Perturbation of fluvial sediment fluxes following the 2008 Wenchuan earthquake

Abstract

Quantifying the removal of co‐seismic landslide material after a large‐magnitude earthquake is central to our understanding of geomorphic recovery from seismic events and the topographic evolution of tectonically active mountain ranges. In order to gain more insight into the fluvial erosion response to co‐seismic landslides, we focus on the sediment fluxes of rivers flowing through the rupture zone of the 2008 Mw 7.9 Wenchuan earthquake in the Longmen Shan of the eastern Tibetan Plateau. Over the post‐seismic period of 2008–2013, we annually collected river sediment samples (0.25–1 mm) at 19 locations and measured the concentration of cosmogenic 10Be in quartz. When compared with published pre‐earthquake data, the 10Be concentrations declined dramatically after the earthquake at all sampling sites, but with significant spatial differences in the amplitude of this decrease, and were starting to increase toward pre‐earthquake level in several basins over the 5‐year survey. Our analysis shows that the amplitude of 10Be decrease is controlled by the amount of landslides directly connected to the river network. Calculations based on 10Be mixing budgets indicate that the sediment flux of the 0.25–1 mm size fraction increased up to sixfold following the Wenchuan earthquake. Our results also suggest that fluvial erosion became supply limited shortly after the earthquake, and predict that it could take a few years to several decades for fluvial sediment fluxes to go back to pre‐earthquake characteristics, depending on catchment properties. We also estimate that it will take at least decades and possibly up to thousands of years to remove the co‐seismic landslide materials from the catchments in the Longmen Shan. Copyright © 2017 John Wiley & Sons, Ltd.

Internal Erosion Controls Failure and Runout of Loose Granular Deposits: Evidence From Flume Tests and Implications for Postseismic Slope Healing

“…Through preliminary experiments in which the particles transported by the water seepage outflowing from the artificial slopes are collected and analyzed (Figures b and d), we identify a threshold size d = 0.5 mm to discriminate between the small and the coarse fractions. This size matches, qualitatively, with that of the eroded particles transported as river‐suspended sediments, which constitute a significant proportion (10%–26%) of the Wenchuan earthquake coseismic debris (G. Li et al, ; W. Wang et al, ). However, different thresholds may result from different grain assemblies having different pore structures.…”

“…Coseismic landslide deposits are very sensitive to hydrological inputs soon after the earthquake (e.g., Hovius et al, ; R. Huang & Fan, ; Marc et al, ; Yu et al, ). However, this high sensitivity is short lived, as landslide‐triggering rainfall thresholds and landslide rates tend to normalize quickly, in about a decade (Marc et al, ; W. Wang et al, ; W. Yang et al, ). Enhanced mass wasting and fluvial sediment evacuation following the Wenchuan earthquake can only account for the depletion of a small proportion of the coseismic debris (W. Wang et al, ; S. Zhang et al, ; S. Zhang & Zhang, ), while most of it stabilized along the slopes, within the orogen.…”

“…However, this high sensitivity is short lived, as landslide‐triggering rainfall thresholds and landslide rates tend to normalize quickly, in about a decade (Marc et al, ; W. Wang et al, ; W. Yang et al, ). Enhanced mass wasting and fluvial sediment evacuation following the Wenchuan earthquake can only account for the depletion of a small proportion of the coseismic debris (W. Wang et al, ; S. Zhang et al, ; S. Zhang & Zhang, ), while most of it stabilized along the slopes, within the orogen. Various mechanisms have been proposed to explain the postseismic healing in place of the coseismic debris, such as progressive revegetation, consolidation, and grain coarsening (e.g., L. Li et al, ; Lin et al, ; Saba et al, ).…”

Topographic Loads Modified by Fluvial Incision Impact Fault Activity in the Longmenshan Thrust Belt, Eastern Margin of the Tibetan Plateau

“…The Wenchuan earthquake altered the landscape in the central LTB moderately to markedly, which enables us to quantitatively characterize the development of geomorphic indices in the earthquake cycle using an integration of multiple types of data, including pre‐earthquake geomorphic indices, coseismic deformation, coseismic geomorphic indices (Parker et al, ; Ren et al, ; Shen et al, ; X. Xu et al, ; C. Xu et al, ), and fluvial incision intensity and transportation ability (Godard et al, ; W. Wang et al, ). Figure illustrates the geomorphic evolution of the central LTB, in terms of the mean hillslope angle in an earthquake cycle: (1) Prior to the earthquake (such as the 2008 Wenchuan event), the hillslope angle of the hanging‐wall block of the thrust (i.e., the Pengguan massif) reaches the critical value.…”

“…Assuming a steady‐state landscape over the seismic cycle (Steer et al, ), the total erosion thickness during the earthquake cycle is ~7.5 m, equal to the total vertical uplift of the Longmenshan relative to the Sichuan basin (X. Xu et al, ), with about one third (~2.7 m) of landslide deposit. The postearthquake erosion increase up to sixfolds (W. Wang et al, ); therefore, the time length for transport of the landslide deposit is ~1/13 (1 / (1 + 2 * 6)) of the earthquake cycle. (4) Following the transport, the rivers incise into the bedrocks, which further steepen the mean hillslope until it reaches the threshold again.…”