On 8 September 2023 at 23:11 DST (22:11 UTC), an earthquake with a moment magnitude of 6.9 and maximum Mercalli intensity of IX (Violent) struck Morocco's Al Haouz Province. The earthquake's epicenter was 73.4 km (45.6 mi) southwest of Marrakesh, near the town of Ighil and the Oukaïmeden ski resort in the Atlas Mountains. It occurred as a result of shallow oblique-thrust faulting beneath the mountain range. At least 2,960 deaths were reported, with most occurring outside Marrakesh. Damage was widespread, and historic landmarks in Marrakesh were destroyed. The earthquake was also felt in Spain, Portugal, and Algeria.
It is the strongest instrumentally recorded earthquake in Morocco, the deadliest in the country since the 1960 Agadir earthquake, and the second-deadliest earthquake of 2023 after the Turkey–Syria earthquakes. Its magnitude also makes it the largest earthquake on the African continent since the 2006 Mw 7.0 Mozambique earthquake and the largest in North Africa since the 1980 Mw 7.1 El Asnam earthquake. Over 2.8 million people from Marrakesh and areas surrounding the Atlas Mountains were affected, including 100,000 children. Following the earthquake, many countries offered humanitarian assistance, and Morocco announced a three-day period of national mourning.
Morocco lies south of a major tectonic boundary between the African and Eurasian plates, the Azores–Gibraltar transform fault. This major fault stretches from the Azores to Gibraltar Strait where it is dominated by right-latera strike-slip movement. In the Gibraltar Arc and Alboran Sea, at the eastern end of the fault it becomes transpressional with the development of large thrust faults. East of the Strait of Gibraltar, in the Alboran Sea, the boundary becomes collisional in type. Most of the seismicity in Morocco is related to movement on that plate boundary, with the greatest seismic hazard in the north of the country close to the boundary. In 2004, the coastal province of Al Hoceima was struck by a magnitude 6.3 earthquake that left 628 people dead and 926 injured. In nearby Algeria, magnitude 7.3 earthquake occurred in 1980 that killed 2,500 people.
The Atlas Mountains are an intracontinental mountain belt that extends 2,000 km (1,200 mi) from Morocco to Tunisia. These mountains formed from a collision during the Cenozoic. The mountain range reaches its highest elevation to the west, in Morocco. The High Atlas, a subrange, formed when an ancient Triassic rift was reactivated. However rather than resuming the rifting process, the reactivation compressed the rift due to the collision in the north. Due to the unusually high topography of the Atlas range, mantle upwelling may have played a role in its orogeny. The crust beneath the Atlas range from 32–40 km (20–25 mi), considered thin and physically impossible to support high elevations exceeding 4,000 m (13,000 ft). Typically, a crustal thickness of 50 km (31 mi) is required, hence mantle upwelling raises the overlying crust.
The seismicity of Morocco is concentrated in its northern region and the Alboran Sea. South of the Rif, seismic activity is sparse but widely distributed across the Middle Atlas, High Atlas, and Anti-Atlas. Seismicity in the Saharan Atlas is limited, and is absent in the Saharan region south of the belt; it is also less active eastwards in Algeria and Tunisia. Previously, the largest earthquake recorded in the Atlas Mountains was a Mw 5.9 earthquake that struck Agadir in 1960. Earthquakes in the Atlas Mountains display focal mechanisms of strike-slip, thrust or a combination of both (oblique-slip).
Measuring Mw 6.9 at a depth of 19 km (12 mi), it is the strongest earthquake recorded by a seismograph with an epicenter in Morocco. Morocco's seismic agency reported a Mwp of 7.2 and focal depth of 8 km (5.0 mi). The tremors were detected by monitoring stations as far away as Egypt.
