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Slump (geology)
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The slump that destroyed Thistle, Utah, by creating an earthen dam that flooded the area
Bentonite Clay along the valley of the Little Missouri River in Theodore Roosevelt National Park (North Unit)
The tilted mounds in slump formations formed when the streams that cut into the cliffs over-steepened them. Heavy Moisture led to land slides of blocks of overhanging earth. Thus the slumps retained their original layering sequence

A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or a rock layer moves a short distance down a slope.[1] Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.

Translational slumps occur when a detached landmass moves along a planar surface.[2] Common planar surfaces of failure include joints or bedding planes, especially where a permeable layer overrides an impermeable surface. Block slumps are a type of translational slump in which one or more related block units move downslope as a relatively coherent mass.

A rotational slump occurs when a slump block, composed of sediment or rock, slides along a concave-upward slip surface with rotation about an axis parallel to the slope.[3] Rotational movement causes the original surface of the block to become less steep, and the top of the slump is rotated backward. This results in internal deformation of the moving mass consisting chiefly of overturned folds called sheath folds.

Slumps have several characteristic features. The cut which forms as the landmass breaks away from the slope is called the scarp and is often cliff-like and concave. In rotational slumps, the main slump block often breaks into a series of secondary slumps and associated scarps to form stair-step pattern of displaced blocks.[4] The upper surface of the blocks are rotated backwards, forming depressions which may accumulate water to create ponds or swampy areas. The surface of the detached mass often remains relatively undisturbed, especially at the top. However, hummocky ridges may form near the toe of the slump. Addition of water and loss of sediment cohesion at the toe may transform slumping material into an earthflow. Transverse cracks at the head scarp drain water, possibly killing vegetation. Transverse ridges, transverse cracks and radial cracks form in displaced material on the foot of the slump.

Slumps frequently form due to removal of a slope base, either from natural or manmade processes. Stream or wave erosion, as well as road construction are common instigators for slumping. It is the removal of the slope's physical support which provokes this mass wasting event. Thorough wetting is a common cause, which explains why slumping is often associated with heavy rainfall, storm events and earthflows. Rain provides lubrication for the material to slide, and increases the self-mass of the material. Both factors increase the rate of slumping. Earthquakes also trigger massive slumps, such as the fatal slumps of Turnagain Heights Subdivision in Anchorage, Alaska. This particular slump was initiated by a magnitude 8.4 earthquake that resulted in liquefaction of the soil. Around 75 houses were destroyed by the Turnagain Slump. Power lines, fences, roads, houses, and other manmade structures may be damaged if in the path of a slump.

The speed of slump varies widely, ranging from meters per second, to meters per year. Sudden slumps usually occur after earthquakes or heavy continuing rains, and can stabilize within a few hours. Most slumps develop over comparatively longer periods, taking months or years to reach stability. An example of a slow-moving slump is the Swift Creek Landslide, a deep-seated rotational slump located on Sumas Mountain, Washington.

Slumps may also occur underwater along the margins of continents and islands, resulting from tidal action or a large seismic event. These submarine slumps can generate disastrous tsunamis. The underwater terrain which encompasses the Hawaiian Islands gains its unusual hummocky topography from the many slumps that have taken place for millions of years.

One of the largest known slumps occurred on the south-eastern edge of the Agulhas Bank south of Africa in the Pliocene or more recently. This so-called Agulhas Slump is 750 km (470 mi) long, 106 km (66 mi) wide, and has a volume of 20,000 km3 (4,800 cu mi). It is a composite slump with proximal and distal allochthonous sediment masses separated by a large glide plane scar.[5]

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Slump (geology)

