An overview of tremor and icequake types during the 2015 Eastern Skaftá jökulhlaup, Iceland

Ice-covered volcanoes regularly cause subglacial floods called jökulhlaups. These may be caused by volcanic eruptions but more often they are associated with geothermal activity beneath the ice. Within this presentation we will study the largest jökulhlaup on record in Iceland using GPS sensors above the flood path, hydrological sensors and geochemical measurements in the affected river and a seismic array in combination with the local seismic network. We detected four different tremor types: (1) Migrating tremor and high-frequency transient events follow the propagation of the flood front. (2) Minute-long tremor bursts occur in the cauldron area once the water has drained from the subglacial lake and (3) each one is followed by hour-long harmonic tremor. (4) Tremor due to more energetic flow in the rapids near Sveinstindur in the Skaftá river. Interestingly, families of icequakes were generated in the area around the cauldron, during the onset of the flood. This grouping into families - and their consecutive activation with time - suggests that we see the gradual collapse of the ice shelf above the subglacial lake seismically. The flood generated five different seismic signal types that can be associated with five different geophysical processes, including the wide spectrum from brittle failure and explosions to boiling and turbulent flow. We will discuss these sources in detail in the presentation.