Seismic Ground Motion and Hazard in the Volcanic Environment: New Insights from the Volcano-Tectonic Unrest in the Reykjanes Peninsula, Iceland

Abstract

The ongoing active volcanic period in the Reykjanes Peninsula, which started in 2021, has been accompanied by intense earthquakes with magnitudes as large as Mw 5.6. The first volcanic eruption began at Fagradalsfjall on March 19, 2021. This was followed by two additional eruptions at Fagradalsfjall, before volcanic activity shifted to Sundhnúkagígar. In this study, focal mechanism inversion is implemented from the accelerometric data recorded by the IceSMN, using the code ISOLA, and the estimated magnitudes are compared with independent estimates from IMO and EMSC. It was observed that our magnitude estimate from the waveform inversion is very close to that of EMSC, and in general larger than the estimate from IMO. However, even though the results are promising, certain events with a low variance reduction give less reliable results, affecting both magnitude and focal mechanism estimates. Moreover, an empirical mixed-effects ground motion model (GMM) for PGA, PGV, and PSA was calibrated for rock sites based on the available recordings and the magnitude estimate from EMSC. This GMM uses the same functional form used by Hernández-Aguirre et al. (2023) [14], but includes approximately six times more data. The new GMM presents larger amplitudes at short distances and a faster attenuation with distance compared to the model by Hernández-Aguirre et al. (2023)[14]. These findings highlight the need for continued refinement of the inversion method to enhance the accuracy of GMM calibration and its applicability in seismic hazard analysis. Finally, a fast and simple approach is proposed to estimate magnitudes from the recorded PGAs using the updated GMM.