Analysis of the Seismicity Recorded before the May 22, 2021 Eruption of Nyiragongo Volcano, Democratic Republic of the Congo

We present an overview of the different seismic precursors of the Nyiragongo volcano eruption on May 22, 2021, as well as a statistical analysis of the seismic swarms recorded during the pre-eruptive period. The analysis of the seismic activity of Nyiragongo volcano during the pre-eruptive period shows that there was a particular seismicity composed of hybrid and/or volcano-tectonic type events that characterized the long-period seismic swarm recorded about a month before the May 2021 eruption. In February 2016, similar seismic activity was observed at Nyiragongo volcano, which led to the opening of a new vent within Nyiragongo's main crater on February 29, 2016, and which remained active until the May 2021 eruption. During the period from January 2016 to May 2021, the lava lake remained very active with significant fluctuation as evidenced by the Real Seismic Amplitude measurement at Rusayo and Kibati stations. The increase and stabilization of the lava lake to a higher level resulted in an increase and accumulation of stress on the flanks of the volcano since the lava lake had already reached a higher critical level. Whether in January 2016, November 2016 or April 2021, we show that this particular type of so-called hybrid seismic events recorded during a swarm at Nyiragongo volcano could lead to an eruption either inside the main crater (February 2016) or on the flanks in May 2021.