One of the ultimate goals in landslide hazard assessment is to predict maximum landslide extension and velocity. Despite much work, the physical processes governing energy dissipation during these ...natural granular flows remain uncertain. Field observations show that large landslides travel over unexpectedly long distances, suggesting low dissipation. Numerical simulations of landslides require a small friction coefficient to reproduce the extension of their deposits. Here, based on analytical and numerical solutions for granular flows constrained by remote-sensing observations, we develop a consistent method to estimate the effective friction coefficient of landslides. This method uses a constant basal friction coefficient that reproduces the first-order landslide properties. We show that friction decreases with increasing volume or, more fundamentally, with increasing sliding velocity. Inspired by frictional weakening mechanisms thought to operate during earthquakes, we propose an empirical velocity-weakening friction law under a unifying phenomenological framework applicable to small and large landslides observed on Earth and beyond.
Entrainment of underlying debris by geophysical flows can significantly increase the flow deposit extent. To study this phenomenon, analog laboratory experiments have been conducted on granular ...column collapse over an inclined channel with and without an erodible bed made of similar granular material. Results show that for slope angles below a critical value θc, between 10° and 16°, the run out distance rf depends only on the initial column height h0 and is unaffected by the presence of an erodible bed. On steeper slopes, the flow dynamics change fundamentally, with a slow propagation phase developing after flow front deceleration, significantly extending the flow duration. This phase has characteristics similar to those of steady uniform flows. Its duration increases with increasing slope angle, column volume, column inclination with respect to the slope and channel width, decreasing column aspect ratio (height over length), and in the presence of an erodible bed. It is independent, however, of the maximum front velocity. The increase in the duration of the slow propagation phase has a crucial effect on flow dynamics and deposition. Over a rigid bed, the development of this phase leads to run out distances rf that depend on both the initial column height h0 and length r0. Over an erodible bed, as the duration of the slow propagation phase increases, the duration of bed excavation increases, leading to a greater increase in the run out distance compared with that over a rigid bed (up to 50%). This effect is even more pronounced as bed compaction decreases.
Key Points
There is a critical angle over which the flow dynamics change
Above the critical angle a slow propagation phase appear after deceleration
Increasing the duration of the slow propagation phase changes the dynamics
We performed seismic waveform inversions and numerical landslide simulations of deep-seated landslides in Japan to understand the dynamic evolution of friction of the landslides. By comparing the ...forces obtained from a numerical simulation to those resolved from seismic waveform inversion, the coefficient of friction during sliding was well-constrained between 0.3 and 0.4 for landslides with volumes of 2–8 ×10
6
m
3
. We obtained similar coefficients of friction for landslides with similar scale and geology, and they are consistent with the empirical relationship between the volume and dynamic coefficient of friction obtained from the past studies. This hybrid method of the numerical simulation and seismic waveform inversion shows the possibility of reproducing or predicting the movement of a large-scale landslide. Our numerical simulation allows us to estimate the velocity distribution for each time step. The maximum velocity at the center of mass is 12–36 m/s and is proportional to the square root of the elevation change at the center of mass of the landslide body, which suggests that they can be estimated from the initial DEMs. About 20% of the total potential energy is transferred to the kinetic energy in our volume range. The combination of the seismic waveform inversion and the numerical simulation helps to obtain the well-constrained dynamic coefficients of friction and velocity distribution during sliding, which will be used in numerical models to estimate the hazard of potential landslides.
We performed numerical simulations of the 2011 deep-seated Akatani landslide in central Japan to understand the dynamic evolution of friction of the landslide. By comparing the forces obtained from ...numerical simulation to those resolved from seismic waveform inversion, the coefficient of the friction during sliding was investigated in the range of 0.1–0.4. The simulation assuming standard Coulomb friction shows that the forces obtained by the seismic waveform inversion are well explained using a constant friction of μ = 0.3. A small difference between the residuals of Coulomb simulation and a velocity-dependent simulation suggests that the coefficient of friction over the volume is well constrained as 0.3 most of time during sliding. It suggests the sudden loss of shearing resistance at the onset of sliding, that is, sudden drop of the initial coefficient of friction in our model, which accelerates the deep-seated landslide. Our numerical simulation calibrated by seismic data provides the evolution of dynamic friction with a reasonable resolution in time, which is difficult to obtain from a conventional runout simulation, or seismic waveform inversion alone.
The force applied to the Earth by the calving of two icebergs at Jakobshavn Isbrae, Greenland, has been quantified. The source force history was recovered by inversion of regional broadband ...seismograms without any a priori constraint on the source time function, in contrast with previous studies. For periods 10–100 s, the three‐component force can be obtained from distant stations alone and is proportional to the closest station seismograms. This inversion makes it possible to quantify changes of the source force direction and amplitude as a function of time and frequency. A detailed comparison with a video of the event was used to identify four forces associated with collision, then bottom‐out and top‐out rotation of the first and second icebergs, and ice mélange motion. Only the two iceberg rotations were identified in previous studies. All four processes are found here to contribute to the force amplitude and variability. Such a complete time‐frequency force history provides unique dynamical constraints for mechanical calving models.
