Quasi-real-time hazard analysis of lava flows prior to the 2021 Fagradalsfjall eruption, Iceland

On 24th February 2021 after a year of unrest, a seismic crisis began on the Reykjanes peninsula, with a series of M3-5 earthquakes affecting the southwest corner of Iceland including Reykjavík. For a month over 3000 earthquakes >M2 occurred on the Reykjanes peninsula, 63 of which were over M4. This was accompanied by crustal inflation with geodetic inverse models inferring a dyke intrusion. In preparation for a potential eruption in the region, probabilistic lava modelling was carried out to identify areas most at risk of lava inundation. The primary input to the model was a long-term volcanic susceptibility map computed several years prior using the QGIS plugin QVAST. The long-term susceptibility map takes geological information such as the location and age of volcanic vents, fissures, and fractures. A probability density function of each feature class is given a different weight through the elicitation of expert judgement and combined through a Poisson point process. To move from long-term to short-term volcanic susceptibility, contemporary seismic information was incorporated with recent events given more importance than older events, with events older than several days removed altogether. The short-term volcanic susceptibility map was then used as an input to the QLavHA whereby areas with a susceptibility higher than a given threshold are considered areas which may host an eruption. Together with a digital elevation model of the area, thermo-rheological parameters of lavas typical to Reykjanes were used to simulate a series of lava flows from vents hosted within the high-susceptibility zones. This methodology was applied twice daily from 24th February up until the start of the eruption on 19th March and results were distributed to the authorities and public. Results were increasingly accurate, with the map that was released just hours prior to the onset of the eruption almost perfectly predicting the final result.