Application of the Nearest Neighbor Method to the Analysis of Volcanic Swarms Using Data From Iceland's Bárðarbunga and Fagradalsfjall Volcanoes

The paper is devoted to the analysis of grouping of volcanic seismicity events, especially in volcanic swarms. Volcanic swarms observed during the eruptions of the Bárðarbunga (2014) and Fagradalsfjall (2021) volcanoes in Iceland were analyzed. In the paper, an attempt was made to apply the nearest neighbor method the stated goal. The method allows identifying groups with different scales of generalized distances. For example, it typically reveals two groups of events in tectonic seismicity and is widely used to identify aftershocks. As a result of the work, significant differences were observed in the shape of the distributions of generalized distances to the nearest neighbor for volcanic seismicity compared to tectonic seismicity. Namely, two types of unimodal distributions were found, one of them is observed mainly before the eruption, and the other during the eruption. The first type is probably caused by the merging of two close distribution modes and reflects the internal heterogeneity of seismicity during such periods. However, the unimodality of distributions makes it difficult to identify events in terms of related (clustered) or independent (background). Based on the results obtained, it can be assumed that before the eruption, the proportion of background seismicity fluctuates around 70%, and during the eruption from 90 to 100%. This may indicate different sources of seismicity at one or another stage of the eruption.