Seismic signatures of the 2015–2021 Skaftárkatlar jökulhlaups

The Skaftá cauldrons, a pair of surface depressions NW of Grímsvötn on the Vatnajökull glacier, signify subglacial lakes created by geothermal heat sources within the underlying bedrock. These subglacial lakes continuously grow in volume and each produce jökulhlaups every 1–5 years at the glacier outlet 35–40 km away. Seismic activity associated with the eight 2015–2021 jökulhlaups from the Skaftá cauldrons was analyzed, reflecting striking similarities. Seismic activity associated with subglacial flood propagation and cauldron deepening was dominated by small transient signals, interpreted as icequakes from ice deformation and hydrofracturing as the water starts to migrate subglacially. Low-amplitude, low-frequency highly repetitive events were discovered during this period for the larger magnitude jökulhlaups, possibly relating to stick-slip motion of the glacier at the bedrock. Seismic tremor events coincide with these large floods, but the origin of the tremor has been disputed. Sustained tremor (1–3 Hz) persists for 1.5–3 days, while high-amplitude tremor bursts (0.5–4 Hz) with durations up to tens of minutes and a strong relationship with increased electrical conductivity in the flood water are observed once most of the water has drained from the cauldrons, exhibiting higher amplitudes during larger jökulhlaups. A probabilistic location method reveals that this 0.5–4 Hz tremor is co-located with the cauldrons and temporally coincides with the end of cauldron subsidence, indicating that the tremor is related to the rapid depressurization of the bedrock and is likely generated by either confined shallow level magmatic activity or enhanced geothermal activity such as vigorous boiling or hydrothermal explosions.