On the evening of September 8th, Morocco experienced a significant seismic event with a magnitude of 6.8. The earthquake’s epicenter was located approximately 70 km from Marrakech and resulted in substantial damage to both buildings and infrastructure, with reports of 1500 to 2000 affected structures (1). This earthquake marked the deadliest seismic event in Morocco since the 1960 Agadir earthquake.
Following the event, many questions have surfaced:
1# Is an earthquake of this magnitude surprising in this area?
This earthquake is exceptional in magnitude, but unfortunately, it is not surprising.
Morocco is located at the boundary of the African and Eurasian tectonic plates (red dotted line, Fig. 1), which are converging each other at a speed of between 4 and 6 mm/year (2). The area where the plates meet is highly seismically active, which has resulted in several earthquakes occurring in the Rif region and the Alboran Sea (such as the Al Hoceima earthquake in 2004).
The area that caused the 2023 earthquake is located several hundred kilometres south of the boundary (red star, Fig. 1), which means that the earthquake occurred away from the tectonic plate boundary. Here studies have identified the presence of faults considered to be active and have estimated a deformation rate of less than 1 mm/year. (2). This difference in displacement speed implies that the accumulation of constraints in the Atlas chain takes longer than in the North to generate an earthquake of this magnitude.
Figure 1: Map by AXA Climate based on data from the USGS for the event and GEM for fault mapping. Black spots correspond to historical seismicity in magnitude of moment (entre 1000 et 2022). The red dotted line corresponds to the boundary between the African and Eurasian plates. (3)
The High Atlas is a mountainous area marked by several reverse faults and characterised as having “moderate risk” by seismic models. A reverse fault is a geological phenomenon that occurs when tectonic plates collide, and the Earth’s crust thickens. Earthquakes occur when rocks abruptly shift to release the stress that has accumulated along these fault lines. The 1960 Agadir earthquake (magnitude 5.8) was the last major known earthquake produced by this system. The analysis of the faults present in this area suggests that future earthquakes could have maximum magnitudes that may exceed magnitude 7 (3).
However, this area it is not prone to earthquakes and there is no equivalent earthquake information for the epicentre region. This is why many people are surprised at the location of this event.
2# What was the impact of this earthquake?
This event had a greater impact than we would have expected!
Following the earthquake, the U.S. Geological Survey (USGS) quickly published a map of the impact of the event from station data, online reports (Did You Feel It) and modelling from the characteristics of the rupture. Here, 2 metrics were used:
• The peak ground acceleration (PGA in g), which is regularly used as a reference by engineers for the design of beams.
• And the macrosismic intensity, which translates the effects of the earthquake to a given point (which we will explain below).
Figure 2: Shakemap in 2023 earthquake intensity (USGS)
The analysis of the USGS intensity map for this earthquake indicates that it was felt as far north as southern Spain and Lisbon (Intensity II to III, corresponding to low vibrations). It also generated V to VI intensity (middle to strong tremors and slight damage) more than 150 km from the epicentre, as in Agadir. Closer to the epicenter intensities above VIII have been estimated, consistent with all images of collapsed buildings by the various media.
Figure 3: The Modified Mercalli Intensity (MMI) estimates the shaking intensity from an earthquake at a specific location by considering its effects on people, objects, and buildings (USGS)
In order to model the effects of earthquakes or their intensity scientists use the analysis of past earthquakes specific to a particular geographical area. The initial estimates obtained for this earthquake suggest that much greater effects were observed than would usually be expected. As a result, it seems that this event has had a greater impact than we would have expected.
#3 Could this event have been predicted?
Despite increasing technologies, it is still impossible to predict the occurrence of an earthquake effectively and accurately today.
regions like California and Mexico are equipped with a warning network to quickly alert population of the arrival of seismic waves. This system is heavily dependent on a dense network of seismic stations, and it enables citizens to receive an alert only a few seconds or minutes before the arrival of the waves (depending on the location of the epicentre).
