Large earthquakes can dramatically reshape mountain landscapes by triggering thousands of landslides and rapidly accelerating erosion. However, why some mountain regions experience intense and long-lasting erosion after large earthquakes while others show only limited responses has remained poorly understood.

A new study published in Geology reveals that topography is the key factor controlling how mountain erosion responds to large earthquakes. By reconstructing the environmental impacts of the great AD 1717 Alpine Fault earthquake (Mw >8.0) in New Zealand, researchers found that differences in landscape steepness, hillslope-to-channel connectivity, and river transport capacity determine whether earthquakes mainly erode shallow surface soils or mobilize deeper materials from hillslopes and bedrock into downstream lakes.

The research team investigated two lake catchments systems, Lake Mapourika and Lake Paringa, located along New Zealand’s Alpine Fault. The lake sediments preserve a continuous record of the earthquake cycle. To reconstruct the erosion history, the researchers combined multiple geochemical tracers—including carbon and nitrogen isotopes, and molecular biomarkers—with analyses of catchment topography and geomorphology. These geochemical fingerprints allow scientists to distinguish organic matter derived from soils of different elevations and depths, as well as from bedrock, providing a detailed record of how erosion processes changed before and after the earthquake.

They found the two catchments showed strikingly different erosion styles, erosion depths, and organic carbon sources, despite their similar climate, vegetation, geology, and tectonic setting. In the steeper Mapourika catchment, where hillslopes are more strongly connected to river channels, deep-seated landslides rapidly transported material from deep soils and bedrock into the lake. By contrast, the gentler Paringa catchment was dominated by shallow soil erosion, resulting in a greater contribution of modern biospheric organic carbon derived from surface soils.

The two catchments also followed markedly different post-earthquake erosion pathways. In the Paringa catchment, sediments deposited immediately after the earthquake were initially derived from soils at high elevations, but over time shifted toward deeper soils from lower elevations. This transition indicates that the dominant erosion process evolved from pre-earthquake surface soil erosion to earthquake-triggered bedrock landsliding. In contrast, the source elevation and erosion depth in the Mapourika catchment remained consistent before and after the earthquake. Because of its steep terrain, deep-seated bedrock landslides already dominated erosion under normal conditions, continuously eroding high-elevation soils and bedrock. The earthquake greatly increased the volume of sediment delivered to the lake, but it did not fundamentally alter the dominant erosion process.

“Earthquakes don't affect every mountain in the same way,” said Dr. WANG Jin, lead author of the study. “We found that the shape of the landscape determines whether an earthquake mainly strips away surface soils or excavates much deeper material. This helps explain why similar earthquakes can leave very different geological footprints.”

The findings will help scientists better predict how mountain landscapes respond to future large earthquakes and assess their impacts on sediment transport, landscape evolution, and the cycling of organic carbon in tectonically active regions. Because earthquakes redistribute enormous amounts of sediment and carbon across mountain landscapes, they influence river systems, long-term landscape evolution, and the transfer of carbon between the Earth's surface and atmosphere. Understanding these processes is therefore essential for evaluating the role of tectonically active mountain belts in global biogeochemical cycles and long-term climate evolution.

Study authors Jamie Howarth and Sean Fitzsimons collect a sediment core from Lake Paringa, New Zealand, for reconstructing the erosion history following large earthquakes. (Credit: Adelaine Moody)

Reference: Wang, J., Howarth, J. D., Jin, Z., McClymont, E. L., Densmore, A. L., Fitzsimons, S. J., Croissant, T., Gröcke, D. R., West, M. D., Harvey, E. L., Garnett, M. H., Zhu, C., & Hilton, R. G. (2026). Topographic regime controls the response of erosion to large earthquakes. Geology, https://doi.org/10.1130/G54785.1