UPDATE: USGS Publishes Aftershock Scenarios for Morocco

In support of the people of Morocco affected by the magnitude 6.8 earthquake, USGS is continuing to provide science to help them make the best decisions for themselves, their families, and their communities.

Based on similar large earthquakes, the region surrounding Oukaïmedene southwest of Marrakech, Morocco is likely to continue to experience more earthquakes, known as aftershocks. About 1 in 20 earthquakes are followed by a similar-sized or larger aftershock within the first week.  

Large aftershocks could cause additional damage, especially in weakened or more poorly constructed structures. The largest aftershock so far in Morocco has been a magnitude 4.9, but magnitude 5 and larger aftershocks are also possible. The chance goes down with time, but large aftershocks can still occur months after the mainshock.

What To Do in the Event of More Earthquakes

According to GeoHazards International, those living in an area with shaking should not go inside visibly damaged structures. Damaged buildings can collapse in aftershocks.

If you are inside a building when you feel an earthquake:

• Drop where you are, cover your head and neck with one arm, get under a sturdy table, and then hold on to the table legs until the shaking stops.
• If you are inside on the ground floor and can easily get out in a few seconds, evacuate to a safe open place covering your head and your neck. Head to an open space where walls and electric poles cannot fall on you.

If you are outdoors when you feel an earthquake:

• Stay outdoors, away from things that could fall on you: electric poles, walls, buildings, or structures, for example.
• Stay away from landslide areas and hillsides with cracks, as aftershocks can cause new landslides and existing landslides to move again.

Aftershock Sequence Scenarios

USGS published three likely scenarios for how the aftershock sequence will evolve over the next month starting September 11, 2023, for this large earthquake. 

The most likely scenario (at 98% probability) is that aftershocks will continue to decrease in frequency, with no aftershocks larger than magnitude 6 within the next month. Moderate to large aftershocks around magnitude 5 are possible and could cause localized damage, particularly in weak structures. Smaller magnitude earthquakes (magnitudes 3 -4) are likely to be felt by people close to the epicenters.

We estimate about a 2% probability of a second scenario, in which one or more additional aftershocks larger than magnitude 6 could occur. In this scenario, none would be larger than the M6.8 mainshock. Aftershocks of this size would cause additional damage and temporarily re-energize the aftershock sequence. These aftershocks would most likely affect the area already impacted by the mainshock.

There is less than 1% chance that an aftershock of the same size or larger than the M6.8 mainshock could occur. While this is a very small probability, such an earthquake would affect communities both in and adjacent to the areas already impacted by the mainshock. Such an earthquake would likely trigger an aftershock sequence of its own.

For continued updates on the Morocco earthquake, see https://earthquake.usgs.gov/earthquakes/eventpage/us7000kufc/executive

Summary

The magnitude 6.8 earthquake in the Atlas Mountains on September 8 has taken a sobering toll on human life and property in a part of western Morocco that is not prone to frequent earthquakes. 

At 11:11 pm local time, the U.S. Geological Survey’s National Earthquake Information Center detected the relatively shallow earthquake, at a depth of about ~26 km, or 16 miles, near the town of Oukaïmedene. This rural area, about 75 km (50 miles) southwest of Marrakech, includes many residences that are vulnerable to shaking. Due to the shallow depth of the event and its proximity to high population centers, many buildings experienced severe shaking that can result in catastrophic failure.  

At the time of posting, news reports indicate that at least 2,000 people have died and thousands more have been injured.

Aftershocks are always a concern to both residents and the first responders who help those affected. In addition to the many smaller aftershocks detected by local agencies, the USGS National Earthquake Information Center has reported one major aftershock of magnitude 4.9, 20 minutes after the main shock. It’s likely that smaller aftershocks in the region below the USGS NEIC’s detection and reporting thresholds will continue to be felt for weeks to come.

In some cases, there may be strong aftershocks. People reported shaking from the initial magnitude 6.8 earthquake throughout Morocco and in neighboring countries, with weak shaking reported as far as Portugal, Spain, and Algeria. 

Though rare, large earthquakes are not completely unknown to western Morocco. In 1960, the 5.9 magnitude Agadir earthquake killed 12,000-15,000 people in coastal western Morocco.  

However, there have been no recorded earthquakes of M6 or larger within 500 km of this event (USGS records for the region date back to 1900). Earthquakes in the magnitude 6 range are more common in the northern part of Morocco near the Mediterranean Sea, where a magnitude 6.4 earthquake struck in February 2004 and a magnitude 6.3 in January 2016. 

The earthquake was the result of oblique reverse faulting on a roughly east-west or northwest-southeast oriented fault, possibly the North Atlas fault, a range front reverse fault that roughly matches the faulting mechanism of the earthquake. As more information becomes available, including satellite-based observations, it will likely be possible to determine the causative fault.  

While the USGS has measured the magnitude of the earthquake to be 6.8, measurements from local authorities or other agencies could differ. Measurements may vary due to different sensors and methods. In addition, early magnitude readings are measured automatically and may ultimately be revised by seismologists. 

For updated information on this earthquake, including a tectonic summary available in English and Arabic, see: https://earthquake.usgs.gov/earthquakes/eventpage/us7000kufc/executive

Reporters with questions can contact: earthquakemedia@usgs.gov