongo volcano

摘要

Active lava lakes at volcanoes can be regarded as open windows to their magmatic systems. The dynamics of such lakes may therefore provide decisive insights into deeper magmatic processes, potentially leading to fundamental theoretical implications and volcano monitoring improvements. Among the rare volcanoes on Earth hosting a persistent lava lake, Nyiragongo in D.R. Congo directly threatens a massive population of roughly 1 million inhabitants. Here we analyze close-range (i.e., summit) and distant (around 17 km) seismic measurements acquired at this African volcano between 2011 and 2018 in order to better understand the seismic signature of the lava lake activity and how it relates to the deeper volcanic processes. Both summit and distant seismic records contain a similar continuous tremor pattern attributable to the lava lake activity. Combining this information with time-lapse camera images and lava lake level measurements confirms the mechanism of gas pistoning at Nyiragongo, which is characterized by short-duration (a few minutes long) and meter-scale level variations during the period of observation. We also characterize the dominant periodicity of this shallow tremor signature of about a few tens of minutes. Because this marked periodic pattern varies during a significant one-month fluctuation of SO2 emissions (estimated from space), we suggest that this particular seismic periodicity corresponds to the convective lake movement driven by the persistent degassing typical of active open-vent volcanoes. Finally, new seismic evidence reveals the effect of deep magmatic intrusion and consequent major pressure changes in the plumbing system, resulting in sudden and large drops of the lava lake level associated with strong degassing. Such transient episodes have similar characteristics to total lava lake drainage associated with flank eruptions already observed at this volcano in 1977 and 2002, or at Kilauea volcano in 2018. (C) 2019 Elsevier B.V. All rights reser