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Protracted river response to medieval earthquakes

Stolle, A.; Schwanghart, W.; Andermann, C.; Bernhardt, A.; Wittmann, H.; Merchel, S.; Rugel, G.; Fort, M.; Adhikari, B. R.; Korup, O.

Mountain rivers respond to strong earthquakes by rapidly aggrading to accommodate excess sediment delivered by co-seismic landslides. Detailed sediment budgets indicate that rivers need several years to decades to recover from seismic disturbances, depending on how recovery is defined. We examine several proxies of river recovery around Pokhara, Nepal’s second largest city. We use a freshly exhumed cohort of floodplain trees in growth position as a geomorphic marker of rapid sedimentation that formed a fan covering 150 km2 in a Lesser Himalayan basin with tens of meters of debris. Radiocarbon dates of these buried trees are consistent with those of nearby valley fills linked to major medieval earthquakes, and allow estimating average rates of sedimentation and re-incision.
We combine high-resolution digital elevation data, geodetic field surveys, aerial photos, and dated re-exhumed tree trunks to reconstruct dated geomorphic marker surfaces. The volumes of sediment lost from beneath these surfaces require net sediment yields of up to 4200 t km –2 yr –1, averaged over some 650 years since the last inferred earthquake. These rates exceed rates of catchment-wide Denudation inferred from concentrations of cosmogenic 10Be in river sands. The lithological composition of active channel-bed load differs from that of local bedrock, confirming that rivers still mostly evacuate medieval valley fills, locally incising at rates of 160 to 220 mm yr –1. Pronounced knickpoints and epigenetic gorges at tributary junctions further document a protracted fluvial response; only the distal portions of the earthquake-derived sediment wedge have been incised to near their base. Our results challenge the notion that mountain rivers recover from earthquakes within years to decades. The valley fills around Pokhara show that even highly erosive Himalayan rivers need at least centuries to millennia to adjust. Our results motivate some rethinking of post-seismic hazard appraisals and infrastructural planning in mountain regions.

Keywords: Fluvial recovery; sediment yield; medieval earthquakes; Nepal

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Publ.-Id: 25734