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Landscape topography is the expression of the dynamic equilibrium between external forcings (for example, climate and tectonics) and the underlying lithology. The magnitude and spatial arrangement of erosional and depositional fluxes dictate the evolution of landforms during both statistical steady state (SS) and transient state (TS) of major landscape reorganization. For SS landscapes, the common expectation is that any point of the landscape has an equal chance to erode below or above the landscape median erosion rate. We show that this is not the case. Afforded by a unique experimental landscape that provided a detailed space-time recording of erosional fluxes and by defining the so-called E50-area curve, we reveal for the first time that there exists a hierarchical pattern of erosion. Specifically, hillslopes and fluvial channels erode more rapidly than the landscape median erosion rate, whereas intervening parts of the landscape in terms of upstream contributing areas (colluvial regime) erode more slowly. We explain this apparent paradox by documenting the dynamic nature of SS landscapes-landscape locations may transition from being a hillslope to being a valley and then to being a fluvial channel due to ridge migration, channel piracy, and small-scale landscape dynamics through time. Under TS conditions caused by increased precipitation, we show that the E50-area curve drastically changes shape during landscape reorganization. Scale-dependent erosional patterns, as observed in this study, suggest benchmarks in evaluating numerical models and interpreting the variability of sampled erosional rates in field landscapes.

Soil moisture is a key preconditioning factor influencing hillslope stability and the initiation of landslides. Direct measurements of soil moisture on a large scale are logistically complicated, expensive, and therefore sparse, resulting in large data gaps. In this study, we calibrate a numerical land surface model to improve our representation of post-monsoon soil drying in landslide-prone Nepal. We use a parameter perturbation experiment to identify optimal parameter sets at three field monitoring sites and evaluate the performance of those optimal parameter sets at each location. This process enables the calibration of key soil hydraulic parameters, in particular a higher hydraulic conductivity and a lower saturation moisture content relative to the default parameter setting. Runs with the calibrated model parameters provide a substantially more accurate (50 % or greater reduction in root mean squared error) soil moisture record than those with the default model parameters, even when calibrated from sites as much as 250 km apart. This process enables meaningful calculation of post-monsoon soil moisture decay at locations with no in situ monitoring, so as to inform a key component of landslide susceptibility mapping in Nepal and other regions where field measurements of soil moisture are limited. [ABSTRACT FROM AUTHOR]

Mapping exposure to landslides is necessary to mitigate risk and reduce vulnerability. Exposure maps can be constructed from building databases, akin to seismic risk assessments, but there has been little investigation of the predictive relationship between building damage and risk to human life from landslides. Our study investigates this relationship globally and in Nepal (47,213 and 5,664 landslides, respectively). While a correlation exists for nationwide totals (R2=0.75), it is near zero for individual events (R2=0.025). It is important not to use building datasets in isolation for landslide exposure maps and disaster planning to avoid unintentionally prioritising building damage over human lives. [ABSTRACT FROM AUTHOR]

The Himalayan orogen exports millions of tons of sediment annually to the Indo‐Gangetic foreland basin, derived from both hinterland and foreland fold‐thrust belts (FTB). Although erosion rates in the hinterland are well‐constrained, erosion rates in the foreland FTB and, by extension, the sediment flux have remained poorly constrained. Here, we quantified erosion rates and sediment flux from the Mohand Range in the northwestern Himalaya by modeling and measuring the cosmogenic radionuclide (CRN) 10Be and 26Al concentrations in modern fluvial sediments. Our model uses local geological and geophysical constraints and accounts for CRN inheritance and sediment recycling, which enables us to determine the relative contributions of the hinterland and foreland FTB sources to the CRN budget of the proximal foreland deposits. Our model predictions closely match measured concentrations for a crustal shortening rate across the Mohand Range of 8.0 ± 0.5 mm yr−1 (i.e., approximately 50% of the total shortening across the Himalaya at this longitude) since 0.75−0.06+0.02 $0.7{5}_{-0.06}^{+0.02}$ Ma. This shortening implies a spatial gradient in erosion rates ranging from 0.42 ± 0.03 to 4.92 ± 0.34 mm yr−1, controlled by the geometry of the underlying structure. This erosion pattern corresponds to an annual sediment recycling of ∼2.0 megatons from the Mohand Range to the downstream Yamuna foreland. Converted to sediment fluxes per unit width along the Himalaya, the foreland FTB accounts for ∼21% ± 5% of the total flux entering the foreland. Because these sediments have lower 10Be concentrations than hinterland‐derived sediment, they would lead to ∼14% overestimation of 10Be‐derived erosion rates, based on Yamuna sediments in the proximal foreland. Plain Language Summary: Cosmogenic radionuclides such as beryllium‐10 (10Be) are important tools for tracking erosion processes at the Earth's surface. However, they are challenging to apply when sediment remains temporarily stored for millions of years before being re‐eroded. We call this sediment recycling. We quantified sediment recycling in the Mohand Range, at the foothills of the northwestern Himalaya, based on the present‐day 10Be concentration in fluvial sediments recycled from tectonically uplifted older foreland deposits. We do this by modeling how the 10Be concentrations change as sediment is first eroded in the Himalayan source region, then deposited in the foreland, and finally eroded again. By comparing modeled and measured concentrations, we find that erosion rates in the Mohand Range vary from 0.42 ± 0.03 to 4.92 ± 0.34 mm yr−1, corresponding to about 2.0 megatons of sediment recycling from this range to the downstream Yamuna foreland. Because the 10Be concentration in recycled sediment is lower than that in sediment eroded from the high Himalaya, admixing high Himalayan sediments with recycled material may result in erosion rates that are higher than the actual erosion rates in the high Himalaya. Key Points: Novel approach for tracking cosmogenic nuclide accumulation in fluvial sediments through each step of a fold‐thrust belt sediment routing system10Be‐derived erosion rates and sediment flux from the Mohand Range in the northwestern HimalayaOnset of Main Frontal Thrust motion and rate of crustal shortening across the Mohand Range constrained by modeling 10Be concentrations in present‐day river sediments [ABSTRACT FROM AUTHOR]

