Abstract:
The source of the famous November 1st, 1755 “Lisbon” earthquake has been constrained to be an up to 200 km long structure in the offshore west of Cabo de S. Vincente, and the magnitude has been estimated in the range of 8.5-9.4. The stress regime argued for this shock would have been an around NNW-SSE-oriented compression. Less well studied is the successive “Meknes” earthquake, which occurred a few days later in Morocco (November 27), and was erroneously confused by the European contemporary reports with a strong aftershock, occurred on November 18, of the main seismic sequence of the November 1st 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 an E-W elongation of the macroseismic field, 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 Prérifaines are active. Through 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 Prérifaines, both part of the local thrust front: the Jebel El 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. Data collected seem therefore to indicate the thrusts of the Rides Prérifaines, 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 Prérifaines would be consistent with this stress regime, which in turn would be similar to the stress field maintained as responsible for the November 1st, 1755 Lisbon earthquake. We therefore attempted an estimate of the change 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 an after-event (triggered earthquake or aftershock) of the November 1st, 1755 Lisbon seismic sequence, or a local distinct earthquake.

Note:
Impact Factor 0,754

Descrizione sintetica del prodotto:
On November 27, 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. 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 November 27. Through a re-evaluation of the historical documents, compared with field analysis and air photo interpretation, we surveyed the sites where geological coseismic effects of the Meknes earthquake were described. Coseismic ground ruptures along active faults point to the E-W-trending thrusts of the Rides Prérifaines as the most likely seismic source. The surficial rupture did not occur directly along the surface emergence of the main thrusts of the Rides, the seismogenic structure, but along splay faults close to it. Activation of these thrusts, located within the elongated macroseismic area, agrees with the N-S-oriented stress regime acting in the region, indicated by focal mechanisms and neotectonic data. To constrain the regional seismotectonic setting we analysed the distribution of seismicity and the moment tensor sum based on available focal mechanisms and tried to evaluate the possible triggering of the Meknes earthquake by the Lisbon main shock. Our study supports the hypothesis that the 1755 was a local event, as indicated also by the localised macroseismic field elongated E-W around Meknes and Fes.

Ulteriori informazioni sul prodotto:
This paper is the first one which integrates historical, structural and geophysical data to study the effects of a historical earthquake which, in 1755, stroke in Northern Morocco, producing enormous damages and casualties. The recognition of this earthquake as a local shock provides new insights on the seismic hazard of this region, surrounding important cities like Meknes and Fes, and world famous archaeological sites like Volubilis. The active tectonics of the area has in fact been poorly investigated up to now, so that seismogenic structures and associated hazard are not well defined yet. The paper was published in Journal of Geodynamics, one of the best reviews in Earth Sciences, which presents high-quality papers focussing on structural geology, geodynamics and tectonics.