Surface fractures generated during the 2021 Reykjanes oblique rifting event (SW Iceland)

Abstract

We use a comprehensive dataset of field observations, high spatial resolution drone orthomosaics and digital terrain models (DTMs) to map, quantify and characterize the extensive ground fracturing related to the 2021 seismo-tectonic and volcanic activity in the Reykjanes Peninsula (Iceland). The dataset, spans an area of about 30 km^2, where we map nearly 20 000 ground cracks with metric to decametric lengths and centimetric extensional offsets, revealing a dominant dextral shear, in agreement with published seismic data. Although striking in a direction similar to the volcanic systems in the Reykjanes Peninsula (N030–040), most fractures appear as en-échelon structures globally aligned along N-S-striking fault zones up to 3–4 km long. By examining the timing of ground fracturing through repeated field observations, seismic data and InSAR images, we associate a fracture zone with most earthquakes of M_omega ge 5.0 that occurred in the month preceding the March 2021 Fagradalsfjall eruption. We describe three preexisting N-S fault zones, with fault segments that were reactivated up to three times during the pre-eruptive seismic activity, while the magma intrusion did not trigger graben-related ground fractures typically observed during magmatic injections. Our depiction of a system dominated by strike-slip tectonic features helps in understanding the geometry and bookshelf-mode of tectonic activity along a diffuse and highly oblique extensional plate boundary. Evidence of transient fracturing is typically quickly lost because of erosion or lava flow burial, highlighting a potential under-representation of diffuse fracturing when studying old tectonic and volcanic systems.