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The 1755 "Meknes" earthquake (Morocco): Field data and geodynamic implications
Abstract
The source of the famous 1 November, 1755 “Lisbon” earthquake has been constrained to be an up to 200 km long structure in the offshore west of Cabo de S. Vincente. The magnitude of this earthquake was estimated in the range of 8.5–9.4. The stress regime argued for this shock would have been characterised by an around NNW–SSE-oriented compression. Less well studied is the successive “Meknes” earthquake, which occurred a few days later in Morocco (27 November), and was erroneously confused by the European contemporary reports with a strong aftershock, occurred on 18 November, of the main seismic sequence of the 1 November earthquake. The Meknes earthquake had destructive effects in the region of Meknes and Fes and along the E–W trending Rides Prérifaines, the main frontal thrust of the Rif. Historical data indicate a macroseismic field closed around the towns of Meknes and Fes.
... Evidences for this crustal deformation are manifested in the southern front of the Rif Range since Quaternary times, due to the reactivation of the Pre-Rif thrust ridges where most of the deformation is manifested (Meghraoui et al. 1996;Bernini et al. 2000;Moratti et al. 2003;Bargach et al. 2004;Chalouan et al. 2006). The latter processes continue in the present with a slip rate of around 0.7 to 4 mm/yr (Chalouan et al. 2014;Poujol et al. 2017;Agharroud et al. 2021) and an uplift rate of around 0.6-2 mm/yr. ...
... However, the rate of deformation can be higher as shown by Chalouan et al (2014) who reported an annual rate of 4-5 mm/yr in the Trhat ridge (Chalouan et al. 2014). Consequently, recent evidence have surfaced regarding the tectonic reactivation the Pre-Rif (Meghraoui et al. 1996;Bernini et al. 2000;Moratti et al. 2003;Poujol et al. 2017), with the lest being the in an important seismic event that struck the region in 1755 (Moratti et al. 2003;Poujol et al. 2017). ...
... However, the rate of deformation can be higher as shown by Chalouan et al (2014) who reported an annual rate of 4-5 mm/yr in the Trhat ridge (Chalouan et al. 2014). Consequently, recent evidence have surfaced regarding the tectonic reactivation the Pre-Rif (Meghraoui et al. 1996;Bernini et al. 2000;Moratti et al. 2003;Poujol et al. 2017), with the lest being the in an important seismic event that struck the region in 1755 (Moratti et al. 2003;Poujol et al. 2017). ...
The Pre-Rif Ridges constitute the southward deformation front of the entire Rif chain, Northern Morocco. Previous investigations showed that evidence in favor of active, recent and neotectonic deformation can be found all over this area. However, the effect of such tectonic activity on surface processes such as gullying and land-sliding was never properly investigated. Therefore, we conducted a statistical analysis using different statistical approaches (comparison tests, frequency-area distributions, frequency ratio) to compare three zones of the study area, with different tectonic attributes (zone A: inactive tectonic processes, zone B: neotectonic deformation and zone C: active tectonic deformation). The first zone (A), which presents little to no evidence of intense deformation, is shown to be very different to the other two areas both in terms of landslides and gully morphometries. It has also gentler slopes which promote less landslides in marly terrains as shown by the factor of safety analyses conducted in this study. The other two segments of the study area are more or less similar with relatively steeper topographies and a higher density of mass movements. Given that both zones are relatively intensely deformed compared to zone 1, one can attribute the abundance of surface processes in these two areas to the tectonic uplift and topographic growth, which play a major role in promoting such processes.
... While geomorphic studies have long demonstrated the tectonic activity of the Rif and the Pre-Rif Ridges (Morel, 1988(Morel, , 1989, it is only recently that dating techniques such as optically stimulated luminescence or in situ-produced cosmogenic nuclides have enabled determination of time constraints on fault activity in the Rif, such as along the Nekor Fault (Poujol et al., 2014) or the Pre-Rif Ridges (Poujol et al., 2017). The latter have also been identified as a probable source for several historical earthquakes in the Fès-Meknès region (Chalouan et al., 2014;El Mrabet, 2005;Moratti et al., 2003;Poujol et al., 2017) but a reappraisal of the morpho-structures associated with the Pre-Rif front, and their relationships with inherited basement structures, is still needed in order to better characterize their segmentation, kinematics, and seismogenic potential. ...
... Along the southern border of the Rif Mountains, the region of Meknès and Fès has experienced almost 10 earthquakes since the eleventh century (Blanc, 2009;Cherkaoui et al., 2017;El Mrabet, 2005;Mourabit et al., 2014;Peláez et al., 2007;Roux, 1934), with three moderate to large events with intensities ranging between VII and VIII in 1045, 1624, and 1755 ( Figure 1). Overshadowed by the large Lisbon Earthquake in the historical archives (e.g., Blanc, 2009), the precise date of the Fès-Meknès Earthquake is unclear, with Spanish and Portuguese sources referring to a shock on November 18, which could be interpreted as an aftershock of the Lisbon Earthquake (Pereira de Sousa, 1919;Roux, 1934;Vogt, 1984), whereas Arab sources document it on November 27, with an epicentral intensity of VIII that was restricted to the Saïss Region (Moratti et al., 2003;Poujol et al., 2017). ...
... The boundary between the Pre-Rif Ridges and the Saïss Basin has long been investigated through geological (Choubert & Faure-Muret, 1962;Faugères, 1978;Morel, 1988Morel, , 1989, structural (Bargach et al., 2004;Frizon de Lamotte et al., 2004;Moratti et al., 2003;Sani et al., 2007), and geodetic (Chalouan et al., 2014;Poujol et al., 2017) approaches. The Pre-Rif Ridges correspond to elongated hills of Mesozoic sedimentary rocks that belong to the Meseta-Atlas cover of the foreland involved in the Late Miocene to Middle Pliocene thrusting of the external Rif (Faugères, 1978;Zizi, 1996). ...
Located at the southern boundary of the Alpine chain in Morocco, the deformation front of the Southern Rif Mountains is a region of moderate tectonic activity, which makes it a good natural laboratory to understand whether, and how, low compressional strains are located on specific structures. Along the ≈80 km-long left-lateral, transpressive and reverse fault zone that runs at the toe of the Pre-Rif Ridges, an analysis of high-resolution digital topography provides new geomorphic lines of evidence supporting Quaternary activity along, 20 km-long fault segments. The fault zone can be divided into the Meknès and the Fès segments, which are constrained at depth by reactivated, NE-trending basement faults, delimitating paleo-grabens associated with the Late Triassic-Jurassic opening of the Atlantic Ocean. For selected sites, we used in situ-produced 36Cl, 10Be and 26Al and high-resolution topography to infer the timing of abandonment of fluvial markers, which suggest incision rates on the order of 0.6-2 mm/yr. Given their lengths, scaling laws suggest that the identified fault segments should root at about 7-12 km-depth, possibly reactivating former basement normal faults and making them potential seismogenic sources capable of generating Mw6+ earthquakes, with return times of the order of several hundreds of years. Our new morphochronological dataset confirms that the Southern Rif deformation front is a key structure that may have accommodated most of the lateral extrusion of the Rif between the Nubia and Iberia tectonic plates.
... We analysed tectonics of the frontal faults from the southern boundary of the Jebel Zerhoune (20 km north of Meknes). This fault considered by Moratti et al. (2003) as a surface rupture originating due to the earthquake of November 18, 1755, two weeks after the Lisbon earthquake. The structural analysis done on the mirror of this fault shows the calcareous crusts and Quaternary formations bear no trace of friction. ...
... The observations were made in two sectors at the western end and in the middle part of this ridge for several reasons. First, because of the outcrop conditions allowing observation, and also because the recent work have focused on the region deserve to be clarified (Moratti et al., 2003;Sani et al., 2007;Chalouan et al., 2007). Indeed, in the west end of the ridge of Zerhoune (Fig. 2 site 1), the E-W trending fault that can be observed along the road Meknes-Moulay Idriss Zerhoune, which crops out as a trench in the same direction. ...
... Indeed, in the west end of the ridge of Zerhoune (Fig. 2 site 1), the E-W trending fault that can be observed along the road Meknes-Moulay Idriss Zerhoune, which crops out as a trench in the same direction. It was considered by Moratti et al. (2003) as a surface rupture due to an earthquake of 18. November 1755. The earthquake, which devastated two cities of Meknes and Fez, occurred few days after the earthquake of 1. November 1755, known as the Lisbon earthquake. ...
... Geodetic studies show that the general convergence rate between Africa and Eurasia is 4.3 ± 0.5 mm/yr with an azimuth of 116 ± 6 • (McClusky et al., 2003). This present-day deformation pattern is consistent with tectonics studies that show Mio-Pliocene NW-SE compression Moratti et al., 2003;Tahayt et al., 2009). However, GPS studies show that the Rif is moving perpendicularly to the general WNW-ESE Africa-Eurasia convergence direction (Fadil et al., 2006;Vernant et al., 2010) (Fig. 1A). ...
... Despite the GPS velocity field and a few pioneering tectonic studies (Aït Meghraoui et al., 1996;Moratti et al., 2003;Rampnoux et al., 1977), the distribution and kinematics of the present-day deformations in the Rif remains poorly constrained. ...
... In the southern Rif, the structural study of Moratti et al. (2003) and Bargach et al. (2004) indicate that the E-W-trending frontal thrust of the Rif is still active with a shortening direction oriented N-S to NW-SE. ...
... Ancient documents compiling observations on the 1 November earthquake and tsunami in Morocco are more numerous than original local accounts. Most of the compiled documents proceed to an unfortunate amalgamation between the 1 November Oceanic earthquake, with tsunami, and a Rifian earthquake dated 18–19 November by all European documents or 27–28 by some Moroccan manuscripts (Levret, 1991; Moratti et al., 2003 ). The isoseismal lines for the 1 November earthquake, as published by Davison (1936) , clearly illustrate the age-old state of confusion between these two earthquakes: he had clearly understood that there were two foci involved in the data gathered, but for want of revisiting the original documentary sources (a difficult process, when no automatic reprographic means were in existence), he was not aware that these described two different events, at different dates (Fig. 1). ...
... Last, again in these two most reliable documents, the town of Taza (Tessa, Tasso, Fafso) is deemed swallowed, with nothing left. The destructions reported by the accounts of the 18–19 November earthquake are thus particularly attached to the Northern flank or the Miocene South-Rifian corridor: both the Jebel Zalagh and Jebel Zerhoun are found north of the two great Moroccan metropolis, Fez and Meknes; Moratti et al. (2003) illustrated convincingly some co-seismic movements on the slopes of these mounts. We do not have any knowledge of such observations close to Taza, the structural position of which, at the border between the pre-Rifian ridges and the Moroccan Meseta, still make it very likely that it was struck by the earthquake, though not to the point of disap- pearance. ...
... Within inland Morocco, the damages attributed to this earthquake actually result from the Rifian earthquake which happened two and a half weeks later (18–19 November), to which Levret (1991) has drawn attention, and re-documented by Moratti et al. (2003). If extensive destructions took place in Fez and Meknes, it cannot have been caused by a marine tsunamigenic earthquake. ...
Tsunami seldom strike the European Atlantic shores. The great Lisbon Earthquake of 1 November 1755 is the main destructive tsunamigenic event recorded. Since the mid-1990's, many simulations of propagation of tsunami waves from variants of the possible seismic source have been conducted. Estimates of run-up in Morocco are seldom included in publications, maybe for want of reliable historical data to control the simulations. This paper revisits some early accounts, transmitted as translations to European Chanceries, Scientific Societies and Newspapers. A critical analysis of the documents leads us to conclude that the Lisbon earthquake was overestimated because of amalgamation with a later Rifian earthquake. Then, the overestimation of the tsunami through worst interpretation of the scant data available appeared only reasonable, while the moderate measurements or interpretations were not given their due attention. In Morocco the amplitude of the tsunami (i.e. height at shoreline minus expected tide level) may not have exceed the measurement given by Godin (1755) for Cadiz, 2.5 m above the calculated astronomical tide, a crest-to-trough amplitude of 5 m at most. This age-old overestimation of both the earthquake and tsunami is detrimental to the evaluation of the risk for coastal people and activities.