Coseismic landslides represent the first stage of a broader cascading sequence of geohazards associated with high‐magnitude continental earthquakes, with the subsequent remobilization of coseismic landslide debris posing a long‐term post‐seismic legacy in mountain regions. Here, we quantify the controls on the hazard posed by landslide remobilization and debris runout, and compare the overlap between areas at risk of runout and the pattern of post‐seismic landslides and debris flows that actually occurred. Focusing on the 2015 Mw 7.8 Gorkha earthquake in Nepal, we show that the extent of the area that could be affected by debris runout remained elevated above coseismic levels 4.5 years after the event. While 150 km2 (0.6% of the study area) was directly impacted by landslides in the earthquake, an additional 614 km2 (2.5%) was left at risk from debris runout, increasing to 777 km2 (3.2%) after the 2019 monsoon. We evaluate how this area evolved by comparing modelled predictions of runout from coseismic landslides to multi‐temporal post‐seismic landslide inventories, and find that 14% (85 km2) of the total modelled potential runout area experienced landslide activity within 4.5 years after the earthquake. This value increases to 32% when modelled runout probability is thresholded, equivalent to 10 km2 of realized runout from a remaining modelled area of 32 km2. Although the proportion of the modelled runout area from coseismic landslides that remains a hazard has decreased through time, the overall runout susceptibility for the study area remains high. This indicates that runout potential is changing both spatially and temporally as a result of changes to the landslide distribution after the earthquake. These findings are particularly important for understanding evolving patterns of cascading hazards following large earthquakes, which is crucial for guiding decision‐making associated with post‐seismic recovery and reconstruction. [ABSTRACT FROM AUTHOR]

We test the spatial correspondence between rock uplift in active normal fault footwalls ill the western USA, and catchment-averaged denudation rates from cosmogenic radionuclide (CRN) analysis. We find that denudation rates vary along strike, depending on the fault length and displacement. For the 18-km-long Sweetwater fault, flint-wall relief is largely inherited from the prefaulting topography, and denudation rates are only partly reflective of fault displacement. For the 130-km-long Wassuk fault, all inherited topography has been removed by erosion in response to fault displacement, Denudation rates, however, show little along-strike consistency anti no correlation with catchment size or morphology. We argue that, in the early stages of fault growth, CRN-derived denudation rates reflect inherited prefaulting relief, with an overprint of fault-controlled erosion. In later stages of fault growth, rates are decoupled from fault displacement through the impact of stochastic landsliding events, and longer term measures of denudation (such as low-temperature thermochronometry) that integrate over multiple earthquakes may provide a better record of spatial variations in rock uplift. doi: 10.1130/G30164A.1

The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic cooling and denudation yet shows little evidence for large-magnitude shortening or accommodation generation in the foreland basin. We address this paradox by using a variety of geomorphic observations to place constraints on the kinematics and slip rates of several large faults that parallel the plateau margin. The Beichuan and Pengguan faults are active, dominantly dextral-slip structures that can be traced continuously for up to 200 km along the plateau margin. Both faults offset fluvial fill terraces that yield inheritance-corrected, cosmogenic [sup.10]Be exposure ages of doi:10.1029/2006TC001987.

