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