... As a result, besides suggesting a likely zonation in deformation complexity (Rodrıǵuez-Pascua et al., 2000), these findings point to a northward location of the potential epicentres; more studies should be conducted, however, at nearby sites to verify this assumption. This is in fact consistent with recent investigations that have emphasised higher seismic activity at the transition between the Rif and the Middle Atlas, known as the South Rif Front (SRF, Fig. 1; Medina and Cherkaoui, 1992;Moratti et al., 2003;Poujol et al., 2017), in response to the convergence between Africa and Iberia since the Cretaceous (Dewey et al., 1989;Lonergan and White, 1997;Fernàndez et al., 2019). Indeed, at least four historical earthquakes of Mw > 5.5 have been reported to have occurred in this transitional zone, not far from Lake Ifrah, over the last millennium (Moratti et al., 2003;Peláez et al., 2007;Poujol et al., 2017;Fig. ...
... This is in fact consistent with recent investigations that have emphasised higher seismic activity at the transition between the Rif and the Middle Atlas, known as the South Rif Front (SRF, Fig. 1; Medina and Cherkaoui, 1992;Moratti et al., 2003;Poujol et al., 2017), in response to the convergence between Africa and Iberia since the Cretaceous (Dewey et al., 1989;Lonergan and White, 1997;Fernàndez et al., 2019). Indeed, at least four historical earthquakes of Mw > 5.5 have been reported to have occurred in this transitional zone, not far from Lake Ifrah, over the last millennium (Moratti et al., 2003;Peláez et al., 2007;Poujol et al., 2017;Fig. 1). ...
Reverse-drags in extensional settings are emblematic structures that may provide imprints of (neo)tectonic pulses, and even evidence of palaeoseismic activity. The direct pieces of evidence of (neo)tectonic activity therein are mostly reported to be subsidiary grabens, synthetic and antithetic normal faults, and/or bed-parallel slip that brittlely deform newly deposited soft-sediments, therefor being classified as soft-sediment deformation structures (SSDSs). Liquidization-induced SSDSs are, reasonably, likely to occur in such tectonically active settings as well. Nonetheless, both SSDSs types have not gained enough interest from structural geologist and sedimentologist communities. We hereby address this gap by investigating brittle and liquidization-induced SSDSs from late Pleistocene-Holocene fluvio-lacustrine beds (Lake Ifrah, northern Morocco). Sedimentation and deformation occur upon a reverse-drag anticline connected to a master normal fault that borders a half-graben structure (Ifrah sub-basin) superimposed on a master basin (Ifrah Basin), itself connected to a major strike-slip fault (the Tizi n'Tretten Fault, TTF). Liquidization-induced SSDSs involve load structures and water-escape structures, whereas brittle deformations encompass shrinkage cracks, subsidiary grabens, neptunian dykes, and bed-parallel slip (BPS) and associated domino-style faults. The deformation mechanism for both SSDS types involves either loss of strength upon liquidization (load structures and water-escape structures) or an increase in applied (internal or external) shear (shrinkage cracks, faults). Facies analysis and conventional trigger assessment allowed deciphering endogenic from exogenic triggers for deformation. Liquidization has been inferred to be seismic-triggered whereby the deformed beds are considered seismites. While BPS deformed brittlely late Pleistocene deposits (probably Marine Isotope Stage 3, MIS-3) and subsidiary grabens and neptunian dykes involve Middle Holocene sediments, they are considered to be linked to two flexural-slip events of the reverse-drag during MIS-3? and the Middle Holocene, respectively.
Scaling relationships between the structural entities of the hanging-wall-master fault-footwall system allowed estimation of the subsidence due to hanging-wall collapses, and thereby the initiation of the Ifrah sub-basin (at least since 260 kyr). The rhomboidal-shaped master basin was suggested to belong to a dilational quadrant of the associated strike-slip fault, the TTF, at a tip-damage region. Accordingly, we highlight structural isolation of a segment of the TTF that is synchronous with the Ifrah Basin initiation.
... The ZeR is a faulted east-west elongated anticline, with a reduced southern limb, where the south-verging Moussaoua-Hamraoua (MHF) reverse faults (Faugères, 1978) splits and bifurcates into other branches of faults to the west and extends to the east where it coincides with the Nesrani sinistral ramp (NF) (Haddaoui, 2000) (Fig. 3b). The MHF is considered by Moratti et al. (2003) as the most cause of the 1755 Meknes earthquake occurring few days after the "Lisbon earthquake", which has damaged the Roman archaeological site of Volubilis (Chatelain, 1968) and destroyed the Moulay Idriss town ( Fig. 3a (SFERN, 1985)). The TaR-KnR is an east-west to WNW-ESE inclined anticline, with a southward vergence. ...
... Journal of African Earth Sciences 167 (2020) 103833 indicate its relative lower activity in the MIM, comparing to those of the MIF or the TKF. Thus, it seems not to be the cause of the 1755 Meknes earthquake as it has been interpreted by Moratti et al. (2003). The obtained morphotectonic results not only provide evidences of active structures accommodating the southwestward escape of the central Rif, but also highlight regional tectonic deformations as well as active salt tectonics, which can be studied under other aspects. ...
The Moulay Idriss Ramp anticlines of the South Rifian Ridges are located in the front of the Moroccan Rif Belt (southern branch of the Arc of Gibraltar) and have accommodated the tectonic escape of this Alpine chain as an active edge, in response to the converging Eurasian and African plates. Nevertheless, recent evidences of this landscape tectonic activity remain poorly defined.
We investigated the variations in tectonic activity and evaluated its influence on the landscape evolution, through the application of geomorphic indices including: the mountain front sinuosity, valley floor width-to-height ratio, drainage basin asymmetry factor, hypsometric curves, normalized steepness index, longitudinal river profile, and χ as an indicator of drainage network disequilibrium.
Morphotectonic study shows that fault related folds, in the area, produce differential uplift with a relatively high degree of tectonic activity along the eastern part of the massif. The doming process accounts for the elevation of the area, in the Dehar en Nsour salt dome, and this in turn, could be explained as being promoted by the Nzala des Oudayas fault system. On the other hand, the Moulay Idriss fault exerts a strong influence on the morphology of the Fert El Bir anticline inducing the Khoumane river deflection, while other streams exhibit rejuvenation and water divide migration.
These new data elucidate the recent tectonic evolution of folds and strike-slip fault systems in the southern front of the Gibraltar-Rif southern edge, which contributes to the movement of the Rif belt with respect of the stable Africa, and sheds new light on the ‘‘Meknes’’ earthquake, confused with the contemporaneous 1755 Lisbon earthquake.
... Levret, 1991). Moratti et al. (2003) report a coseismic rupture within the Meknes area (Fig. 1B), but document very little field observations with no age constraints. By modeling the static stress variation induced by three potential source of the 1755 Lisbon Earthquake, the authors consider that no significant (Koulali et al., 2011). ...
... Although our estimated magnitude (Mw 6.5-7) is higher than the maximum intensity derived magnitude given for the 1755 CE earthquake (Mw 5.2 after Peláez et al., 2007) and the instrumental seismicity recorded in the region for the last century, it seems more consistent with the occurrence of a surface rupture (despite of the uncertainty about the depth of the hypocenter) and the description of the Arab and European historical reports mentioning a "shorter in time but stronger earthquake than the last one (i.e. November 1st)", "many collapsed houses" in Fes and "total destruction (…) and many dead" in Meknes (Levret, 1991 and references therein;SFERN, 1985;Moratti et al., 2003). ...
... Levret, 1991). Moratti et al. (2003) report a coseismic rupture within the Meknes area (Fig. 1B), but document very little field observations with no age constraints. By modeling the static stress variation induced by three potential source of the 1755 Lisbon Earthquake, the authors consider that no significant (Koulali et al., 2011). ...
... Although our estimated magnitude (Mw 6.5-7) is higher than the maximum intensity derived magnitude given for the 1755 CE earthquake (Mw 5.2 after Peláez et al., 2007) and the instrumental seismicity recorded in the region for the last century, it seems more consistent with the occurrence of a surface rupture (despite of the uncertainty about the depth of the hypocenter) and the description of the Arab and European historical reports mentioning a "shorter in time but stronger earthquake than the last one (i.e. November 1st)", "many collapsed houses" in Fes and "total destruction (…) and many dead" in Meknes (Levret, 1991 and references therein;SFERN, 1985;Moratti et al., 2003). ...
... Levret, 1991). Moratti et al. (2003) report a coseismic rupture within the Meknes area (Fig. 1B), but document very little field observations with no age constraints. By modeling the static stress variation induced by three potential source of the 1755 Lisbon Earthquake, the authors consider that no significant (Koulali et al., 2011). ...
... Although our estimated magnitude (Mw 6.5-7) is higher than the maximum intensity derived magnitude given for the 1755 CE earthquake (Mw 5.2 after Peláez et al., 2007) and the instrumental seismicity recorded in the region for the last century, it seems more consistent with the occurrence of a surface rupture (despite of the uncertainty about the depth of the hypocenter) and the description of the Arab and European historical reports mentioning a "shorter in time but stronger earthquake than the last one (i.e. November 1st)", "many collapsed houses" in Fes and "total destruction (…) and many dead" in Meknes (Levret, 1991 and references therein;SFERN, 1985;Moratti et al., 2003). ...
In this paper, we present the first estimate of the Holocene deformation along the southern front of Gibraltar
arc (Morocco) and the first field constraints on the local 1755 CE Fes-Meknes surface rupturing earthquake
which could be associated to the “Great Lisbon Earthquake” (M > 8.5) in November 1st, 1755. Using
satellite imagery, aerial photographs and field investigations, we carried out a morphotectonic study along
the ~ 150 km-long Southern Rif Front (SRF) to identify the most recent evidences of tectonic activity. Analyzed
offset alluvial deposits confirm that (i) the last ~ 5 ka cumulative deformation leading to a slip rate
of ~ 3.5 ± 1 mm/yr for this segment of the SRF is consistent with the GPS derived horizontal shortening rate
of 2–4 mm/yr and (ii) a recent major earthquake ruptured a ~ 30 km-long segment along the SRF. Based on
deposits dating and historical seismicity we propose that this seismic event occurred in 1755 as a local earthquake.
Even though this 1755 local event cannot be considered as a strong aftershock of the main Lisbon
seismic event (M > 8.5), their temporal closeness, their occurrence under the same convergent stress regime
(~ NNW-SSE-oriented compression) and the fact that Fes-Meknes area was strongly shaken during the Lisbon
earthquake, raises the question of the possible triggering of the Fes earthquake. Anyway, our new results
suggest that most of the Nubia-Rif belt convergence is accommodated by the SRF, making it potentially the
most destructive structure of the Rif.
... This velocity pattern is associated with shortening at the center of the model, which corresponds to the approximate southward termination of the Rif block (Fadil et al., 2006;Vernant et al., 2010;Koulali et al., 2011). Geological observations indicate active thrust-fault and shortening structures in the southern Rif -western Gharb Basin region (Moratti et al., 2003;Chalouan et al., 2006), corroborating the localized shortening observed in upper crust in the numerical models (Figs. 4 and 6B). Model velocities south of the Gharb Basin under-predict the GPS data, indicating either a possible southward extension of the basal Fig. 6. ...