We use three large normal fault arrays in the northeastern Basin and Range Province, western United States, to document catchment development and relief production during fault growth. Fault slip and slip rates increase systematically along strike from zero at the fault tips. Catchment relief and across-strike range width both increase as slip accumulates but reach maximum values at a distance of ~15 km from the fault tips and remain uniform along strike over much of the footwalls. Catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5-6 along strike. We infer that catchments first elongate in the across-strike direction as slip accumulates and the range half-width increases. Once the half-width reaches its maximum value, continued catchment growth is possible only by along-strike capture, which increases outlet spacing but not relief. The close correspondence between catchment relief and range half-width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone. Thus, the limiting factor in footwall development is the width of the range, which is controlled by two external agents: the geometry and spacing of the major faults, and the elevations of base level on both flanks. Keywords: normal faults, topography, tectonic geomorphology, landscape evolution, Idaho, Montana.

[1] An integrated study of topography, bathymetry, high-resolution aeromagnetic data, and structural observations demonstrates significant Cenozoic fault activity in SE Ireland. Tectonically generated knickpoints and reddened fault breccias along topographic escarpments that are underlain by greywacke bedrock and trend oblique to the regional Caledonian strike provide evidence for fault displacement. Near-offshore faults with similar geometry produce present-day bathymetric scarps and localized tectonic topography in the inverted Kish Bank Basin. The integration of offshore high resolution aeromagnetic data and structural interpretation of the Kish Bank Basin provides evidence for dextral transtension on NNW trending faults and sinistral transpression on ENE trending faults bounding a lower Paleozoic to Carboniferous basement block. These faults correlate onshore with previously recognized Caledonian faults producing topographic offsets and surface uplift. To the north, offshore structures can be traced onshore and cut exposures of the Caledonian Leinster Granite. Structural analysis of these outcrops indicates post-Variscan deformation. A major fault on the NW margin of the batholith cuts a major erosion surface developed on Carboniferous carbonate rocks, and nonmarine Miocene deposits are preserved above this surface. Fault kinematics provide evidence of two paleostress systems: (1) NW-SE [[sigma].sub.1], subvertical [[sigma].sub.2] and NE-SW [[sigma].sub.3] followed by (2) a clockwise swing of [[sigma].sub.1] to NNW-SSE. Timing of deformation in both stress systems is probably post-Oligocene age. The mechanism driving this deformation is likely ridge-push. Much is already known about Cenozoic tectonics and exhumation from offshore basins. This study shows that onshore Ireland has also been affected by significant tectonic activity and exhumation during the Cenozoic. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8030 Structural Geology: Microstructures; 8110 Tectonophysics: Continental tectonics--general (0905); 8168 Tectonophysics: Stresses--general; 9335 Information Related to Geographic Region: Europe; KEYWORDS: Ireland. Cenozoic, reactivation, uplift, knickpoints.

Coseismic landslides are a major hazard associated with large earthquakes in mountainous regions. Despite growing evidence for their widespread impacts and persistence, current understanding of the evolution of landsliding over time after large earthquakes, the hazard that these landslides pose, and their role in the mountain sediment cascade remains limited. To address this, we present the first systematic multi‐temporal landslide inventory to span the full rupture area of a large continental earthquake across the pre‐, co‐ and post‐seismic periods. We focus on the 3.5 years after the 2015 Mw 7.8 Gorkha earthquake in Nepal and show that throughout this period both the number and area of mapped landslides have remained higher than on the day of the earthquake itself. We document systematic upslope and northward shifts in the density of landsliding through time. Areas where landslides have persisted tend to cluster in space, but those areas that have returned to pre‐earthquake conditions are more dispersed. While both pre‐ and coseismic landslide locations tend to persist within mapped post‐earthquake inventories, a wider population of newly activated but spatially dispersed landslides has developed after the earthquake. This is particularly important for post‐earthquake recovery plans that are typically based on hazard assessments conducted immediately after the earthquake and thus do not consider the evolving landslide hazard. We show that recovery back to pre‐earthquake landsliding rates is fundamentally dependent on how that recovery is defined and measured. Clarity around this definition is particularly important for informing a comprehensive approach to post‐earthquake landslide hazard and risk. Key Points: Both the number and area of mapped landslides remain significantly higher than coseismic levels >3.5 years after the earthquakeLocations with persistent post‐earthquake landslides are clustered while newly activated spatially dispersed landsliding has also developedDifferences in observed post‐seismic hillslope recovery are fundamentally dependent on how recovery is defined and measured [ABSTRACT FROM AUTHOR]