Africa-Eurasia Plate Boundary Research Articles

Paleostress evolution of the Outer Kwanza basin (offshore Angola); comparison with the Congo basin and implications for the tectonic history of the Central segment of the West Africa passive margin

This study aims at reconstructing the paleostress history of the Outer (offshore) Kwanza basin (West African passive margin) and at comparing it to stress results acquired further north in the Congo basin. Three oriented borehole cores provided by TotalEnergies and reaching the syn-rift, Barremian-Aptian pre-salt carbonates offshore Angola were investigated. Paleopiezometry based on the Stylolite Roughness Inversion Technique (SRIT) and Calcite Twin Inversion Technique (CSIT) was combined with fracture analysis, U–Pb geochronology of carbonates and burial modelling to unravel the orientations and magnitudes of horizontal and vertical stresses affecting the pre-salt carbonates over time. Calcite twins were measured from a primary sparite matrix, and the inversion process unravelled a polyphase stress history, comprising ∼ E-W and NE-SW extensional trends that we associate to the rifting (130-112 Ma) that led to the opening of the South Atlantic ocean. The ∼ E-W extension is consistent with the early occurrence of N-S striking normal faults which developed in relation to the reactivation of inherited basement structures. This ∼ E-W extension evolved during the Barremian-Aptian (?) into the dominant regional NE-SW extension marked by large-scale NW-SE striking normal faults. The stress history also comprises compressional and strike-slip stress regimes associated with a ∼N-S trending σ1 which can be related to the transfer of orogenic stresses from the distant Africa-Eurasia plate boundary at ∼67-60 Ma. Finally, compressional and strike-slip stress regimes associated with a ENE-WSW to ∼E-W trending σ1 dominated since at least ∼17-15 Ma (possibly ∼34 Ma); they are interpreted as the expression of the mid-Atlantic ridge push. These (paleo)stress results are compared and combined with earlier paleostress reconstructions in the northern offshore Lower Congo basin (also belonging to the Central segment of the margin) and in the onshore Congo basin in order to refine the stress record and the timing of tectonic events since the early Cretaceous, thus providing unprecedented constraints on the tectonic history of the West Africa passive margin. This tectonic history includes both extensional and compressional events, and was driven mainly by far-field stresses, either gravitational or tectonic in origin, which are related to interactions between the African plate and surrounding plates.

Monchique alkaline magmatic intrusion (SW Iberia): Geophysical modeling and relationship with active seismicity and hydrothermalism

Monchique is a prominent 902 m topographic high in SW Iberia, which stands out in the general flat landscape of southern Portugal. It lies to the north of the Africa-Eurasia plate boundary, which locally accommodates a slow oblique convergence (∼5 mm/yr). Monchique comprises alkaline magmatic rocks of Late Cretaceous age, intruded in a post-rift context. It hosts the most active seismic cluster in mainland Portugal and important hydrothermal activity.This work investigates the relationship between the alkaline intrusion, local seismicity and hydrothermalism.We present magnetic and gravity modeling based on new drone-borne magnetic data and ground gravity data. New magnetic mapping of Monchique shows a ∼ 15 km long dipolar anomaly with 7–8 km wavelength and 2000 nT amplitude. 3D magnetic inversion models the main Monchique intrusion as a high-susceptibility body, 15 km long and 6 km wide, located below the Monchique mountain and extending 5–7 km depth. 2D forward modeling and geological interpretation further support the existence of ENE-WSW oriented dike-like gabbroic bodies that may extend deeper, around which syenite units have later emplaced.We relocate the seismicity using NonLinLoc and a 3D regional tomographic model, and find that earthquakes align along four main lineations that radiate outwards from the intrusion. We also find that most earthquakes cluster between 8 and 18 km depth, below the magmatic intrusion. The b-value at the core of the seismic cluster is higher than at the surrounding region, possibly related to the local hydrothermalism. We present five new focal mechanisms that are compatible with the regional stress field, supporting a regional tectonic control.The emplacement of the Monchique alkaline intrusion left fractures in the lithosphere that currently act as preferred pathways for fluids. In the context of the present-day stress field, the enhanced fracturing and fluid circulation facilitate the localization of small-magnitude earthquakes.

