Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath

Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05 UTC and propagating to the NW in the following 2 min. The bulk sliding mass was 7–16 × 1010 kg, equivalent to a collapsed volume of 35–80 × 106 m3. The sliding mass was displaced downslope by 1260 ± 250 m. At short periods, a seismic tremor was observed for 30 min before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3 Hz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2 min before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8 h after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term "afterslides" for this subsequent, declining slope activity after a large landslide

The role of infrequently mobile boulders in modulating landscape evolution and geomorphic hazards

A landscape’s sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation? Boulders are produced on eroding hillslopes by landsliding, rockfall, and/or exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal sediment transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization

Detection of debris-flow events from seismic signals using Benford’s law

The first step in building an early warning system using seismic signals is to automatically identify events of interest. Here, the first digit distribution of seismic signals generated by debris flows and other surface processes was calculated to validate compliance with Benford's law (BL). A detector model for debris flow events was introduced based on amplitude range and goodness of fit of BL. We show that seismic signals generated by debris flows, landslides, and bedload transport follow the BL. These events release more energy and last longer than rockfalls, which do not follow BL. In the test dataset with 1224 samples, the accuracy of the detector model in identifying debris flow events was 0.75

Site Dependence of Fluvial Incision Rate Scaling With Timescale

Global measurements of incision rate typically show a negative scaling with the timescale over which they were averaged, a phenomenon referred to as the “Sadler effect.” This time dependency is thought to result from hiatus periods between incision phases, which leads to a power law scaling of incision rate with timescale. Alternatively, the “Sadler effect” has been argued to be a consequence of the mobility of the modern river bed, where the timescale dependency of incision rates arises from a bias due to the choice of the reference system. In this case, incision rates should be independent of the timescale, provided that the correct reference system is chosen. It is unclear which model best explains the “Sadler effect,” and, if a timescale dependency exists, which mathematical formulation can be used to describe it. Here, we present a compilation of 581 bedrock incision rates from 34 studies, averaged over timescales ranging from single floods to millions of years. We constrain the functional relationship between incision rate and timescale and show that time‐independent incision rate is inconsistent with the global data. Using a power law dependence, a single constant power is inconsistent with the distribution of observed exponents. Therefore, the scaling exponent is site dependent. Consequently, incision rates measured over contrasting timescales cannot be meaningfully compared between different field sites without properly considering the “Sadler effect.” We explore the controls on the variable exponents and propose an empirical equation to correct observed incision rates for their timescale dependency.Plain Language Summary: The rate at which rivers cut into their own bed (incision) typically decreases with the age of past river surfaces used to infer it. This phenomenon, previously described for numerous geological processes, has been traditionally attributed to be a result of an unsteady incision process over the time of investigation. Alternatively, it has been argued that it is a consequence of a measurement bias that can occur when the modern river bed is used as a reference point. To test which of these contrasting hypotheses is valid, we designed specific tests for the competing models, yielding statistical criteria that can be used against actual data. We compiled data on river incision from 34 papers and compared them to the tests. A bias due to the choice of the modern river bed as reference point cannot explain the observations. Instead, we find a site‐specific dependence of incision on timescale. Thus, when comparing incision rates measured at different sites, time dependency needs to be corrected for. Using the field data, we offer a simple empirical equation that can be utilized for such a correction.Key Points: Fluvial bedrock incision rates decrease with the timescale over which they are averaged. Among the examined models, a power law model with a site‐dependent exponent is consistent with 26 previously published field data sets. An empirical equation is proposed to remove the Sadler effect and make incision rates measured at different timescales comparable.Israel Science Foundation (ISF) http://dx.doi.org/10.13039/501100003977Ben Gurion University of the Negev http://dx.doi.org/10.13039/501100014833National Cooperative for the Disposal of Radioactive Waste (NAGRA

