Strong ground motion in the epicentral area of the 2020-2021 earthquake swarm in the Reykjanes Peninsula, Iceland

Abstract

The Geldingadalur eruption in the Reykjanes Peninsula on 19 March 2021 was preceded by several earthquakes of volcano-tectonic origin throughout 2020 and 2021. Seven earthquakes with magnitude M≥5 took place during the swarm, all of them recorded by the Icelandic Strong Motion Network operated by the Earthquake Engineering Research Centre of the University of Iceland. In this paper we present salient features of strong ground motion in the epicentral area caused by the swarm. Interestingly, earthquakes as small as M5.0 caused peak ground acceleration (PGA) larger than the 475-year return period PGA at a town near the epicentral area. At two recording stations, unusually high energy content at vibration periods <0.3s was detected, with spectral accelerations exceeding the design values. The largest recorded horizontal PGA was ~0.4g at Krýsuvík, station, which is the strongest PGA recorded in Iceland since the MW6.3 2008 Ölfus Earthquake. For this station we present horizontal-to-vertical spectral ratios indicating likely site-effects. We also compare the attenuation of PGA of the largest event of the sequence with two groundmotion prediction equations (GMPEs). The recorded PGA attenuation is well captured by a local GMPE.