We investigate a seismic crisis that occurred in the western Gulf of Corinth (Greece) between December 2020 and February 2021. This area is the main focus of the Corinth Rift Laboratory (CRL) network, and has been closely monitored with local seismological and geodetic networks for 20 yr. The 2020–2021 seismic crisis evolved in three stages: It started with an Mw 4.6 event near the northern shore of the Gulf, opposite of Aigion, then migrated eastward toward Trizonia Island after an Mw 5.0 event, and eventually culminated with an Mw 5.3 event, ∼3  km northeast of the Psathopyrgos fault. Aftershocks gradually migrated westward, triggering another cluster near the junction with the Rion–Patras fault. Moment tensor inversion revealed mainly normal faulting; however, some strike-slip mechanisms also exist, composing a complex tectonic regime in this region dominated by east–west normal faults. We employ seismic and geodetic observations to constrain the geometry and kinematics of the structures that hosted the major events. We discuss possible triggering mechanisms of the second and third stages of the sequence, including fluids migration and aseismic creep, and propose potential implications of the Mw 5.3 mainshock for the seismic hazard of the region.

We assess the accuracy and the precision of the TanDEM-X digital elevation model (DEM) of the western Gulf of Corinth, Greece. We use a dense set of accurate ground coordinates obtained by kinematic GNSS observations. Between 2001 and 2019, 148 surveys were made, at a 1 s sampling rate, along highways, roads, and tracks, with a total traveled distance of ~25,000 km. The data are processed with the on-line Canadian Spatial Reference System precise point positioning software. From the output files, we select 885,252 coordinates from epochs with theoretical uncertainty below 0.1 m in horizontal and 0.2 m in vertical. Using specific calibration surveys we estimate the mean vertical accuracy of the GNSS coordinates at 0.2 m. Resampling the DEM by a factor of ten allows one to compare it with the GNSS in pixels of metric size, smaller than the width of the roads, even the small trails. The best fit is obtained by shifting the DEM by 0.47 ± 0.03 m upward, 0.10 ± 0.1 m westward, and 0.36 ± 0.1 m southward. Those values are twenty times below the nominal resolution of the DEM. Once the shift is corrected, the root mean square deviation between TanDEM-X DEM and GNSS elevations is 1.125 m. In forest and urban areas, the shift between the DEM and the GNSS increases by ~0.5 m. The metric accuracy of the TanDEM-X DEM paves the way for new applications for long-term deformation monitoring of this area.

<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>

Moderate‐to‐large earthquakes in rifts may occur on leading boundary faults or inner antithetic faults. Here we show a rare case of the 2020–2021 seismic sequence in the Corinth rift, that culminated in the shallow rupture of the antithetic fault, neither preceded nor followed by the leading fault rupture. The hypocenter of the largest shock (Mw 5.3 of 17 February 2021) was located at ∼8 km depth. However, seismic waveform data, supported by satellite‐geodetic and tide gauge measurements, pointed to rupture at shallow depth (∼3 km), where no earthquakes were previously observed. We show that the earthquake most probably ruptured two orthogonal, conjugate fault segments: a weak nucleation phase occurred in the microseismically highly active sub‐horizontal detachment layer, followed – a few seconds later – by a larger, shallow moment release on a high‐angle, south‐dipping normal fault. The latter is the Mornos offshore fault, antithetic to the leading, north‐dipping Psathopyrgos fault. Our study presents the first instrumental/observational evidence of a very shallow Mw 5+ event in this rift – and one of the few reported worldwide. The depth limit of the main shallow slip patch coincides with the expected crossing of the Mornos fault with the Psathopyrgos fault, stressing the importance of fault segmentation and rooting inherited from the rift history. This unusual shallow slip in a depth range with little background seismicity and few aftershocks needs to be further investigated by dynamic modeling as a possible prototype of hazardous events in rift environments.

