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The Cherichira aqueduct, originating from Roman times, supplied the city of Kairouan, Tunisia, with water and has had alternating phases of damage and repair after the Roman and during the Aghlabid and Fatimid era. A crucial section of the lifeline is the Cherichira aqueduct bridge (CAB), and scholars have discussed the possibility that earthquake ground motions caused damage which disrupted the water supply of Kairouan. However, little was known about the dynamic behavior of the bridge and its vulnerability to earthquake ground motions. Computer-aided design based on a detailed laser scan model of the remains of the bridge and published data were used to reconstruct the CAB of the Aghlabid period. Subsequently converted into a discrete element model, the digital version of the CAB was subjected to analytic ground motion signals and full 3D simulations of local earthquakes. The CAB model shows a fundamental eigenfrequency close to 1 Hz in the direction transverse to its trend, and single-component ground motions in this direction with peak particle velocities above 1.0 m/s cause damage to the top of the CAB. Among the earthquake scenarios with full 3D ground motions applied, only the activation of a nearby thrust fault caused distinct damage. While fractures in the ruins of the CAB cutting through the upper part of the bridge which includes the water canal are a likely cause for disrupting the water flow and are similar to the damage pattern produced in the model calculations, a solely seismogenic cause of the total collapse of some parts of the CAB cannot be verified by the simulations.

The Lebanese Restraining Bend is an active bend along the Dead Sea Transform Fault in the eastern Mediterranean region where several destructive earthquakes happened throughout history. In this paper, we assess the gross features of seismic hazard of the Lebanese Restraining Bend by applying a neo-deterministic method that involves the generation of synthetic seismograms distributed on a regular grid over the study area. We use the regional seismicity, seismic source zones, focal mechanism solutions, and velocity structural models. We present maps of ground displacement, velocity, and acceleration. This is the first study that generates neo-deterministic seismic hazard maps for the Lebanese Restraining Bend using representative ground motion modelling. Our results show that displacement values of 15-30 cm and velocity values of 30-60 cm/s can be expected along most of Lebanon. In addition, 0.15-0.30 g acceleration values can dominate most of the Lebanese territory and surrounding areas. It is evident from these results that the study area in general and Lebanon in particular constitute a high seismic hazard area, which necessitates further attention from the authorities regarding the precaution measures needed to mitigate the effects of potential catastrophic seismic events; in addition, more detailed investigations are needed at local scale for specific sites of interest.

GDR Rift colloquium 2022Collection of abstracts. This conference was an opportunity to review current research in the Rift or on the Rift,and share and discuss during targeted workshops. These workshops were the place to initiate new projects, launch new collaborations, enrich our sometimes too disciplinary visions, and exchange. The colloquium was also a moment dedicated to a joint reflection on target areas, federating observatories allowing the acquisition, sharing, and distribution of interdisciplinary data over the long term. Specific workshops were proposed to identify and build these target areas with our Rift partners. We gathered 70 students and researchers, and 36 abstracts were presented either during oral presentation or with posters.

The North Anatolian Fault experienced large earthquakes with 250–400 years recurrence time. In the Marmara Sea region,the 1999 (Mw=7.4) and the 1912 (Mw =7.4) earthquake ruptures bound the Central Marmara Sea fault segment. Usinghistorical-instrumental seismicity catalogue and paleoseismic results (≃ 2000-year database), the mapped fault segments, faultkinematic and GPS data, we compute the paleoseismic-seismic moment rate and geodetic moment rate. A clear discrepancyappears between the moment rates and implies a signifcant delay in the seismic slip along the fault in the Marmara Sea. Therich database allows us to identify and model the size of the seismic gap and related fault segment and estimate the momentrate defcit. Our modelling suggest that the locked Central Marmara Sea fault segment (even including a creeping section)bears a moment rate defcit 6.4 × 1017 N.m./year that corresponds to Mw ≃ 7.4 for a future earthquake with an average≃ 3.25 m coseismic slip. Taking into account the uncertainty in the strain accumulation along the 130-km-long Central faultsegment, our estimate of the seismic slip defcit being ≃ 10 mm/year implies that the size of the future earthquake rangesbetween Mw=7.4 and 7.5.Reference:[1] Meghraoui, M., Toussaint, R. & Aksoy, M.E. The slip deficit on the North Anatolian Fault (Turkey) in the Marmara Sea: insights from paleoseismicity, seismicity and geodetic data. Med. Geosc. Rev. 3, 45–56 (2021). https://doi.org/10.1007/s42990-021-00053-w