Catalogue of source mechanisms and overview of present-day stress fields in the western region of the Africa–Eurasia plate boundary

Catalogue of source mechanisms and overview of present-day stress fields in the western region of the Africa–Eurasia plate boundary

Offshore seismicity clusters in the West Iberian Margin illustrated by 2 decades of events

Abstract. An analysis of 2 decades (2003–2022) of seismicity recorded by the Spanish and Portuguese seismic networks along the West Iberian passive margin has resulted in a better understanding of the distribution of moderate seismic activity in this intraplate submarine area. The study provides a precise trend of specific alignments inferred from the density maps of seismicity, giving an accurate depiction of event distribution along two wide stripes that extend for 700 km through the ocean floor in the WNW–ESE direction. These bands are parallel to the Africa–Eurasia plate boundary but are distinctly separated from its related seismicity by approximately 300 and 700 km, respectively. This is a sufficient distance to be considered intraplate activity. When trying to relate this seismicity to structural and geophysical features, a conclusive picture does not emerge. The earthquakes occur indiscriminately across thinned continental, hyperextended, and exhumed mantle rift domains. They fade out in proximity to undisputed oceanic crust, but some events extend beyond. The hypocentral depths signal a considerable number of events nucleating in the upper mantle. The focal mechanisms, although scarce, are predominantly strike-slip. Considering these observations, hypotheses ranging from subduction initiation and development of strained corridors to local structures of the margin are discussed in order to explain this relatively anomalous seismicity. However, some of them lack convincing arguments, while others are too vague. None of them are flawless, suggesting that several factors may be at play. Despite being one of the most probed passive margins in the world, the present geodynamic status of the West Iberian Margin manifested in its modern seismicity remains unknown. Interpreting these data within a global tectonic plate framework, together with the potential addition of seafloor seismometers, may provide the key to understanding this activity along one of the most archetypical margins of the Atlantic Ocean.

Paleostress evolution of the West Africa passive margin: New insights from calcite twinning paleopiezometry in the deeply buried syn-rift TOCA formation (Lower Congo basin)

Accurate characterization of hydrocarbon reservoirs requires a good understanding of the paleostress history of their host sedimentary basin. This study aims at reconstructing the paleoburial and paleostress history of the syn-rift Barremian TOCA Fm. in the Lower Congo basin (West African margin). Two oriented borehole cores drilled offshore Congo were investigated using stress inversion techniques applied to calcite mechanical twins to constrain paleostress orientations and magnitudes. The inversion of calcite twins was performed on a widespread early diagenetic cement (~129 to ~113 Ma) and revealed that the TOCA Fm. has undergone a complex polyphase stress history, including (1) extensional stress regimes with vertical σ1 and horizontal σ3 trending NE-SW and WNW-ESE (N100) associated with the rifting episode preceding the opening of the South Atlantic (130 to 112 Ma), and with mean differential stresses of ~47 MPa for (σ1-σ3) and ~ 20 MPa for (σ2-σ3). The early N100 extension is associated with the development of normal faults striking N-S that likely reactivated inherited basement structures. The direction of extension evolved during the Barremian into the main NE-SW extension marked by the dominant normal faults striking NW-SE. (2) Compressional and strike-slip stress regimes with horizontal σ1 oriented ~N-S possibly related to the far-field intraplate transmission of orogenic stresses generated at the distant Africa-Eurasia plate boundary at ~67–60 Ma, and with mean differential stresses of 40 MPa for (σ1-σ3) and 28 MPa for (σ2-σ3). (3) Compressional and strike-slip stress regimes with horizontal σ1 oriented ENE-WSW to ~E-W that we tentatively relate to the mid-Atlantic ridge push and prevailing since ~15 Ma onwards, associated with mean differential stresses of ~45 MPa for (σ1-σ3) and ~ 18 MPa for (σ2-σ3). Our results reveal that the TOCA Fm. recorded mainly far-field stress effects that could be related to interactions between the African plate and surrounding plates, and that divergent passive margins may experience a complex tectonic history including both extensional and compressional events.

