High-Resolution Microseismicity Provides Insights into Ring-Fault Geometry at the Re-inflating Bárðarbunga Caldera, Iceland

In 2014-15, the subglacial Bárðarbunga caldera collapsed, subsiding 65 metres as magma flowed out from beneath it to feed a fissure eruption at Holuhraun. Subsequently, the caldera has been re-inflating, indicating recharge of the crustal magma reservoir. Sustained seismicity along the caldera ring faults – but with reversed focal mechanism polarity compared to the eruption period – further supports its ongoing resurgence. In summer 2021, 2024 and 2025 we installed temporary broadband seismic arrays on the ice cap above Bárðarbunga, to provide improved constraints on earthquake hypocentres and focal mechanisms.

We use QuakeMigrate to produce catalogues of microseismicity, with 8,500 and 19,500 events located in the campaigns in 2021 and 2024, respectively. The magnitude of completeness, M_C is ~ -1. Relative relocation reveals a sharply defined ring fault, consistent in geometry with geodetic constraints obtained during the 2014-15 collapse, thus providing strong evidence that the same structure is being reactivated as the caldera re-inflates. Tightly constrained focal mechanisms show excellent agreement with the local ring-fault geometry defined by the relocated microseismicity, and steep dip-slip faulting corresponding to uplift of the caldera floor. Low frequency earthquakes observed between 15 - 25 km depth in the normally ductile part of the crust below Bárðarbunga, and at around 6 km depth below the caldera, signify activity in the deeper plumbing system of the volcano, which may indicate magma ascent pathways. These events contribute to excellent ray coverage for tomography, which we will use to image the shallow melt reservoir and its geometry relative to the ring-fault.