Moment tensors of the earthquake swarms associated with the 2021 Fagradalsfjall volcanic eruption (SW Iceland)

Abstract

<p><span lang="en-US">The seismic moment tensor (MT) is a universal tool for describing earthquake source mechanisms which enables a description of both a shear-slip and an opening or closing of a fracture. A linear relationship between data, which are the displacement amplitudes of the direct P- and SH waves, and the source parameters makes the MT the most favorable description of the source mechanism a priori. Observation geometry plays a key role in resolution of the mechanism. For success, observations should be available from many different directions surrounding the focus. This is the case of intense earthquake-swarm activity on the Reykjanes Peninsula in 2020 to 2021 preceding the Fagradalsfjall volcanic eruption (March 2021), as a local network REYKJANET complemented the network of </span><span lang="en-GB">the </span><span lang="en-GB">University of Cambridge</span><span lang="en-US"> and by the </span><span lang="en-US">n</span><span lang="en-US">ear</span><span lang="en-US"> stations of the regional SIL network covered well the whole area of the peninsula; a total of 33 stations are used. For a sufficient resolution of the source mechanism, availability of accurate enough response of the medium – the Green’s function (GF) – is demanded. For this purpose we used </span><span lang="en-GB">a 1-D</span><span lang="en-US"> velocity model </span><span lang="en-GB">derived by Vogfjörd et al.(2002) for the Reykjanes</span><span lang="en-US"> Peninsula. For the GF construction, we applied the ray theory (the ANRAY package), where especially in the task of two-point ray-tracing a smoothness of the velocity model is essential; hence the </span><span lang="en-GB">Vogfjörd et al. (2002) model</span><span lang="en-US"> was smoothed. Apart from the smoothness, we paid attention to the topmost P-wave velocity to match the synthetic incidence angle and the observed one. In the result, the topmost velocity was modified (both up and down) for most of the stations used. Substantial effort has been devoted to facilitate a batch processing of the events, first of all it means to allow the ANRAY to work in a semi-automatic regime, which is not its inherent mode of work. As a result, we have a comprehensive tool for retrieval of moment tensors on the Reykjanes Peninsula, particularly in the in 2020/2021 earthquake-swarm activity. We demonstrate its usefulness by treating a set of several tens of events. Source mechanisms of some of them are compared to those obtained by ISOLA, a standard software for processing of regional earthquakes, which however applies different data and different approach</span><span lang="en-GB">. Our set of MTs retrieved provides a deeper insight into the mechanism of faulting due to dyke propagation </span><span lang="en-US">in the region in question from the beginning of 2020 to March of 2021.</span></p>