Array seismologic insights into Skaft jkulhlaups (GLOFs), Iceland, 2014-2016.

GLOFs, glacier lake outburst floods or by their local type name jkulhlaups sometimes travel and propagate subglacially. Usually, these subglacial floods can be observed only indirectly using records of the related glaciers surface deformation derived from GNSS and InSAR techniques. However, these floods can also be monitored by the seismic signals they generate: Flood noise by turbulent water or carried sediments, ice quakes, and others. At Icelands Vatnajkull glacier, floods from the Skaft cauldrons travel more than 40km below the ice before feeding into the Skaft river. To study the propagation and the physics of the jkulhlaups, temporal seismic arrays were installed close to the glacier terminus, i.e., west and southwest of Vatnajkull between 2014 and 2016. To locate the generated tremor along the flood propagation path, we used two seismic methods: frequency-wavenumber-analysis (fk-analysis) and match field processing (MFP). In previous works, we were able to track the subglacial propagation of the flood tremors seismically for four GLOF events occurring between 2014 and 2016. For the discharge-highest flood occuring in October 2015 we were also able to identify seismic signals related to the rising water level downstream at rapids outside of the glacier. These findings can be directly related to hydrometric and GNSS observations, measured outside and on the glacier respectively. We find that the tremor can be related to the vertical deformation of the glacier after the flood fronts passing: an important insight allowing to reconstruct the glaciers deformation seismically. Our results highlight the potential of seismic methods for monitoring subglacial floods, especially in combination with GNSS and hydrometric observations. We emphasize that such multi-method approaches can improve the understanding and quantification of flood characteristics such as their volume, origin, propagation and timing which are critical to effectively provide early warning of subglacial floods.