On September 8th, 2023, at 11:11pm Morocco suffered a catastrophic earthquake in the al-Haouz province of the Atlas Mountains, the highest mountains of Morocco with peaks of over 13,000 feet, near the town of Oukaimedene in Morocco’s western region. This earthquake lasted for more than twenty seconds, and more than 2,900 lives were lost with over 5,500 injured. It was rated at a magnitude of 6.9 on the Richter scale, making it the most powerful earthquake in recorded history for that region and the deadliest in Morocco since the 1960 Agadir earthquake that was rated at a 5.8 on the Richter scale and killed 12,000-15,000 while injuring another 12,000. This recent earthquake had an epicenter 45 miles to the southwest of Marrakech and was felt as far away as Portugal and Algeria. The United States Geological Survey gave it a maximum Modified Mercalli intensity of IX, or violent with an extended region around the epicenter was rated at intensity VIII, or severe, and felt by about 157,000 people. Over 811,000 people experienced it as intensity VII (very strong) while over three million people in cities as far away as Marrakech and Taroudant felt it as an intensity VI strong shaking. It was followed up by a magnitude 4.9 aftershock twenty minutes later.
A more common area for earthquakes of this magnitude would be approximately 342 miles to the north along the Azores-Gibraltar Transform Fault, which is the boundary between the African Plate and the Eurasian Plate. This region is generally active due to a four to ten millimeter per year (compared to the San Andreas fault along the United States west coast which shifts approximately fifty millimeters per year) northward convergence of the two plates, which began about fifty million years ago and was responsible for the closing of the Tethys Sea. Most activity in this area is along the Hellenic subduction zone of southern Greece. The site of this epicenter, however, sits in the northwest portion of the African Plate in the center of the Atlas Mountains. The Atlas Mountains extend 1,200 miles, southwest to northeast from Morocco to Tunisia and were formed from a collision during the Cenozoic era due to an ancient Triassic rift, however instead of extensional forces pulling the land apart, the land was uplifted due to northern collisional features along the African and Eurasian Plates.
This earthquake happened when a reverse fault occurred between the Morocco and Iberia microplates, both being parts of the African plate. This reverse fault lies to the north of the mountains and in this case, the edge that sits toward the mountains slid over the other, pushing
up toward the peaks. A reverse fault is one in which compression pushes two blocks of rock against each other and the block of rock on one side of the fault moves up and over the other side of the rock. One side of that rock is called the hanging wall while the other side is called the footwall, terms coined by minors who would hang their lanterns on the hanging wall and walk around on the footwall. At a reverse fault it is the hanging wall that moves upward. In a normal fault it would be the opposite. Other well known reverse faults were responsible for the formation of the Swiss Alps in Switzerland and the Longmen mountains in China.
In this particular area and for this event, focal mechanisms indicate an oblique-thrust fault, which is shown by both dip-slip and strike-slip motion. The fault rupture area was estimated to be about nineteen miles by sixteen miles on an east-northeast to west-southwest striking, north-northwest dipping thrust fault. The epicenter was placed at a depth of 11.2 miles by the United States Geological Survey while Morocco’s seismic agency reported a depth of 5 miles. Generally, slip was reportedly observed between 9.3 miles and 22 miles in depth. Mehdi Zare, who is a professor at the International Institute of Earthquake Engineering and Seismology in Tehran, said that movement was on two levels, shallow and deeper down. This resulted in a slipping and folding known as decollement. Decollement is defined by SLB as a fault surface parallel to a mechanically weak layer that detaches or separates deformed rocks above from undeformed or differently deformed rocks below. He noted this decollement shallowly at 0.6 to 2.5 miles and deeper in the middle crust at 6.2 to 12.4 miles. He also believes the this process started at the deeper layers and moved towards the surface.
This idea of decollement could explain the intensity of this recent quake. With deeper quakes, the shaking can be felt at a much further distance but are not generally as strong while shallow quakes give much more of an intense jolt on the surface. This event gave both, causing unthinkable damage on the surface and felt far and wide. Near the epicenter, it has been suggested that the upward moving hanging wall was raised by up to 5.6 feet along a nineteen-mile section of the fault. At this location, nearly all structures, mostly made of unreinforced brick and masonry, were destroyed. Towns nearly twenty miles from the epicenter were almost completely destroyed with one area losing as many as 2,000 of its residents. Forty five miles away in Marrakech, where building standards are more modern, they still lost many historic structures to the quake.
Satellite data has shown that there was a twenty-centimeter uplift on the surface near the epicenter with seven centimeters of subsidence to the south. Deformation of the surface was observed in an area of thirty one miles to the east and west and sixty two miles to the north and south.
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