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In , a slump is a type of process involving the rotational downslope movement of unconsolidated , , or rock along a concave-upward curved rupture surface, often resulting in a relatively intact upper block that tilts backward as it slides. This movement separates the sliding mass from stable underlying material and typically occurs on steep slopes where gravitational forces exceed the of the material. Slumps are distinguished by their rotational mechanism, which produces features such as a steep head scarp at the top of the slide and a hummocky or lobate toe at the base, with the slide surface often spanning from tens to thousands of square meters in area. Slumps are commonly triggered by factors that reduce , including heavy rainfall or that saturates the soil and increases , thereby diminishing frictional resistance along potential failure planes. Other causes include undercutting of the base by from rivers, waves, or streams, as well as seismic activity from earthquakes that momentarily weakens material cohesion. Human activities, such as steepening during construction of roads, buildings, or coastal developments, can also initiate slumps by altering natural and loading the with additional weight. These events are prevalent in mountainous, hilly, and coastal regions worldwide, where weak, clay-rich soils or fractured provide zones of low . Notable characteristics of slumps include the preservation of surface features on the displaced block, such as or structures, which may appear tilted or "drunken" in the case of trees; additionally, seepage often emerges at the toe, forming springs. While slumps can occur as isolated events, they frequently form complexes of multiple rotational slides, accelerating over time if not stabilized. Prominent examples include the 1995 La Conchita landslide in , a rotational slump triggered by rainfall that damaged homes and infrastructure, and recurring coastal slumps at , illustrating the hazards in tectonically active areas.

Definition and Characteristics

Definition

A slump is a form of mass wasting characterized by the downward and outward rotational movement of a coherent mass of rock, sediment, or soil along a curved, concave-upward failure surface, with the material largely retaining its original internal structure during displacement. This process typically involves thick deposits of unconsolidated or weakly consolidated materials, often exceeding 10 meters in thickness, sliding as a single unit rather than disintegrating. Unlike other types of mass wasting, such as debris flows or rockfalls, where the material behaves fluidly or fragments upon movement, a slump emphasizes the coherence and rotational nature of the displaced mass, distinguishing it from translational slides that occur along planar surfaces. , the broader category encompassing slumps, refers to any gravity-driven downslope relocation of without the involvement of running or .

Key Characteristics

Slumps are characterized by the coherent movement of a mass of or rock as a relatively intact block down a , with minimal internal deformation during the initial phase of displacement. This movement typically occurs along a curved, spoon-shaped rupture surface, resulting in rotational sliding where the upper portion tilts backward relative to the lower portion. In some cases, the motion can be translational, involving planar sliding with less rotation, but the overall style preserves the structural integrity of the material block. The of slumps generally ranges from slow to moderate, spanning centimeters per day in creeping phases to several per second during rapid failure events, influenced by factors such as material cohesion and steepness. Initial movement is often gradual, accelerating only under specific conditions, which distinguishes slumps from faster, more fluid mass-wasting processes. Slumps commonly develop on slopes with angles between 20 and 40 degrees, though they can occur on gentler or steeper inclines depending on material properties, and involve volumes from a few cubic in small terrestrial examples to thousands of cubic kilometers in large events. This wide scale reflects the variability in geological settings, from coastal bluffs to continental margins. Key morphological deformations associated with slumps include the formation of a steep head scarp at the upper edge, where the failure plane intersects the surface, creating a crescent-shaped depression, and a bulge at the lower margin, where accumulated compresses and mounds up. Minor faulting and cracking may also occur within the slump mass, particularly along the margins, due to differential stresses during displacement.

