Large earthquakes can cause huge devastation in mountains by triggering landslides. But what happens to the rocks, mud and sand when the shaking stops? New research by the Department of Geography answers this question as Dr Bob Hilton explains.
What did you find in your research?
In the months to years that follow an earthquake, rocks and sediment moved by landslides can still cause serious issues, filling rivers and causing more widespread damage.
Our research examined this in detail following the devastating 2008 Wenchuan earthquake, which triggered more than 57,150 landslides in the Longmen Shan mountains in Sichuan in China.
We found that it will take decades to centuries for rivers to remove fine sediment (mud and sand) from earthquake-triggered landslides.
Our results demonstrate for the first time that the memory of rivers to earthquakes is longest where the area of landsliding is high, and where the climate results in lower-intensity rainfall and river flows that are less efficient at removing the excess sediment.
How did you carry out your research?
We used detailed, daily measurements from rivers of the amount of water (water discharge) and the amount of mud and sand (suspended sediment discharge) at 16 gauging stations in the Longmen Shan mountains. These river catchments cover an area more than 40 times that of Greater London.
We were able to use data from 2006 (two years before the Wenchuan earthquake) until 2013 (five years after the earthquake) collected by the Chinese Bureau of Hydrology.
The data is extremely detailed, and we were able to track the immediate changes to river loads after the earthquake. We also mapped landslides triggered by the earthquake using satellite imagery. These landslide maps were used to calculate the volume of mud and sand delivered by earthquake-triggered landslides to the rivers.
Why is this research important?
Rocks and sediment moved by earthquake-triggered landslides cause hazards, filling rivers, affecting water resources and hydro-electric power generation. Loose sediment can also cause more damage if new landslides occur.
If we want to understand better how to manage these hazards following large earthquakes, it is essential to know what controls the lifespan of sediment moved by landslides in mountain river catchments.
Until our study, there was only one other detailed example from the 1999 Chi-Chi earthquake in Taiwan. In the river catchments impacted by Chi-Chi, it took around six years to remove fine sediment, which is quite quick. Until now it has not been clear whether this was an exceptional case because Taiwan experiences intense tropical storms. With our new data, we show that the Chi-Chi case is part of a range of responses of rivers to large earthquakes. Some catchments in the Longmen Shan also had very short memories of the earthquake
However, if rainfall intensity is low, and earthquake-triggered landsliding is high then fine sediment may stay in river catchments for more than 80 years.
What are the next steps?
We’ve been able to study the input and transport of fine material (mud and sand sized), but what we still need to do is better understand the coarser material (e.g. pebbles and cobbles) which travels on the river bed. This material is much harder to measure in rivers, but can do a huge amount of damage. If the fine sediment stays in these catchments for decades, the coarse sediment is likely to stay in catchments for much longer periods of time.
Our study provides a new framework to tackle this question. Places with high amounts of landsliding following an earthquake and lower intensity of rainfall may have very long memories of large earthquakes, and pose a significant challenge to hazard management.
Who funded the research and who did you collaborate with?
The research was funded by the Royal Society, the National Science Foundation of China, the Chinese Academy of Sciences and the US National Science Foundation.
The project was part of Jin Wang’s PhD research, who is working at the Institute of Earth Environment, Chinese Academy of Science Xi’an, and the Department of Geography, Durham University.
The research is part of an international collaboration between Durham researchers (Dr Bob Hilton and Prof. Alex Densmore), Prof. Zhangdong Jin (Xi’an) and Prof. Josh West of University of Southern California (USC), and involved other PhD students at Xi’an and USC.
Source: Durham University