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On 2 December 2020 10:54 UTC a shallow earthquake of MW (NOA) = 4.6 occurred near the village of Kallithea (to the east of Thiva), central Greece, which, despite its modest size, was locally damaging. Using InSAR and GNSS data, we mapped a permanent change on the ground surface, i.e., a subsidence of 7 cm. Our geodetic inversion modelling indicates that the rupture occurred on a WNW–ESE striking, SSW-dipping normal fault, with a dip-angle of ~ 54°. The maximum slip value was 0.35 m, which was reached at a depth of about 1100 m. The analysis of broadband seismological data also provided kinematic source parameters such as moment magnitude MW = 4.6 (± 0.1), rupture area 6.3 km2 and mean slip 0.16 m, which agree with the values obtained from the geodetic model. The effects of the earthquake were disproportionate to its moderate magnitude, probably due to its unusually shallow depth (slip centroid at 1.1 km) and the high efficiency of the earthquake (radiation efficiency η = 0.62). The geodetic data inversion also indicates that within the uncertainty limits of the technique, three scenarios are possible (a) the earthquake responsible for the mapped surface deformation may have occurred on a ~ 2-km long, blind normal fault different from the well-known active Kallithea normal fault or (b) could have occurred along a secondary fault that branches off the Kallithea fault or (c) it may have occurred along the Kallithea fault itself, but with its geometrical configuration could not be modelled with available data. We have also concluded that with a high dip-angle Kallithea Fault forward model it is not possible to fit the geodetic data. The rupture initiated at a very shallow depth (1.1 km) and it could not propagate deeper possibly because of a structural barrier down-dip. The 2020 event near Kallithea highlighted the structural complexity in this region of the Asopos Rift valley as the reactivation of the WNW–ESE structures indicates their significant role in strain accommodation and that they still represent a seismic hazard for this region. [ABSTRACT FROM AUTHOR]

A strong, shallow earthquake occurred near Heraklion (Crete, Greece) on 27 September 2021. The earthquake produced significant ground deformation in the vicinity of Arkalochori village but without any evidence for surface ruptures of primary origin. We used geodetic (InSAR and GNSS) data to map motions of the Earth's surface that occurred during and shortly after the earthquake. A 14 cm subsidence of the GNSS station ARKL and a maximum of 19 cm distance from the SAR satellite were recorded. The measured surface displacements were used to constrain the rupture geometry and slip distribution at depth. Our best-fitting inversion model suggests that the rupture occurred on a 13 km-long planar normal fault striking N195° E dipping 55° to the northwest, with major slip occurring to the east and updip of the hypocentre. The fault tip is located 1.2 km beneath the surface. The maximum coseismic slip occurred in the uppermost crust, in the depth interval of 4–6 km. A decrease in the fault offsets toward the Earth's surface is likely caused by an increased frictional resistance of the shallow layers to rapid coseismic slip. Satellite observations made in the first month after the earthquake detected no post-seismic deformation (i.e., below one fringe or 2.8 cm). The seismic fault may be identified with the Avli (Lagouta) segment of the NNE-SSW striking, west-dipping, 23 km-long neotectonic Kastelli Fault Zone (KFZ). Part of the rupture occurred along the Kastelli segment, indicating a fault segment linkage and a history of overlapping ruptures along KFZ. Based on geological data and footwall topography we estimate an average slip rate between 0.17–0.26 mm/yr for the KFZ. The Arkalochori earthquake is a paradigm example for the on-going extension of Heraklion basin (central Crete) in the WNW-ESE direction, which is almost orthogonal to the E-W Messara graben and other active faults along the south coast of Crete. [ABSTRACT FROM AUTHOR]