Recognizing megatsunamis in Mediterranean deep sea sediments based on the massive deposits of the 365 CE Crete\xa0event

The Mediterranean Sea hosts two subduction systems along the convergent Africa-Eurasia plate boundary that have produced strong ground shaking and generated tsunamis. Based on historical descriptions and sedimentary records, one of these events, in 365 CE, impacted a broad geographical area, including tsunami evidence for distances of 700–800 km from the source event, qualifying it as a ‘megatsunami’. Understanding how megatsunamis are produced, and where they are more likely, requires a better understanding of the different secondary processes linked to these events such as massive slope failures, multiple turbidity current generation, and basin seiching. Our sedimentary records from an extensive collection of cores located in distal and disconnected basins, identify turbidites which are analyzed using granulometry, elemental (XRF), micropaleontological, and geochemical data in order to reconstruct their coastal or marine source. The results show that the 365 CE basin floor sediments are a mixture of inner shelf and slope materials. The tsunami wave produced multiple far-field slope failures that resulted in stacked basal turbidites. It also caused transport of continent-derived organic carbon and deposition over basal turbidites and into isolated basins of the deep ocean. The composition of sediment in isolated basins suggests their deposition by large-scale sheet like flows similar to what has been caused by the Tohoku earthquake associated tsunamis. This is significant for rectifying and resolving where risk is greatest and how cross-basin tsunamis are generated. Based on these results, estimates of the underlying deposits from the same locations were interpreted as possible older megatsunamis.

Archaeoseismological study of the Cherichira aqueduct bridge, Kairouan, Tunisia

In the past, several destructive earthquakes have occurred in the North African Atlas Mountain ranges located along the Africa–Eurasia plate boundary. Although the region is rich with impressive archaeological sites, including those in modern Tunisia, few comprehensive archaeoseismological studies have been conducted. Historic sources account at least three damaging earthquakes in the Kairouan area in central Tunisia between AD 859 and 1041. Little is known about which faults triggered these earthquakes or the size of these events. The water supply of the city of Kairouan depended on a 32-km-long aqueduct with a large bridge (now partially collapsed) at the confluence of the de Mouta and Cherichira rivers. The original bridge of Roman construction was retrofitted twice during the Aghlabid period (AD 800–903) and probably in AD 995 during the Fatimid period. The ruined section of the bridge shows damage which might be related to the AD 859 earthquake shaking. Here, we present a detailed study of the history, the status and the damage of the Cherichira aqueduct bridge using previous historic accounts and written works, a 3D laser scan model, local geological and seismological characteristics, and include results of radiocarbon dating and a timeline of events. In addition to earthquake ground motions, we consider severe flash floods on the bridge as a potential cause of the damage. We estimate the severity of such flash floods and develop a model with 18 earthquake scenarios on local reverse and strike-slip faults with magnitudes between MW 6.1 and 7.2. While a few damage patterns might be indicative of flooding, most damage can be attributed to earthquakes. It is highly probable that the earthquake in AD 859 caused enough damage to the Aghlabid bridge to render it dysfunctional; however, to resolve the question of whether another earthquake in AD 911 or 1041 caused the complete destruction of the previously retrofitted aqueduct by the Fatimids requires dating of additional sections of the bridge.

Active tectonics and GPS data analysis of the Maghrebian thrust belt and Africa-Eurasia plate convergence in Tunisia

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.

Eurasia–Africa Plate Boundary Affected by a South Atlantic Asthenospheric Channel in the Gulf of Cadiz Region?