New field investigations along the East Tunisian coastline reveal sedimentary deposits and damaged localities that may account for a catastrophic event during late Holocene. North of Sfax - Thyna city (at Henchir El Majdoul site) ~3.4 m high cliff coastal marine and alluvial terraces show a 20 to 50-cm-thick chaotic layer with sandy coarse gravels mixed with limestone beach-rocks, reworked blocks, broken shells of marine and lagoon gastropods and lamellibranch mollusks, organic matter, and Roman pottery. The chaotic layer truncates a succession of sandy-silty paleosol, covers Roman settlements and is overlain by fire remains and a relatively thin (~10 cm) sandy-silty aeolian unit and ~1-m-thick alluvial deposits. Charcoal samples collected at 10 cm below and 4 cm above the catastrophic deposits provide radiocarbon dating that brackets a catastrophic event between 286 and 370 CE (2s). Beside the damaged Roman site of Thyna, other localities of the east Tunisian coastline such as Neapolis (Nabeul) near Tunis, Hadrumete (Sousse), Meninx-town in Girba (Djerba), Wadi Ennouili (Gulf of Gabes), and Sabratha (in Libya) experienced major damage and abandonment of sites in Fifth century. The extent of damage from northern Libya to northern Tunisia at the Fourth century and radiocarbon dating, added to the 2.6 m thick turbidite deposits west of Malta correlate with the major tsunamigenic earthquake of 21 July 365 (Mw ~ 8) in west Crete (Greece). Numerical modelling of the tsunami caused by an earthquake in the Hellenic Arc subduction zone suggests more than 3.5 m high tsunami waves propagation affecting the Tunisia coastline, resulting in a run-up consistent with the stratigraphic evidence presented here. The catastrophic deposits, offshore-onshore correlations, archeological damage and modelling of tsunami waves suggest a new, higher-resolution, assessment of the tsunami hazard leading to a better estimate of tsunami risk on the eastern coast of Tunisia.

Northern Algeria experienced moderate and large earthquakes (with Mw > 6) during the last decades due to the convergence between the African and Eurasian plates. We conduct the joint analysis of multi-temporal SAR-dataset (1995 to 2021), combined with the GPS velocities (2007 to 2018) and seismotectonic studies in the Chlef-El Asnam and Zemmouri Mitidja regions of the Tell Atlas. The multidisciplinary approach adopted in this study has the advantage of integrating the interseismic (paleoseismology, tectonic geomorphology), the coseismic and postseismic (airborne geodesy) crustal deformation. The multi-temporal interferometry is performed using the standard method for persistent scatterers (StaMPS/MTI software) applied to ERS1/2, ENVISAT and Sentinel SAR images, all from C-band dataset on descending and ascending orbits. The GPS velocities are modeled and re-interpreted from previous works in order to fit the tectonic block sub-division and related major faulting geometry. The seismicity rate and associated major earthquakes such as the El Asnam in 10/10/1980 (Mw 7.1) and Zemmouri-Boumerdes in 05/21/2003 (Mw 6.8) mark the seismotectonic characteristics of the Tell Atlas. The achieved data analysis and results of the joint InSAR, GPS and seismotectonics reveal that large areas with active deformation undergo uplifting and shortening with a consistent tectonic, geodetic and seismicity rate ranging between 2 and 3 mm/yr.