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We identify the source of the Mw = 5.6 earthquake that hit west-central Epirus on 21 March 2020 00:49:52 UTC. We use Sentinel-1 synthetic aperture radar interferograms tied to one permanent Global Navigation Satellite System (GNSS) station (GARD). We model the source by inverting the INSAR displacement data. The inversion model suggests a shallow source on a low-angle fault (39°) dipping towards east with a centroid depth of 8.5 km. The seismic moment deduced from our model agrees with those of the published seismic moment tensors. This geometry is compatible with reverse-slip motion along the west-verging Margariti thrust fault that accommodates part of the convergence within the collision zone between Apulia and Eurasia. We also processed new GNSS data and estimate a total convergence rate between Apulia and Eurasia of 8.9 mm yr−1, of which the shortening of the crust between the Epirus coastal GNSS stations and station PAXO in the Ionian Sea (across the Ionian Thrust) is equivalent to ~50% of it or 4.6 mm yr−1. By back-slip modelling we found that a 60-km wide deformation zone takes up nearly most of the convergence between Apulia-Eurasia, trending N318°E. Its central axis runs along the southwest coast of Corfu, along the northeast coast of Paxoi, heading toward the northern extremity of the Lefkada island. The island of Paxoi appears kinematically as part of the Apulian plate. [ABSTRACT FROM AUTHOR]

We identify the source of the Mw = 6.4 earthquake that rocked north-central Albania on November 26, 2019 02:54 UTC. We use synthetic aperture radar interferograms tied to the time series of coordinates of two permanent Global Navigation Satellite System (GNSS) stations (DUR2 and TIR2). We model the source by inverting the displacement data. Assuming in our model a half-space elastic medium and uniform slip along a rectangular fault surface, we invert the 104 picked measurements on a couple of ascending and descending interferograms to calculate the parameters of the fault. All inversions made with different input parameters converge towards a stable and robust solution with root mean square (r.m.s.) residual of 5.4 mm, thus ~1/5 of a fringe. They reveal that the earthquake occurred deep in the crust on a low-angle fault (23°) dipping towards east with a centroid at 16.5 km depth. The best-fitting length and width of the fault are 22 and 13 km, and the reverse slip, 0.55 m. The seismic moment deduced from our model agrees with those of the published seismic moment tensors. This geometry is compatible with a blind thrust fault that may root on the main basal thrust, i.e., along the thrust front that separates Adria–Apulia from Eurasia. It is notable that there is a 123 ns yr−1 active shortening of the crust between the GNSS stations DUR2-TIR2 (equivalent to a shortening rate of 3.6 mm yr−1), and roughly in the east–west direction. Given this amount of strain the recurrence time of M6+ earthquakes along this fault should be of the order of 150 years. [ABSTRACT FROM AUTHOR]

We present a joint analysis of seismological and Global Navigation Satellite System (GNSS) data of the seismic sequence that started on October 25, 2018 with a shallow M w = 6.7 earthquake offshore Zakynthos (Ionian Sea, Greece). The inversion of the GNSS data shows the activation of a N-S striking thrust/oblique-slip fault at the African-Aegean plate interface with a length of ∼26 km, and depth shallower than 15 km. The fault-plane geometry is well constrained by GNSS with a low-dip angle (23°), and dip-direction towards east. This is consistent with the distribution of the relocated aftershocks (1811 events). Our analysis indicates that the October 25, 2018 event ruptured the Hellenic megathrust. This event highlights the high degree of seismic coupling in the western region of the Hellenic Arc. It also highlights the "strong" nature of the subducting slab with the occurrence of "locked" patches under the Ionian seafloor that fail during large, reverse/oblique-slip earthquakes. The latest, shallow seismic events of the western Hellenic arc (1976 Zakynthos, 1997 Strofades, 2018 Zakynthos) point to a sequence of low-angle events along the plate interface with most of co-seismic deformation taken up by the upper (Aegean) plate. The GNSS data also point to a similar pattern between the co-seismic strain release and the long-term (interseismic) strain accumulation along the west Hellenic Arc. [ABSTRACT FROM AUTHOR]