The Gulf of Cadiz has been affected by a long and complex geodynamic evolution. The lithospheric structure is poorly understood in this region, and it also shows a diffuse seismicity, spanning over a broad area. The Canary Archipelago has been extensively studied. Nevertheless, there are fundamental topics that are still under debate. No studies have addressed or suggested the possibility of a plausible geodynamic connection between both remarkable locations. In this study we integrate total tectonic subsidence (TTS), Curie point depth (CPD), Bouguer gravity anomaly, and seismic information. TTS shows the existence of a basement bulge in the area of the Canary Archipelago that extends to the north, and in the area of Madeira Island, which extends in a more subtle way to the north, too. Likewise, the CPD reaches the shallowest values in the same location at the Canary Archipelago. These two aspects suggest a cause-effect relationship between TTS and CPD at this specific area. Gravity data and CPD show a linear feature, which links the NW of the Canary Archipelago and the Gulf of Cadiz. The data we manage in this work show remarkable clues as: (a) the absence of a similar signal in the TTS, (b) the fact that CPD it is rather constant along this track, (c) CPD amplitude also almost doubles the values obtained on Canary Archipelago, and (d) the existence of a clear correlation between seismogenic depths and CPD, which points to the existence of a correlation between seismicity and the thermal architecture of the lithosphere. All of these evidences support the presence of a lithospheric thinning between Canary Islands and Gulf of Cadiz area, and in turn, the presence of an asthenospheric channel which feeds and alter locally the Eurasia–Africa Plate Boundary.

Reanalysis of the 1761 transatlantic tsunami

Abstract. The segment of the Africa–Eurasia plate boundary between the Gloria Fault and the Strait of Gibraltar has been the setting of significant tsunamigenic earthquakes. However, their precise location and rupture mechanism remain poorly understood. The investigation of each event contributes to a better understanding of the structure of this diffuse plate boundary and ultimately leads to a better evaluation of the seismic and tsunami hazard. The 31 March 1761 event is one of the few known transatlantic tsunamis. Macroseismic data and tsunami travel times were used in previous studies to assess its source area. However, no one discussed the geological source of this event. In this study, we present a reappraisal of tsunami data to show that the observations data set is compatible with a geological source close to Coral Patch and Ampere seamounts. We constrain the rupture mechanism with plate kinematics and the tectonic setting of the area. This study favours the hypothesis that the 1761 event occurred in the southwest of the likely location of the 1 November 1755 earthquake in a slow deforming compressive regime driven by the dextral transpressive collision between Africa and Eurasia.

Field evidence of Quaternary seismites in the Mostaganem-Relizane (western Algeria) region: seismotectonic implication

Northwestern Algeria, Tell Atlas chain, belongs to the converging Africa-Eurasia plate boundary. Several active faults have been previously identified and several earthquakes occurred in the past. In the present study, seismites are observed in the Quaternary deposits. The identified seismites include injection sand dykes, pillar structures, pillow structures, load-cast structures, water escape structures, sismoslumps, thixotropic wedges, and thixotropic bowls. The following arguments support their seismic origin: (i) presence of active faults able of producing strong earthquakes, (ii) the granulometric characteristics of the deposits are favorable to liquefaction, (iii) the observed features, mainly those related to water escape structures, are comparable to those observed in modern earthquakes. Therefore, such features are evidence of the occurrence of earthquakes of M > 5.5 magnitude in this study area, which may occur in the future.

The active faults of the Mitidja basin (North Central Algeria): what does the seismic history of the region tell us? A review

The Africa–Eurasia plate boundary runs along Northern Algeria, which is thus an active seismic zone. However, surface ruptures from large earthquakes are rarely observed, which makes it difficult to define the characteristic parameters of any active fault. In this paper, we combine the results obtained in recent investigations of historical and instrumental seismicity with field observations and available geological maps and measurements of neotectonic ruptures to help comprehend the complexity of the active deformation of the Mitidja basin (MB). Our analyses reveal the tectonic characteristics of the Mitidja basin and indicate that the seismic activity is essentially concentrated along the system of the boundary faults. The general structural shape of the seismogenic Mitidja basin suggests that the southern fault system (Blida faults) would mimic the northern fault system (Sahel fault), and we observe a highest seismicity rate at the junction point of the NW–SE fault and the south thrust fault system.

