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PRESSurE

Perturbations of Earth Surface Processes by Large Earthquakes

Strong earthquakes cause transient perturbations of the near Earth’s surface system. These include the widespread landsliding and subsequent mass movement and the loading of rivers with sediments. In addition, rock mass is shattered during the event, forming cracks that affect rock strength and hydrological conductivity. Often overlooked in the immediate aftermath of an earthquake, these perturbations can represent a major part of the overall disaster with an impact that can last for years before restoring to background conditions. Thus, the relaxation phase is part of the seismically induced change by an earthquake and need to be monitored in order to understand the full impact of earthquakes on the Earth system. Early June 2015, shortly after the April 2015 Mw7.9 Gorkha earthquake, we installed an twelve hydrological stations covering all rivers draining the epicentral area. At each station we sample daily river water for suspended sediment analysis, as well as geochemistry of the dissolved load. Samples are filtered and packed in Nepal and then shipped to the sediment lab for further analysis. The sampling network is complemented by an array of seismometers, repeated satellite imagery observations and on-side stage high recording and is optimized for the monitoring of Earth surface processes (landsliding, mass wasting river processes, debris flows) and for the monitoring of properties of the shallow subsurface by coda analysis. The monitoring network is the first and most complete observatory to monitor the perturbation of Earth surface process by a major earthquake. The main questions of the project: Inter-seismic erosion vs earthquake induced erosion and what is the role of earthquakes in the Himalayan landscape evolution.

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