Correlation of volcanic activity and S-wave attenuation anomalies in the Reykjanes Peninsula, Iceland

Increased attenuation of seismic S-waves propagating beneath a volcano is one of the most important seismic indicators of magma or partially melted rocks. We studied the attenuation in the Reykjanes peninsula, Southwest Iceland and its local anomalies in relation to the Fagradalsfjall eruption in March 2021.  

The Reykjanes Peninsula (situated on the rift between Eurasian and North American tectonic plates) is characterized by intensive volcanism that forms its unique geological structure and generates seismic swarm activity. Since 2013, it has been monitored by the REYKJANET network. Seismic activity intensified from December 2019 and lasted until the eruption of Fagradalsfjall volcano in March 2021. Seismicity during this period was distributed along the whole peninsula, not only in the vicinity of the eruption site. These data give us a unique opportunity to study the attenuation of seismic S-waves waves and their frequency dependence and to identify anomalies of attenuation.

The formula for mean attenuation is derived by estimating maximum seismic amplitudes as a function of earthquake magnitude accounting for hypocentral distance and station constants that reflect local conditions beneath the stations. It was derived for the vertical and horizontal components of S waves using the ground displacement, velocity and acceleration. Significant frequency dependence of attenuation was found with the attenuation coefficient proportional to the logarithm of the frequency. This explains different attenuation of the maximum amplitudes for stronger and weaker earthquakes, which have different prevailing frequencies. It was also found that the attenuation is not homogeneous in the entire area covered by REYKJANET (approximately 35 km x 15 km). The attenuation showed significant changes in time. Strong S-wave attenuation was detected for rays passing through the Krýsuvík volcanic system during the year 2020. This may indicate the presence of partially melted rocks at shallow depth. The attenuation beneath the eruption site at Fagradalsfjall was not anomalous during the year 2020; the anomalous values were only detected at the time of eruption.

 This study was supported by the NASPMON project.