Co-eruptive crustal deformation changes associated with the 2021 Fagradalsfjall eruption (SW-Iceland)

Ground deformation during an eruption may help to interpret physical processes related to the plumbing system. Here, we report the co-eruptive deformation of the Fagradalsfjall (SW-Iceland) eruption in 2021, that occurred in an oblique rift zone. The eruption lasted six months, 19 March - 21 September, following several weeks of intense seismic activity. The spatial and temporal analysis of Global Navigation Satellite System, GNSS, and Interferometric Synthetic Aperture Radar, InSAR, (by Sentinel-1) observations of ground displacements locate changes in the deformation pattern during the eruptive period. Three temporal changes in the subsidence rates and horizontal motion towards the eruptive area are identified: the first period, T1, 19 March–12 May; the second period, T2, 12 May–30 July, and the third period, T3, 30 July–21 September. The maximum deformation rate (20 mm/yr in line-of-sight) is observed in T2 and coincides with the average effusive rate increase (from 8 m³/s in March–April to 9–13 m³/s in May, Pedersen et al., 2022). We jointly inverted the GNSS and InSAR data to place constraints on the size and location of the source of subsidence during the six-month eruption. Initial modelling result (InSAR and GNSS) indicates a point-source at mid-crust level, 9.5-10.5 km depth and a volume decrease of 18-21 × 10⁶ m³. The deflation volume estimated is significantly lower than that of the lava flow field, with a bulk volume of 150 ± 3 × 10⁶ m³of lava (Pedersen et al., 2022). A residual signal is observed in our model, centered above the source location and around the eruptive center. Both the residual signal and the lower-than-expected volume change suggest additional inflow from a deeper source in agreement with evidence of physical mixing of magma from a mantle supply after the start of the eruption (Halldórsson et al., 2022). A link to a deeper source influences the influx rate of the magma and, consequently, the magma available. Both local seismicity rate and seismic moment release gradually decreases between the onset of the eruption and late April. Afterwards, both show a relatively constant rate, until the end of the eruption.

 

 

 

 

Pedersen, G., Belart, Joaquín M.C., Óskarsson, B., Gudmundsson, M. et al (2022). Volume, Effusion Rate, and Lava Transport During the 2021 Fagradalsfjall Eruption: Results From Near Real‐Time Photogrammetric Monitoring. Geophys. Res. Lett., 49. 10.1029/2021GL097125.

 

Halldórsson, S.A., Marshall, E.W., Caracciolo, A. et al. (2022). Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland. Nature 609, 529–534 https://doi.org/10.1038/s41586-022-04981-x