<p>During the Neogene, the Aegean domain underwent intense deformation, leading to a thinning by a factor of two or more of the Alpine orogenic prism. Today, tectonic velocity gradients are still among the fastest in Europe due to the Anatolian extrusion induced by the Arabian indentation and by the Hellenic slab retreat. The present-day deformation essentially localizes in the subduction backstop. With respect to the central Aegean, which is almost stable today, this still-thick buttress has remained at a much earlier and brittle deformation stage. This is particularly the case in the ~east–west-extending External Hellenides (Southern Greece), shaped by a series of major NNW–SSE-oriented normal faults.</p> <ul> <li aria-level="1">How has the crustal deformation been accommodated by the various fault systems present in the Peloponnese since the Paleogene?</li> <li aria-level="1">Which of those fault systems are still active today?</li> <li aria-level="1">To what extent can boundary forces such as the Hellenic slab pull be sufficient to explain this extension?</li> </ul> <p>Thanks to a significant increase in the GNSS network density in the Peloponnese, we present an updated local strain field. The resulting strain confirms the ~east–west sprawl of the External Hellenides, with extension also, to a lesser extent, in the other directions. Through identifying low-angle detachments by field and satellite morpho-structural analysis, we show that this spreading has been occurring since the Pliocene, mostly by reusing décollement layers of the Alpine nappes as extensional structures. We suggest that the main high-angle normal faults existing in the Peloponnese correspond to a localization of the extension in the weakest azimuth dictated by the Alpine backbone. We propose that this surface sprawl results not only from the Hellenic slab retreat but also from the exhumation of the deep Peloponnesian stacked units, and the subsequent crustal gravity collapse.</p>

<p>The dynamics of fault slip in the upper hundreds of meters of Earth’s crust has long been an open question, as their behavior differs from classical elastic dislocation models and their observation still raises challenges. Here, we analyze centimeter-scale ground resolution aerial optical images of the surface ruptures associated with the 8 Mw ≥ 5.0 sub-surface earthquakes that stroke during the Reykjanes seismo-tectonic unrest, starting on February 24, 2021, and ending with the start of an eruption at Fagradasfjall on March 19, 2021. For four major earthquakes, we apply a sub-pixel correlation technique of pre-, syn- and post-crisis aerial and drone orthomosaics to describe the displacement field on surface blocks. We find that surface offsets reached up to 50 cm, with almost pure dextral strike-slip in a NS direction. These orientations contrast with the overall NE-SW-oriented extensional structures originating from magmatic intrusions and appear as a bookshelf faulting system conjugated to the left-lateral strike-slip plate boundary, oriented ~N070.</p><p>On hard grounds (e.g.: lava flows), shallow ruptures reached the surface, reactivating pre-existing structures and displaying an en-échelon succession of hectometric-sized fractures. We believe these ruptures are representative of medium-sized faults behavior in the last few hundred meters of the crust. On soft grounds, however, the rupture was only betrayed by meter-sized en-échelon systems, evidenced by thousands of discrete sub-metric surface fractures we were able to observe in the field and map from the orthomosaics. The sharp deformation gradient we imaged indicates that the dislocation drastically decreased above ten to a few tens of meters below the surface. In this layer, diffuse deformation takes on most of the slip deficit, mainly through inelastic processes. As a result, evidence of the February 2021 earthquake did not endure erosion for more than a few months. Except for an isolated sinkhole which allowed us to assume that one fault pre-existed, there were no markers of its presence before the earthquake. We emphasize that this issue must frequently lead to an underestimation of the seismic hazard when performed from surface traces.</p>

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On 30 October 2020 at 11:51 UT, a magnitude 7.0 earthquake occurred in the Dodecanese sea (37.84°N, 26.81°E, 10 km depth) and generated a tsunami with an observed run-up of more than 1 meter on the Turkish coasts. Both the earthquake and the tsunami produced acoustic and gravity waves that propagated upward, triggering coseismic and co-tsunamic ionospheric disturbances. This paper presents a multi-instrumental study of the ionospheric impact of the earthquake and related tsunami based on ionosonde data, ground-based Global Navigation Satellite Systems (GNSS) data and data from DORIS beacons received by Jason3 in the Mediterranean region. Our study focuses on the Total Electron Content to describe the propagation of Medium Scale Travelling Ionospheric Disturbances (MSTIDs), possibly related to gravity waves triggered by the earthquake and tsunami. We use simultaneous vertical ionosonde soundings to study the interactions between the upper and lower atmosphere, highlighting the detection of acoustic waves generated by the seismic Rayleigh waves reaching the ionosonde locations and propagating vertically up to the ionosphere. The results of this study provide a detailed picture of the Lithosphere-AtmosphereIonosphere coupling in the scarcely investigated Mediterranean region and for a relatively weak earthquake.