New Findings by Earth Scientists from Potsdam and Berlin Published in Science
Earth scientists Dr. Wolfgang Schwanghart and Prof. Oliver Korup, PhD, from the Natural Hazards Group at the University of Potsdam investigate extreme events in the Earth’s recent history. In a study recently published in the science journal Science, Schwanghart and Korup were part of an international team who investigated thick deposits in the Pokhara valley in the Himalayan country Nepal. One of the co-authors is Dr. Philipp Hoelzmann, who is a researcher at the Institute of Geographical Sciences, Freie Universität Berlin. Detailed studies now show for the first time that three major earthquakes in the Middle Ages resulted in upheavals with great impact in the countryside around Nepal’s second-largest city. The seismic waves triggered landslides that displaced tons of debris from the Annapurna Massif over 60 kilometers, filling at least a dozen valleys up to 100 meters high. Comparable landscape changes have not been made by recent earthquakes, which illustrates the enormous destructive potential of large earthquakes on the "Roof of the World."
In the spring of 2015, an earthquake with a magnitude of 7.4 on the Richter scale struck Nepal, killing nearly 9,000 people. More than 20,000 individuals were injured, and more than half a million homes were destroyed. Thousands of landslides buried villages and roads and blocked rivers. From previous studies scientists know that large earthquakes can lead to increased landslides on slopes for years to come. The accumulated debris also affects rivers and increases the risk of flooding. These long-term effects hinder the reconstruction of the infrastructure and the return to life as it was before the quake.
Now the scientists have found evidence that the impact of individual strong quakes can also shape the landscape and the processes that form it centuries later. The group of mainly young investigators combined sedimentological and geochemical surveys, such as radiocarbon dating and statistical modeling. They show that the city of Pokhara, one of the most important tourist destinations in Nepal, is built upon the rubble of three medieval earthquakes with magnitudes greater than eight. There is some historical written documentation for the quakes in 1100, 1255, and 1344. They triggered devastating landslides and flood events, which initiating from the glaciated Annapurna Massif, buried more than 60 kilometers of the original river network, clogged many tributaries, and damned many lakes, some of which constitutes the charm of the countryside around Pokhara today.
Although these medieval earthquakes had been documented, their impact on the landscape and the residents in the central Himalaya has long been a mystery. The recent study led by Wolfgang Schwanghart is therefore an important step toward understanding thick deposits such as those in Pokhara as independent archives of past earthquakes. Using carefully documented sedimentological descriptions and extensive datings, it would be possible in the future to better assess not only the timeframe, but also the consequences of historically or geologically documented strong earthquakes.
According to the findings, even today the rivers in the Pokhara Valley are still adapting to the catastrophic amounts of medieval sediments. Rivers dig into the deposits, shift rapidly, and are difficult to build bridges over, while collapsing sidewalls of deeply incised canyons move dangerously close to rows of houses. Meanwhile underground cavities frequently cause sinkholes. In 2012 more than 70 people were killed by a flash flood triggered by a landslide in the source region of the medieval deposits. The countryside around Pokhara still reacts fast even today, and part of the associated natural hazard potential for the local population was already influenced by strong earthquakes more than 900 years ago.