SUMMARY The westernmost segment of the North Anatolian fault in NW Turkey lies mostly offshore, in the Sea of Marmara and the Gulf of Saros (NE Aegean), respectively to the E and W of a 45 km inland central portion. The 9 August 1912 Mürefte-Şarköy (Ms 7.4) and 13 September 1912 (Ms 6.8) earthquakes occurred along this segment. To date, the segment was studied mostly onshore although estimated magnitude and location suggest an offshore extension. Recent studies show the eastern rupture extension in the Sea of Marmara, while its western counterpart in the Gulf of Saros remains less documented. Here we use new observations from high-resolution marine geophysical data (multibeam bathymetry, side-scan-sonar, and seismic reflection profiles), to constrain the offshore 1912 ruptures in the Gulf of Saros. Detailed mapping of the subaqueous fine-scale morphology and structure of the fault provides a new insight for the western limit of the two 1912 surface ruptures. Distribution of fresh scarps, 3-D structural reconstructions, the complexity of fault segments, and the recent seismicity, altogether suggest that the western termination of the 1912 rupture(s) ends 37 km offshore in the Gulf of Saros. Following the 1999 Kocaeli earthquake, in the eastern Sea of Marmara, the unruptured segment length between the 1999 and 1912 ruptures became a critical issue, because of its implication for future earthquakes in the so-called Marmara seismic gap. If a 150–160 km total rupture length for the two 1912 earthquakes is assumed, a western rupture termination point at the inner Saros basin margin means that the eastern extension of the 9 August earthquake rupture reached the Central Marmara Basin. This outcome necessarily has implications for the seismic hazard in the Marmara coastal area that includes the Istanbul metropolitan area.

The North Anatolian Fault experienced large earthquakes with 250–400 years recurrence time. In the Marmara Sea region, the 1999 (Mw = 7.4) and the 1912 (Mw = 7.4) earthquake ruptures bound the Central Marmara Sea fault segment. Using historical-instrumental seismicity catalogue and paleoseismic results (≃ 2000-year database), the mapped fault segments, fault kinematic and GPS data, we compute the paleoseismic-seismic moment rate and geodetic moment rate. A clear discrepancy appears between the moment rates and implies a significant delay in the seismic slip along the fault in the Marmara Sea. The rich database allows us to identify and model the size of the seismic gap and related fault segment and estimate the moment rate deficit. Our modelling suggest that the locked Central Marmara Sea fault segment (even including a creeping section) bears a moment rate deficit $${\dot{M}}_{d}$$ = 6.4 × 1017 N.m./year that corresponds to Mw ≃ 7.4 for a future earthquake with an average ≃ 3.25 m coseismic slip. Taking into account the uncertainty in the strain accumulation along the 130-km-long Central fault segment, our estimate of the seismic slip deficit being ≃ 10 mm/year implies that the size of the future earthquake ranges between Mw = 7.4 and 7.5.

The seismotectonics of Western Africa show the occurrence of major earthquakes (e.g., 1636 southwestern Ghana, 1855 offshore Monrovia, 1939 offshore Accra, and 1983 Gaoual-Guinea) and prominent offshore transform faults. However, there is no analysis that links the continental active tectonics with the oceanic fault zones of the Gulf of Guinea. We study the active tectonics by firstly mapping the main transform faults using a combination of bathymetric, gravimetric, and magnetic data. The data analysis associates regional seismicity (historical and instrumental) with focal mechanisms as extracted from the recently published seismotectonic map of Africa. We identify active transform faults, the Chain (CFZ), Romanche (RFZ), Saint Paul (SPFZ), and Arkhangelskiy (AFZ) fault zones. We also calculate strain rates on these faults from Late Cretaceous (–85 Ma) to present time using paleomagnetic data and infer slip rates from the seismic moment data. The strain rates show a first stable trend around 2 cm/year and then accelerate to 4 cm/year in the last 10 million years. The comparison of Late Quaternary strain rates with geodetic strain rates shows an accumulation of seismic energy that could lead to the initiation of an Mw 7–7.5 earthquakes on the Saint Paul transform fault. Our seismotectonic analysis clearly links oceanic and continental tectonics, with about a 20°–30° anticlockwise fault trend rotation for CFZ, RFZ, and SPFZ. The potential for the occurrence of large earthquakes in the Gulf of Guinea should be taken into account for a realistic regional seismic and tsunami hazard of the Gulf of Guinea.

A special collection in Tectonics on geodynamics in the Mediterranean region celebrates the life and work of the renowned Italian geologist Renato Funiciello.

