Surface fractures and deformation of the magnitude 5.6 earthquake near Reykjavík on 20 October 2020

On 20 October 2020, Reykjavík was rocked by the largest earthquake in southwest Iceland in over a decade when a magnitude 5.6 event occurred only 25 km from the city. The earthquake caused movement on multiple surface fractures, distributed over an 8-km-long north-south oriented area, indicating the location of the underlaying right-lateral strike-slip fault rupture. We mapped the coseismic surface fractures and deformation using Sentinel-1 and TerraSAR-X InSAR data, selecting with a new method the best pre- and post-earthquake SAR scenes from analyzing the tropospheric signals on each SAR image. This method does not require masking out deformed areas when determining the InSAR covariance structure and thus yields better earthquake source estimations. As the InSAR data are primarily sensitive to east-west and vertical displacements, we additionally used split-beam interferometry to obtain more information about north-south displacements. For this, we used burst-overlap interferometry (BOI), in the case of Sentinel-1 data, and multiple-aperture interferometry (MAI) on the TerraSAR-X data. Together with the standard InSAR data, we estimated the full 3D coseismic surface displacement field of the earthquake. The results show that most of the fractures had limited surface offsets, apart from a 2-3 km long north-south trending segment just north of the epicenter that was right-laterally offset by about 15 cm. Source modeling of the earthquake shows that the deformation is consistent with a near vertical north-south striking fault with up to ~30 cm of slip located at roughly 3 km depth below the surface. The estimated geodetic moment of the model amounts to a magnitude 5.6 earthquake, consistent with seismological estimates. Most of the modeled fault slip and mapped surface fractures are located north of the earthquake epicenter, indicating that the earthquake ruptured unilaterally from south to north, which agrees with the more severe surface effects and shaking reported from near the northern end of the earthquake rupture.