Formation and Causes

Geological Processes

In slumps, failure initiates when the acting on a potential slip surface exceeds the of the material, leading to downslope movement along a discrete basal plane. This basal slip surface typically forms as a curved, spoon-shaped rupture plane within relatively homogeneous sediments or soils, allowing the overlying mass to rotate and slide as a semi-coherent unit. The angle of repose, which represents the maximum stable slope angle for unconsolidated materials (often 25–40 degrees depending on and cohesion), plays a key role, as slopes approaching or exceeding this angle concentrate at the base. Gravity provides the primary driving force, pulling material downslope parallel to the slope face, while geometry determines the distribution of stresses; oversteepening—such as through undercutting by or loading—increases the component of acting tangentially to the slope, promoting the development of a failure plane parallel to the ground surface. As the slope angle steepens beyond equilibrium, the normal force (perpendicular to the slope) decreases relative to the , reducing frictional resistance and facilitating rupture along the basal surface. This process often occurs in slopes of 20–40 degrees, where the geometry amplifies without requiring extreme inclinations. The progression begins with initial detachment along the basal slip surface, where the upper block tilts backward relative to the slope, followed by rotational sliding that displaces the mass downslope. This movement maintains much of the mass's coherence during the primary phase, though it may evolve into secondary toppling or fragmentation if the rupture deepens or extends. Velocities can range from extremely slow (less than 16 mm per year) to rapid, though slumps are typically slow-moving. A quantitative measure of stability in slumps is the (FS), defined as the ratio of resisting forces (primarily from cohesion and ) to driving forces ( from ). FS=resisting forcesdriving forcesFS = \frac{\text{resisting forces}}{\text{driving forces}} An FS greater than 1 indicates stability, while values below 1 signal impending ; in slumps, this ratio decreases as slope steepening or material weakening shifts the balance, often analyzed for homogeneous slopes prone to rotational slips.

Triggering Factors

Heavy rainfall is a primary natural trigger for slumps, as it infiltrates slopes and increases , reducing the shear strength of the underlying material and promoting rotational failure. Earthquakes can also initiate slumps by inducing seismic shaking that temporarily exceeds the frictional resistance along potential failure planes, particularly in areas with pre-existing instabilities. In periglacial environments, glacial melt or freeze-thaw cycles weaken materials by altering content and promoting ground thaw, leading to retrogressive thaw slumps in regions. Human activities often exacerbate slump susceptibility through direct modifications to slope geometry and hydrology. Slope undercutting, such as from road construction or stream channelization, removes basal support and steepens the angle, tipping marginally stable slopes into failure. Loading the upper slope with fill material for development increases gravitational stress, while deforestation diminishes root cohesion that binds soil particles, thereby elevating the risk of slumps during subsequent rainfall events. Certain material properties predispose slopes to slumping when combined with triggers, notably in unconsolidated sediments. Clay-rich soils, which exhibit low and high plasticity, lose stability rapidly upon saturation, as water facilitates particle and reduces intergranular . Slumps frequently exhibit temporal patterns tied to environmental cycles, occurring seasonally during periods of intense or that align with peak water loading. Since 2000, has contributed to heightened slump frequency in various regions through intensified storm events and accelerated thaw, with studies documenting increased frequency of shallow landslides, including rotational types, in alpine areas. As of 2025, ongoing degradation has led to further increases in retrogressive thaw slumps across the , with comprehensive inventories documenting thousands of active sites.

Types of Slumps

In geological classifications such as those by Varnes (1978) and the USGS, slumps are characteristically rotational forms of mass wasting. Translational movements, while coherent and block-like, are typically classified as slides rather than slumps.

Rotational Slumps

Rotational slumps involve the backward rotation of a coherent mass of soil, regolith, or rock along a curved, concave-upward slip surface that resembles a spoon shape. This movement occurs around an axis parallel to the slope, distinguishing it kinematically from planar sliding by producing a rotational displacement where the upper portion of the failing mass tilts backward relative to the slope face. The slip surface typically develops in materials with sufficient cohesion to maintain block integrity during rotation, such as unjointed or weakly jointed strata. These slumps commonly form in overconsolidated clays, where high at depth contrasts with weaker surface layers, or in weak rock layers like or that yield a cylindrical plane. Such conditions are prevalent in coastal environments, where wave undercutting exposes and destabilizes the lower , and in riverbank settings, where fluvial removes basal support and promotes rotational . For instance, rotational slumps frequently occur along cliffs in sedimentary sequences, as observed in coastal exposures. Characteristic deformation features include upward and backward tilting at the slump head, which exposes the failure plane and creates a steep headscarp, while the toe compresses into a bulbous lobe. At the crown, extensional stresses often generate normal faults, forming grabens (down-dropped blocks) and horsts (upthrown blocks) that widen the failure zone and accommodate the rotational spread. These internal structures enhance the overall coherence of the slump block during movement. In terrestrial settings, rotational slumps represent a prevalent form of in cohesive materials.

Translational Slides

Translational slides involve the downslope movement of a coherent of material as rigid blocks along a planar or gently inclined surface, typically with minimal internal deformation or . This mechanism contrasts with rotational slumps by emphasizing straight-line displacement, where the sliding retains its overall integrity as it translates downslope. Often, multiple parallel planes develop, resulting in step-like or stairstep patterns of displacement that facilitate blocky, terraced failures. These slides commonly occur in settings dominated by bedded sedimentary rocks, where failure planes align with surfaces, or in glacial deposits that exhibit layered or jointed structures susceptible to planar shearing. They are frequent in areas with jointed , such as fractured or formations, where discontinuities like joints or faults serve as preferential slip surfaces. In glacial environments, translational slides in are often triggered by thaw or saturation, leading to coherent block movement along shallow planes within the unconsolidated sediment. Deformation in translational slides is characterized by the preservation of original stratigraphic layers within the displaced blocks, with little to no tilting or folding, allowing the internal structure of the material to remain largely intact. This results in the formation of block fields at the base of the slope, where detached units accumulate as discrete, angular masses, or prominent stairstep scarps along the failure path. Such features highlight the block-like nature of the movement, distinguishing it as a form of coherent mass wasting. Examples of translational slides often involve large intact blocks spanning hundreds of meters in width, as seen in the Stob Coire Sgriodain landslide in , where jointed rock masses slid along planar discontinuities over significant distances. These events underscore the potential scale, with failure volumes reaching thousands of cubic meters while maintaining block integrity.

Submarine Slumps

Submarine slumps represent a form of on the seafloor, analogous to terrestrial slumps in their coherent block movement along planes but distinctly influenced by overlying . The hydrostatic pressure from significantly reduces the effective normal on sediment particles, thereby lowering and enabling on gentle slopes often below 5° or even 1°. This reduction in effective facilitates instability without requiring steep , contrasting with settings where gravity alone dominates. Additionally, excess within can further diminish frictional resistance, promoting progressive . These events are frequently triggered by sediment overloading on continental slopes, where rapid deposition from rivers, currents, or glacial inputs exceeds the slope's capacity to support the accumulating . Other common initiators include earthquakes, which account for approximately 40% of documented cases, as well as rapid rates and gas dissociation. slumps often manifest as rotational or translational movements but incorporate hydrodynamic effects like hydroplaning, where a lubricating fluid layer at the base allows extended runout distances, with medians around 8 km. They predominantly occur in marine settings such as deep-sea fans at the base of continental slopes, prograding deltas, and flanks of volcanic islands, where high rates promote . Volumes can be enormous, exceeding 10,000 km³ in exceptional cases, such as the Agulhas Slump off , which spans over 750 km in length and involves seismic triggering on a sheared margin. The 1929 Grand Banks event, with a volume of about 200 km³, exemplifies smaller but still significant slumps on continental slopes. Distinctive morphological features include prominent headwall scarps, which appear as steep escarpments (up to 100 m high) in bathymetric data, marking the rupture zone and often exhibiting U-shaped profiles. These scarps are visible through multibeam sonar mapping and indicate retrogressive failure propagation. Post-slump, the mobilized debris frequently disintegrates to generate turbidity currents, which erode channels and redistribute fine sediments across basins, amplifying the event's downslope impact. Submarine slumps are especially prevalent in seismically active zones, where tectonic shaking lowers , with global catalogs documenting over 500 such failures based on extensive seafloor surveys. Enhanced mapping efforts since 2010, including multibeam echosounders, have revealed thousands of smaller slump features worldwide, underscoring their frequency—potentially numbering in the thousands annually for minor events—though large-volume slumps (>1 km³) recur less often, on scales of millennia. For instance, recent surveys off alone identified nearly 1,500 landslides.