We study the Tenes Abou El Hassan (TAEH) fault-related fold of the Tell Atlas (the site of the 1922 Mw 6.2 earthquake) using field investigations combined with satellite image processing. The 1922 earthquake is also revisited through previous reports, publications and contemporaneous newspapers in order to locate the area of maximum ground motion and its relation to the active TAEH fault-related fold. The analysis of high-resolution (0.5 m) satellite imagery (panchromatic Pleiades tri-stereo images) allows the accurate study in tectonic geomorphology and the characterization of active and seismogenic deformation. High-resolution DEMs obtained from Pleiades images highlight alluvial and marine terraces with cumulative uplift during the Upper Pleistocene–Holocene period and allowed the identification of geomorphological markers along the active fold with recognition of complex tectonic structures. The fault-related fold is modelled using both elastic (Okada 1992) and kinematic (Trishear) approaches using balanced cross-sections that reveal 1.2 mm/year shortening rate during the Upper Pleistocene–Holocene. Uplift rate calculated using marine and alluvial terrace altitudes is ~0.3 mm/year during the Upper Pleistocene along the Allalah Quaternary basin and the coastal zone reaching ~1 mm/year during the Holocene. The earthquake geology and correlation between surface deformation and seismotectonic modelling provides additional constraints on the 1922 earthquake fault parameters and related seismic hazard assessment in the Tell Atlas of Algeria.

For any seismic hazard study, a reliable, homogenized, and complete seismic catalog is required. The Algerian seismicity catalog has been recently updated by retrieving and reappraising many historical events. The Algerian seismic network has also been densified up to about 80 seismic stations covering the Tell Atlas which is the most active area of northern Algeria for monitoring of the seismic activity reducing the magnitude threshold. Recently, we have launched archeoseismological studies to retrieve past strong earthquakes that have affected Roman sites located along the Tell Atlas. Here, we proceed with tectonic investigations around selected sites where significant observed damage were identified. On the other hand, paleoseismological investigations were conducted along the El Asnam fault (now Chlef) following the large Ms 7.3 earthquake of 1980. Paleoseismic studies combined with archeoseismological results provide the dating of past earthquakes and contribute to the completeness of the seismicity catalog.

This study focuses on the stress transfer using the Coulomb Failure Function (CFF) modeling and related poroelastic effect of four major earthquake sequences that occurred in different active zones of Africa: the May to July 1990 Sudan earthquake sequence, the May 2018 to June 2019 Mayotte-Comores earthquake swarm sequence, the 1980–2003 El Asnam-Zemmouri (Algeria) earthquake sequence, and the 1994–2016 Al Hoceima (Morocco) earthquake sequence. We observe the relationship between the stress transfer caused by mainshock fault ruptures and the post-seismic deformation controlled by the aftershock distribution. Based on other case studies, our hypothesis is that all seismic sequences are apparently controlled by the increase in pore fluid pressure caused by co-seismic phase and fluid-drained short-term post-seismic response. The poroelastic properties of any seismogenic zone appear to depend on the undrained and drained fluid conditions. The comparison between the 1990 Sudan sequence and the 1994–2016 Al Hoceima (Morocco) stress modeling shows that for most sequences the poroelastic response of the first mainshock play an important role in the occurrence of the second mainshock. Similar observations can be made for the 2018–2019 Mayotte-Comores and the 1980–2003 El Asnam-Zemmouri (Algeria) earthquake sequences. By comparing the case studies, we find that the value of fluid diffusivity controls the timing of earthquake sequences (e.g., more than three times larger between the Al Hoceima and Sudan earthquakes). The constraint of fault interactions with CFF modeling and fluid diffusivity allows a better estimate of the seismic hazard assessment.

