| Summary: | The Skaftár cauldrons, a pair of surface depressions on the Vatnajökull glacier, NW of Grímsvötn, signify subglacial lakes caused by geothermal heat sources within the underlying bedrock. These subglacial lakes continuously grow in volume and produce fast-rising jökulhlaups at the glacier outlet 35-40 km away every 1-5 years. Seismic tremor events coincide with these large floods, but the exact tremor source has been unknown. This thesis provides new insight into common seismic features observed during eight jökulhlaups of varying magnitudes from 2015 to 2021, in comparison with hydrological and GPS data. Activity during the first phase of a jökulhlaup, which spans subglacial flood propagation and cauldron deepening, is very similar between all eight jökulhlaups examined. The seismic record is dominated by small transient signals, interpreted as icequakes from surface crevassing and hydrofracturing as the water propagates subglacially. Low-amplitude, low-frequency highly repetitive events were discovered during this period during several jökulhlaups, possibly relating to stick-slip motion of the glacier at the bedrock. The second phase, once most of the water has drained from the cauldrons, exhibits regional tremor. Probabilistic location methods reveal that this tremor is co-located with the cauldrons. Sustained tremor, persisting for 1.5-3 days, is interpreted as enhanced geothermal boiling or non-eruptive magma migration due to rapid depressurization of the subglacial hydrothermal system. Spasmodic, high amplitude tremor bursts with durations of 10s of minutes and a strong relationship with increased electrical conductivity also occur during this period and are interpreted as confined hydrothermal explosions. Skaftárkatlar eru sigkatlar í vestanverðum Vatnajökli, norðvestan Grímsvatna. Slíkir sigkatlar myndast þar sem öflug jarðhitasvæði bræða þykkan jökulís. Jarðhitavatn safnast saman við botn jökulsins. Vatnsþrýstingur vex með tímanum vegna hækkandi vatnsstöðu, þar til vatnið brýst undan viðkomandi ...
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