The volcano-tectonic unrest in the Reykjanes Peninsula in Iceland in 2021 and the new seismic and strong-motion arrays in Southwest Iceland
Authors: Hanna Blanck, Dirk Roessler, Bernd Weber, Benedikt Halldorsson and Kristín S. Vogfjord
Conference - extended abstract: 3rd EUROPEAN CONFERENCE ON EARTHQUAKE ENGINEERING & SEISMOLOGY BUCHAREST, ROMANIA, 2022
Full text in proceedings: https://data.gempa.de/dirk/Proceedings_3ECEES_2022_blank.pdf
Abstract: An intense period of volcano-tectonic unrest in the Reykjanes Peninsula Oblique Rift
(RPOR) zone in Southwest Iceland commenced on 24/02/2021 with a M! 5.64 earthquake in the
central part of the Peninsula, followed by a drastic increase in seismic activity over the western-
central RPOR and clear deformation signals associated with a dike intrusion. The sequence that
saw six NS-striking earthquakes of M ! 5 and larger along the ∼10 km long NE-SW striking dike
intrusion culminated on 19/3/2021 in an eruption in Mt. Fagradalsfjall near the centre of the dike.
Thereafter, magnitudes and intensity decreased along with generally ceasing seismicity. The
movement of the magma front repeatedly caused remarkable accelerations of seismicity with
increases in the frequency of phase detections, known to reduce the sensitivity and reliability of
real-time hypocenter location estimates of regional networks. This potentially had practical
implications due to the proximity of the unrest to the capital region of Reykjavik (15-25 km NE)
and the town of Grindavik (6 km SW). Therefore, both the improved seismic monitoring of the
advancing magma front was essential to cope with the high seismicity as well as mapping the
spatial differences in ground motion amplitudes inside the closest town of Grindavik, 6 km SW
of the volcanic eruption. We therefore set up a new seismic and strong-motion array consisting
of 6 stations in Grindavik (on 12 March 2021) that streamed data in real-time to a local SeisComP
server. Calibration of the array processing involved tuning Gempa’s interactive and automatic
LAMBDA and AUTOLAMBDA modules applying the Progressive Multi Channel Correlation
and FK-analysis methods. We calculated back-azimuth and slowness values for known
earthquakes and compared them with official permanent network locations. For most events, the
back-azimuths deviate by less than 10° and on average, the distribution of residuals is Gaussian.
The ground motions in Grindavik itself show resonance at 3 Hz, typical of lava-layers in the
region. In addition, a new seismic array was deployed on 23 March 202 in the mountains about 6
km E of the dike intrusion, improving hypocentral locations of small events and tracking the
magma migration in the sub-surface.