New field investigations along the East Tunisian near Sfax coastline reveal sedimentary deposits that may account for a catastrophic event. The sedimentary unit is made of sand coarse gravels, limestone beach-rock, mixed with broken shells of marine gastropods and lamellibranch mollusks, bones and organic matter. Near Thyna, at El Amra site located north of Sfax city, 3.2 m to 3.6 m high late Quaternary coastal terraces are spread over the coastline; they contain a catastrophic deposit that often cover archeological sites of the Roman period. The stratigraphic units show a succession of sandy-silty paleosol truncated by 40 to 70-cm-thick catastrophic unit which is covered in some sites by fire remains overlain by a relatively thin (~10 cm) sandy-silty aeolian unit. The sedimentary succession ends with about 1-m-thick of alluvial deposits and paleosol units. Charcoal samples collected at 10 cm below and 4 cm above the catastrophic units provide radiocarbon dating 236 - 385 cal AD and 249 – 541 cal AD (2s), respectively. Radiocarbon ages bracket the catastrophic unit that may refer to the major tsunamigenic earthquake of 21 July 365 (Mw ~ 8) in west Crete (Greece) reported to have inundated coastlines of Sabratha in Libya and Alexandria in Egypt. The nonlinear shallow water Tsunami-HySEA code is used to perform numerical modelling using 2 different seismic sources comparable to that of the AD 365 Crete earthquake. They feature 2 principal mechanisms that accommodate the Nubia-Aegean convergence along the Hellenic Arc, namely a shallowly dipping thrust-faulting on the subduction interface, as well as a steeper splay faulting in the overriding material. The maximum tsunami wave heights distribution calculated along the Tunisia coast peak in both cases at about 3 meters. The run-up caused by these sources, also considering that we have used uniform slip on the causative fault, can be significantly higher. This proves that the tsunami waves may have reached Tunisia where several coastal cities where severely damaged and reported to have stopped their economic activity. With the identification of the 365 tsunami deposits in eastern coast of Tunisia, the tsunami hazard and risk associated with a major earthquake from the western Hellenic subduction zone cannot be ruled out.

The kinematic of tectonic motions between the African (Sahara) platform and the Maghrebian thrust belt remained unexplored since the onset of space geodesy. Here, we use data of 6 permanent GNSS stations located north and south of the Atlas thrust belt in Algeria to constrain shortening and transpression at the tectonic boundary. The permanent GPS data and results are obtained from the network in Algeria operative from 2013 to 2019, presented with the results of the REGAT network in Algeria since 2007. The south Atlas suture zone constitutes the limit between African (Sahara) shield domain considered as a stable continental interior and the Sahara Atlas that belong to the Alpine orogeny. The tectonic boundary is marked by a E-W to ENE-WSW, en echelon fold belt system with deformed Plio-Quaternary formations to the North and flat laying Mesozoic and Tertiary sedimentary units south of the suture zone. The GNSS data are processed using Gamit-GlobK and results show tectonic motions with a predominant 5 to 6 mm/yr velocities trending NNW-SSE to NW-SE (westward) in the Sahara Platform. The GPS velocities show uniform trend in the African platform from which we infer 0.5 to 1.0 mm/yr convergence across the south Atlas suture zone. The intraplate convergence is attested by the moderate but permanent seismic activity at the tectonic boundary.

The 3 April 2017 MW 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the African plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershock sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 1,000 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (MW 4.6 on the 5 April 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of focal mechanisms obtained from the mainshock and aftershock database are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes with two lobes with 4 to 6 cm coseismic slip on a NW-SE elongated and 30-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~ 1 m maximum slip on a fault plane striking 315°, dipping 45°, -80° rake and with Mo 7.12 1018 Nm Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo belt classifies the intraplate region as an active plate interior.

International audience; The region of northeastern France is affected by low-magnitude background seismicity, with the rare occurrence of moderate earthquakes, which gives this region a non-negligible seismic risk. We provide an overview of the seismicity and seismotectonics of this intraplate domain and of its subregions: the Upper-Rhine Graben (URG), the external range and foreland of Jura, the Vosges, northern France and southern Belgium. Previously published catalogues over historical and instrumental times are used, and the epicentral distribution of earthquakes is compared to known tectonic structures, and the recently computed deformation field. Although no large earthquakes with M w > 6.0 occurred since the 1356 Basel seismic event (Io IX, MKS), the recent identification of active faults suggests periods of high seismic strain rates in the past. The origin of the seismic activity in each of these sub-regions, characterized by low to very-low strain rates, is attributed to pre-existing faults reactivated under specific natural or anthropogenic conditions.