Upscaling Sediment‐Flux‐Dependent Fluvial Bedrock Incision to Long Timescales

Fluvial bedrock incision is driven by the impact of moving bedload particles. Mechanistic, sediment‐flux‐dependent incision models have been proposed, but the stream power incision model (SPIM) is frequently used to model landscape evolution over large spatial and temporal scales. This disconnect between the mechanistic understanding of fluvial bedrock incision on the process scale, and the way it is modeled on long time scales presents one of the current challenges in quantitative geomorphology. Here, a mechanistic model of fluvial bedrock incision that is rooted in current process understanding is explicitly upscaled to long time scales by integrating over the distribution of discharge. The model predicts a channel long profile form equivalent to the one yielded by the SPIM, but explicitly resolves the effects of channel width, cross‐sectional shape, bedrock erodibility, and discharge variability. The channel long profile chiefly depends on the mechanics of bedload transport, rather than bedrock incision. In addition to the imposed boundary conditions specifying the upstream supply of water and sediment, and the incision rate, the model includes four free parameters, describing the at‐a‐station hydraulic geometry of channel width, the dependence of bedload transport capacity on channel width, the threshold discharge of bedload motion, and reach‐scale cover dynamics. For certain parameter combinations, no solutions exist. However, by adjusting the free parameters, one or several solutions can usually be found. The controls on and the feedbacks between the free parameters have so far been little studied, but may exert important controls on bedrock channel morphology and dynamics.Plain Language Summary: Bedrock erosion by rivers is driven by the impact of moving sediment particles, chipping away tiny pieces of rocks in their passage. Sediment transport occurs infrequently, during floods. Over thousands of years, this slow process shapes the river, sometimes leading to the creation of spectacular landforms such as gorges. Mechanistic models of fluvial bedrock erosion explicitly take into account the effects of moving sediment particles, while models used for long time scales do not. Here, the connection between mechanistic and long‐term models is made explicit by integrating a mechanistic model over the entire distribution of floods, yielding solutions for the long‐term erosion rate and the channel bed slope. Some of these solutions are similar to those used previously, but other solutions are also possible, showing the rich dynamic behavior that rivers can exhibit. The solutions make explicit the role of lithology, channel width, and discharge variability, which were previously hidden in a single lumped calibration parameter.Key Points: Analytical solution from explicit upscaling of a sediment‐flux‐dependent fluvial bedrock incision model to long time scales. The model includes solutions similar to those obtained in the stream power paradigm, in addition to other possible solutions. The model explicitly resolves forcing behavior and highlights potential dynamic feedbacks that have so far not been considered.GF

Hillslope Sediment Supply Limits Alluvial Valley Width

River‐valley morphology preserves information on tectonic and climatic conditions that shape landscapes. Observations suggest that river discharge and valley‐wall lithology are the main controls on valley width. Yet, current models based on these observations fail to explain the full range of cross‐sectional valley shapes in nature, suggesting hitherto unquantified controls on valley width. In particular, current models cannot explain the existence of paired terrace sequences that form under cyclic climate forcing. Paired river terraces are staircases of abandoned floodplains on both valley sides, and hence preserve past valley widths. Their formation requires alternating phases of predominantly river incision and predominantly lateral planation, plus progressive valley narrowing. While cyclic Quaternary climate changes can explain shifts between incision and lateral erosion, the driving mechanism of valley narrowing is unknown. Here, we extract valley geometries from climatically formed, alluvial river‐terrace sequences and show that across our dataset, the total cumulative terrace height (here: total valley height) explains 90%–99% of the variance in valley width at the terrace sites. This finding suggests that valley height, or a parameter that scales linearly with valley height, controls valley width in addition to river discharge and lithology. To explain this valley‐width‐height relationship, we reformulate existing valley‐width models and suggest that, when adjusting to new boundary conditions, alluvial valleys evolve to a width at which sediment removal from valley walls matches lateral sediment supply from hillslope erosion. Such a hillslope‐channel coupling is not captured in current valley‐evolution models. Our model can explain the existence of paired terrace sequences under cyclic climate forcing and relates valley width to measurable field parameters. Therefore, it facilitates the reconstruction of past climatic and tectonic conditions from valley topography.Plain Language Summary: Little is known on how valleys widen and what sets their width. Therefore, it remains difficult to model the wealth of valley geometries that occur in nature and to predict how valleys adjust to environmental changes. Paired river terraces are staircases of abandoned valley floors that preserve valley widths of the past. The formation of river‐terrace sequences requires changes between vertical river incision and lateral river erosion of valley walls. Moreover, to preserve terraces on both sides of the river, the valley has to narrow over time. While cyclic climate changes during the Quaternary can explain the alternations between vertical incision and lateral erosion, they cannot explain why those valleys narrow. Here we investigate past valley geometries in paired, climatically formed river terraces. We find a negative linear relationship between valley width and valley height. We propose that this relationship reflects a balance between sediment that is moved from hillslopes into the channel and the capacity of the river to remove this sediment. Higher valley walls contribute more sediment that protects the wall from further widening. By including this hillslope‐erosion term, valley‐formation models can reproduce paired river terraces, and allow us to work toward “reading” climatic conditions from valley geometries.Key Points: Valley width in alluvial terraces is inversely proportional to valley height. We suggest sediment supply from river‐independent hillslope erosion limits valley width. The coupling of hillslopes and river channels demands revision of current valley‐evolution models.EC H2020 PRIORITY “Excellent science” H2020 Marie Skłodowska‐Curie Actions http://dx.doi.org/10.13039/100010665https://doi.org/10.5880/fidgeo.2022.02

