Nepal: Varying impact of earthquake- and monsoon-induced landslides

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Using nearly 50 years of satellite data and records stretching back millennia, scientists determine the relative frequency—and the erosional power—of monsoon- and earthquake-induced landslides in Nepal

By Katherine Kornei

Valleys in the Himalayas are regularly rocked by earthquakes and pummeled by monsoon rains, resulting in fractured bedrock and waterlogged soils that are in turn conducive to landslides. Researchers now have used nearly 50 years of satellite data and records of millennia-old landslides to investigate the relative impacts of earthquake- and monsoon-induced landsliding in Nepal. The team calculated that earthquake- and monsoon-induced landslides contributed to the overall erosion budget in differing proportions, depending on the timescale—years, centuries, or millennia—being considered. The researchers also found a pronounced uptick in landslides during the 2015 monsoon season, an effect they attribute to the 25 April 2015 Gorkha earthquake, which weakened and destabilized slopes.

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Odin Marc, a geomorphologist at Eidgenössische Technische Hochschule (ETH) Zurich in Switzerland, and his colleagues collected archival satellite data—Landsat images from 1972 to 2014 and RapidEye images from 2010 to 2018—and applied a landslide-mapping algorithm to the images. This algorithm looked for changes in vegetation patterns consistent with plant material being removed to reveal the rock underneath.

Marc and his team found thousands of landslides. The scientists estimated the landslides’ areas and, using scaling relations, their approximate volumes. “The depth of a landslide is related to its size,” said Marc, who was at the University of Strasbourg in France when this research was completed. “When you have a bigger area, you know it’s deeper.” By also including measurements from the literature of massive landslides thousands of years old, Marc and his colleagues calculated the relative frequencies of landslides of various sizes and their associated erosion rates in millimeters per year. By integrating these erosion values over time, Marc and his colleagues determined average erosion rates for different time durations (e.g., years, centuries, millennia). The researchers repeated this process for two categories of landslides: earthquake-induced landslides that occurred during the M7.8 Gorkha earthquake on 25 April 2015 and monsoon-induced landslides that occurred each year during the monsoon season from June to October.

Marc and his colleagues showed that the earthquake-triggered landslides caused tenfold to twentyfold more erosion than an average year’s worth of monsoon-induced landsliding. But the picture changed when the researchers considered longer time intervals: Earthquakes, being rare events, contributed only modestly—about 1 part in 10—to the overall erosion budget on timescales of centuries, the scientists found. And on geological timescales of thousands or tens of thousands of years, a new type of landsliding—caused by the collapse of the entire flank of a mountain—emerged as an important erosion mechanism. These results provide an explanation for the apparent increase in the average erosion rate with sampling time obtained from field measurements. “It is clear that if we observe a larger area and wait longer, [we will] have more chances to catch very big landslides,” said Marc. These findings were published last month in Earth Surface Dynamics.

“This is an important result,” said Manny Gabet, a geomorphologist at San José State University in California not involved in the research. “Erosion rates in the Himalayas are typically estimated through the analyses of river sediment, which, because of the relatively short sampling periods, typically miss the material produced by infrequent earthquakes.”

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