Seismic monitoring of co-eruptive volcanic processes during the 2021 Fagradalsfjall Eruption, Iceland, using two small-aperture arrays

The 2021 Fagradalsfjall eruption in the Reykjanes peninsula, Iceland, was marked by episodes with varying volcanic activity. Our study focuses on the period from eruption start on the 19th March 2021 until the 2nd May 2021. This phase was marked by relatively continuous lava flows and non-periodic lava fountaining observed at up to 12 different vents, increasing in intensity throughout the observation period. Seismic tremor emanating from co-eruptive processes like for example lava fountaining, collapse of crater walls and magma and lava migration is non-impulsive, often with emergent onsets and no defined phase arrivals. Thus it is difficult to locate the tremor sources with traditional network based methods. We show that using small aperture arrays it is possible to locate and monitor several tremor sources that were active simultaneously, providing good spatial resolution on the details of the eruptive fissure. We investigate how array processing of 3-component data can assist with the determination of different seismic wave types and lead to a better understanding of the underlying volcanic processes. We find that seismic arrays are well suited to monitor the location, type and strength of volcanic processes that are active simultaneously. This can have important implications for volcanic hazard monitoring, especially when visual monitoring with webcams is difficult for example due to remoteness or poor visibility.