Seismological observations of the 2011 Nabro, Eritrea eruption: implications for eruptive deep closure and volcano-tectonic interactions

Understanding the eruptive processes of an active volcano is integral to eruption prediction and hazard mitigation, and co-eruptive earthquakes are potentially a vital indicator. However, co-eruptive seismicity is hard to detect and interpret given the elevated level of tremors, scarcity of seismic stations, and large explosive events. We used a single station at Nabro Volcano, Eritrea, starting on 23 June, 11 days after the onset, which provided a suitable dataset to study co-eruptive seismicity and volcano-tectonic interactions. We detected and classified the events according to their waveform features: short-duration mid-crustal events, long-duration events, and events clustered at approximately an S-P traveltime difference consistent with the vent location, subdivided into the events with large P/S ratios and the ordinary ones. Overall, the daily Nabro seismicity rate has a strong relationship to the daily SO ₂ emission rate. We observed seismic patterns linked to the vent openness and eruptive styles. Seismically recorded events at the vent decrease when the strength of the ash eruption increases, which suggests that stronger ash eruption unclogs the vent, reduces stress accumulation in the plumbing system, and discourages earthquake nucleation. We also discovered a strong linkage between the SO ₂ eruption and a seismically inferred deep magma reservoir. Activation of deep-seated earthquakes accompanying a decline in SO ₂ flux is observed in two transitional ash eruptions. In addition, we observe an abrupt increase in microseismicity near Lake Afrera following the cessation of the eruption, which suggests the interactions between the volcano and the magmatic-enriched rift zone. The observation, in summary, reveals the closure of an open system, which is closely tracked by collapse-related mid-crustal seismicity.