Seismic mountain building: Landslides associated with the 2008 Wenchuan earthquake in the context of a generalized model for earthquake volume balance

Here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the M-w 7.9 Wenchuan earthquake in central China. Coseismic landslides were mapped from high-resolution remote imagery using an automated algorithm and manual delineation, which allow us to distinguish clustered landslides that can bias landslide volume calculations. Employing a power-law landslide area-volume relation, we find that the volume of landslide-associated mass wasting (approximate to 2.8+0.9/-0.7 km(3)) is lower than previously estimated (approximate to 5.7-15.2 km(3)) and comparable to the volume of rock uplift (approximate to 2.61.2 km(3)) during the Wenchuan earthquake. If fluvial evacuation removes landslide debris within the earthquake cycle, then the volume addition from coseismic uplift will be effectively offset by landslide erosion. If all earthquakes in the region followed this volume budget pattern, the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography. To provide a framework for addressing this question, we explore a group of scaling relations to assess earthquake volume balance. We predict coseismic uplift volumes for thrust-fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude, M-w. By coupling this scaling relation with landslide volume-M-w scaling, we obtain an earthquake volume balance relation in terms of moment magnitude M-w, which is consistent with the revised Wenchuan landslide volumes and observations from the 1999 Chi-Chi earthquake in Taiwan. Incorporating the Gutenburg-Richter frequency-M-w relation, we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area. This model yields reasonable rates of crustal thickening from coseismic deformation (e.g., approximate to 0.1-0.5 km Ma(-1) in tectonically active convergent settings), and implies that moderate magnitude earthquakes (M-w approximate to 6-7) are likely responsible for most of the coseismic contribution to rock uplift because of their smaller landslide-associated volume reduction. Our first-order model does not consider a range of factors (e.g., lithology, climate conditions, epicentral depth, and tectonic setting), nor does it account for viscoelastic effects or isostatic responses to erosion, and there are important large uncertainties on the scaling relationships used to quantify coseismic deformation. Nevertheless, our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building.</p

Connectivity of earthquake-triggered landslideswith the fluvial network: Implicationsfor landslide sediment transport afterthe 2008 Wenchuan earthquake

Evaluating the influence of earthquakes on erosion, landscape evolution, and sediment-related hazards requires understanding fluvial transport of material liberated in earthquake-triggered landslides. The location of landslides relative to river channels is expected to play an important role in postearthquake sediment dynamics. In this study, we assess the position of landslides triggered by the Mw 7.9 Wenchuan earthquake, aiming to understand the relationship between landslides and the fluvial network of the steep Longmen Shan mountain range. Combining a landslide inventory map and geomorphic analysis, we quantify landslide-channel connectivity in terms of the number of landslides, landslide area, and landslide volume estimated from scaling relationships. We observe a strong spatial variability in landslide-channel connectivity, with volumetric connectivity (&xi;) ranging from ~20% to ~90% for different catchments. This variability is linked to topographic effects that set local channel densities, seismic effects (including seismogenic faulting) that regulate landslide size, and substrate effects that may influence both channelization and landslide size. Altogether, we estimate that the volume of landslides connected to channels comprises 43 + 9/ 7% of the total coseismic landslide volume. Following the Wenchuan earthquake, fine-grained (&lt;~0.25 mm) suspended sediment yield across the Longmen Shan catchments is positively correlated to catchment-wide landslide density, but this correlation is statistically indistinguishable whether or not connectivity is considered. The weaker-than-expected influence of connectivity on suspended sediment yield may be related to mobilization of fine-grained landslide material that resides in hillslope domains, i.e., not directly connected to river channels. In contrast, transport of the coarser fraction (which makes up &gt;90% of the total landslide volume) may be more significantly affected by landslide locations.</p

Dilution of Be-10 in detrital quartz by earthquake-induced landslides: Implications for determining denudation rates and potential to provide insights into landslide sediment dynamics

The concentration of Be-10 in detrital quartz (Be-10(qtz)) from river sediments is now widely used to quantify catchment-wide denudation rates but may also be sensitive to inputs from bedrock landslides that deliver sediment with low Be-10(qtz). Major landslide-triggering events can provide large amounts of low-concentration material to rivers in mountain catchments, but changes in river sediment Be-10(qtz) due to such events have not yet been measured directly. Here we examine the impact of widespread landslides triggered by the 2008 Wenchuan earthquake on Be-10(qtz) in sediment samples from the Min Jiang river basin, in Sichuan, China. Landslide deposit material associated with the Wenchuan earthquake has consistently lower Be-10(qtz) than in river sediment prior to the earthquake. River sediment Be-10(qtz) decreased significantly following the earthquake downstream of areas of high coseismic landslide occurrence (i.e., with greater than similar to 0.3% of the upstream catchment area affected by landslides), because of input of the Be-10-depleted landslide material, but showed no systematic changes where landslide occurrence was low. Changes in river sediment Be-10(qtz) concentration were largest in small first-order catchments but were still significant in large river basins with areas of 10(4)-10(5) km(2). Spatial and temporal variability in river sediment Be-10(qtz) has important implications for inferring representative denudation rates in tectonically active, landslide-dominated environments, even in large basins. Although the dilution of Be-10(qtz) in river sediment by landslide inputs may complicate interpretation of denudation rates, it also may provide a possible opportunity to track the transport of landslide sediment. The associated uncertainties are large, but in the Wenchuan case, calculations based on Be-10 mixing proportions suggest that river sediment fluxes in the 2-3 years following the earthquake increased by a similar order of magnitude in the 0.25-1 mm and the &lt;0.25 mm size fractions, as determined from Be-10(qtz) mixing calculations and hydrological gauging, respectively. Such information could provide new insight into sediment transfer, with implications for secondary sediment-related hazards and for understanding the removal of mass from mountains.</p