... Although the predicted flexure location corresponds to that of the Gharb Basin, the timing does not. Geological and stratigraphic data indicate a deposition phase from Middle Miocene to Pleistocene (e.g., Iribarren et al., 2009), associated with the development of the foreland thrust-and- fold belt of the Rif orogen (e.g., Chalouan et al., 2008), and followed by recent shortening along its southern front (Moratti et al., 2003;Chalouan et al., 2006). Our model does not account for the complex crustal structure resulting from this Rif orogeny that produced the marked topography of the Rif Mountains and, likely, an associated crustal root (Gil et al., 2014) that is not present in our model initial stage. ...
... However, this small orogenic wedge, characterized by well recognized active boundaries and only involving shallow structures (e.g., Rebai et al., 1992;Buforn et al., 1995;Maldonado et al., 1999), should not be confused with the much larger Morocco microplate. On the other hand, a relative motion between the Betic-Rif wedge and the Morocco microplate is well documented by the compressional deformation recognized at the border between these two blocks (e.g., Moratti et al., 2003;Bargach et al., 2004;Medialdea et al., 2004). ...
... The roughly E-W extension, recognised from southeastern Spain to eastern Rif through the Alboran Sea (e.g., Buforn et al., 1995Buforn et al., , 2004Medina, 1995;Ait Brahim et al., 2002;Martinez-Martinez et al., 2006;Reicherter and Peters, 2005;Gracia et al., 2006) most probably occurs in the wake of the extruding Betic-Rif wedge. The compressional fronts recognized in the Atlantic offshore of Gibraltar, at the western border of the Betic-Rif wedge (e.g., Maldonado et al., 1999;Moratti et al., 2003;Bargach et al., 2004;Medialdea et al., 2004;Gutscher et al., 2006;Thiebot and Gutscher, 2006) mark the zone where this wedge overthrusts the Morocco microplate. ...
... Faugères (1978) considered Jbel Zalagh to be a southward very steeply dipping monoclinal structure, bounded to the south and to the north by two almost vertical N708E-oriented faults. The northern fault has been considered by Moratti et al. (2003) as an active fault related to a NNE -SSW compression that produced a sharp scarp (Fig. 4c), affecting recent deposits, and responsible for the seismic activity of the region. Structural analysis of this sector shows that Jbel Zalagh is, rather, a close to tight and faulted fold (Fig. 4). ...
... The N708E fault of the northern side has a high dip (708 -808 north or south) and a main horizontal sinistral slip with a minor vertical component that downthrows the northern block. This fault is of Quaternary age (Moratti et al. 2003), and has been active after the development of the fold. The N1708E -N308E sinistral and N1358E -1508E dextral faults are even more recent, as they affect the N708E fault and show Quaternary slope deposits on the southern side of Jbel Zalagh. ...
The Rif Cordillera is a part of the Alpine orogenic arc in the Western Mediterranean, which was developed by the interaction of the westward motion of the Alboran Domain between the converging Eurasian and African plates. The Prerif Ridges, located along the southwestern front of the Rif, are south-vergent folds that are in places associated with faults affecting Jurassic to Quaternary sedimentary rocks and slope breccias that evidence the deformations that were active over the Neogene-Quaternary period. The different southward or southwestward displacement of each Prerif Ridge is related to the development of frontal and lateral ramps, which may or may not reach the surface. Oblique shortening may be explained by southwestward escape of large tectonic wedges, bounded by large strike-slip faults: the North-Middle Atlas fault which extends northward into the Alboran Sea, the Fez-Tissa-Taineste fault, the Bou Draa-Sidi Fili fault, the Jebha fault and the Fatties fault. The relative displacement of these tectonic wedges toward the SW may explain the NNE-SSW to ENE-WSW compression observed in the Rif front and in the northern part of its Meseta-Atlas foreland.
... Both the northern and southern borders of the High Atlas are east –westtrending active thrusts (Dutour & Ferrandini 1985; Froitzheim et al. 1988; Jacobshagen et al. 1988; Zouine et al. 1996; Piccardi et al. 2001). Also active are the east – west-trending 'Rides Prérifaines' , which are the expression of the frontal thrust of the Rif in the Meknes– Fez area (Moratti et al. 2003; Piccardi et al. 2004 ). The NE – SWtrending faults, limiting the Middle Atlas, underwent , and still are subject to, sinistral transpression under the same compression (e.g. ...
... High contents of CH 4 –N 2 coupled with the presence of significant amounts of mantle 3 He are typical of gases discharging in active transpressive structures, such as the San Andreas fault (Kennedy et al. 1997), without the presence of shallow active hydrothermal systems. In the Meknes area two main fault systems intersect: (1) the active east – west-trending thrusts of the Rides Prérifaines (Moratti et al. 2003; Piccardi et al. 2004) and (2) the NE – SW-trending mainly sinistral transpressive structures represented in outcrop by the Sidi Fili fault and the Agourai- Moulay Yacoub fault, along which Triassic evaporites crop out (Fig. 10). This surface pattern, together with the presence of many thermal sources and active travertine deposits, argues for a connection at depth between the NE– SW-trending fault system and the east – west-trending active thrusts of the Rides Prérifaines, and may account for the distribution of 3 He anomalies over a wide area along the frontal thrust of the Rif in the Meknes area (Fig. 10). ...
This paper presents the results of an extensive chemical and isotopic investigation on
natural thermal and cold fluids (spring waters and associated gases) discharging throughout the
main geological domains of Morocco. The chemical features of the thermal springs are mainly
dependent on rock dissolution involving Triassic evaporite formations, producing either Na–Cl
or Ca–SO4 composition, although mixing with shallower connate high-saline waters in
Neogene post-orogenic sedimentary layers cannot be ruled out. Only in the Moroccan Meseta
and Anti-Atlas domains have spring discharges probably undergone equilibration as a result of
water–rock interaction in granites. Of the chemical and isotopic features of the gas seeps,
3He/4He ratios and d13C–CO2 values indicate the occurrence of a significant contribution of
mantle-derived gas, especially at Oulme`s (Moroccan Meseta) and Tinejdad-Erfoud (Anti-
Atlas), where associated waters are found to equilibrate at relatively high temperatures
(c. 130 8C). These areas are also characterized by the presence of Pliocene to Quaternary basaltic
volcanic rocks. Thermal discharges located along the Rif front and related to the NE–SW-oriented
main strike-slip faults are associated with a CH4- and/or N2-rich gas phase, derived respectively
from a crustal or an atmospheric source. Some of them have significant contents of 3He that could
indicate the rising of mantle fluids. Such a striking isotopic signature, which is not related to any
recent volcanism visible at surface, is likely to be associated with cooling magma at depth related
to transpressive fault systems. Similarly, in the northeastern area, the small, although significant,
enrichment of 3He in the gas discharges seeping out along the Nekor seismic active fault and
related to Pliocene–Quaternary basalts also suggests a deep-seated (magmatic) contribution.
The distribution of thermal discharges is strongly related to the main active tectonic structures
of Morocco. Moreover, this study indicates the presence of deep active tectonic structures in
areas until now considered as stable. In particular, the NE–SW-trending Nekor fault may be
part of a major system that extends to the Moroccan Meseta and into the Smaala–Oulme`s fault
system, thus emerging as a deep structure with crustal significance.
... However, this small orogenic wedge, characterized by well recognized active boundaries and only involving shallow structures (e.g., Rebai et al., 1992;Buforn et al., 1995;Maldonado et al., 1999), should not be confused with the much larger Morocco microplate. On the other hand, a relative motion between the Betic-Rif wedge and the Morocco microplate is well documented by the compressional deformation recognized at the border between these two blocks (e.g., Moratti et al., 2003;Bargach et al., 2004;Medialdea et al., 2004). ...
... The roughly E-W extension, recognised from southeastern Spain to eastern Rif through the Alboran Sea (e.g., Buforn et al., 1995Buforn et al., , 2004Medina, 1995;Ait Brahim et al., 2002;Martinez-Martinez et al., 2006;Reicherter and Peters, 2005;Gracia et al., 2006) most probably occurs in the wake of the extruding Betic-Rif wedge. The compressional fronts recognized in the Atlantic offshore of Gibraltar, at the western border of the Betic-Rif wedge (e.g., Maldonado et al., 1999;Moratti et al., 2003;Bargach et al., 2004;Medialdea et al., 2004;Gutscher et al., 2006;Thiebot and Gutscher, 2006) mark the zone where this wedge overthrusts the Morocco microplate. ...
The post-Tortonian deformation pattern in the Central Mediterranean can be satisfactorily interpreted in the framework of
a SSW-NNE to SW-NE convergence between Africa and Eurasia (Mantovani et al., 1992 and this volume). However, this hypothesis is not in line with the most widely used Africa-Eurasia kinematic models, based
on the analysis of North Atlantic kinematic data, which predict a SE-NW to S-N motion between Africa and Eurasia. Here it
is argued that these models might be not reliable, since are based on the assumption that Eurasia is a unique coherent block
from the Atlantic ridges to the Pacific trenches, which can hardly account for the significant intraplate deformation occurring
in Western Europe, and in particular, in the Iberian peninsula and surrounding regions. This evidence would imply that the
kinematic data observed along the Mid-Atlantic ridges just North of Azores and those on the Azores-Gibraltar belt cannot be
used to constrain the relative motions of Eurasia with respect to North America and Africa respectively. It is shown by quantitative
computations that the remaining kinematic data in the North Atlantic can be reconciled, within errors, with a NNE to NEward
motion of Africa with respect to Eurasia, in the Central Mediterranean. This kinematic pattern does not involve any significant
difficulty in explaining the tectonic pattern in the other sectors of the Mediterranean area and along the boundaries of the
Eurasian continent.
... While the full extent of the high intensities in this earthquake is not known, the magnitude is likely to have been in the range M6.5-7. Moratti et al., (2003) propose that this earthquake was located on an E-W, northerly dipping reverse fault outcropping about 15km to the north of Meknes (ie away from the city) but within 5km north of Fez but this does not reconcile with the relative levels of damage at the two cities, or with the level of destruction at Meknes (which suggests fault rupture in the vicinity of the city) and the question as to the causative fault of this earthquake should probably be left open. ...
... This is consistent with the proposal of Vilanova and Fonseca (2003) that the LTV fault was triggered by the 1755 Nov 1 st rupture, within the same episode. The location of the 1761 offshore earthquake is not fully resolved but if we consider the more plausible location represented on Moratti et al. (2003) is orthogonal to the displacement implied by the majority of the fault sources for the 1755 earthquake, and based on this mechanism it is not easy to see how stress transfer would have triggered the failure. However if the underlying mechanism involved a sinistral displacement along a NE-SW fault, as is typical of Morocco, this would have had the potential to be triggered by E-W stress reduction within the region to the east of the main 1755 fault rupture. ...
... However, they are locally well developed and unambiguous (Fig. 11), and affect different units allowing indirect control of the results. Our structural observations, as well as those done by previous authors (Faugeres 1978Faugeres , 1981; Aı¨tAı¨t Chotin 1984, 1989; Morel 1988 Morel , 1989 Moratti et al. 2003; Aı¨tAı¨t Brahim et al. 2002; Bargach et al. 2004 ), allow reconstructing two major compressive phases. It should also be noted that, from other structural considerations and without detailed structural analyses, Levy and Tilloy (1952), Tilloy (1955, and Choubert and Faure-Muret (1962) indirectly attained similar conclusions. ...
... At the leading edge of the Rif chain the Late Pliocene lacustrine limestones are involved in deformation, thus demonstrating continuous compressive tectonic activity up to very recent times (Moulay Yacoub area, Draa el Merga and Aicha Mouguetaya structure; Figs. 5, 12, site 4). Active faults have been detected in the area of Jebel Zalagh, and the associated focal mechanisms show a still active N–S-trending compressive stress field (Figs. 5, 12; Moratti et al. 2003 and references therein). ...
