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Volcano monitoring requires simple techniques to rapidly identify the cause of volcanic unrest. The so-called RSAM (real-time seismic amplitude measurements) technique, used in many observatories, is a good example of extracting information from seismograms with minimal processing. Built on a similar principle, the more recent seismic amplitude ratio analysis (SARA) technique allows locating migrating seismicity at high frequency (> 2 Hz, e.g., due to dike intrusions) under certain assumptions. However, such analysis generally requires a dense distribution of stations close to the seismic sources (depending on the magnitude) and/or station sites undisturbed by human activity. In a more straightforward and qualitative approach, computing amplitude ratios between station pairs can also allow for the detection of temporal and (2D) spatial changes of volcanic activity. In this work, we adopt such a simplified approach of SARA in order to characterize seismic tremors originating from two open-vent neighboring volcanoes, Nyiragongo and Nyamulagira, in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo (DRC). In contrast with previous studies, we focus here on the low-frequency band (0.3–1 Hz), free from anthropogenic noise and sensitive to shallow volcanic tremors linked to intermittent or permanent intra-crater eruptive activity recorded through the large-aperture local network. We apply for the first time the SARA methodology for volcanic sources predominantly generating surface waves and propagating over long distances. The analysis is performed on more than two years of continuous seismic data. Seismic amplitude analysis in this frequency band is strongly influenced by the short-period microseisms originating from nearby Lake Kivu. Despite this diurnal to seasonal amplitude variability, SARA successfully detects continuous volcanic tremor activity and its arrest at both volcanoes. In light of these findings, we discuss the applicability of the method to the continuous, real-time detection, and characterization of long-period shallow volcanic tremor sources in this region.

The Virunga Volcanic Province (VVP) represents the most active zone of volcanism in the western branch of the East African Rift System. While the VVP’s two historically active volcanoes, Nyamulagira and Nyiragongo, have built scoria cones and lava flows in the adjacent lava fields, several small phreatomagmatic eruptive centers lie along Lake Kivu’s northern shoreline, highlighting the potential for explosive magma-water interaction. Their presence in the densely urbanized Sake-Goma-Gisenyi area necessitates an assessment of their eruptive mechanisms and chronology. Some of these eruptive centers possess multiple vents, and depositional contacts suggest distinct eruptive phases within a single structure. Depositional facies range from polymict tuff breccia to tuff and loose lapilli, often impacted by blocks and volcanic bombs. Along with the presence of dilute pyroclastic density current (PDC) deposits, indicators of magma-water interaction include the presence of fine palagonitized ash, ash aggregates, cross-bedding, and ballistic impact sags. We estimate that at least 15 phreatomagmatic eruptions occurred in the Holocene, during which Lake Kivu rose to its current water level. Radiocarbon dates of five paleosols in the top of volcanic tuff deposits range between ∼2500 and ∼150 cal. year BP and suggest centennial- to millennial-scale recurrence of phreatomagmatic activity. A vast part of the currently urbanized zone on the northern shoreline of Lake Kivu was most likely impacted by products from phreatomagmatic activity, including PDC events, during the Late Holocene, highlighting the need to consider explosive magma-water interaction as a potential scenario in future risk assessments.