Investigation of Tremor Events Coinciding with Skaftár-Cauldrons Glacial Floods from 2015 to 2021

The Skaftár cauldrons are a pair of depressions at the surface of the Vatnajökull glacier that signify subglacial lakes caused by geothermal heat sources at the underlying bedrock. These subglacial lakes continuously grow in volume and produce fast-rising jökulhlaups (glacial lake outburst floods) at the glacier outlet 35-40 km away. Seismic tremor events coincide with these large floods, but the exact source of this tremor is unknown. To investigate the origin and characteristics of these tremors, network covariance matrix spectral width, real-time seismic amplitude measurements (RSAM), and spectral analyses were conducted for eight jökulhlaups spanning the years 2015 to 2021, with joint interpretation alongside hydrological and GPS data. Seismic data was aquired by the Icelandic Meteorological Office's network. The several-day time period spanning the flood propagation and deepening of the ice cauldrons was dominated by small icequakes along the subglacial flood path and emergent low frequency, temporally regular earthquakes with repetitive waveforms. Once most of the water had drained from the cauldrons, strong tremor bursts with a duration on the order of 10s of minutes and a dominant frequency range between 0.5 and 3 Hz were recorded by seismic stations both on and off of the glacier. This tremor occurred during seven out of the eight floods examined, exhibiting fewer than 10 clear tremor bursts per flood over approximately a 24-hour period. Preliminary results show that the amplitude of the tremor bursts correlates to the magnitude of the flood, with the strongest tremor occurring in 2015, during the largest recent flood from the eastern cauldron. Due to the similarities shared between the shape of this tremor’s seismic envelope and that of the tremor during hydrothermal explosions, we interpret the tremor seen after the Skaftá cauldrons have drained as steam explosions facilitated by a drop in the overlying water pressure.