Large earthquakes could trigger thousands of landslides and cause huge devastation in mountains.The mountain landscape is a result of battle between tectonic uplift, river incision and bedrock landsliding.

The tectonic uplift builds topography, while the river incision cuts downward into the bed, deepening the active channel, and the landslide removes the sediment where it occurs. The erosion of landslide dominates the process of removing sediment from the watersheds, therefore, where it happens is important for the landscape. If landslides occur repeatedly in particular parts of a mountain range, then they will dominate the landscape evolution of that section and could leave a fingerprint in the topography.

An international research team from Institute of Earth Environment, Chinese Academy of Sciences and Durham University worked with colleagues from Victoria University of Wellington, University of Otago and NERC Radiocarbon Facility discovered that erosion after the large earthquakes preferred to removing sediments from high elevations by triggering landslides, while the landslides cause by the intense rainfall erode sediment from the lower elevation.The research was recently published in the Science Advances.

The research team collected sediment cores from Lake Paringa and soil samples on New Zealand's west coast of South Island. This sediment cores recorded four Mw>7.6 earthquakes in the last 1000 years.

To examine where the sediment comes from, the researchers used a range of geochemistry techniques, which included the carbon and nitrogen isotopes and a specific compound of organic matter in the sediments ('biomarkers'). Their shifts after each large earthquake suggest the sediment sources had been changed by the large earthquakes.

Using the relationship between the organic geochemistry proxies and the soil elevation and depth, they calculated that the earthquake triggered landslide eroded organic matter from elevations around 700 meters, while the sediments were eroded from a lower elevation around 450 meters when there was no earthquake.

These results are the first time to prove that repeated large earthquakes can consistently focus erosion at high elevations.

The research is important for the understanding the dynamics of landscape evolution in mountain ranges. The results suggest that large earthquakes trigger landslides at the elevations of drainage divide, which is the boundary of two rivers/tributaries. Therefore, large earthquakes are the primary process driving drainage divide migration at the mountain ranges, which is a key process changing the landscape and the river networks. The research team demonstrates that extreme events, such as earthquakes and storms, may exert a first-order influence on landscape evolution.

"The large earthquakes played important roles on modifying the mountain landscape”, said Dr. WANG Jin.

The work was supported by grants from National Natural Science Foundation of China (41991322), Durham University COFUND Junior Research Fellowship, Natural Environment Research Council Standard Grant (NE/P013538/1),and Rutherford Foundation Postdoctoral Fellowship.

Collecting lake sediment core from the Lake Paringa, New Zealand (Imaged by WANG, Jin)

Contact: Bai Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn