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 Pre´rifaines, the main frontal thrust of the Rif.Historical data indicate a macroseismic field closed around the towns of Meknes and Fes. Our structural–geological fieldwork and remote sensing analysis in the epicentral area of the Meknes earthquake, along the local major recent faults, indicate that the E–W-oriented thrusts of the Rides Pre´- rifaines are active.Throu gh a re-examination of historical sources compared with field work and air photo interpretation, we could individuate the traces of coseismic surface faulting of the 1755 Meknes earthquake in two areas of the Rides Pre´rifaines, both part of the local thrust front: the Jebel Zerhoun area and the Jebel Zalagh area.Tectonic data on the Quaternary stress fields derived from our fieldwork and from literature, consistently with the revised focal mechanisms in the region, indicate active shortening oriented NNW–SSE to N–S in northern Morocco.The data collected seem therefore to indicate the thrusts of the Rides Pre´rifaines, located within the macroseismic area of the Meknes earthquake, as the most probable seismic source of that event.As such, the activation of the thrusts of the Rides Pre´rifaines would be consistent with this stress regime, which in turn would be similar to the stress field maintained as responsible for the 1 November, 1755 Lisbon earthquake.W e also attempted an estimate of the change due to the Lisbon earthquake of the Coulomb Failure Function (CFF) on the Meknes structure, as identified in this paper, in order to evaluate if the Meknes earthquake could have been induced by the 1 November, 1755 Lisbon earthquake, or a local distinct earthquake.Our modelling suggests that the latter hypothesis is the more likely one.