Figure 4: Principle of the early warning system ShakeAlert for California (USGS)
In the absence of a reliable and dense warning system, probabilistic seismic hazard maps are established which can be used for risk reduction, enabling areas to establish mitigation strategies and emergency response plans. The Global Earthquake model is a global hazard map that provides a view of the earthquake threat worldwide.
Seismic agitation is the possibility of being exposed to seismic shocks of given characteristics (Peak Ground Acceleration (PGA) or intensity) for a specified period of time. The benchmark for this map is set by determining a 10% probability of exceeding a PGA level over the next 50 years. Hazard maps (4) allow engineers to have an indication for improving the resilience of populations and buildings to earthquakes.
Figure 5: Hazard map with PGA of 475 years of return period for Morocco (Global Earthquake Model)
#4 What risk reduction strategies for such events?
While it remains impossible to reduce the risk altogether, it is possible to prepare for these risks by establishing financial support with risk transfer solutions. The objective is to help the most vulnerable populations and support the reconstruction of damaged areas. For example, the Morocco Integrated Disaster Risk Management and Resilience Program (5) has prepared for an earthquake scenario in 2 ways:
- The Morocco’s Fund for the Fight against Natural Catastrophes (FLCN) is a national resilience fund. As of March 2022, the fund had supported 180 disaster risk reduction projects, for a total investment volume of US$304 million. Completed structural projects have supported more than 174,000 direct beneficiaries across the national territory.
- The adoption in 2018 of an innovative disaster risk insurance regime. This programme introduced a private insurance scheme covering close to 9 million people and established a public solidarity fund (FSEC), separate from the FLCN, targeted at the poorest and most vulnerable households (an estimated 6 million people). The combined private and public schemes can provide about US$100 million in compensation every year.
Figure 6: Map of maximum intensity per municipality based on the USGS shakemap for the 2023 event and used for the FSEC coverage (AXA Climate)
Since 2019, the risk insurance regime structure is being supported by AXA Climate’s scientific team through our risk modelling capacity. For this coverage, the intensity provided by the USGS shake map was used as trigger, as this parameter directly gives an indication on the impact of the earthquake (Figure 6).
In addition, risk reduction also involves the implementation of different strategies:
- Communication and response exercises to prepare people to respond to an event and train rescue teams to coordinate. For example, following the earthquake that devastated Mexico City in 1985, the country organised several national exercises.
- The implementation of construction techniques to increase the resilience of buildings. Following the 2015 earthquake that struck Nepal, programmes have been developed to renovate existing buildings to make them more resilient (programmes UN and World Bank).
5# Is there an impact of climate change on earthquake generation?
The most devastating earthquakes ever observed were all of tectonic origin. They come from a sudden release of energy from the stress accumulated in the rocks by different mechanisms linked to the movement of plates. However, climate change and human activities can have impacts on the accumulation of these constraints, and thus indirectly contribute to the triggering of earthquakes: this is called induced seismicity.
Induced seismicity can be caused by:
- The injection of fluids deep underground, as in geothermal injection. This can result in an increase in the interstitial pressure of rocks, causing the rupture of a pre-existing fault. In general, this seismicity is of low magnitude and is monitored.
- The change in hydrostatic pressure, as in the setting up of a dam, or the variations of water volumes caused by the increased frequency and severity of heavy rainfall events.
Figure 7: Map of induced seismicity and its origins (6)
More directly, climate change causes glaciers to melt. The loss of this mass results in a reduction of the pressure on the Earth’s crust. In regions where there were once massive ice sheets, this release of pressure can cause the Earth’s crust to rebound or uplift, this is called “glacial isostatic adjustment“, which in some cases can result in a change in the stresses on existing faults.
Scientists published the findings of their studies showing that the 1958 earthquake that struck Alaska (magnitude 7.8) formed very close to the location of the maximum isostatic adjustment or land movement that occurred due to the melting of the Glacier Bay Icefield (> 3000 m3 since 1770). The uplifting of the land that occurred due to the loss of ice would have led to an increase in constraints on the Fairweather fault (8).
It’s important to note that there may be a link between climate change and the occurrence of earthquakes, but this concerns only a small fraction of the seismicity observed globally. The main drivers of significant earthquakes remain geological and tectonic forces.
