The 1755 “Meknes” earthquake (Morocco): field data and geodynamic implications
Introduction
The 1 November, 1755 “Lisbon” earthquake, accompanied by a large tsunami wave over a large part of the Atlantic coast, has been the most devastating event affecting the Atlantic and Mediterranean area and one of the strongest in history. Its magnitude has been estimated in the range of 8.5–9.4 (Machado, 1966, Lopez Arroya & Udias, 1972, Abe, 1979, Sousa Moreira, 1983, Martins & Mendes Victor, 1990, Baptista et al., 1998a, Baptista et al., 1998b). Its source has been constrained to be a structure, up to 200 km long, in the offshore west of Cabo de S. Vincente (Fig. 1), and two main hypotheses have been put forward to localise the epicentral area of the 1 November earthquake: (1) the area of Marques de Pombal (MP in Fig. 1), where active reverse faults have been identified by seismic investigation (Zitellini et al., 1999, Zitellini et al., 2001, Gracia et al., 2001); (2) the area of Gorringe Bank (GB in Fig. 1), where compressive deformations distributed over a 200-330 Km wide zone have been recognised (Hayward et al., 1999). This latter was also the epicentre of the 28 February 1969 earthquake (Mw=7.5–8.1). The shock of 1969, for the similarities of the two macroseismic fields (Fig. 1), has been assumed as an analogue of the 1 November 1755 earthquake (Levret, 1991 and references therein).
A few days after the main Lisbon earthquake a strong shock affected Morocco. Historical data indicate a macroseismic field closed around the towns of Meknes and Fes (Levret, 1991; Fig. 1). The most destructive effects were in the town of Meknes, from where it is referred to as the “Meknes” earthquake. European and Arab sources disagree about the date of the occurrence of this latter event. While contemporaneous Arab sources report unanimously 27 November for this occurrence (SFERN, 1985, Levret, 1991 and references therein; El Mrabet Azaroula, 1991), European historical sources report only about damages in Morocco due to an aftershock, which occurred on 18 November for which the same epicentre of the 1 November main shock was estimated (Pereira de Sousa, 1919–1928).
In this paper, we analyse the active faults of the Meknes area aiming at the identification of the possible seismogenic structure of that earthquake and discussing its relationships with the tectonics of the area. We also perform a computation of the stress field change induced by the Lisbon earthquake, in order to evaluate its possible influences in the Meknes area. This paper should also provide a better characterisation of the seismotectonic and geodynamic behaviour of the area in relation with the western Eurasia–Africa plate boundary.
Section snippets
Seismotectonics of the Alboran Sea and Gibraltar Arc
The western sector of the Eurasia–Africa plate boundary, running along the transform dextral Gloria Fault System, which relies the Azores triple junction to the Gibraltar Arc, is clearly defined by both bathymetry and clustering of seismicity (e.g. Udias et al., 1976, Grimison & Chen, 1986, Grimison & Chen, 1988, Buforn et al., 1988a, Hayward et al., 1999). In the Gibraltar Arc and Alboran Sea area, the plate boundary is less well defined and seismicity spreads over a larger area (McKenzie, 1972
The Rif-Atlas System
The Rif-Atlas System of Morocco consists of three Alpine chains delineating a big Z-shaped structure. The NE–SW-trending Middle Atlas relies the E–W-oriented Rif and High Atlas chains, respectively developed to the north and to the south of it (Fig. 3a and b).
The E–W-trending High Atlas bounds to the south the Moroccan Meseta, the Middle Atlas and the Oran Meseta. Its central and eastern sectors constituted the southern branch of the Triassic/Early Jurassic Atlasic basin, aborted and inverted
The 1755 Meknes earthquake and effects
The 1755 Meknes earthquake macroseimic area is located in correspondence of the Rides Prérifaines and of the northern part of the Meknes Basin (Levret, 1991) (Fig. 1, Fig. 3).
We performed a field survey in the Rif-Atlas system aimed at the identification of the major active faults. The main thrust fronts of the Rides Prérifaines and of the High Atlas show clear evidence of activity and tend to thrust onto the NE–SW-oriented Middle Atlas (Piccardi et al., 2001) (Fig. 3a and b).
Stress field variation of the 1755 Lisbon earthquake
The European historical sources report only a destructive earthquake after the Lisbon main shock. According to Levret (1991) this event was “an aftershock…on November 18 at 10 p.m.” that “was felt in both Portugal and Morocco”. We localised the Meknes source in Morocco so that we cannot regard it as an aftershock of the Lisbon earthquake in a classical sense. However the same Meknes earthquake could be considered as an after-effect of the Lisbon earthquake in the sense that it could be a
Discussion and conclusion
On 27 November 1755 a strong earthquake stroke the area of Meknes, in front of the E–W-trending Rides Prérifaines, the local external frontal thrust of the Rif, the southern branch of the Gibraltar Arc.
European historical sources confused the 1755 Meknes earthquake with an aftershock, occurred on November 18, of the main Lisbon event, while the Arab sources clearly date it to 27 November. Through a re-evaluation of the historical documents, compared with field analysis and air photo
Acknowledgments
Authors wish to thank Carlos Sanz de Galdeano, Ahmed Chalouan and an anonymous referee for critical review of the manuscript. The work was supported by CNR (Italy) and CNRST (Morocco) funds in the framework of the bilateral project “Structural analysis, active tectonics and fluid geochemistry along the Middle Atlas Shear Zone” (Italian responsible Giovanna Moratti, Moroccan responsible Mohammed Dahmani).
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