Mountain Shape, Not Just Earthquake Size, Determines Erosion Aftermath

Large earthquakes can dramatically reshape mountain landscapes by triggering thousands of landslides and rapidly accelerating erosion, but scientists have long struggled to explain why some mountain regions see intense, long-lasting erosion afterward while others show only limited change. A new study published in the journal Geology finds that the shape of the terrain itself, not just the earthquake, determines how a mountain range responds.

Researchers from the Institute of Earth Environment at the Chinese Academy of Sciences reconstructed the environmental impact of the magnitude-8-plus 1717 earthquake on New Zealand's Alpine Fault by studying sediment cores from two lake catchments, Lake Mapourika and Lake Paringa. Despite sharing similar climate, vegetation, geology and tectonic setting, the two catchments showed strikingly different erosion styles. The team combined carbon and nitrogen isotopes and molecular biomarkers with analyses of catchment topography to trace whether sediment reaching the lakes came from shallow surface soils or deeper bedrock.

In the steeper Mapourika catchment, where hillslopes are more strongly connected to river channels, deep-seated landslides quickly carried material from deep soils and bedrock into the lake, and the dominant erosion process stayed the same before and after the earthquake. In the gentler Paringa catchment, shallow soil erosion dominated at first, but sediment sources shifted over time from high-elevation surface soils to deeper, lower-elevation material, marking a switch from ordinary surface erosion to earthquake-triggered bedrock landsliding.

"Earthquakes don't affect every mountain in the same way," said Wang Jin, the study's lead author. "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 researchers say the findings could help predict how mountain landscapes will respond to future large earthquakes and how the resulting erosion moves sediment and carbon through river systems.