Two phases of aseismic afterslip following the March 2021 Damasi, Greece, normal faulting earthquakes retrieved from InSAR measurements

We investigated the post-seismic period of the March 2021 Damasi-Tyrnavos (Thessaly, Greece) normal fault earthquakes by applying the multi-temporal interferometric Small Baseline Subset method. We processed 68 ascending Sentinel-1 acquisitions between 2020/03/15 and 2022/09/12. Our results identified three areas on the hanging wall of the ruptured faults showing non-linear deformation trends (systematic motion away from the satellite), and another area, on the footwall (systematic motion towards the satellite), interpreted as due to a post-seismic effect. Inversion of the InSAR data indicated the occurrence of afterslip co-planar to the sequence's two largest fault planes (M 6.3 and M 6.0, respectively). Most of the afterslip, with a peak of about 0.2 m, occurred on the fault corresponding to the 4 March 2021 event, at a depth of 7.5 km, while the fault corresponding to the M 6.3 event only showed very shallow adjustments and minor features at the border of the coseismic pattern. The transient uplift affected the footwall of the 3 March 2021 event and may indicate that the rupture nearly reached the surface towards the SW of the epicenter. The afterslip showed a fast phase lasting between March and August 2021 (5 months) and a second phase from March 2022 up to September 2022. A correlation between afterslip and relocated hypocenters indicates that most of the afterslip was aseismic. The moment release of the afterslip (fast phase) is about 7% that of the mainshocks.