Bedload transport controls bedrock erosion under sediment-starved conditions

Fluvial bedrock incision constrains the pace of mountainous landscape evolution. Bedrock erosion processes have been described with incision models that are widely applied in river-reach and catchment-scale studies. However, so far no linked field data set at the process scale had been published that permits the assessment of model plausibility and accuracy. Here, we evaluate the predictive power of various incision models using independent data on hydraulics, bedload transport and erosion recorded on an artificial bedrock slab installed in a steep bedrock stream section for a single bedload transport event. The influence of transported bedload on the erosion rate (the "tools effect") is shown to be dominant, while other sediment effects are of minor importance. Hence, a simple temporally distributed incision model, in which erosion rate is proportional to bedload transport rate, is proposed for transient local studies under detachment-limited conditions. This model can be site-calibrated with temporally lumped bedload and erosion data and its applicability can be assessed by visual inspection of the study site. For the event at hand, basic discharge-based models, such as derivatives of the stream power model family, are adequate to reproduce the overall trend of the observed erosion rate. This may be relevant for long-term studies of landscape evolution without specific interest in transient local behavior. However, it remains to be seen whether the same model calibration can reliably predict erosion in future events

From Process to Centuries: Upscaling Field-Calibrated Models of Fluvial Bedrock Erosion

Fluvial bedrock erosion formulas lack validation over space and time. We explore the performance of field-calibrated models at the patch-scale (<1m2) and from minutes to centuries. At the hour to annual scales (in 1-min resolution), we verify predictions using linked discharge, bedload transport and at-a-point erosion, together with spatial erosion from a mountain streambed. Local and spatial erosion linearly scale with bedload mass. The unit stream power model (USP) fails to describe erosion dynamics without a threshold for its onset. Extrapolating over the decadal scale (14 years of discharge and bedload data), scaled models predict up to 12% of erosion for two exceptional floods. Erosion predictions for a bi-centennial discharge varied over four orders of magnitude (extrapolated from 32.5 years discharge and 16 years bedload data at 10-min resolution). Bi-centennial erosion predictions summing up to 1 m for bedload models versus 0.1 m for USP highlight the likely dominance of large events in setting long-term erosion under sediment-starved conditions.ISSN:0094-8276ISSN:1944-800

Hillslope Sediment Supply Limits Alluvial Valley Width

Abstract River‐valley morphology preserves information on tectonic and climatic conditions that shape landscapes. Observations suggest that river discharge and valley‐wall lithology are the main controls on valley width. Yet, current models based on these observations fail to explain the full range of cross‐sectional valley shapes in nature, suggesting hitherto unquantified controls on valley width. In particular, current models cannot explain the existence of paired terrace sequences that form under cyclic climate forcing. Paired river terraces are staircases of abandoned floodplains on both valley sides, and hence preserve past valley widths. Their formation requires alternating phases of predominantly river incision and predominantly lateral planation, plus progressive valley narrowing. While cyclic Quaternary climate changes can explain shifts between incision and lateral erosion, the driving mechanism of valley narrowing is unknown. Here, we extract valley geometries from climatically formed, alluvial river‐terrace sequences and show that across our dataset, the total cumulative terrace height (here: total valley height) explains 90%–99% of the variance in valley width at the terrace sites. This finding suggests that valley height, or a parameter that scales linearly with valley height, controls valley width in addition to river discharge and lithology. To explain this valley‐width‐height relationship, we reformulate existing valley‐width models and suggest that, when adjusting to new boundary conditions, alluvial valleys evolve to a width at which sediment removal from valley walls matches lateral sediment supply from hillslope erosion. Such a hillslope‐channel coupling is not captured in current valley‐evolution models. Our model can explain the existence of paired terrace sequences under cyclic climate forcing and relates valley width to measurable field parameters. Therefore, it facilitates the reconstruction of past climatic and tectonic conditions from valley topography