Crustal structure of the Eurasia–Africa plate boundary across the Gloria Fault, North Atlantic Ocean

Crustal structure of the Eurasia–Africa plate boundary across the Gloria Fault, North Atlantic Ocean

A Seismic Risk Simulator for Iberia

Abstract Iberia is positioned close to the Eurasia–Africa plate boundary. The return period between events is long, due to the moderate plate convergence rate. In the past, many Iberian cities have been severely damaged by earthquakes. A large record of historic events such as the 1344, 1531, 1722, 1755, and 1859 earthquakes were mentioned in coeval writings. A new seismic risk simulator, SImulador de Risco sismiCO (SIRCO), is proposed to estimate the seismic risk in Portugal. This methodology could also be transversally applied to the Iberian Peninsula. Two historic events, that of 1722 in the Algarve ( M w ∼7) and that of 1531 in Lisbon ( M w ∼6), have been modeled using SIRCO. These results have been compared with the results obtained with other seismic risk estimator software (ERSTA) and with the isoseismal maps obtained from historical data. A direct comparison between models suggests that SIRCO performs efficiently in estimating seismic risk for a large part of the Iberian Peninsula. The calculation time is acceptable as, currently, the whole process takes approximately 18–20 min. It could therefore be used as an almost real‐time tool. This makes SIRCO a valuable tool for Portuguese authorities for planning, prevention, and response.

Evolution of the gulf of Cadiz margin and southwest Portugal contourite depositional system: Tectonic, sedimentary and paleoceanographic implications from IODP expedition 339

The contourite depositional system (CDS) along the southwestern Iberian Margin (SIM), within the Gulf of Cadiz and offshore areas of western Portugal bear the unmistakable signal of Mediterranean Outflow Water (MOW) exiting the Strait of Gibraltar. This locality records key information concerning the effects of tectonic activity on margin sedimentation, the effects of MOW dynamics on Atlantic circulation, and how these factors may have influenced global climate. Over the last four decades, numerous studies have been conducted on the late Miocene, Pliocene and Quaternary sedimentary stacking pattern of Neogene basins along the SIM for both academic and resources exploration purposes. However, understanding of the region rests primarily on basic seismic stratigraphy calibrated with limited data from only a few exploration wells. The Integrated Ocean Drilling Program (IODP) Expedition 339 recently drilled five sites in the Gulf of Cadiz and two sites on the western Iberian margin. The integration of core and borehole data with other geophysical databases leads us to propose a new stratigraphic framework. Interpretation of IODP Exp. 339 data along with that from industry sources and onshore outcrop analysis helps refine our understanding of the SIM's sedimentary evolution.We identify significant changes in sedimentation style and dominant sedimentary processes, coupled with widespread depositional hiatuses along the SIM within the Cadiz, Sanlucar, Doñana, Algarve and Alentejo basins. Following the 4.5Ma cessation of a previous phase of tectonic activity related to the Miocene–Pliocene boundary, tectonics continued to influence margin development, downslope sediment transport and CDS evolution. Sedimentary features indicate tectonic pulses of about 0.8–0.9Ma duration with a pronounced overprint of ~2–2.5Ma cycles. These more protracted cycles relate to the westward rollback of subducted lithosphere at the convergent Africa-Eurasia plate boundary as its previous NW–SE compressional regime shifted to a WNW–ESE direction. Two major compressional events affecting to the Neogene basins at 3.2–3Ma and 2–2.3Ma help constrain the three main stages of CDS evolution. The stages include: 1) the initial-drift stage (5.33–3.2Ma) with a weak MOW, 2) a transitional-drift stage (3.2–2Ma) and 3) a growth-drift stage (2Ma-present time) with enhanced MOW circulation into the Atlantic and associated contourite development due to greater bottom-current velocity. Two minor Pleistocene discontinuities at 0.7–0.9Ma and 0.3–0.6Ma record the effects of renewed tectonic activity on basin evolution, appearing most prominently in the Doñana basin. Several discontinuities bounding major and minor units appear on seismic profiles. Quaternary records offer the clearest example of this, with major units of about 0.8–0.9Ma and sub-units of 0.4–0.5Ma. Sedimentation is controlled by a combination of tectonics, sediment supply, sea-level and climate. This research identifies time scales of tectonic controls on deep-marine sedimentation, specifically over periods of 2.5–>0.4Ma. Shorter-term climatic (orbital) mechanisms control sedimentation at time scales of ≤0.4Ma. The role of bottom water circulation and associated processes in shaping the seafloor and controlling the sedimentary stacking pattern on continental margins has to be seriously reconsidered in future multidisciplinary studies. This is not only because of the common occurrence of sandy contourite deposits in deep water setting and their economic interest for hydrocarbon exploration, but principally because they archive the heartbeat of the interior Earth and therefore have important sedimentary and paleoceanographic implications.

