Reflections on the 2023 Marrakesh-Safi earthquake
27-09-2023 | Posted by Joaquín Martí
September 2023 will stand as a sombre chapter in the annals of North Africa, marked by the cruel hand of natural disasters. On the fateful 8th day of the month, at 22:11 UTC, the Marrakesh-Safi region of Morocco was jolted by an earthquake of moment magnitude Mw 6.8–6.9, with a maximum Mercalli intensity of IX. The epicentre of this seismic event laid 73 kilometres southwest of Marrakesh.
Two days later, the coastal Libyan city of Derna, home to approximately 100,000 souls, bore the brunt of torrential flooding unleashed by Storm Daniel. The flooding was further exacerbated by the failure of two dams constructed towards the 70’s, upstream from the city, originally intended to tame the waters flowing along Wadi Derna. Both natural catastrophes left a huge trail of destruction, with thousands of fatalities caused by the earthquake and tens of thousands by the floods.
The earthquake was the strongest instrumentally recorded event ever felt in Morocco. Indeed, it appears to be the strongest in Moroccan history, surpassed only by the upper estimates of the 1755 Meknes earthquake, at Mw 6.5–7.0. However, in terms of loss of life, it will not reach the level of the tragic 1960 Agadir earthquake. Despite its lower magnitude of 5.8, that devastating event, due to its shallow hypocentre in close proximity to the city, claimed the lives of 12,000-15,000 people, roughly a third of Agadir’s population at the time. Note that a one-unit increase in Mw represents a 32-fold increase in released energy.
Let us recall the concepts of seismic activity, hazard, vulnerability, and risk. Activity is the capacity to generate earthquakes. Hazard is the probability of experiencing each level of motions at a site over a certain period. Vulnerability is the probability of triggering a failure for each level of input motions. And risk is the convolution of hazard and vulnerability, weighted with the ensuing consequences.
In the case of the Moroccan earthquake, the seismic activity of the Marrakesh region is only moderate, thus explaining the relative lack of precedents of this scale of motions. In this situation, the hazard is also expected to be low: there is a low annual probability of experiencing high levels of motions at any location in the region.
The crux of the matter lay in vulnerability. Despite enhancements to building codes following the 1960 Agadir earthquake, the affected area included mountain villages characterized by substandard construction, rendering them incapable of withstanding the seismic forces that ultimately led to their tragic collapse, claiming the lives of their inhabitants. The consequences that enter the risk calculations are their deaths.
In the face of such calamities, our hearts weigh heavily. Moreover, instances like those mentioned above resonate deeply with us, given our expertise in areas such as evaluation of the seismic hazard, earthquake engineering, stability of earth embankments, and flooding calculations.