The Rides Prerifaines (RP) of Morocco constitute the leading edge of the Rif chain. They involve a Triassic–Palaeocene succession
deposited on a peneplained Palaeozoic fold belt and accumulated in basins delimited by NE–SW-trending normal fault systems.
A significant hiatus separates an overlying Middle Miocene–Upper Miocene foredeep sequence. The reconstruction of the complex
structural evolution of the RP during the later compressive phases that affected the Rif chain since Middle Miocene time has
been the aim of this paper. We integrated field structural analyses, seismic line interpretation, and analogue modelling in
order to evaluate the control exerted by the Late Triassic–Jurassic normal fault systems onto the later compressive tectonics.
The maximum compression direction associated with the first compressive phase is roughly NE–SW to ENE–WSW oriented. During
this phase the Mesozoic basin fill was scooped-out from the graben and the main décollement level were the Triassic evaporites.
Since Pliocene times the maximum compression direction was oriented roughly N–S. During this phase the RP assumed their present
structural setting. The earlier normal faults delimiting the Mesozoic graben were reactivated in a strike–slip mode also involving
the Palaeozoic basement. The analogue modelling experiments demonstrated that the basement reactivation promoted salt tectonics
and favoured fluid circulation.
... However , this small orogenic wedge, characterized by well recognized active boundaries and only involving shallow structures (e.g., Rebai et al., 1992; Buforn et al., 1995; Maldonado et al., 1999), should not be confused with the much larger Morocco microplate . On the other hand, a relative motion between the Betic-Rif wedge and the Morocco microplate is well documented by the compressional deformation recognized at the border between these two blocks (e.g., Moratti et al., 2003; Bargach et al., 2004; Medialdea et al., 2004). The Mediterranean evidence described in this section and the arguments reported by Mantovani et al. (1997 Mantovani et al. ( , 2002 Mantovani et al. ( , 2006a) and Mantovani (2005) suggest that in the last few million years Nubia and Eurasia have undergone a SSW-NNE convergence. ...
... The roughly E-W extension , recognised from southeastern Spain to eastern Rif through the Alboran Sea (e.g., Buforn et al., 1995 Buforn et al., , 2004 Medina, 1995; Ait Brahim et al., 2002; Martinez-Martinez et al., 2006; Reicherter and Peters, 2005; Gracia et al., 2006) most probably occurs in the wake of the extruding Betic-Rif wedge. The compressional fronts recognized in the Atlantic offshore of Gibraltar, at the western border of the Betic-Rif wedge (e.g., Maldonado et al., 1999; Moratti et al., 2003; Bargach et al., 2004; Medialdea et al., 2004; Gutscher et al., 2006; Thiebot and Gutscher, 2006 ) mark the zone where this wedge overthrusts the Morocco microplate. ...
It is argued that the Plio-Quaternary deformation pattern in the Mediterranean region is compatible with a SSWNNE
convergence between Africa (Nubia) and Eurasia and that the significant difference between this kinematics
and the one provided by global models (SSE-NNW convergence e.g., the NUVEL-1) may be due to the fact
that those models interpret North Atlantic data by adopting an oversimplified two-plate configuration, which
cannot account for the occurrence of significant seismotectonic activity inside the presumed Nubia and Eurasia
blocks. It is shown that the adoption of a new plate configuration involving the Iberia and Morocco microplates,
strongly suggested by geological and seismotectonic evidence, makes it possible to identify a kinematic model
compatible within errors with the constraints recognized in the Mediterranean region and with the NUVEL-1
North Atlantic data set. Some considerations are made about why the present-day Nubia-Eurasia kinematic models
inferred from geodetic observations are significantly different from long-term models, such as model NUVEL-
1 and the one proposed in this work.
... After the death of Idris I in 791, the city was progressively abandoned until the Almoravid period (11 th -12 th centuries). The destruction of the site is likely related to the earthquake in 1755 [6]. The house of Venus was built in several phases: the first house dates from the second half of the first century while most of the mosaics and baths were added in the second century [3]. ...
The Volubilis project has created an opportunity for the transfer and sharing of knowledge in the field of heritage conservation between teams from both shores of the Mediterranean. The ambition of this project, supported by the region Centre-Val-de-Loire (France), is to develop a set of tools to preserve and develop the archaeological site of Volubilis. This Moroccan and global iconic ancient heritage is exposed to damage that endangers its durability.
The project started in September 2015 and has two main goals. The first one is to carry out a historic inventory of the materials used for the construction of the site and their environment in order to understand the origin of the damage and propose sustainable solutions for remediation that meet the requirements of the ICOMOS Charter, in particular concerning the choice of compatible materials for restoration (mortar and stone). The second one is to promote the site towards local, national and international tourists via historical, scientific and technical media to highlight the cultural importance of this heritage. This project brings together two French partners, the Multidisciplinary Institute of Engineering Research Systems, Mechanics and Energetics (PRISME - University of Orléans) and the Mission Val de Loire, and three Moroccan partners, the Ecole Nationale Supérieure d’Arts et Métiers (ENSAM-Meknes), Moulay Ismail University and the conservation authorities of the archaeological site, Volubilis Conservation.
... Thus this strong correlation existing between magnetic anomalies and the regional structures shows that this digital mapping based on an aeromagnetic survey can be used for other regions where access is difficult, in order to apply some corrections to various existing geologic maps. Field data permit locating the detected features corresponding to NE to SW-trending folds characteristic of the Paleozoic basement (Durand-Delga 1960-1962Faug?res 1978;Ait Brahim 1983;Leblanc and Olivier 1984;Hoepffner 1994;Haddaoui 2000;Ben Abbou 2001;Zizi 2002;Moratti et al. 2003;Moratti and Chalouan 2007;Sani et al. 2007;Habibou et al. 2012;Ouarhache et al. 2012) and verifying the existence of the NE-SW fault system, corresponding essentially to normal faults, inherited from the Late Variscan. It should be pointed out that even with this strong correlation of processed magnetic maps with field geology, it was not possible to detect some of the folded basaltic sills occurring in this region, probably because of their small thickness and the spacing of flight lines (500 m). ...
The aim of this work is to interpret the geologic structures of the Agourai area (Paleozoic and Mesozoic structures) from processed magnetic maps. The detected magnetic anomalies from different standard methods used in aeromagnetism (Residual map, RTP map, horizontal gradient map) were compared to geologic structures and permit enhancing the mapping quality of some areas, and thus defining many geologic features. Existing geologic maps and geologic field studies allow interpreting some detected anomalies. It was thus possible to define the limits between the Paleozoic basement and the Mesozoic cover, to determine magnetic anomalies according to NE-SW trends compatible with the regional geologic structures and finally to detect a NE to SW-oriented fault system in the Mesozoic cover of the Agourai Plateau. Despite the reliability of this approach, some folded basaltic sills occurring in this region were not well detected, probably because of their reduced thickness.
... They also pointed out the existence of another N-S compressive phase from the Pliocene entailing a generalized shortening of the area. Moreover, the active tectonics in the southern ridges are related with present-day seismic activity (Moratti et al., 2003). ...
The Volubilis Basin is located between two structural arcs formed by the Prerif Ridges that developed during and after sedimentation. The arcs correspond with W- to WSW-verging anticline culminations, limited, to the north by a NE-SW strike-slip lateral ramp. Sedimentary infill took place during two stages of ridge formation and propagation. The first stage occurred in the Middle Miocene-early Tortonian? and was determined by the deposition of the Nappe Prérifaine in the northern part of the basin, and continental and marine sediments over the Prerif Ridges. The second one, Late Miocene in age (Tortonian-Messinian), corresponds to the sedimentation of calcarenites and bioclastic limestones at the basin edges, with a lateral transition to white and blue marls towards the center of the basin. There is clear evidence of synsedimentary deformation, suggesting the interaction of sedimentation and tectonics. Geophysical data allow us to characterize the stratigraphic architecture of the Volubilis Basin and the geometry of the top of the Paleozoic basement. An approximately N-S Tortonian-Messinian asymmetric depocenter is located close to the front of the eastern arc. This research illustrates the nucleation, progressive thrust bending and segmentation, and the propagation of folds interacting with sedimentation. Thrust nucleation agrees with Paleozoic basement highs under the detachment surface. The progressive development of these tectonic structures conditioned the formation, segmentation and final continentalization of the Volubilis Basin, which can be considered as a piggy-back basin.
... However, no regional right-lateral strike-slip faults are reported where we locate the western Rif block boundary, suggesting that this boundary could be interpreted as a broad shear zone. The southern Rif block boundary corresponds to mapped Quaternary thrust faults along the southern edge of the Rif (Moratti et al., 2003). The northern and eastern boundaries of the Rif block are poorly constrained by the GPS results. ...
Surface deformation in Morocco, derived from five years of global positioning system (GPS) survey observations of a 22-station network, four continuously recording GPS (CGPS) stations, and four International GNSS Service (IGS) stations in Iberia, indicates roughly southward motion (˜3 mm/yr) of the Rif Mountains, Morocco, relative to stable Africa. Motion of the Rif is approximately normal to the direction of Africa-Eurasia relative motion, which is predominantly strike slip, and results in shortening of the Rif and subsequent crustal extension of the adjacent Alboran Sea region. The sense, and the N-S asymmetry of the observed deformation (i.e., no evidence for north-directed shortening in the Betic Mountains north of the Alboran Sea) cannot be easily explained in terms of crustal plate interactions, suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western Mediterranean. The model that best fits the observations involves delamination and southward rollback of the African lithospheric mantle under the Alboran and Rif domains.
... The southern Alboran block boundary follows the E-W oriented thrust faults of the Pre-Rif-ridges (Fig. 1), where our model predicts left-lateral strike-slip of 1.9 mm/yr with a compressional component varying between 2.1 and 2.8 mm/yr. This result is consistent with shortening reported form tectonic studies (Chalouan and Michard, 2004;Moratti et al., 2003). Our block model extends the mapped thrust faults bounding the southern Rif offshore to the south of the Gulf of Cadiz, where our model indicates slow left-lateral strike slip (1.1 mm/yr) with an insignificant fault-normal component, consistent with the low level of seismicity on this fault in comparison to the N BAR block boundary (Fig. 1). ...
We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling to investigate present day deformation along the Africa–Iberia plate boundary zone in the western Mediterranean region. The GPS velocity field shows southwestward motion of the central part of the Rif Mountains in northern Morocco with respect to Africa varying between 3.5 and 4.0mm/yr, consistent with prior published results. Stations in the southwestern part of the Betic Mountains of southern Spain move west–southwest with respect to Eurasia (∼2–3mm/yr). The western component of Betics motion is consistent with partial transfer of Nubia–Eurasia plate motion into the southern Betics. The southward component of Betics motion with respect to Iberia is kinematically consistent with south to southwest motion of the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Our preferred plate boundary geometry includes one block between Iberia and Africa including the SW Betics, Alboran Sea, and central Rif. This geometry provides a good fit to the observed motions, suggesting a wide transpressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by the Gloria–Azores fault system to the West and the Rif–Tell lineament to the East. Block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE–SW trending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E–W seismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateral motion with compression extending East along the Algerian Tell. We interpret differential motion in the Rif–Alboran–Betic system to be driven both by surface processes related the Africa–Eurasia oblique convergence and sub-crustal dynamic processes associated with the long history of subduction of the Neotethys ocean lithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshore fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazard for the West Mediterranean region.