The earthquake had a focal mechanism indicating oblique-thrust faulting beneath the High Atlas. The rupture occurred on a steep-dipping oblique-reverse fault striking northwest or a shallow-dipping oblique-reverse fault striking east. The USGS estimated the fault rupture area to be 30 km (19 mi) by 25 km (16 mi) on an east-northeast striking, north–northwest dipping fault. Slip was generally observed at 15 km (9.3 mi) to 35 km (22 mi) depth, but mostly concentrated around the hypocentre within an elliptical slip patch 30 km (19 mi) long by 25 km (16 mi) wide. A maximum displacement of 1.9 m (6 ft 3 in) was observed at 25 km (16 mi) depth while there was little to no slip above 15 km (9.3 mi) depth. Many east-west and northeast–southwest strike-slip and thrust faults occur in the High Atlas. Since 1900, there has not been a Mw 6.0 or larger earthquake within 500 km (310 mi) of the recent earthquake's epicenter; but nine Mw 5.0 and larger events have occurred to its east. In another finite fault model published by Italy's National Institute of Geophysics and Volcanology, the focal depth was determined at 24.7 km (15.3 mi) beneath the High Atlas. The focal mechanism of this model displayed reverse and left-lateral faulting. Slip occurred in an elliptical area along an east-northeast–west-southwest trending fault dipping 69° north–northwest. A peak slip of 2 m (6 ft 7 in) occurred at 23.3 km (14.5 mi).
Geodetic modeling suggests the earthquake originated from within the lower crust and ruptured up to the middle crust. The deeper depth and greater remoteness from populated areas compared to the earthquake that struck Agadir in 1960 meant it caused fewer casualties and damage. The range of depth where slip occurred is unusually deep for crustal earthquakes as they tend to occur shallower than 15 km (9.3 mi) depth. Fluid and magma associated with the mantle plume beneath the High Atlas may have intruded via a fault and pervade across, bringing it closer to rupture.
No surface faulting occurred hence the causative fault responsible could not be identified, however the focal mechanism suggests rupture on a steep north-dipping plane or shallow south-dipping plane. The USGS finite fault is aligned with the former solution. Two dominant shallow-dipping thrust systems, the North and South Atlas faults, occur in the western High Atlas. Their fault geometries contradict that of the USGS finite fault's preferred steep-dipping plane. Other unmapped faults within the range, including the Tizi n'Test Fault, have surface projections that match closer to the USGS finite fault. If the rupture occurred on the shallow south-dipping plane, a possible source is a low-angle detachment beneath the High Atlas. Geologists have previously interpreted low-angle faults in the region in past studies. For the steep north-dipping plane, the possible source are unmapped or blind thrust faults. The Tizi n'Test Fault, a north-dipping fault where no recent activity has been recorded, may be a possible source of the earthquake. Cornell University geologist Judith Hubbard said the fault was active 300 million years ago during the formation of Pangaea and later, its fragmentation. Ancient faults, such as the Tizi n'Test Fault, create zones of strain within the crust and could reactivate, such as the case in Morocco.
Vertical movement of the land surface detected by repeat observations of the Sentinel-1 satellite is consistent with movement on a blind thrust fault dipping north. An analysis of satellite data obtained from Daichi-2 by the Geospatial Information Authority of Japan revealed a 20 cm (7.9 in) surface uplift around the epicenter and 7 cm (2.8 in) of subsidence to the south. Surface deformation was observed around the epicenter across a 50 km (31 mi) area trending east–west, and 100 km (62 mi) trending north–south.
According to the United States Geological Survey's PAGER service, the earthquake had a maximum Modified Mercalli intensity of IX (Violent). Intensity VIII (Severe) shaking was felt by approximately 157,000 residents, including the town of Azgour and villages surrounding the Atlas Mountains. Intensity VII (Very Strong) shaking was felt by over 811,000 people, with intensity VI (Strong) shaking felt by 3.2 million residents, including in the cities of Marrakesh, Taroudant and Ouarzazate. Shaking of intensity V (Moderate) was felt in Agadir, Beni Mellal, and Safi, with intensity IV (Light) shaking being felt in Casablanca. According to the European-Mediterranean Seismological Centre, it was also felt in Portugal, Spain, Mauritania, Algeria, Western Sahara and along the coast of the Strait of Gibraltar.