The landslide problem

Abstract

The synonymous use of the general term "landslide", with a built-in reference to a sliding motion, for all varieties of mass-transport deposits (MTD), which include slides, slumps, debrites, topples, creeps, debris avalanches etc. in subaerial, sublacustrine, submarine, and extraterrestrial environments has created a multitude of conceptual and nomenclatural problems. In addition, concepts of triggers and long-runout mechanisms of mass movements are loosely applied without rigor. These problems have enormous implications for studies in process sedimentology, sequence stratigraphy, palaeogeography, petroleum geology, and engineering geology. Therefore, the objective of this critical review is to identify key problems and to provide conceptual clarity and possible solutions. Specific issues are the following: (1) According to "limit equilibrium analyses" in soil mechanics, sediment failure with a sliding motion is initiated over a shear surface when the factor of safety for slope stability (F) is less than 1. However, the term landslide is not meaningful for debris flows with a flowing motion.(2) Sliding motion can be measured in oriented core and outcrop, but such measurement is not practical on seismic profiles or radar images. (3) Although 79 MTD types exist in the geological and engineering literature, only slides, slumps, and debrites are viable depositional facies for interpreting ancient stratigraphic records. (4) The use of the term landslide for highvelocity debris avalanches is inappropriate because velocities of mass-transport processes cannot be determined in the rock record. (5) Of the 21 potential triggering mechanisms of sediment failures, frequent short-term events that last for only a few minutes to several hours or days (e.g., earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more relevant in controlling deposition of deep-water sands than sporadic long-term events that last for thousands to millions of years (e.g., sea-level lowstands). (6) The comparison of H/L (fall height/runout distance) ratios of MTD in subaerial environments with H/L ratios of MTD in submarine and extraterrestrial environments is incongruous because of differences in data sources (e.g., outcrop vs. seismic or radar images). (7) Slides represent the pre-transport disposition of strata and their reservoir quality (i.e., porosity and permeability) of the provenance region, whereas debrites reflect post-transport depositional texture and reservoir quality. However, both sandy slides and sandy debrites could generate blocky wireline (gamma-ray) log motifs. Therefore, reservoir characterization of deep-water strata must be based on direct examination of the rocks and related process-specific facies interpretations, not on wireline logs or on seismic profiles and related process-vague facies interpretations. A solution to these problems is to apply the term "landslide" solely to cases in which a sliding motion can be empirically determined. Otherwise, a general term MTD is appropriate. This decree is not just ...

Giant sublacustrine landslide in the Cretaceous Songliao Basin, NE China

“…Slopes, whether subaerial or subaqueous, are inherently unstable because the sediments deposited on them are subjected to gravitational forces along an inclined surface (Posamentier & Martinsen, ). Moreover, subaqueous landslides can be much larger than those on land (Cruden, ; Pope et al, ; Shanmugam, ). Several possible mechanisms have been proposed to account for the vast subaqueous landslides (Galloway et al, ; Lewis, ).…”

“…Consequently, identifying a single process responsible for triggering a collapse is very difficult (Moscardelli, Wood, & Mann, ). Two or more of these processes could be acting at the same time (Moscardelli et al, ; Ogiesoba & Hammes, ; Shanmugam, ).…”

Inferring the high velocity of landslides in Valles Marineris on Mars from morphological analysis

“…Maximum velocities are attained at the break of slope, followed by a decrease of velocity been visualized in great detail with sidescan sonar and multibeam echosounder, whereas corings have provided rich samples of oceanic sediments, allowing to distinguish genuine landside deposits from other sediments such as turbidites (Locat and Lee 2002). The ratio H/L for subaqueous landslide deposits may reach extremely low values, demonstrating that subaqueous landslides are more mobile than their subaerial counterparts (e.g., Locat and Lee 2002;De Blasio et al 2006;Shanmugam 2015). Because water exerts a strong drag hindrance on a fast-moving landslide (Eq.…”

“…Because water exerts a strong drag hindrance on a fast-moving landslide (Eq. (6) in Appendix 2 shows that the drag increases with the square of the velocity), it follows that submarine mass flows travel exploiting a phenomenon like hydroplaning (Mohrig et al 1998) capable of outcompeting the drag force (e.g., De Blasio 2011b;Shanmugam 2015).…”

Comparing thixotropic and Herschel–Bulkley parameterizations for continuum models of avalanches and subaqueous debris flows

“…Real-world debris flows include additional complexity as they erode and entrain material along the bottom and sides of the slope with the downstream flow. We take these issues as motivational for the present work and refer the reader to the recent review of Delannay et al (2017) for further insight on granular flows and Shanmugam (2015) for heterogenous flows. The fundamentals physics of such flows is presented in Iverson (1997).…”