... http://www.ign.es/ign/es/IGN The papers employed in this work that include magnitude reevaluations are those by Samardjieva et al. (1998), Bezzeghoud and Buforn (1999), Badal et al. (2000), Braunmiller et al. (2002, Buforn and Coca (2002), Moratti et al. (2003), Rueda and Mezcua (2002), Stich et al. (2003), Mezcua et al. (2004, and Rueda and Mezcua (2005). We must point out the paper by Johnston (1996b), which included an assessment of the moment magnitude from seismic intensity observations of the 1755 Lisbon earthquake, the most energetic and destructive shock in the region of study (M W 8.7, I max = X) (e.g., Levret 1991; Baptista et al. 1998; Martínez Solares and López Arroyo 2004). ...
A homogeneous and reliable earthquake catalog is highly desirable in seismic hazard studies. This work, the compilation of as complete and homogeneous a main Moroccan earthquake catalog as possible, has been developed in the context of a project to compute the probabilistic seismic hazard in this region. The result shown here is the outcome of fruitful teamwork among several Spanish, Moroccan, and Algerian institutions.
Previous earthquake catalogs that specifically cover this region ( e.g. Tadili and Ramdani 1983; Cherkaoui 1986; Benouar 1994), although employed in seismic hazard assessments (Benouar et al. 1996; Jimenez et al. 1999), did not span the desirable time interval. Many of them did not include pre-1900 events, i.e. , large shocks that occurred in the historical period. Moreover, a real magnitude unification process was not performed.
Our initial goal was to catalog all known events from every available published source for the area between 27° to 37°N and 15°W to 1°E, including the southernmost part of Spain and Portugal and the western region of Algeria. We obtained a uniform catalog, using for this purpose several empirical relationships among reported magnitudes, macroseismic intensity, and moment magnitude. Finally, we removed all dependent events, as well as earthquakes with magnitudes smaller than M W 3.0. The final catalog covers the period from 1045 to 2005 and includes 1,739 mainshocks. It can be downloaded in a self-explanatory Excel file from the University of Jaen Web site at http://www.ujaen.es/investiga/rnm217/moroccan_catalog.xls. Tabulated data include agency, origin time, epicenter (coordinates, depth, and location), reported magnitude or maximum intensity and unified magnitude.
The analyzed region (figure 1) includes the southernmost part of the Iberian Peninsula, the western portion of the Mediterranean Sea (Alboran Sea), the central-eastern area of the Atlantic Ocean, and the northwestern area of Africa (basically Morocco), located in the …
... In both cases, strikeslip focal mechanisms and shallow depths have been proposed (Stich et al., 2005). The frontal area of the Rif, where the cities of Fes and Meknes are located (Fig. 2), suffered the effects of a major earthquake in 1755 (Moratti et al., 2003), almost simultaneous to the earthquake (and related tsunami) that destroyed much of Lisbon (November 1, 1755). InSAR studies based on radar interferometry (Akoglu et al., 2006; Biggs et al., 2006) carried out in the area indicate the presence of ground motion in hard rock sectors (mainly limestone), yet scarcely provide results in the soft rock sectors where the epicenter was located. ...
Seismic hazard is associated with recent and present fault activity in mountain ranges. In the Betic-Rif alpine mountain chain, tectonic activity started in the Cretaceous, and topographic uplift continues since Tortonian times as a consequence of the NW–SE oblique convergence between Africa and Eurasia. The deformation is active and produces seismicity that sometimes has catastrophic consequences. The Al Hoceima earthquake (February 24, 2004), considered one of the largest earthquakes ever recorded instrumentally in the westernmost Mediterranean (M = 6.3), caused great damage in the region. Seismological studies agree that the main shock was situated on land, at the limit between the External and Internal Zones of the Rif, at a depth of 10–14 km. The focal mechanism points to a strike-slip solution with a NW–SE oriented P axis, quite similar to those of the significant 1994 earthquake swarm located to the north. The epicenter aftershocks distribution would signal the presence of a N–S oriented sinistral fault, activated by the NW–SE regional compression associated to plate boundary convergence. In this setting, the seismogenic fault ruptures related to these seismic events are expected to have reached the Earth's surface. However, detailed field work carried out 1 month after the earthquake does not evidence any N–S strike-slip coseismic fault in the epicentral area. The main observed effects were landslides, damages to constructions, and locally open cracks indicating an unexpected NW–SE extension. Scarce N–S faults are normal, the main ones being located several kilometers away from the epicentral area. To explain this apparent contradiction between geological and seismological observations, we propose a decoupled tectonic model with crustal detachments that separate a deep brittle crust from an upper crust undergoing uplift, and the development of large folds and normal faults. This geological setting, common to internal zones of cordilleras, may need to be taken into account in future paleoseismicity studies and in the assessment of seismic hazard.
... they assume all observed strain is elastic, and depend on how the deformation is accommodated within the regions evidenced by our kinematic analysis. The southern Alboran-Rif block boundary corresponds to mapped Quaternary thrust faults along the southern edge of the Rif (Moratti et al., 2003). The northern and eastern boundaries of the Rif block are poorly constrained by the GPS results. ...
We present GPS observations in Morocco and adjacent areas of Spain from 15 continuous (CGPS) and 31 survey-mode (SGPS) sites extending from the stable part of the Nubian plate to central Spain. We determine a robust velocity field for the W Mediterranean that we use to constrain models for the Iberia-Nubia plate boundary. South of the High Atlas Mountain system, GPS motions are consistent with Nubia plate motions from prior geodetic studies. We constrain shortening in the Atlas system to <1.5 mm/yr, 95% confidence level. North of the Atlas Mountains, the GPS velocities indicate Nubia motion with respect to Eurasia, but also a component of motion normal to the direction of Nubia-Eurasia motion, consisting of southward translation of the Rif Mountains in N Morocco at rates exceeding 5 mm/yr. This southward motion appears to be directly related to Miocene opening of the Alboran Sea. The Betic Mountain system north of the Alboran Sea is characterized by WNW motion with respect to Eurasia at ∼1–2 mm/yr, paralleling Nubia-Eurasia relative motion. In addition, sites located in the Betics north of the southerly moving Rif Mountains also indicate a component of southerly motion with respect to Eurasia. We interpret this as indicating that deformation associated with Nubia-Eurasia plate motion extends into the southern Betics, but also that the Betic system may be affected by the same processes that are causing southward motion of the Rif Mountains south of the Alboran Sea. Kinematic modeling indicates that plate boundary geometries that include a boundary through the Straits of Gibraltar are most compatible with the component of motion in the direction of relative plate motion, but that two additional blocks (Alboran-Rif block, Betic Mountain block), independent of both Nubia and Eurasia are needed to account for the motions of the Rif and Betic Mountains normal to the direction of relative plate motion. We speculate that the southward motions of the Alboran-Rif and Betic blocks may be related to mantle flow, possibly induced by southward rollback of the subducted Nubian plate beneath the Alboran Sea and Rif Mountains.
Northeastern Morocco is made up of several units belonging to the Alpine belt and its foreland. Miocene to plio-quaternary volcanic rocks with variable mineralogy and geochemistry dominate the geology of this region. The presence of active faults in different directions explains the high tectonic instability and the high frequency of earthquakes. This study contributes to the effort of understanding the geothermal potential of the Northeast of Morocco. Heat source and permeability are both key factors in the geothermal process. Indeed , lineaments analysis constrains the structures and their directions and indicates severely faulted zones, which are the most promising areas for geo-thermal exploration. For this purpose, we used Landsat data combined with geological and structural maps available in this region. Different image processing techniques were applied including band ratio (6/2) and directional filters. To validate the results, we conducted a comparative study between linear structures, available geological data, and previous studies. Results of the automatic extraction method of lineaments from Landsat 8 OLI/TIRS indicate three main lineament systems: 1) a NE-SW system ranging from N40 to N70; 2) an N-S system ranging from N10 to N45; 3) an EW to WNW-ESE systems ranging from N80 to N120. Most of lineaments extracted are localized in Kebdana, Amejjaou, Nador and Melilla regions. Compared to previous studies, the NE-SW system is consistent with an extensive period (Tor-tonian to Pliocene); the NW-SE system is consistent with the last compressive episode (Pliocene); the N-S system is consistent with the first compressive period (Late/End Tortonian).
Le présent travail consiste à faire l’étude des déformations tectoniques plio-quaternaires de la partie frontale de la chaîne rifaine, en particulier les Rides prérifaines et le
bassin du Saïs. La partie frontale est formée par la superposition d’écailles avec des plis et
failles, nées par le déplacement des unités prérifaines vers l’WSW pendant le Miocène
supérieur.
Les rides prérifaines chevauchent les formations plio-quaternaires du bassin du Saïs et
du Gharb. La sismicité actuelle dans cette région confirme l’activité des déformations
tectoniques.
Les Rides prérifaines et les formations plio-quaternaires du bassin du Saïs sont
déformées par des plis à axe parallèle aux Rides, par des chevauchements à vergence sud et
par des décrochements NE-SW à ENE-WSW senestres. Les formations lacustres plio-quaternaires sont formées de conglomérats à galets striés, calcaires et marnes. Elles sont
redressées ou renversées au contact des Rides.
Nous avons déterminé les paléocontraintes de ces déformations sur les fractures et sur
les galets striés, en utilisant des méthodes numériques récentes. Les résultats obtenus mettent
en évidence deux types de raccourcissements : N-S à NNW-SSE, avec des ellipsoïdes de
contraintes prolates, lié directement à la convergence Afrique – Eurasie ; et NE-SW à ENE-WSW résultant de l’échappement du Rif central vers le SW.
Tandis que le front de la chaîne est en compression, le bassin d’avant-pays est en
extension, probablement liée à une flexure de la croûte sous la charge de la chaîne du Rif.
Mots clés : Rif ; Plio-Quaternaire ; Tectonique ; Galets striés ; Géodynamique.
The 1755 Lisbon earthquake is still poorly understood due to its offshore location and complex macroseismic intensity pattern. Gutenberg and Richter (1949) tentatively assigned a magnitude between 8¾ and 9 judging from an estimated perceptibility radius of 2500 km. More recent attempts to estimate the magnitude from isoseismal areas led to results in the 8.5–8.7 range. These values have been adopted in several studies of the seismic hazard of southwest Iberia.
In this article, I use the available macroseismic data—a total of 1206 data points from Portugal, Spain, and Morocco—to reassess the magnitude of the 1755 Lisbon earthquake. Because a training set of instrumental earthquakes was not available, I apply the technique of Bakun and Wentworth (1997) in conjunction with the ground‐motion model of Atkinson and Wald (2007), which was selected through comparison with the 1969 M 7.8 Gorringe bank earthquake data. I obtain a moment magnitude of 7.7±0.5, significantly lower than previous results. The epicentral location obtained with this analysis is offshore southwest Iberia—as expected given the tsunami effects—but poorly constrained.
Based on the pattern of intensity data outliers, I suggest that the source was complex and spatially distributed, with part of the rupture taking place onshore or inshore. I propose an explanation for the large tsunamigenic power of the earthquake, which invokes the basal rupture of the Gulf of Cadiz accretionary prism. Finally, I discuss the implications for hazard assessment of the type of complex rupture proposed.
Resumen. El terremoto del 11 de mayo de 1624 está considerado como una de las peores catástrofes naturales de la historia de Marruecos. El seísmo se produjo en una de las zonas sísmicas más activas y pobladas del país, provocando la pérdida de vidas humanas y daños considerables en Fez y sus alrededores.
El terremoto de 1624 se produjo en un momento crítico de la historia de Marruecos (decadencia de la dinastía saadí y guerras internas), lo cual explica la ausencia de informaciones suficientes sobre los
daños. Se ha consultado fuentes primarias marroquíes y europeas. Se ha llevado a cabo una revisión completa de testimonios históricos y revistas que describen este terremoto. En una segunda etapa,
hemos intentado relacionar esos efectos con el contexto geológico local y los datos sismológicos recientes. Y finalmente reexaminando las fuentes históricas de la distribución de la intensidad sísmica.
