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During eruptions, volcanoes produce air-pressure waves inaudible for the human ear called infrasound, which are very helpful for detecting early signs of magma at the surface. Compared to violent ash-rich explosions, recording more discreet atmospheric disturbances from effusive eruptions remains a practical challenge. Wind and human activity are other powerful sources of unwanted infrasound noise masking volcanic ones. At Nyiragongo volcano (D.R. Congo), close to a one-million urban area, the drainage of the world’s largest lava lake concomitant with short-duration lava flows on its flank and the renewal of an effusive eruption within its crater a few months later were a series of major volcanic events in 2021, all monitored with infrasound sensors. First, we explore these records for characterizing the temporal and spatial evolution of the flank eruption on May 22, 2021. In a second step, we show evidence of the infrasonic rumbling of a nascent lava lake starting a few months later detected up to the volcano observatory facilities in Goma city center about 17 km from Nyiragongo’s crater. These results show remarkable local recordings of eruptive infrasound from rural and city-based stations and have significant implication for optimizing future monitoring efforts in a harsh field environment., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Reliable estimation of earthquake source parameters is fundamental to improve our understanding of earthquake source physics and for ground-motion modeling in seismic hazard assessment. Nowadays, methods traditionally used for investigating the source parameters of earthquakes with Mw≥3, such as spectral fitting or spectral ratio approaches, are also extensively applied to smaller magnitude events because of the increase in the number of stations and the more common borehole installations. However, when working with recordings of such minor and microearthquakes, significant limitations of the usable frequency range spanned by the spectra arise. At the lower end, signal-to-noise ratio constraints limit the usage of low frequencies, whereas at the upper end, the sampling rates of typical seismological networks as well as high-frequency attenuation can be limiting factors. In addition, earthquake source parameters determined from ground-motion spectra are known to exhibit potentially serious trade-offs, in particular the corner frequency and high-frequency attenuation. In this study, we go beyond the typical discussion of these trade-offs using simplistic spectral models by investigating the impact of the background wave propagation model on the source parameter trade-offs as well as its effect on the feasibility of obtaining useful source parameters by means of spectral fitting for minor and microevents. The analysis takes advantage of ad hoc simulated synthetic seismograms with well-defined underlying background propagation models and considers increasing complications in these models (intrinsic and scattering attenuation). The results show that with given realistic background models and usable frequency bands, the source parameter estimation for minor and microevents can be significantly biased, and not surprisingly, this bias is mainly affecting the estimation of the corner frequency. We highlight the inherent limitations of joint spectral fitting approaches for the determination of source parameters from minor and microearthquakes, which should always be viewed with great caution when physically interpreted.