The seismic signature of rapid inflation at Askja volcano, Iceland

Askja is a large central volcano that sits within the Northern Volcanic Zone (NVZ) of Iceland, less than 50 km from the Vatnajökull ice cap. It has a long history of magmatic activity, which includes a large Plinian (explosive) eruption in 1875 that was accompanied by a massive caldera collapse. This excavated an 11 square km area some 250+ m deep that now hosts the crater lake Öskjuvatn, and deposited tephra as far away as Norway and Sweden. Its last eruption in 1961 involved three eruptive sequences that all initiated from lava fountains to the north of Öskjuvatn and produced lava flows up to 8 km in length. Subsequent levelling surveys have documented a lengthy period of surface subsidence that has continued, albeit at a slightly decreasing rate over time, until August 2021, when a switch to uplift was detected by both InSAR and GPS measurements.

The significance of the switch from subsidence to uplift was not immediately obvious; the peak uplift is located on the western edge of the lake, some 5+ km from the locus of the 1961 eruption where historic measurements of surface displacement have been made. It was therefore unclear whether it represented a fundamental change in the volcanic plumbing system that may lead to a significant eruption, or a localised intrusion that would not result in any further activity. However, in the >18 months since the switch to reinflation, the surface has uplifted by over 50 cm to date, which is more than the total subsidence experienced since the 1961 eruption. Furthermore, a backbone array of seismic stations operated by the Cambridge Volcano Seismology Group (CVSG), which has been in place for the last 15+ years in the NVZ, detected an ongoing increase in microseismicity associated with the surface uplift. In February and March 2023, the frozen surface of Öskjuvatn melted in its entirety, potentially indicating significant interaction between melt intrusion at shallow depths and the hydrothermal system.

There appears to be little doubt that a major subsurface magmatic event is currently underway beneath Askja, but exactly how it will evolve in future is unknown. It is therefore critical to significantly improve our observational capabilities in and around the Askja caldera. The current backbone array only has one seismic station that sits on the region of uplift identified by InSAR, and another four within the outer caldera edge. While this has allowed us to detect the increase in earthquake activity, we have not been able to track how the melt has migrated and accumulated at shallow depth to produce the surface uplift, nor do we have the resolution to image the shallow melt intrusion using seismic tomography methods.

To exploit the opportunity described above, the goal of this project is to deploy a dense array of seismic stations within the Askja caldera for ~12 months, starting in July 2023. The timing is essential, given the unpredictability of volcanic processes, and the fact that Askja is only accessible in summer months (July-September). As a result of the deployment, station density will be greatly increased, from >5 km to 1-2 km spacing. This dramatic improvement will allow us to detect an order of magnitude more earthquakes, and image the shallow subsurface region in unprecedented detail. The new dataset will allow us to address key questions regarding the the shallow magmatic intrusion, including (i) what is its depth and lateral extent?; (ii) is it being fed from directly below, where current seismic imaging suggests there exists a deeper source of melt?; (iii) is there any evidence of developing dykes, which provide important clues about how the shallow melt body is evolving, and may precede eruptive activity?; (iv) can we illuminate any interaction or contact between the magmatic and hydrothermal system? The latter question is particularly significant in light of a potentially large volcanic eruption if rising magmatic material interacts with Lake Öskjuvatn.