24 December 2024–The rapid, decade-long depletion of groundwater in Morocco’s Haouz plain may have affected the accumulation of tectonic stress that led to the deadly 8 September 2023 magnitude 6.8 Al Haouz earthquake, according to a new study published in the Bulletin of the Seismological Society of America.
Bhaskar Kundu of NIT Rourkela and colleagues estimate the changes in crustal loading as a result of groundwater extraction may account for about 10% to 14% of the ongoing tectonic stress accumulation in this region.
“This indicates that stress accumulation in the slowly deforming High Atlas Mountains was significantly influenced by anthropogenic groundwater extraction, potentially playing a significant role in triggering the Al Haouz earthquake,” said Kundu.
The 2023 earthquake produced strong shaking and was the deadliest earthquake ever recorded by instruments in Morocco, killing about 3,000 people according to the country’s Interior Ministry. The region of the High Atlas Mountains, where tectonic plates converge, experiences low to moderate seismicity. But stress accumulates much more slowly in the area between earthquakes, compared to other well-known convergence areas such as the Himalayan-Tibetan mountain belt.
The accumulated fault slip in this slow-deforming region is significantly lower compared to the maximum coseismic slip that the research team modeled for the 2023 Al Haouz earthquake, said Kundu. “This significant discrepancy raises critical questions about how such a devastating magnitude 6.8 earthquake nucleated on the Tizi n’Test fault in a region with relatively low tectonic deformation rates.”
The potential impacts of groundwater extraction on crustal loading and fault stress have been studied across the globe, Kundu noted.
“The core premise of such research exploration is that, although faulting mechanisms are primarily driven by tectonic forces, crustal loading and unloading processes can significantly influence interseismic strain accumulation,” he explained. “Notable examples include regions like California, India, and Iran, where seismic activity has been correlated with substantial groundwater depletion.”
In their earlier investigation of the 2015 magnitude 7.8 Gorkha earthquake in Nepal Himalaya, for instance, Kundu and colleagues found some evidence that groundwater extraction and the resulting changes in crustal loading “may have modestly advanced the earthquake clock” for that event, he said.
Intensive and rapid groundwater depletion in the region of the 2023 Moroccan earthquake, mostly due to expanded agriculture, combined with a lack of rainfall to replenish underground aquifers, led to a decline in groundwater levels of approximately 0.9 meters per year between 2002 and 2019. The most affected areas saw a 37-meter drop in the water table over this period.
“This unusual fact, coupled with regular tectonic processes, prompted us to investigate beyond the typical tectonic causes associated with such seismic events,” Kundu said.
The researchers used a combination of geodetic data, satellite observations, modeling of the water table and a poromechanical model of the crust under different groundwater conditions to look for connections between groundwater extraction, changes in crustal loading and the Al Haouz earthquake.
While their findings suggest that groundwater extraction played some role in the region’s tectonic stress accumulation, the researchers stress that more work needs to be done to confirm this conclusion. Specifically, more direct observations of the water table and groundwater withdrawal patterns, and a better understanding of strain accumulation in the region, would “refine our understanding of how human activities modulate seismic risk in the High Atlas Mountains and other regions with similar characteristics,” said Kundu.
This paper is part of an upcoming BSSA special issue on earthquakes in slowly deforming mountain belts.