Earthquakes in northern Africa, while not frequent, are not unexpected. Morocco is positioned at the juncture of a slow-motion tectonic crash between the African and Eurasian plates. Over millions of years, the movements have crumpled the landscape, raised the Atlas Mountains and crafted a complex network of fractures through the region.
The rate of collision near Morocco is fairly slow, with the plates colliding at a mere 4 to 6 millimeters per year, which means earthquakes do not happen often in this region. For comparison, the land around the San Andreas Fault shifts some 50 millimeters each year. But over many years, the slow movement near Africa’s northern coast can build enough stress to cause violent quakes, including Friday night’s deadly temblor in Morocco.
Yet the complex tectonics of this region are poorly understood, according to Judith Hubbard, a geologist at Cornell University. The collision deforms the landscape across multiple, interconnected zones rather than the kind of single defined fault of the Pacific Northwest. And the slow passage of the plates makes it tough to measure movement and identify the region’s most earthquake-prone faults.
Other processes deep underground might also be influencing the stresses near the surface. “The current tectonic stresses are therefore only part of the story,” Dr. Hubbard said.
Scientists are still pinning down many details about this latest event, including the precise fault responsible for the devastation.
Historical earthquakes offer few answers to that question, according to Dr. Hubbard. “There is no information about any earthquakes on any of these faults,” she said. Even the fault that caused Morocco’s deadliest earthquake in recent history — a 5.8-magnitude temblor in 1960 that killed 12,000 people — “is still largely unknown,” she said.
Another challenging detail to study is an earthquake’s depth, Dr. Hubbard said. Scientists often initially assign a depth based on rough estimates, later refining these values as more data emerges. Estimates by various seismic agencies highlight the uncertainty: The U.S. Geological Survey’s early assessment placed the depth of the Morocco quake at 11.5 miles, then updated it to 16.3 miles. The European-Mediterranean Seismological Centre currently places the depth at 7.2 miles.
The depth of an earthquake affects the intensity and spread of shaking. The shallower an earthquake when it strikes, the more intense a jolt it will cause at the surface. The shaking from a deeper earthquakes may not be as strong, but it can be felt across a wider swath of the surface, Dr. Hubbard said.
Another important factor is the direction in which an earthquake fractures the ground, which may explain where seismic energy is focused on the surface. The effect is similar to being blasted by sound while standing in front of a speaker, compared with the muffled sound heard if standing behind it.
As Jascha Polet, a seismologist and an emeritus professor at California State Polytechnic University, noted, the pattern of aftershocks hints that the Moroccan earthquake fractured to the northeast in the direction of Marrakesh, which likely intensified the destructive shaking in the city.