What You Need to Know
Recent research from Ethiopia reveals that seasonal rains significantly affect the earth’s crust, challenging the notion of its rigidity. A team from Mada Walabu University demonstrated that increased water mass during rainy seasons causes slight downward shifts in the crust, a phenomenon observed through advanced GPS and satellite data analysis, providing crucial insights into geological dynamics
Africa. Many have long believed that the earth’s crust is solid and unaffected by events occurring above it. However, what is happening in Ethiopia adds new scientific evidence that it is a flexible entity that rises and falls with changes in the loads placed upon its surface, whether from ice, oceans, or seasonal rains.
This phenomenon has been previously observed in areas of ice melt in Canada and Scandinavia, as well as in major river basins like the Amazon. However, observing it in Ethiopia posed a significant scientific challenge due to the country’s location in a tectonically active region, the Great African Rift, where signals from water interact with the effects of earthquakes, faults, and volcanoes.
Thus, separating the impact of rainfall from these complex tectonic activities was a daunting task, successfully accomplished by a research team from the College of Natural and Computational Sciences at Mada Walabu University in Ethiopia, led by Assistant Professor Dr. Abdisah Kawo Koji. The results of this achievement were announced in a study published in the “Journal of African Earth Sciences.”
Kawo Koji explains the phenomenon with a simplified analogy, stating in exclusive remarks to Al Jazeera Net: “If we imagine the earth’s crust as a sponge mattress, and seasonal water as a person sitting or standing on it, during the heavy rain season, rivers, lakes, soil, and groundwater fill up, increasing the water mass above the crust, causing it to sink slightly (subsidence). After the rainy season ends, water begins to seep, evaporate, or flow away, reducing the weight above the crust, allowing it to rebound upward again (elastic rise).”
This process is scientifically known as “elastic loading of surface mass,” where the earth’s lithosphere behaves like a flexible plate over a viscous and elastic mantle. According to this concept, any temporal change in the mass present on the earth’s surface, such as rainfall, surface water accumulation, soil moisture, and groundwater recharge, results in corresponding elastic responses in the earth’s crust.
When Does the Phenomenon Occur?
These changes occur in Ethiopia during the rainy season, as a strong seasonal rainfall system dominates, particularly during the Kiremt (June-September) and Belg (February-May) seasons.
During these two periods, heavy rainfall leads to significant seasonal increases in water storage, filling lakes and reservoirs, increasing river discharge, accumulating soil moisture, and recharging groundwater.
This additional water mass exerts an increasing load on the earth’s crust, resulting in a limited downward vertical displacement.
The study indicates that confirming this subsidence—no more than a few millimeters—is indeed caused by seasonal rains was not easy. However, the methodology employed by Kawo Koji and his team, which relied on a joint analysis of GPS data and GRACE satellite data, a joint space project between the United States and Germany to monitor changes in earth gravity, provided conclusive evidence.
Kawo Koji states: “GPS data provided near-instantaneous monitoring of the earth’s crust response after rainfall, as it measures the earth’s movement directly at fixed points, recording any subsidence or uplift resulting from the additional weight of water very close to the time of its fall.”
He adds: “In contrast, GRACE satellites operate differently; they do not measure the crust’s movement itself but monitor gravity changes associated with increases or decreases in water mass on a large scale. Since a significant portion of rainfall needs time to infiltrate the soil, recharge groundwater, and accumulate in water basins, the signal detected by these satellites typically appears after the rainy season, specifically in October and November, when the water storage process is complete.”
Three Reasons for the Uniqueness of the Ethiopian Case
Although the phenomenon is geologically known, the impact of water in Ethiopia appears stronger and more pronounced compared to other regions of the world for three main reasons, as Kawo Koji enumerates: the intensity and regularity of seasonal rains, the presence of major drainage basins capable of storing vast amounts of water, and the rapid response of shallow groundwater to rainfall.
To separate the impact of natural seasonal rains from the effects of human activities on crustal movement, such as the construction of the Grand Ethiopian Renaissance Dam, scientists rely on a comprehensive mix of long-term monitoring and scientific modeling.
Kawo Koji states that this process “begins with studying the behavior of the earth’s crust during rainy seasons in the years preceding any human intervention, aiming to determine the natural pattern of earth movement.”
He explains that “by using rainfall records and satellite data, primarily from GRACE, a so-called ‘baseline’ of natural seasonal movement is drawn, reflecting the expected response of the earth’s crust to increased water resulting from rainfall alone.
After that, these reference values are compared with actual measurements recorded by GPS stations, where any unusual changes—such as sudden subsidence or uplift exceeding natural rates—indicate effects resulting from human activity.”
The analysis, according to Kawo Koji, is not limited to direct monitoring but also includes using hydrological models that simulate water movement in soil and rivers based solely on climatic factors. If actual measurements show increases in water storage or crustal deformation that do not align with what these models predict, it serves as a clear indicator of human intervention, such as dam construction or extensive groundwater pumping.
Researchers also rely on what is known as the “spatial fingerprint” of earth movement, as natural rainfall causes widespread and geographically distributed deformations, while the impacts of human activities tend to be local and concentrated, as seen in areas surrounding the Grand Ethiopian Renaissance Dam.
In the final stage, the expected impact of these human activities is calculated and subtracted from GPS measurements, leaving only the signal associated with natural rainfall.
Kawo Koji concludes: “With this integrated methodology, scientists can accurately distinguish between crustal movement resulting from natural processes and those associated with human intervention, providing an important scientific basis for understanding tectonic activity, improving water resource management, and assessing geological risks in the region.
Ethiopia is located in the East African Rift, a tectonically active region where geological processes are complex. The interplay of seasonal rains and tectonic activities has long been a subject of study, as researchers seek to understand how natural phenomena influence the earth’s crust. This recent study sheds light on the unique geological responses observed in Ethiopia, contributing to the broader understanding of tectonic movements globally.
