Imaging the Shallow Structure Beneath Askja Volcano, Iceland, With Ambient Noise Tomography

Abstract

Abstract Askja is a large central volcano in the Northern Volcanic Zone (NVZ) of Iceland which last erupted effusively in 1961 and subsequently experienced steady deflation from the 1970s until August 2021, when GPS and InSAR measurements confirmed that it had begun re‐inflating. A dense (1–2 km station spacing) network of broadband seismometers and nodes were deployed in the Askja caldera in response to this re‐inflation. Ambient noise tomography results reveal a shallow (2 km beneath the surface) low velocity zone underneath the center of uplift that we interpret to be a shallow magma storage region. This low velocity zone is also coincident with a region of positive (+3%) radial anisotropy, which contrasts with a radial anisotropy of −6% in the top 3–5 km of the crust elsewhere in the NVZ, likely due to pervasive vertically aligned fractures in the near surface caused by ongoing rifting. We interpret this significant change to be caused by the magma storage region being made up of horizontal sills. Effective media modeling was used to estimate a bulk sill aspect ratio of 0.2 and a melt fraction of 5%. This shallow melt zone is interpreted to be part of a two‐stage magma storage system with a deeper zone of melt (7 km beneath the surface) feeding the shallower one. This has significant implications for hazard because the shallow storage region can act as a source of either evolved or primitive melt, allowing for larger volume or potentially explosive eruptions. Plain Language Summary Askja volcano is in the Northern Volcanic Zone of Iceland, a section of the mid‐Atlantic ridge—where tectonic plates diverge. It is an active volcano that last erupted in 1961, and has recently been showing signs of unrest in the form of surface uplift that started in August 2021. This study uses data recorded by seismometers in order to image the top 10 km of the crust beneath Askja and determine how magma is stored and transported in volcanic plumbing systems. We exploit ambient seismic noise data, which comes from continuous low‐amplitude ground motion primarily generated by ocean waves and is well suited to imaging the shallow crust. From the new imaging results, we infer the presence of a shallow magma storage region about 2 km beneath the caldera floor that we hypothesize is made up of sills, which are horizontal elongate pockets of melt. Based on the eruption history of Askja, it appears that these sills have a variety of compositions, which can influence the nature and timing of eruptive activity. As such, our results can contribute to the understanding of volcanic hazard associated with Askja. Key Points New high resolution and radial anisotropy models of Askja's shallow plumbing system are obtained from ambient noise tomography A shallow melt storage region centered 2 km below the current center of uplift is inferred from low and positive radial anisotropy Effective media modeling results suggest that the melt region is composed of horizontal sills with a best fitting aspect ratio of 0.2