A new analysis of high-resolution multibeam and seismic reflection data, collected during several oceanographic expeditions starting from 1999, allowed us to compile an updated morphotectonic map of the North Anatolian Fault below the Sea of Marmara. We reconstructed kinematics and geometries of individual fault segments, active at the time scale of 10 ka, an interval which includes several earthquake cycles, taking as stratigraphic marker the base of the latest marine transgression. Given the high deformation rates relative to sediment supply, most active tectonic structures have a morphological expression at the seafloor, even in presence of composite fault geometries and/or overprinting due to mass-wasting or turbidite deposits. In the frame of the right-lateral strike-slip domain characterizing the North Anatolian fault system, three types of deformation are observed: almost pure strike-slip faults, oriented mainly E–W; NE/SW-aligned axes of transpressive structures; NW/SE-oriented trans-tensional depressions. Fault segmentation occurs at different scales, but main segments develop along three major right-lateral oversteps, which delimit main fault branches, from east to west: (i) the transtensive Cinarcik segment; (ii) the Central (East and West) segments; and (iii) the westernmost Tekirdag segment. A quantitative morphometric analysis of the shallow deformation patterns observed by seafloor morphology maps and high-resolution seismic reflection profiles along the entire basin allowed to determine nature and cumulative lengths of individual fault segments. These data were used as inputs for empirical relationships, to estimate maximum expected Moment Magnitudes, obtaining values in the range of 6.8–7.4 for the Central, and 6.9–7.1 for the Cinarcik and Tekirdag segments, respectively. We discuss these findings considering analyses of historical catalogues and available paleoseismological studies for the Sea of Marmara region to formulate reliable seismic hazard scenarios.

Major earthquakes display a significant tsunamigenic seismic activity (with Mw > 7) that may affect Africa continent. These events are from the far-field tectonics plates or even nearby tectonics boundaries. The main target of this study was to choose the most dangerous tsunamigenic zones according to the historical tsunami events and to update the tsunami hazard map of Africa published in UNISDR (2009). This was done by using nested bathymetry grids to reevaluate the maximum wave height using high-resolution (15 arcs per second) data near the African coastlines. Mirone version 2.10 software is used in these calculations. Four tsunamigenic zones sources affecting the African coastal zones have been tested at Andaman-Sumatra subduction zone, Makran trench zone, Western and Eastern Hellenic arcs. These tsunamigenic source zones were responsible for huge tsunamis generated from large historical earthquakes on 26 December 2004; 27 November 1945; 8 August 1303; and 21 July 365. Two to 4 m was the calculated maximum wave height resulted from the scenario 1 which arrived to the coasts of Tanzania, South Africa, and South Madagascar, while scenario 2 resulted in maximum wave height of 1–2 m toward the Somalian coast. The scenarios 3 and 4 were responsible for the maximum wave height of 2–4 m at the Egyptian and Libyan coasts. Preparing an early warning system will be required necessarily for the whole of Africa to overcome possible future high tsunami risk to the African coastal cities.

The Hasbaya fault is a 50-km-long fault branch of the Dead Sea Transform Fault within the Lebanese Restraining Bend. It lies within the intersection zone of Mount Lebanon and Anti-Lebanon chains, along the western flank of Mount Hermon. It is little studied, its tectonic behavior is unknown, and its role among the other active fault branches of the restraining bend is the least defined. It was mapped as a discontinuous array of fault segments, with minor displacements and tectonically undisturbed basaltic cover. We studied the Hasbaya fault with detailed field mapping, and combining geomorphology, structural geology, and active tectonic investigations. The results reveal little and localized faulting along the entire length of the fault, with no signs of tectonic correlation between the drainage network evolution and the tectonic activity of the fault. The absence of active-tectonic surface features along the Hasbaya fault trend indicates that it does not manifest enough surface evidence to be classified as a tectonically active structure within the Lebanese Restraining Bend, and subsequently does not appear to be a major source of seismic hazard in the region. It may be related to the subsurface structure that controls the monocline that forms the western flank of Mount Hermon, or it may represent the remnant of an old suture zone between Mount Lebanon and Anti-Lebanon as a part of the regional compression imposed across the restraining bend. Our results shed light on the role of the non-active Hasbaya fault in a zone of active faults within a restraining bend, which may provide a case with broader implications on comparable tectonic settings worldwide.

