Seismic controls on contemporary sediment export in the Siret river catchment, Romania

While differences in catchment sediment yield (SY, [t km− 2 y− 1]) are generally attributed to topography, lithology, climate and land use, recent studies have highlighted that also seismic activity may have an important impact on SY. Nonetheless, relatively little is known about the importance of this factor and the processes and mechanisms explaining its influence. Therefore, this study explores the role of seismic activity in explaining spatial and temporal variation in sediment export within the Siret Basin (Romania, 45,000 km2), a catchment with a large variability in seismic activity. on previously unpublished long-term (> 30 years) SY measurements for 38 subcatchments of the Siret, we analyze the correlation between average SY, seismic activity and various other catchment characteristics. Our results showed that spatial variation in average SY was indeed strongly correlated with the degree of seismic activity in each catchment (R2 = 0.74). Also catchment lithology explained an important part of the differences in SY (R2 = 0.67). The combination of these two factors accounted for about 80% of the observed variation in SY, while other factors (e.g. topography, land use, climate, and runoff) did not significantly contribute to the explained variance in average SY. lore the impact of a specific earthquake event on sediment export, we analyzed daily variations in suspended sediment concentrations of 10 subcatchments, five years before and after an earthquake of Mw = 7.4 that affected the Vrancea region in 1977 and triggered a substantial number of landslides. Only one catchment showed a clear (3-fold) increase in sediment concentrations at unit discharge. For the other nine catchments, no consistent increase could be observed. This indicates that the impact of seismic activity on average SY is mainly indirect and not associated with sudden pulses of sediments, caused by earthquake-triggered landslides. Potential mechanisms that could explain such indirect responses are discussed.