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment, especially where recurrence times overstep historical records. We show that most turbidites emplaced at the Africa-Eurasia plate boundary off west Algeria over the past ∼8 k.y. correlate across sites fed by independent sedimentary sources, requiring a regional trigger. Correlation with paleoseismic data inland and ground motion predictions support that M ∼7 earthquakes have triggered the turbidites. The bimodal distribution of paleo-events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults: 13 paleo-earthquakes underpin clusters of 3–6 events with recurrence intervals of ∼300–600 yr, separated by periods of quiescence of ∼1.6 k.y. without major events on other faults over the study area. This implies broad phases of strain loading alternating with phases of strain release. Our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models

Preface to the special issue “Seismotectonics and Seismic hazards in North Africa”

North Africa region from Egypt to Morocco experienced several damaging earthquakes in the past, and the largest recorded seismic event reached Mw 7.3 in 1980 at El Asnam (now Chlef) in the Tell Atlas of Algeria. This reality is linked with the regional situation along the Africa–Eurasia plate boundary, or in proximity of the Hellenic Arc plate boundary, or near the midRed Sea spreading floor. This special issue in seismotectonics and relationships with the seismic hazards includes different contributions in seismotectonics, seismology, active faulting, and seismic hazard and risk assessment in North Africa. Specifically, nine articles address the earthquake activity and its effect at the crustal level, Earth surface, and building design. The issue begins with the article of Bezzeghoud et al. that describe the earthquake distribution along the Africa–Eurasia plate boundary and gives the characteristics and seismic strain rate of the tectonically active regions from the Gloria transform fault to NW Africa. Using the tomographic investigations, Timoulali and coauthors explore the crustal structure of the Rif Mountains (Morocco), Alboran Sea, and surrounding areas. A fairly exhaustive account of the earthquake distribution and related active tectonic structures by Bahrouni and coauthors reveals the potential for large earthquakes in Tunisia. Badawy and coauthors describe the seismic activity of east North Africa using relocation procedure and source mechanisms of main continental earthquakes in Egypt. The volume switches into the study of historical earthquakes in Algeria by Ferdi and Harbi using epigraphic documents. Benhamouche and coauthors investigate the surface effects and liquefaction features of past earthquakes in the coastal region of Jijel (east Algeria) that experienced the damaging 1856 earthquakes with tsunami (Io IX, EMS). The evaluation of earthquake hazards and risks represents a natural continuation of previous studies on the seismotectonics of North Africa. The comprehensive analysis of the seismic hazards using an updated deterministic approach provides new results and emphasizes the need for the integration of geological and seismological data along the whole North Africa (Mourabit et al.). Gherboudj et al. present a probabilistic seismic hazard assessment as applied to the Algiers city. Finally, the vulnerability of building design to destructive seismic events is explored through conceptual frameworks and application in Algeria by Mebarki et al. J Seismol (2014) 18:203–204 DOI 10.1007/s10950-014-9424-5

