Topography controls how mountains respond to large earthquakes

Recent research published in Geology highlights how mountain topography plays a crucial role in determining how earthquakes reshape mountain landscapes. By studying the aftermath of the A.D. 1717 Alpine Fault earthquake in New Zealand, scientists from the Institute of Earth Environment at the Chinese Academy of Sciences discovered that the manner in which mountains erode following large earthquakes is primarily dictated by landscape steepness, the connectivity between hillslopes and channels, and the capacity of rivers to transport materials.

The researchers examined two lake catchments, Lake Mapourika and Lake Paringa, both situated along New Zealand's Alpine Fault. These lakes hold a continuous record of the earthquake cycle within their sediments. By analyzing multiple geochemical tracers, such as carbon and nitrogen isotopes, alongside detailed topographical and geomorphological studies, the team was able to trace the origins and paths of the eroded materials before and after the earthquake.

In the steeper, more connected Lake Mapourika catchment, deep-seated landslides were the dominant erosion mechanism both before and after the earthquake. These landslides efficiently carried material from both deep soils and bedrock into the lake. Conversely, in the gentler, less connected Lake Paringa catchment, shallow soil erosion predominated in the immediate aftermath, leading to a higher contribution of organic carbon from recent surface soils into the lake.

The study also revealed differing post-earthquake erosion pathways in the two catchments. For Lake Paringa, sediments initially derived from high-elevation soils shifted over time to include deeper soils from lower elevations, indicating an evolution from surface soil erosion to bedrock landsliding triggered by the earthquake. In contrast, Lake Mapourika maintained consistent erosion characteristics, as its steep terrain continuously eroded high-elevation soils and bedrock, and the earthquake simply increased the volume of sediment delivered to the lake without fundamentally altering the dominant erosion process.

Lead author Dr. Wang Jin explained that the shape of the landscape is the key factor in determining how an earthquake impacts a mountain region. Steeper, more connected landscapes tend to see deep bedrock landslides dominating both pre- and post-earthquake conditions, while gentler landscapes may initially erode shallow soils but could transition to deeper landsliding following an earthquake.

Understanding these topographic controls on earthquake-induced erosion is essential for predicting how mountain landscapes will respond to future large earthquakes. This knowledge is crucial for assessing the impacts on sediment transport, landscape evolution, and the cycling of organic carbon in tectonically active regions. Such insights are vital for evaluating the role of mountain belts in global biogeochemical cycles and long-term climate evolution.