The Upper Rhine Graben (URG) undergoes continuous microseismicity recorded by seismic monitoring networks and moderate-magnitude earthquakes, such as in the zone north of the Alpine front, which includes the Jura thrust front, the Vosges, the Black Forest, the Swabian Jura and the Alsace plain. The surface velocity field is a good indication of the occurrence and location of strain gradients likely associated with seismogenic structures. To explore that possibility, we use long time-series of displacements measured from 2002 to 2018 with the dense GURN network (GNSS URG Network). The processing based on double differences of phase measurements offers an up-to-date surface velocity field of the URG and surrounding areas. The surface velocity field relative to the Eurasia reference frame (ITR2014) points out some areas where coherent movements are present for groups of stations. The heterogeneous velocity field across the Rhine Graben (Vosges, URG, Black Forest and Swabian Jura) displays velocity values below 0.2 mm yr−1. We compute a deformation field from the GNSS velocities to link the deformation to the spatial distribution of earthquakes in the Rhine Graben. The earthquakes are concentrated south of an east–west line passing through Strasbourg to the Alpine front. Instrumental and historical seismicity presents spatial similarities in the Vosges and Swabian Jura. Considering the consistent northward movement observed in the Alpine front, we suggest that the strong seismicity in the south of the URG is caused by the shortening of the Alps. North of the URG, the seismicity is more dispersed where the velocity field does not show coherent movements.

The Atlas Mountains of Tunisia belong to the seismically active zone of the Africa (Nubia) - Eurasia plate boundary in the central-western Mediterranean. We study the eastern section of the Maghrebian thrust belt using a subset (6 days) from each of 5 years (2014 to 2018) of permanent (survey-mode) GPS data and active tectonics. WNW to NNW-trending velocities express crustal motion and geodetic strain fields from the Sahara Platform to the Tell Atlas, consistent with African plate convergence. To the south, the velocities and trajectories indicate nearly WNW-ESE-trending right-lateral motion of the Sahara fault-related Atlas fold belt with respect to the Sahara Platform. Farther north and northeast, the significant decrease in velocities between the Eastern Platform (Sahel), Central Atlas and Tell Atlas and the clockwise rotation mark the NNW-trending shortening deformation associated with local ENE-WSW extension visible in the Quaternary grabens. The velocity field and strain distribution associated with the active E-W- to WNW-ESE-trending right-lateral faulting and NE-SW fault-related folds illustrate the transpression tectonics and support the identification of four tectonic domains north of the Africa-Nubia Platform in Tunisia. The transpression reduces northward when reaching the Central and Tell Atlas. These results change our perception of the Africa-Eurasia plate boundary previously located along the western Mediterranean coastline.

The Maghrebian tectonic domain in North Africa is here examined in the light of the recent GPS and seismotectonic results. The region includes the plate boundary in the western Mediterranean previously characterized by transpression and block rotation. The crustal deformation is documented along the Atlas Mountains in terms of the displacement field, with strain partitioning largely controlled by plate motions. The tectonic and seismotectonic analysis is based on our published data on shortening directions of Quaternary faulting and folding compared with present-day seismotectonic characteristics (earthquake moment tensors) of significant seismic events that allow an estimate of local and regional deformation rates in North Africa. Shortening directions oriented NE-SW to NW-SE for the Pliocene and Quaternary, respectively, and the S shape of the Quaternary anticline axes are in agreement with the 2°/Myr to 4°/Myr clockwise rotation obtained from paleomagnetic results on small tectonic blocks in the Tell Atlas. The continuous GPS data and results are obtained from the network in Morocco operative 1999 to 2006, the REGAT network in Algeria since 2007, and the network in Tunisia with data collected from 2014 to 2018. In addition, we add the most recent GPS results in southern Spain and southern Italy. The NW-SE to NNW-SSE 5 ±1.5 mm/yr convergence velocity and strain distribution of the Maghrebian tectonic domain is controlled by crustal block tectonics driven by E-W trending right-lateral faulting and NE-SW thrust-related folding. The correlation between the active transpression tectonic structures and velocity field shows a geodynamic framework consistent with the oblique plate convergence of Africa towards Eurasia.