Occurrence and impact of characteristic earthquakes in northern Algeria

Earthquakes constitute the natural hazard that is one of the main natural threats to the northern part of Algeria because of its geographical setting at the Eurasia–Africa plate boundary. Several active multi-segment reverse faults have been identified near urban areas that may rupture during characteristic earthquakes and produce earthquakes of magnitudes ≥7.0. Characteristic earthquakes are extreme seismic events characterized by long return periods, which can have great societal impact. Earthquakes in northern Algeria are destructive for two main reasons: firstly, the shallow character of the faults and secondly, the vulnerability of the building stock built essentially prior to the implementation of seismic design codes that take into account the level of the seismic hazard. That is why even moderate earthquakes are disastrous in this area.

Active faulting and transpression tectonics along the plate boundary in North Africa

We present a synthesis of the active tectonics of the northern Atlas Mountains, and suggest a kinematic model of transpression and block rotation that illustrates the mechanics of this section of the Africa–Eurasia plate boundary. Neotectonic structures and significant shallow seismicity (with Mw >5.0) indicate that coeval E-W-trending, right-lateral faulting and NE-SW, thrust-related folding result from oblique convergence at the plate boundary, which forms a transpressional system. The strain distribution obtained from fault–fold structures and P axes of focal mechanism solutions, and the geodetic (NUVEL-1 and GPS) convergence show that the shortening and convergence directions are not coaxial. The transpressional strain is partitioned along the strike and the quantitative description of the displacement field yields a compression-to-transcurrence ratio varying from 33% near Gibraltar, to 50% along the Tunisian Atlas. Shortening directions oriented NNE and NNW for the Pliocene and Quaternary, respectively, and the S shape of the Quaternary anticline axes, are in agreement with the 2.24˚/Myr to 3.9˚/Myr modeled clockwise rotation of the small tectonic blocks and with the paleomagnetic data. The convergence between Africa and Eurasia is absorbed along the Atlas Mountains at the upper crustal level, by means of thrusting above decollement systems, which are controlled by subdued transcurrent faults. The Tell Atlas of northwest Algeria, which has experienced numerous large earthquakes with respect to the other regions, is interpreted as a restraining bend that localizes the strain distribution along the plate boundary.

Turbidite paleoseismology in the Calabrian Arc Subduction Complex (Ionian Sea)

The Calabrian Arc subduction system is part of the Africa–Eurasia plate boundary, is one of the most seismically active regions in the Mediterranean Sea, and has been struck repeatedly by destructive historical earthquakes. In this study, we investigate the effects of historical earthquakes on abyssal marine sedimentation through the analysis of the turbidite record. We collected gravity cores in tectonically controlled basins where the eastern Mediterranean pelagic sequence is interbedded with resedimented units. Textural, micropaleontological, geochemical, and mineralogical signatures reveal three turbidite events in the last millennium. We dated the turbidite sequences from two different cores using different radiometric methods, whereas the average time interval between successive turbidite beds was estimated from pelagic sediment thickness and sedimentation rates; chronologies were refined through age modeling that provided age ranges (2σ) of each turbidite bed. The results suggest that turbidite emplacement was triggered by three historical earthquakes recorded in the area (i.e., the 1908, 1693, and 1169 events); their magnitude, epicentral location, and associated tsunamis support causative faults located in the Ionian Sea. The source for all the turbidites, as inferred from their mineralogy, is the metamorphic basement outcropping in southern Calabria and/or northeastern Sicily. Turbidite composition and cable breaks for the 1908 event have been used to infer likely traveling paths and seismogenic faults in the subduction system. Our findings suggest that Ionian Sea turbidites represent more than 80% of sedimentation and may be seabed archives of paleo‐earthquakes capable of reconstructing seismicity back in time, during several earthquake cycles.