Intra-Crater Eruption Dynamics at Nyiragongo (D.R. Congo), 2002–2021

Nyiragongo is one of the rare volcanoes on Earth hosting a lava lake. However, the understanding of its plumbing and lava lake systems remains limited, with, until recently, only sporadic or time-limited historical observations and measurements. Combining dense accurate lava lake and crater floor level measurements based on 1,703 satellite radar images and topographic reconstructions using photogrammetry, we obtain the first reliable picture and time evolution of intra-crater erupted lava volumes between the two last flank eruptions in January 2002 and May 2021. The filling of the crater by lava, initiated in 2002 and continued up to May 2021, is seen as an evidence of a long-term pressure build up of the magmatic system. This filling occurs through irregular pulsatory episodes of rising lava lake level, some of which overflow and solidify on the surrounding crater floor. Pauses of stable molten lava lake level and sudden numerous level drops also marked the summit's eruptive activity. The joint analysis with seismic records available since 2015 revealed that the largest lava lake drops are synchronous with seismic swarms associated with deep magma intrusions, generally preceded by an increase of extrusion rate within the crater. The appearance of a spatter cone in the summit crater in 2016, most likely superficially branched to the lava lake, was a clear marker of the change in eruption dynamics. This first long-term time series of Nyiragongo's crater topography between two hazardous flank eruptions might further help to better decipher Nyiragongo's past and future behavior using multi-parameter observations

Detecting sources of shallow tremor at neighboring volcanoes in the Virunga Volcanic Province using seismic amplitude ratio analysis (SARA)

Volcano monitoring requires easy to implement and interpret techniques allowing scientists to rapidly understand the cause of volcanic unrest. The so-called RSAM technique (Real-time Seismic Amplitude Measurements), 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 Ratios Analysis (SARA) technique allows locating migrating seismicity at high frequency (> 2 Hz, e.g., due to dike intrusions) under certain assumptions made regarding the seismic wave propagation. 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 ratio between station pairs can also allow detecting 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 Congo (DRC). We focus here in a low frequency band (0.3-1 Hz), free from anthropogenic noise and previously known to exhibit shallow volcanic tremors linked to intermittent or permanent intracrateral eruptive activity. The analysis is performed on more than two years of continuous seismic data. This allows to identify that seismic amplitude analysis in this frequency band is also strongly influenced by the lake microseisms originating from the nearby lake Kivu. Despite this diurnal to seasonal unwanted seismic noise, the seismic amplitude ratio procedure successfully detects shutdown and continuous volcanic tremor activity at both volcanoes. In light of these findings, we discuss the applicability of the SARA method applied to the continuous, real-time detection and characterization of long-period shallow volcanic tremor sources in this region

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