Widespread ground cracks generated during the 2021 Reykjanes oblique rifting event (SW Iceland)

Abstract

<p>The Reykjanes Peninsula has recently been subject to a seismo-tectonic unrest triggering widespread ground cracks. This started with a strong seismic swarm from 24 February to 17 March 2021 and culminated in a volcanic eruption on March 19, terminating an 800 years quiescence period in the region. The Peninsula hosts four overlapping and highly oblique rift zones. The structural relations between the plate boundary (N070), the rift zones (N030 to N040) and the barely visible fault zones oriented N175 are challenging to assess, as most structures, beside the rifts, are poorly preserved or absent in the landscape. </p><p>To get the full picture of the fracture field generated by the 2021 Reykjanes rifting event, we collected an unprecedented amount of structural data, mapping almost the entire fresh fracture field. Field observations show widespread ground cracks in up to ~7 km distance from the intrusion area with en-echelon metrical segments with a right-lateral sense of shear. Most of these structures are not visible anymore, either covered by lava flows or eroded due to weathering. They are unique testimony of the strong seismicity preceding the eruption and would have remained unnoticed if not caught up by our fixed-wing drone, surveying an area of ~30 km<sup>2</sup>. We used the resulting high-resolution (<5 cm) orthomosaics and DEMs to study three main NS-oriented fracture zones of 3 to 4 kilometers long, mostly generated by ten earthquakes ranging from M5 to M5.6. Results show metric to decametric en-echelon structures with cracks of very limited extension, even in the vicinity of the eruption site. Two of the three main fracture zones clearly show fault reactivation, suggesting episodicity in the rifting processes. Apart from local sinkholes, the third area has probably also been reactivated, but the loose ground composition did not preserve previous structures.</p><p>We further used high-resolution optical image correlation technique to analyze aerial photos and drone imagery acquired before and after the large earthquakes sequence in the three fracture zones. Results show clear NS-oriented shear structures with a right-lateral sense of motion of up to 50 cm. This is in good agreement with moment tensors we computed from waveform data at seismic stations up to 1000 km distance. We observe consistent non-double-couple mechanisms, with tension-crack components oriented northwest-southeast. The orientations suggest strike-slip faulting with nodal planes oriented in the same direction as the main fault traces. We also found that the three fracture zones have sigmoid shapes and their overall extension bounds the near-field deformation of the plate boundary. These sigmoids may suggest a local high geothermal gradient and elasto-plastic deformation affecting the Reykjanes Peninsula, that further decreases toward the South Icelandic Seismic Zone.</p>