The Nyiragongo and Nyamulagira volcanoes are two neighboring volcanoes located in the western part of the Virunga volcanic chain and have remained the only active volcanoes of this chain for several decades. The seismic activity at these two volcanoes was

The Nyiragongo and Nyamulagira volcanoes are two neighboring volcanoes located in the western part of the Virunga volcanic chain and have remained the only active volcanoes of this chain for several decades. The seismic activity at these two volcanoes was evaluated for the period from 2011 to 2016 in order to observe the chronology of seismic events related to volcanic activity that occurs when these volcanoes present a particular specific states. Using softwares of seismic signal processing such as SWAM and SEISAN, and field observations, we found five events during this time period: (1) the eruption of the Nyamulagira volcano in November 2011, (2) the magmatic intrusion at Nyamulagira in April 2014, (3) the appearance of a lava lake at Nyamulagira in June 2014, (4) the appearance of a small eruptive vent in the Nyiragongo crater in February 2016 and (5) the magmatic intrusion at Nyiragongo in November 2016. From this observed chronology, it has been revealed that the seismic activity during the lava lake period is relatively low compared to that observed in the period without lava lake

Classification of Volcanic Earthquakes Observed at Nyamulagira and Nyiragongo Volcanoes Reference on Frequency Analyses and its Implication in Volcano Monitoring

Identification of the types of seismic signals is the basic of the seismo-volcano analysis, because it allows to have information on the magmatic activity prevailing in the crater. Modern analysis for this work identifies the frequency observed in the evaluation of the signals that can lead to a proper classification following groups arranged in specific spectral bands. This frequency analysis was applied to seismic data that preceded the eruption of Nyamulagira Volcano of 6 November 2011 and the persisted lava lake within Nyiragongo Volcano. The data analyzed covered a period of two years, from January 2010 until November 2011. The work consisted to analyze the frequency content of the signals and to deduce the type of the signals reference to the general classification made by Lahr et al. [1], Miller et al. [2] and White et al. [3]. The work resulted on a classification ranging signals into 5 groups including: (1) Earthquakes whose frequency band row from 10 Hz and over, which were identified as volcano-tectonic earthquakes or A-Types, (2) Earthquakes whose frequency band  row between 3 and 10 Hz identified as Hybrid earthquakes or B-Type earthquakes, (3) Earthquakes whose frequency band row between 1 and 3 Hz identified as Long Periods earthquakes or C-type earthquakes, (4) ) Earthquakes whose frequency band row between 0.25 and 1Hz identified as Very Long Period, VLP earthquakes and (5) Continuous signals marking the same values as the Long Period earthquakes identified as volcanic tremors.It was eventually found that swarms of Long period and hybrid earthquakes are those related to magmatic intrusions, and thus can lead to volcanic eruptions. At other hand, there are only VLP and A-type earthquakes which are observed in the period of persisted Lava Lake. This classification can be used in the monitoring of the volcanoes.

Holocene phreatomagmatic eruptions alongside the densely populated northern shoreline of Lake Kivu, East African Rift: timing and hazard implications

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 ~2,500 and ~150 cal. yr BP, and suggest centennial- to millenial-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

Seismic activity related to the June 2014 New lava Lake apparition at Nyamulagira volcano in the Western Branch of the East African Rift

peer reviewedA lava lake activity is observed at Nyamulagira volcano during this last decade. The training process of this lava lake began in 2012 by the release of gas fumes that was been continuously observed in the crater of the volcano. However no change in seismic activity was observed compared to the usual activity of the volcano until April 2014. On 22 June 2014, an activity of glow was observed by the Goma Volcano Observatory and the inhabitants of the city of Goma. On July 3rd 2014, the United States's organization NASA (National Aeronautics and Space Administration) noted this situation by its satellite detection and published on his Web site the apparition of a new lava lake in the crater of Nyamulagira. Nyamulagira volcano (in its known history) logged again a lave lake from 1921 to 1938. Here are analyzed the seismic activity which preceded this new event at mount Nyamulagira. It was found that this event was been preceded by a significant swarm activity of Long Period earthquakes in April 2014, six hybrid earthquakes and volcanic tremors in June 2014. The April 2014 swarm of low frequency earthquakes lasted about four days and was been located in the NorthEastern part of the Nyamulagira crater (at the place where appeared the new lava lake) and was been interpreted as expressed by the precursor movements of the opening of the crater. The six hybrid earthquakes were been interpreted also as the events that led to a falling movement of the land masses and the opening of the crater. After the visibility of the lava lake in June 2014, the activity of LP events reduced, no swarm and hybrid events were been recorded from this period, but the number of Volcano-Tectonic events remained constant