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.