La región del epicentro del terremoto de 1624 estuvo probablemente situada a lo largo de los relieves montañosos alineados E-W que constituyen las Rides Prérifaines. Para la evaluación de la magnitud del seísmo se han utilizado las relaciones empíricas establecidas para Marruecos.
Es evidente que la comprensión de este terremoto podrá ayudar a mejorar la evaluación del riesgo sísmico de la región.
Palabras clave: Sismicidad histórica; datos macrosísmicos; parámetros del seísmo; tectónica; Fez; Marruecos.
The 11 May 1624 Fez earthquake is one of the worst natural disasters in the history of Morocco, and caused serious damage in the city of Fez and its surroundings. It occurred in one of the most active seismic zones in the country, and in an area that is now densely populated. The 1624 earthquake occurred at a crucial time in the history of Morocco (decline of the Saadian dynasty and outbreak of internal wars) which explains the lack of sufficient information on the extensive devastation that resulted from the earthquake. We consulted several Moroccan and European sources, whenever possible first hand. A complete historical review of testimonies and journals of the time describing this particular earthquake has been carried out. Correlation these effects with the local geological setting and the most recent seismological data shows that the focal region of the 1624 Fez earthquake is likely to be located somewhere along the E–W trending Rides Prérifaines, the main frontal thrust of the Rif. An evaluation of the magnitude of the earthquake is proposed, on the basis of existing empirical relationships. Understanding this earthquake may improve the seismic hazard assessment of the region.
In the South Rifian ridges (SRR), the dominated structures correspond to the faulted anticline characteristic of a foreland orogeny context, front of the Rif Alpine belt. These anticlines correspond to thrust propagation folds. Geometric model of these structures shows that the normal faults have controlled the Mesozoic sedimentation during extensive episodes and participated in determining areas of thrusting during Miocene compressional phases. However, the normal fault strike which is relative to the direction of the shortening determined the geometry of diverse folds whether into the frontal ramps, lateral, or oblique. In the meantime, the systematic fracturing study in the Jurassic limestone beds, in different parts of the folds with axes oriented E-W, NW-SE, and NE-SW, permits to propose a relative fracturing chronology and tries investigating the relationship between folding and fracturing. The three main fracture families, oblique, transversal, and axial, appear simultaneously during the amplification of the fold. The simple shear in the limb contributes the latest to the folding reactivation and the density of the intensification of these microfractures. Likewise, given the important downslope fold limb dip of the ramp propagation folds, theoretically the shear intensity is more important, and micro-fractures are more important in the downslope fold limb, thus the uphill one.
This work aim to assess the risk of tsunami along the Moroccan Atlantic coast. The approach is based on the risk scenario which combines the hazard numerical modelling, spatio-temporal, stakes and criteria of the physical and human vulnerability. This study consists firstly in assessing the exposure of the Moroccan Atlantic coast to tsunami risk at the regional scale. The approach based on coupling the numerical modeling of hazard of tsunami similar to 1755 and regional land use database. It provide a set of geo-risk indicators aggregated in several meshes on the ground (municipal grid square with side of 5km), make possible to draw a comparative analysis of the risk across Morocco. Secondly it involves the risk of loss of life in case of major tsunami at the city of El Jadida, expressed in terms of geo-risk indices. Human vulnerability is expressed by a probability of loss of life. These probabilities are given by matrices based on data from marine submersion numerical modeling and vulnerability criteria determined through empirical curves damage. Risk indices obtained include the spatio-temporal distribution of the population, in the hourly step for two seasons. [etc.]
The Miocene basin evolution of southeastern Spain and eastern Morocco is linked to a "shear zone" elongated from SW across the Alboran Sea. In Spain the magmatism is mostly calc-alkaline (or K-rich calc-alkaline). Most of the products are locatred on strike-slip faults (Almeria-Cabo de Gata). Lavas of dacitic compositions are interpreted as products of crustal anatexis. During Messinian time, lamproites are erupted over an extended area. Later (Plio-Quaternary), alkali basalts are located near Cartagena. In Morocco, calc-alkaline magmatism is not as developed as in Spain; late Tortonian-Messinian volcanoes (Gourougou, Guilliz) have erupted of shoshonitic lavas. Alkali basalts are abundant and appear from the end of Messinian to Quaternary all over northwestern Africa. In the studied area, there are no chronological nor geochemical polarity of the magmatism according to the existence of a Miocene subduction. The association of the magmatism with tectonics and basin evolution shows that it is linked with their aperture. The structure of the lithosphere, as it appears from the geophysical data, shows the existence of two different crusts, separated by the western part of the "shear zone". Trans-Alboran calc-alkaline magmatism is clearly correlated with the activity of this "shear zone", from Miocene to present time.
The building of the Alpine Rif belt (southern limb of the Betic-Rif orocline) is restored, mostly based on the Tertiary stratigraphic and metamorphic data set. The Betic-Rif Internal zones derive from an exotic Alboran Terrane partly involved in a S-dipping Betic subduction during the Late Cretaceous ?-Eocene. Incipient collision of the terrane against Iberia triggered back-thrust tectonics south of the Internal mountain belt during the latest Eocene-Oligocene. A N-dipping Maghrebian subduction developed from that time up to Middle Miocene, responsible for the rifting of the internal Alboran Terrane. Docking of the extending Alboran Terrane onto the North African margin occurred during the Neogene through the closure of the Maghrebian Flysch oceanic trough, with southwestward growth of the external accretionary prism, and foredeep subsidence. Subduction zone westward roll back associated with delamination of the dense lithosphere seem to account for the Betic-Rif late orogenic evolution.
In this study, 19 focal mechanisms of Moroccan earthquakes are presented. These mechanisms give an idea of the modalities of convergence between the African and Eurasian plates in the western Mediterranean and east Atlantic areas. In the Rif, High Atlas and Atlantic areas, almost all the mechanisms correspond to strike-slip or to reverse faulting. The pressure axes are oriented N-S to NW-SE. The review of the neotectonic data available for the area shows that the mechanisms reflect the continuation of the Plio-Quaternary tectonic activity. The solutions are also in agreement with the plate-kinematic studies, which suggest a north-northwestwards motion of Africa towards Europe, around a pole located in the Atlantic, south of the Canary Islands. -from English summary
Dans cette étude, 19 mécanismes au foyer de séismes marocains sont présentés, parmi lesquels 13 sont nouveaux et 6 sont révisés. Ces mécanismes, en plus de ceux étudiés par d'autres auteurs, permettent d'avoir une idée sur les modalités de la convergence entre les plaques africaine et eurasiatique dans la partie ouest-méditerranéenne et est-atlantique. Ainsi, dans le Rif, le Haut Atlas et la région atlantique, presque tous les mécanismes correspondent à des décrochements ou à des failles inverses. Des solutions en faille normale sont également obtenues. Les axes de pression sont en majorité orientés N-S a NW-SE. La revue des données néotectoniques disponibles pour la région montre que ces mécanismes reflètent la poursuite de l'activité tectonique plio-quaternaire"
In this study we aim to understand the effect of static and dynamic
stress changes in promoting earthquake failures on secondary faults.
Toward this goal we solve the equation of motion of a spring-slider
dynamic system including inertia and using rate- and state-dependent
constitutive laws. We separately investigate the dynamic response of
this fault analog system to a sudden stress change represented either as
a stress step or as a stress pulse, which are used to model permanent
(static) and transient (dynamic) stress perturbations. The induced
earthquake failure does not occur immediately at the application of the
coseismic stress change, but it is delayed in time: we define this time
interval as the triggering delay. For a given stress perturbation, we
analyze the dependence of triggering delays on different system
conditions and constitutive parameters. Our results clearly show that
the effects of static and dynamic stress changes are quite different.
While a static stress change is able to advance as well as to delay an
induced instability depending on its sign, a dynamic stress pulse is
only able to promote a nearly instantaneous failure, provided its
amplitude is positive and large enough with respect to the direct effect
of friction. In other words, dynamic stress changes can only cause
nearly instantaneous failures, without any relevant triggering delay.
These results should be considered in interpreting the seismicity rate
changes caused by large earthquakes at least as long as seismic events
are interpreted as sliding instabilities obeying rate- and
state-dependent friction laws.
This is not an abstract but a short comment.
This paper is important because it proposes, for the first time, that in the Betic-Rif Orogen the Miocene tectonic transport is mainly westward and not northward (Betics) and Southward (Rif) as previously assumed. In addition, the paper emphasizes, also for the first time, the role of subduction and slab retreat in the development of the Betic-Rif Arc.
The M_w = 7.4 Izmit, Turkey earthquake triggered widespread regional seismicity in Greece over a study region extending from 400 km to nearly 1000 km away from the epicenter. Small events began immediately after the passage of the mainshock surface waves suggesting that the transient stresses of the seismic waves were the trigger. The increase in cataloged earthquakes in ordinary continental crust is a new observation and is statistically significant at the 95% level. Unlike the previous example of distant triggering during the Landers earthquake, the activated seismicity occurred entirely in non-volcanic areas. The Greek sites were triggered by waves with amplitudes at least a factor of 3 lower than the observed triggering threshold for Imperial Valley. We speculate that dynamic triggering on a regional-scale results in countrywide episodes of increased seismicity, or “superswarms”, in regions with low triggering thresholds such as Greece.
The tsunami generated by the 1755.01.11 earthquake affected mainly the coasts of the Iberian Peninsula and Northwest Morocco and was observed all over the North Atlantic coasts. The catastrophic dimensions of that phenomenon had a tremendous impact on the city of Lisbon and on several villages along the south coast of Portugal. The earthquake was felt all over Europe and the seismic intensity was estimated as X–XI (Mercalli Intensity Scale) at Lisbon and Southwest Portugal (Cape S. Vicente). The most destructive waves were observed along the coast of Portugal, specially in Lisbon, in the area of the S. Vicente Cape, along the Gulf of Cadiz and Northwest Morocco.
The southwestern part of the western Mediterranean Alboran Basin, including part of the Alboran ridge (Xaouen Bank), was investigated through the analysis of 28 intersecting multichannel seismic lines. The seismic stratigraphy is tied to the Amoco well El-Jebha 1. Five seismic units or subunits are described from the Quaternary to the middle (and lower?) Miocene. The acoustic basement is interpreted to be mainly Paleozoic and Triassic metamorphic rocks of the Alboran Domain nappes, and, in places, middle Miocene-Messinian calc-alkalic volcanics. In the depocenters, the thickness of the sedimentary infill (mostly clays and turbidites) exceeds 9 km. Normal faults of middle Miocene-Tortonian age are broadly parallel to the coast, and dip either seaward or landward. They were mostly inverted during pre- and post-Messinian episodes of compression, which formed a set of en echelon, north-verging faulted folds in the Alboran ridge area, in relation with sinistral movement along the offshore projection of the Jebha fault. After Pliocene subsidence, a final episode of compression reactivated the earlier folds and pushed the Alboran ridge onto the Moroccan slope. The complex structural history suggests many structural and stratigraphic potential hydrocarbon traps. A high-resolution seismic survey could lead to the definition of new exploration plays.