The North Anatolian Fault experienced large earthquakes with 250 to 400 years recurrence time. In the Marmara Sea region the 1999 (Mw = 7.4) and the 1912 (Mw = 7.4) earthquake ruptures bound the Central Marmara Sea fault segment. Using historical-instrumental catalogue and paleoseismic results (≃ 2000-year database), the mapped fault segments, fault kinematic and GPS data, we compute the paleoseismic-seismic moment rate and geodetic moment rate. The geodetic moment rate is obtained by projecting the measured surface displacements to estimate the strain rate, and evaluating the associated elastic stress rate over a regular spatial grid. The paleoseismic-seismic moment rate is obtained by summing the moment tensors over regions of the spatial grid and periods of time. A clear discrepancy appears between the moment rates and implies a significant delay in the seismic slip along the fault. The rich database allows us to identify the size of the seismic gap and related fault segment and estimate the moment rate deficit. Our modeling suggest that the locked Central Marmara Sea fault segment even including a creeping section bears a moment rate deficit = 6.4*1017 N.m./yr. that corresponds to Mw ≃ 7.4 for a future earthquake with an average ≃ 3.25 m coseismic slip. Taking into account the uncertainty in the strain accumulation along the 130-km-long Central fault segment, our estimate of the seismic slip deficit being ≃ 10 mm/yr implies the size of the future earthquake ranges between Mw = 7.4 and 7.5.

The plate boundary in the western Mediterranean includes the Tunisian Atlas Mountains. We study the active deformation of this area using GPS data collected from 2014 to 2018. WNW to NNW trending velocities express the crustal motion and geodetic strain field from the Sahara platform to the Tell Atlas, consistent with the African plate convergence. To the south, the velocities indicate a nearly WNW-ESE trending right-lateral motion of the Sahara fault-related fold belt with respect to the Sahara Platform. Further north and northeast, the significant decrease in velocities between the Eastern Platform and Central – Tell Atlas marks the NNW trending shortening deformation associated with local ENE – WSW extension visible in the Quaternary grabens. The velocity field and strain distribution associated with the active E-W trending right-lateral faulting and NE-SW fault-related folds sustain the existence of three main tectonic blocks and related transpression tectonics. The velocity field and pattern of active deformation in Tunisia document the oblique plate convergence of Africa towards Eurasia.

The April 3, 2017 MW 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the Nubian plate and in a seismogenic region previously considered as stable. Our objective is to combine several approaches (field and remote sensing investigations) in order to adopt a multidisciplinary strategy so as to enhance our understanding of earthquake occurrence in intraplate southern Africa. We analyse the mainshock and aftershocks sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 900 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (MW 4.6 on the April 5, 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of the focal mechanisms produced results that are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes (a pair of ascending images 2017-03-30 and 2017-04-11) with two lobes with 3.86 cm–5.15 cm coseismic slip on a NW-SE elongated and 40-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~50 cm maximum slip on a fault plane striking 315°, dipping 45°, −80° rake and with Mo 3.68 × 1018 Nm. Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo Mobile Belt classifies the intraplate region as an active plate interior.

We study the sedimentary record of past tsunamis along the coastal area west of Alexandria (NW Egypt) taking into account the occurrence of major historical earthquakes in the eastern Mediterranean. The two selected sites at Kefr Saber (∼32 km west of Marsa-Matrouh city) and ∼ 10 km northwest of El Alamein village are coastal lagoons protected by 2–20 m-high dunes parallel to the shoreline. Field data were collected by (1) coastal geomorphology along estuaries, wedge-protected and dune-protected lagoons; and (2) identification and spatial distribution of paleotsunamis deposits using five trenches (1.5 m-depth) at Kefr Saber and twelve cores (1 to 2.5 m-depth) at El Alamein. Detailed logging of sedimentary sections was conducted using X-rays, grain size and sorting, total organic and inorganic matter, bulk mineralogy, magnetic susceptibility, and radiocarbon dating to identify past tsunamis records. Generally of low energy, the stratigraphic succession made of coastal lagoon and alluvial deposits includes intercalated high-energy deposits made of mixed fine and coarse sand with broken shells, interpreted as catastrophic layers correlated with tsunami deposits. Radiocarbon dating of 46 samples consist in mixed old (>13 000 BP) and young (<5500 BP), dated charcoal and shells in sedimentary units correlate with the 24 June AD 1870 (Mw 7.5), 8 August AD 1303 (Mw ∼ 8) and 21 July AD 365 (Mw 8–8.5) large tsunamigenic earthquakes that caused inundation along the Alexandria and northern Egyptian shoreline. Our results point out the size and recurrence of past tsunamis and the potential for future tsunami hazards on the Egyptian coastline and the eastern Mediterranean regions.

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