Key Points
Precise force history estimation without any constraint on the source shape
Broadband inversion
Complexity of the force related to different phenomena in the calving sequence
We simulate here the emplacement of the debris avalanche generated by the last flank collapse event of Montagne Pelée volcano (30–45 ka), Martinique, Lesser Antilles. Our objective is to assess the ...maximum distance (i.e., runout) that can be reached by this type of debris avalanche as a function of the volume involved. Numerical simulations are performed using two complementary depth-averaged thin-layer continuum models because no complete models were available in the literature. The first model, SHALTOP, accurately describes dry granular flows over a 3D topography and may be easily extended to describe submarine avalanches. The second model, HYSEA, describes the subaerial and submarine parts of the avalanche as well as its interaction with the water column. However, HYSEA less accurately describes the thin-layer approximation on the 3D topography. Simulations were undertaken testing different empirical friction laws and debris avalanche volume flows. Our study suggests that large collapses (~25 km
3
) probably occurred in several times with successive volumes smaller than about 5 km
3
entering the sea. This result provides new constraints on the emplacement processes of debris avalanches associated with these collapses which can drastically change the related hazard assessment such as the generated tsunami, in a region known for its seismic and volcanic risks.
Axial valleys are found along most slow‐spreading mid‐ocean ridges and are one of the most prominent topographic features on Earth. In this paper, we present the first deep‐tow swath bathymetry for ...the axial valley walls of the Mid‐Atlantic Ridge. These data allow us to analyze axial valley wall morphology with a very high resolution (0.5 to 1 m compared to ≥ 50 m for shipboard multibeam bathymetry), revealing the role played by landslides. Slow‐spreading ridge axial valleys also commonly expose mantle‐derived serpentinized peridotites in the footwalls of large offset normal faults (detachments). In our map of the Ashadze area (lat. 13°N), ultramafic outcrops have an average slope of 18° and behave as sliding deformable rock masses, with little fragmentation. By contrast, the basaltic seafloor in the Krasnov area (lat. 16°38′N) has an average slope of 32° and the erosion of the steep basaltic rock faces leads to extensive fragmentation, forming debris with morphologies consistent with noncohesive granular flow. Comparison with laboratory experiments suggests that the repose angle for this basaltic debris is > 25°. We discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes. We propose that, along axial valley walls where serpentinized peridotites are exposed by detachment faults, mass wasting results in average slopes ≤ 20°, even in places where the emergence angle of the detachment is larger.
Key pointsLandslides shape slopes of axial valley at the Mid‐Atlantic RidgeMass‐wasting is different in serpentinized peridotite and in basaltsLandslides limit overall slopes of ultramafic outcrops to about 20 degrees
Since the 2000s, Greenland ice sheet mass loss has been accelerating, followed by increasing numbers of glacial earthquakes (GEs) at near-grounded glaciers. GEs are caused by calving of km-scale ...icebergs which capsize against the terminus. Seismic record inversion allows a reconstruction of the history of GE sources which captures capsize dynamics through iceberg-to-terminus contact. When compared with a catalog of contact forces from an iceberg capsize model, seismic force history accurately computes calving volumes while the earthquake magnitude fails to uniquely characterize iceberg size, giving errors up to 1 km3. Calving determined from GEs recorded ateight glaciers in 1993–2013 accounts for up to 21% of the associated discharge and 6% of the Greenland mass loss. The proportion of discharge attributed to capsizing calving may be underestimated by at least 10% as numerous events could not be identified by standard seismic detections (Olsen and Nettles, 2018). While calving production tends to stabilize in East Greenland, Western glaciers have released more and larger icebergs since 2010 and have become major contributors to Greenland dynamic discharge. Production of GEs and calving behavior are controlled by glacier geometry with bigger icebergs being produced when the terminus advances in deepening water. We illustrate how GEs can help in partitioning and monitoring Greenland mass loss and characterizing capsize dynamics.
A key point of landslide hazard assessment is the estimation of their runout. Empirical relations linking angle of reach to volume can be used relatively easily, but they are generally associated ...with large uncertainties as they do not consider the topographic specificity of a given study site. On the contrary, numerical simulations provide more detailed results on the deposits morphology, but their rheological parameters can be difficult to constrain. Simulating all possible values can be time consuming and incompatible with operational requirements of rapid estimations. We propose and compare three operational methods to derive scaling power laws relating the landslide travel distance to the destabilized volume. The first one relies only on empirical relations, the second one on numerical simulations with back-analysis, and the third one combines both approaches. Their efficiency is tested on three case studies: the Samperre cliff collapses in Martinique, Lesser Antilles (0.5 to 4×106 m3), the Frank Slide rock avalanche (36×106 m3) and the Samperre cliff collapses in Martinique, Lesser Antilles (0.5 to 4×106 m3) the Fei Tsui debris slide in Hong Kong (0.014×106 m3). Purely numerical estimations yield the smallest uncertainty, but the uncertainty on rheological parameters is difficult to quantify. Combining numerical and empirical approaches allows to reduce the uncertainty of estimation by up to 50%, in comparison to purely empirical estimations. However, it may also induces a bias in the estimation, though observations always lie in the 95% prediction intervals. We also show that empirical estimations fail to model properly the dependence between volume and travel distance, particularly for small landslides (<20,000 <0.02×106 m3).
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