Earthquakes fault plane solutions (FPSs) are routinely computed on the basis of various techniques and are reported in the literature with a wide range of formats and conventions. Although the equations relating the various parameters are well known and relatively simple, their practical application often arise to numerical singularities and indeterminations that sometimes are not well known by the authors and thus may result in wrong or inaccurate reporting of parameters. Such inaccuracies and mistakes affect about 40% of the published data we have examined to test our programs. Moreover the current use, in the seismological community, of at least two different coordinate systems to represent the Cartesian components of vectorial and tensorial quantities is a further cause of confusion. In order to simplify the management of such data, we have prepared a structured package of FORTRAN 77 subroutines performing almost all of the possible computations and conversions among different parameters and coordinate systems. The package has been extensively tested with the data of a revised database of FPS of Italy and surrounding regions (presented in a companion paper) as well as of CMT solutions included in the Harvard catalog.
Out-of-sequence thrusts (OOST) are those thrusts which do not obey the foreland propagating or in-sequence deformation style. They include both isolated thrusts which develop hindward of the thrust front and sequences of break-back thrusts which propagate from the foreland to the hinterland. Two end-members of a series of OOST types are recognized: (1) older in-sequence thrusts which are reactivated along their entire length and (2) completely new thrusts which propagate through already deformed thrust sheets. Between the two end-members are thrusts composed partially of reactivated in-sequence thrust sequences and partially of new, entirely out-of-sequence segments. OOSTs can be initiated for a variety of reasons including: (1) keeping the orogenic wedge at critical taper, (2) break-back sequences from the suture zone in the overriding plate, (3) ramping to overcome a sticking point which inhibits in-sequence thrust propagation, and (4) during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust. Many different types of thrust behavior including gravity sliding, plucking, and derivation of isolated horses from ramps may mimic some of the characteristics of OOSTs. Consequently, it may be difficult to conclusively prove an OOST origin for a complex thrust geometry.
The focal mechanisms and depths of 10 moderately sized earthquakes along the Azores-Gibraltar plate boundary were analyzed by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases. The data, together with a compilation of results reported for very large events from the past 30 years and for 2 recent events in 1983, place new constraints on the present-day deformation of this boundary separating the Eurasian and African plates. -from Authors
Three hundred and twenty earthquakes recorded in the course of 5 weeks in 1989 in the central Rif of northern Morocco provided no evidence for activity on the Nekor fault (the principal geological structure in the region) itself; most of the seismicity is located northwest of the Nekor fault. Most of the 41 fault-plane solutions show mainly strike-slip motion (left-lateral on N-S striking planes) with P axes trending N 145 deg. This deformation is consistent with the N-S trending normal faults caused by the E-W Quaternary extension and with the present relative NW-SE convergence between Africa and Europe. The active deformation around the eastern Rif is better represented by a distributed shear zone than by a strike-slip fault.
A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent powerful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.
The continuous uplift of the High Atlas alpine belt controls sedimentary dynamics of Neogene molasses and Quaternary geomorphic processes in the Central Haouz intramontane basin. New data show that the studied area has always been under compression from Early to Late Quaternary. The deformations, due to reverse faulting or downbending, are located in the vicinity or in continuation to N 50 to N 70 E and N 120 E faults, which constitute the main trends of the High Atlas of Marrakesh.-from English summary
The Middle Atlas basin (Morocco) was formed at the boundary of the Moroccan and Oranese Mesetas during the early stages of the opening of the Atlantic ocean. Structural shapes indicate two main structures: rhombohedral-shaped depocenters filled up with predominantly carbonate sedimentary rocks of Middle Jurassic age; straight ridges forming synsedimentary anticlines that bound depocentres. These structures evoke a clear image of "positive flower structure' that forms in convergent strike-slip. In order to understand these various structures and the interrelationships among them, we compare our observations with microtectonic and mathematic analog models of strain/stress distribution: (1) at the ends of a strike-slip; (2) in both a releasing and restraining stepover discontuity along strike-slip faults. We propose a multiple releasing and restraining stepover model of convergent sinistral strike-slip faults, first along an ENE trend from Toarcian to Bajocian, and then, along a NE trend since the Bathonian times. -from English summary
The earthquake of February 28, 1969, which occurred about 500 km west of the Strait of Gibraltar, was felt over the entire Iberian Peninsula, in a wide region of Morocco, and south to the Canary Islands. It had a long sequence of aftershocks continuing for at least 10 months, but, nevertheless, most of the energy seems to have been liberated in the main shock of which the mb was 7.4.
The source mechanism solution indicates a fault plane striking N 67°W and dipping 68°SW, with motion principally of the strike-slip type. There also is some overthrusting. The horizontal extent of faulting is of the order of 90 km.
The analysis of static stress changes due to co-seismic dislocations has been widely applied in recent years to study the variation in failure stress on well-known segmented seismogenic faults. These studies assert that earthquakes induce changes in static stress on neighboring faults that may delay, advance, or trigger impending earthquakes. So far, most of the investigations have focused on California strike-slip faults and, more recently, on thrust faults. In this work, we study the static stress changes in an extensional tectonic setting. Following Okada (1992), we calculate static stress changes caused by normal-faulting earthquakes in the southern Apennines (Italy) using a 3D dislocation model in an elastic half-space. We first studied the static stress changes produced by the 1980 Irpinia earthquake. This normal-faulting event consisted of four fault segments that ruptured together within 40 sec. The last two subevents occurred 20 and 40 sec after the nucleation on two normal faults parallel to the main one. Because the rupture of these subvents are separated by a sufficiently long time interval, we assume that, before each subevent, the dynamic stress has reached its static configuration. Therefore, we analyze the static stress changes caused by each subevent in order to determine the variations of the initial conditions for subsequent ruptures. Moreover, we applied our procedure to study the static stress changes after the 1980 Irpinia earthquake on the fault zone where the 1990 and 1991 Potenza earthquakes (M ≈ 5) occurred. Our calculations indicate that, even if the amount of stress change depends on the fault geometry assumed for the 20-sec subevent of the Irpinia fault, the static stress increased on the Potenza fault after the 1980 Irpinia earthquake. This event also increased the static stress on seismogenic faults that are believed to have ruptured in the 1732 and 1857 earthquakes (M ≈ 6.5). In these calculations, we have considered the effect of a regional extensional stress field whose minimum principal axis is horizontal and oriented 216° N. The results show that the 1980 Irpinia earthquake increased the static stress by roughly 1 bar on the 1732 fault zone. Right lateral strike-slip faults oriented nearly E-W, such as the Potenza fault, are favorably oriented with respect to the total stress resulting from the extensional regional stress field and the coseismic stress induced by the Irpinia earthquake. Finally, we investigate the static stress changes caused by several historical earthquakes that occurred along the southern Apennines seismogenic belt. Most of the large historical earthquakes occurred in areas of static stress increase, yielding evidence for fault interaction.
A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. Several practical suggestions to avoid mathematical singularities and computational instabilities are presented. -from Author
The Triassic rifting of Pangea and the formation of passive margins in the Central Atlantic ocean were strongly controlled by the preexistent crustal structures. The deformation of the upper continental crust was guided by the Triassic reactivation of the old structures, during the development of an abnormal heat flow. The presence of Triassic basins, their sedimentary filling and the thermal regime lead to distinguish, in Morocco, two rift zones separated by the Meseta: i) the Atlantic rift, located within the continental margin and the Essaouira-Agadir coastal basins, and ii) the "Atlasic rift', composed of the central High Atlas and the Middle Atlas. Regionally, these rifts show an en-echelon pattern, between two sinistral major faults: the Newfoundland and the South Atlasic faults. Later on, from the Liassic to the Middle Jurassic, the Atlantic rift widened and gave way to the Atlantic oceanic crust. The "Atlasic rift' aborted and was dissected into numerous small basins. -from English summary
We study the Tell and Rif neotectonic structures and the geodynamics of the Africa-Eurasia plate boundary in the western Mediterranean. Pliocene and Quaternary deformations west of the Gibraltar strait show NE-SW fault-related folds related to a N-S to NW-SE compressional movements. Neogene and Quaternary basins which are elongated in a E-W direction and reflect a N-S shortening direction, are superimposed on the inherited Alpine geological structures. Based on geological and geophysical data (seismicity and magnetic anomalies) in the Alboran sea, the E-W dextral transcurrent Yussuf fault appears to control the distribution of the NE-SW trending Quaternary folds associated with thrust faults, blind or sometimes emergent, similar to that of El Asnam, and extends inland across the Cheliff basin. Recent folding in the Rif is parallel to the main frontal thrust but is also associated to the main left-lateral reverse faults of Jebha and N'kor. The recent tectonics of the Tell and Rif continental domain and surrounding areas (Alboran sea and Atlantic) are interpreted as a transpressive system which results from right lateral movements along the Azores-Gibraltar transform zone and the convergence of Africa toward Iberia (Eurasia). The active tectonics in agreement with the main seismicity support the existing deforming zone (called GALTEL) of 50 km to 100 km wide, extending from Gorringe, across the southern Alboran sea to the Tell Atlas. A kinematic model explains the interaction between dextral transcurrent faults and left lateral faults with block rotations, no lateral extension (pinned model) and lateral displacement in the Rif mountains. Shortening and lateral movement rates during the Quaternary are estimated to be 2.0-2.5 mm/year, and block rotation rate of 2.5-3.9 °/Ma, then the plate boundary seems to correspond to a transpression zone with active thrusts controlled by dextral transcurrent faults, the deformation being widely distributed due to preexisting Alpine structures.
This region is part of the lithospheric plate boundary between Africa and Eurasia and shows a continuous seismic activity of moderate magnitude (M <5) and the occurrence of large earthquakes separated by long time intervals. The distribution of epicenters for the period 1965 to 1985 shows that earthquakes are located south of the Cadiz-Alicante fault and that they can be associated with observed geological faults. An important activity at intermediate depth (30 km < h ≃ 150 km) is located between Granada and Malaga and is separated from the very deep (h ≃ 650 km) earthquakes also occurring in the region. Eight fault-plane solutions have been determined that can be related to the tectonics of the area. The mechanics of the intermediate depth shocks can be explained in terms of a subduction zone from Africa in a NW direction that is under compression. -from Authors
L’histoire tectono-sédimentaire du Rif (Maroc) et de son Avant-pays, du Tortonien inférieur jusqu’à l’actuel, est contrôlée par des effets de deux phénomènes: la convergence de l’Afrique vers l’Europe et l’expulsion du Rif vers le SW.
Dans le détail, ces deux processus ne sont pas aussi tranchés, il peut y avoir passage d’un système à l’autre dans le temps et dans l’espace.
Les directions moyennes du raccourcissement dépendent de la prépondérance de l’un ou de l’autre.
Depuis le Pliocène, l’expulsion vers le SW est bloquée et de ce fait, seuls les effets de la convergence se font sentir. Ils se traduisent par une direction moyenne NW-SE à NS du raccourcissement.
The analysis of fault plane solutions of earthquakes is carried out in most seismotectonic studies to characterize the tectonic deformation styles and to estimate strain and stress directions in the investigated areas. Nevertheless the data available in the literature, are reported with different formats and notations and, in most cases, only in papery form, so that they are not suitable to be handled by computer procedures and graphic packages. Sometimes the data are reported with typographical errors, inaccuracies and inconsistencies that make them almost useless for other investigators. In some cases several solutions, often very different among each other, are given for the same earthquake by different authors thus requiring a choice be made among them.We have tried to solve some of these problems, in building a comprehensive database, on a Microsoft ACCESS platform, including most of the mechanisms (presently about 5000) published for the Italian region and more generally for the Mediterranean area.We tested the perpendicularity of nodal planes and/or P and T axes of all solutions and, when both axes and planes are given, even their mutual consistency. Moreover from the comparison of planes and axes we were able to detect and sometimes to correct misprints and other types of errors. All the parameters are recomputed uniformly and consistently, keeping track of all the corrections made. We also established an automatic procedure, based on several criteria, to choose the most “representative” solution when more than one is available for the same earthquake.The MS-ACCESS application also allows to making selections on the earthquake data, to display the plot of the mechanisms and to export data files suitable to be handled by graphic software and user written procedures.
We use the thin-sheet plane-stress approach to study the present-day dynamic behavior of the plate boundary between Eurasia and Africa along the Azores–Gibraltar region. This plate boundary, which extends from the Azores triple junction to the Gibraltar strait, shows a tectonic regime that changes from transtension in the west to transpression in the east, with a strike–slip motion in its central segment. Seismological data reveal that the western and central segments are currently marked by a linear series of earthquakes indicating that the plate boundary is located in a narrow zone. In contrast, the eastern segment is not so well defined and deformation spreads over a much broader area. To apply the thin-sheet approach, we combined heat flow, elevation and crustal thickness data to calculate the steady-state geotherm and the total strength of the lithosphere. Several models with different fault friction coefficients and geometries at the eastern segment of the plate boundary were tested. Results are compared with the maximum compressive stress directions from the World Stress Map, and the calculated seismic strain rates and slip vectors from earthquake data. The best fitting models are consistent with the rotation pole of Argus et al. [D.F. Argus et al., J. Geophys. Res. 94 (1989) 5585–5602], and show that the rheological behavior of the plate boundary must necessarily change from the western and central segments to the eastern segment. The diffuse character of the plate boundary east of the Gorringe Bank is dominated by the transition from oceanic to continental lithosphere, the weakness of the Alboran domain, and the convergence between the African and the Eurasian plates. The displacement of the Alboran domain relative to the African plate may play a major role in stress propagation through the Iberian Peninsula and its Atlantic margin.
Centroid-moment tensor inversion of fundamental-mode surface waves, recorded at regional distance, enables retrieval of earthquake source geometry with high reliability. In particular, this is true also for events too small to be analyzed using the standard Harvard CMT scheme, which utilizes long-period body waves recorded teleseismically in the waveform fitting. We routinely apply the Regional Centroid-Moment Tensor (RCMT) algorithm to study intermediate-magnitude earthquakes of the European and Mediterranean region, and maintain a catalog that now spans 6 years. We report on recent developments, in particular the addition of focal mechanisms for earthquakes that took place during the last two years. We select events with a preliminary (bulletin) magnitude lower than 5.5, since these may not be routinely analyzed using the standard CMT algorithm. For several of these events we have determined moment magnitudes as high as M_W = 6.3. In the most favorable situations, especially for events in central Italia recorded by nearby seismographs in a range of azimuths, we can analyze earthquakes with M_W as low as 4.0. For events with magnitudes around 5.5, RCMTs agree very well with standard Harvard CMTs. The RCMT catalog now covers the period 1997-2002 and lists some 400 solutions. Among them are the recent Northern Sicily, September 6, 2002, event (M_W=5.9) with some aftershocks, and the Molise events (M_W 5.7 and 5.8) that occurred in October-November 2002. We also report on the rapid determination of RCMTs shortly after occurrence of an earthquake. Quick RCMTs are based on waveform data from a few seismographs available in quasi-real time. These solutions are always reviewed before dissemination. Ana posteriori comparison of Quick solutions with the revised Catalog determinations attests to their reliability. Our focal-mechanism data are used mainly for the study of the strain and stress distribution, but also for seismotectonic analysis of areas disturbed by earthquakes immediately after their occurrence. Some specific recent examples will be discussed.
It is possible to use the waveform data not only to derive the source mechanism of an earthquake but also to establish the hypocentral coordinates of the 'best point source' (the centroid of the stress glut density) at a given frequency. Thus two classical problems of seismology are combined into a single procedure. Given an estimate of the origin time, epicentral coordinates and depth, an initial moment tensor is derived using one of the variations of the method described in detail by Gilbert and Dziewonski (1975). This set of parameters represents the starting values for an iterative procedure in which perturbations to the elements of the moment tensor are found simultaneously with changes in the hypocentral parameters. In general, the method is stable, and convergence rapid. Although the approach is a general one, we present it here in the context of the analysis of long-period body wave data recorded by the instruments of the SRO and ASRO digital network. -Authors
Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE-SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a Bank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE-SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa.
Neotectonic structures distributed from the Goringe bank to the Tell Atlas mountains show that the Africa-Eurasia plate boundary in the western Mediterranean corresponds to an east-west trending deforming zone (Goringe-Alboran-Tell). Main Pliocene and Quaternary tectonic structures of the northern Atlas Mountains are northeast-southwest striking fault-related folds, similar to that of El Asnam, and are related to north-south to northwest-southeast compressional movements. Active tectonics in agreement with the main seismicity distribution and focal mechanisms also support an oblique plate convergence and the existence of a 50 100-km-wide transpression zone from Goringe, across Alboran, to the Tell Atlas mountains. A kinematic model shows the interaction between dextral transcurrent faults and left-lateral faults with block rotations and no lateral extension (pinned model). Shortening and lateral movement rates during the Quaternary are estimated to be 1.0 2.3 mm/yr, and block rotation rate is estimated as 2.5° 3.9° /m.y. The plate boundary may correspond to a transpression zone, active thrust faults being controlled by deep-seated dextral transcurrent faults.
Several features of the Alboran Sea suggest that it may have been a high
collisional ridge in Paleogene time that subsequently underwent
extensional-collapse, driving radial thrusting around the Gibraltar arc.
(1) The basin is underlain by thin (13-20 km) continental crust, has an
east-west-trending horst and graben morphology, was the locus of Neogene
volcanism, and has subsided 2-4 km since the middle Miocene. (2)
Extension and subsidence in the basin coincided in time with outwardly
directed thrusting in the surrounding mountain chains. (3) Africa and
Europe were converging slowly during this period, so extension must have
been driven by internally generated forces. (4) Onshore, rocks
metamorphosed at 40 km depth are exposed beneath major low-angle normal
faults that separate them from low-grade rocks above. (5) Emplacement of
solid bodies of Iherzolite at asthenospheric temperature into the base
of the collisional edifice in late Oligocene time suggests detachment of
the lithospheric root beneath the collision zone. This would have
increased the surface elevation and the potential energy of the system
and would have favored extensional collapse of the ridge.
The Neogene Taza-Guercif Basin, in north-eastern Morocco, is located in correspondence of the eastern termination of the Rifian arc and is aligned along with the Middle Atlas Shear Zone. Since Tortonian the tectonic evolution of the Taza-Guercif Basin was tied to repeated advancements of the Rifian front and to the transpressive tectonics of the Middle Atlas Shear Zone. The major structures in this basin consist of thrust-related anticlines, aligned along with the Middle Atlas structures, and second-order normal faults, elongated according to the Rifian trend. The latter are well developed mainly in the north-western sector of the basin (Bab Stout Area). Apart from the Bab Stout Area, the basin is dominated by compressive and transpressive structures (Guercif Transpressive Area). The shortening directions responsible for the development of the major and minor compressive structures in the basin vary from NNW to NNE, but no superposition relationships have been found. This change of the paleo-stress field orientations is here ascribed to a progressive deformation under the constant push between Africa and Europe.An evolutionary model for the development of these two structural areas in relation to the tectonics of the Rif and of the Middle Atlas is here proposed. In this model, the sinistral transpressive Middle Atlas Shear Zone is regarded as the boundary between the Moroccan Meseta microplate, to the west, and the African indenter to the east, which is moving faster to the north.
Seismic reflection profiles and magnetic intensity measurements were collected across the southern continental margin of Spain and the Alboran basin between Spain and Africa. Correlation of the distinct seismic stratigraphy observed in the profiles to stratigraphic information obtained from cores at Deep Sea Drilling Project site 121 allows effective dating of tectonic events. The Alboran Sea basin occupies a zone of motion between the African and Iberian lithospheric plates that probably began to form by extension in late Miocene time (Tortonian). At the end of Miocene time (end of Messinian) profiles show that an angular unconformity was cut, and then the strata were block faulted before subsequent deposition. The erosion of the unconformity probably resulted from lowering of Mediterranean sea level by evaporation when the previous channel between the Mediterranean and Atlantic was closed. Continued extension probably caused the block faulting and, eventually the opening of the present channel to the Atlantic through the Strait of Gibraltar and the reflooding of the Mediterranean. Minor tectonic movements at the end of Calabrian time (early Pleistocene) apparently resulted in minor faulting, extensive transgression in southeastern Spain, and major changes in the sedimentary environment of the Alboran basin. Active faulting observed at five locations on seismic profiles seems to form a NNE zone of transcurrent movement across the Alboran Sea. This inferred fault trend is coincident with some bathymetric, magnetic and seismicity trends and colinear with active faults that have been mapped on-shore in Morocco and Spain. The faults were probably caused by stresses related to plate movements, and their direction was modified by inherited fractures in the lithosphere that floors the Alboran Sea.
The terranes which outcrop to the north and south of the westward extension of the Jebha Fault show considerable differences in their nature and structural organisation. The pre-Oligocene basement is composed of synorogenic synclines filled with detrital deposits. In the North, the synclinal troughs are oriented North-South and filled with siliciclastic material. The synclinal structures on top of the western extension of the Jebha Fault are orientated N 140° and are also filled with the same material. South of this westward extension of the Jebha Fault, the orientation of the synclinal structures is north-south; they are filled with bioclastic calcareous material.The synorogenic synclines, situated on top of the Jebha-Arbaoua limit show characteristics of the detached sedimentary cover by their position and orientation. This cover formed the Arbaoua-Jebha lateral ramp during its movement towards the west. Developement of this ramp is explained by the crustal discontinuity inherited from the period of rifting.The resulting palaeomorphological relief allows the difference between the zones situated at each part of the limit to be understood, in particular, it permits the understanding of the differences in the detrital material, in its role as a barrier opposing the arrival of bioclastic calcareous material from the African margin (situated southwards of the limit).The Arbaoua-Jebha lateral ramp appears as a major and original feature of the Rift chain.
Several recent tectonic models for the Alpine deformation in the Rif-Betic Cordillera have proposed some form of uplift of the hiterland of the thrust belt (presently the Alboran Sea), which as it collapsed and extended, drove thrusting in the more external zones. This thrusting episode occurred predominantly during the Middle and Upper Miocene; consequently if the tectonic model is correct, nasin fill of the same age underlying the Alboran Sea should be extensively affected by large scale extensional features. This is not the case in the Western Alboran Basin where seismic reflection and well data indicate that little if any Upper Miocene extension has occurred, and deformation of the Miocene section is predominantly by shale diapirism. Although the crust under the Alboran sea is thinned the timing of the thinning is unknown. It may either be attributed to a pre-Miocene extensional phase, or to delamination of the lower crust perhaps caused by subduction during the Oligocene.
Major tectonic units of Spain have been investigated by deep seismic sounding experiments since 1974 to determine crustal structures and to delineate their differences. These areas are the central part of the Hercynian Meseta, and the Alpine chains: the Betic Cordillera in the south, including the Balearic promontory and the Alboran Sea, and the Pyrenees in the north. The main features of the crust and the upper mantle along a NNE-SSW cross-section from the Pyrenees to the Alboran Sea are described. The crust under the Meseta is typical of Hercynian areas found elsewhere in Europe, with an average thickness of 31 km, whereas the two Alpine regions are characterized by very large lateral inhomogeneities, such as rapid thickening of the crust to 50 and 40 km under the Pyrenees and the Betics, respectively. The deep-reaching E-W-trending North Pyrenean fault has a throw of 10-15 km at the base of the crust. A Pn velocity of 8.1 km s-1 is found under the entire Iberian Peninsula. In the Alboran Sea, strongly varying thicknesses of sediments, shallow variable depths to the Moho (~ 13 km under the Alboran ridge), and strong variations of Pn velocity between 7.5 and 8.2 km s-1 have been found.