Morphological Features

Surface Expressions

Surface expressions of slumps manifest as distinct topographic changes on slopes, primarily visible through fieldwork observations or techniques. The primary features include a crown scarp, which forms a steep headwall or at the upper edge of the failure, marking the initiation point of the slump. Below this, the main slump block appears as a coherent of material that has rotated or tilted downward along a curved rupture surface, often exhibiting back-tilting in rotational slumps. At the downslope end, the toe of rupture typically shows bulging or overriding of the original ground surface, where accumulated material creates a compressed or lobe. Secondary landforms further characterize these expressions, such as transverse ridges that develop perpendicular to the direction of movement due to compression at the margins of the slump block. Tension cracks often form parallel to the crown scarp or within the slump mass, resulting from extensional forces during displacement. Small ponds may accumulate in topographic depressions created by the slump, while hummocky terrain—irregular mounds and hollows—arises from partial disruption and chaotic settling of the displaced blocks. Identification of these surface features relies on , which reveals arcuate or crescent-shaped scarps and overall topographic disruptions, and , which detects subtle elevation changes in vegetated or obscured areas for more precise mapping. Rotational and translational slumps may exhibit varying degrees of block rotation influencing the prominence of these tilted surfaces and ridges. Over time, initial slump expressions evolve through and , with sharp scarps and bulges smoothing out while relict headwalls persist as subdued escarpments.

Internal Structures

The internal structures of slumps are characterized by a variety of deformational features that arise from the shear and gravitational forces acting on unconsolidated or semi-consolidated sediments during downslope movement. Particularly in slumps involving soft sediments, sheath folds—defined as highly non-cylindrical, elongated, and tubular structures formed primarily through simple shear—are prominent in high-strain zones within slump masses, often exhibiting curved hinge lines that sweep through arcs exceeding 90 degrees. Recumbent folds, with their near-horizontal axial planes, and thrust faults, which develop as low-angle compressional structures, further contribute to the complex fabric, reflecting progressive deformation in the moving mass. Stratigraphic disruption within slumps manifests as contorted bedding planes that are intensely folded or faulted, leading to discontinuous layers and significant thickness variations, such as up to 50% shortening in some intervals. In competent layers, boudinage occurs where more rigid strata break into isolated segments separated by ductile matrix, while weaker materials often preserve some coherence, allowing partial retention of original bedding attitudes amid the overall disruption. These internal features are typically revealed through subsurface analysis techniques, including drilling cores that expose fold axes, slip planes, and fault offsets in direct samples, and geophysical profiling methods such as seismic reflection surveys, which image the three-dimensional architecture of deformational fabrics even in submarine settings. The nature of internal deformation varies by slump type; rotational slumps exhibit more intense folding and thrusting due to the rotational along curved slip surfaces, promoting widespread ductile fabrics, whereas translational slumps tend to maintain greater block integrity with minimal internal disruption along planar slip surfaces.

Comparison to Other Mass Wasting Processes

Differences from Landslides

Slumps represent a specific subtype of , classified under the broader category of processes that involve the downslope movement of rock, , or earth under . In the seminal Varnes (1958) classification system, are delineated by material type (rock, , earth) and movement type (falls, slides, flows, spreads, topples), with slumps categorized as rotational slides—a form of sliding movement. This places slumps within the "slides" subcategory, distinct from other varieties such as flows or rockfalls, which often exhibit greater fragmentation and less structured motion. A primary mechanistic distinction lies in the material behavior and failure geometry: slumps involve the coherent displacement of a mass along a well-defined, concave-upward slip surface, preserving the integrity of the moving block with minimal internal disruption. In contrast, many landslides, particularly rockfalls or , feature tumbling, rolling, or avalanching of fragmented material without a continuous slip plane, leading to significant breakup and scattering. This coherence in slumps arises from the rotational nature of the movement, often around an axis parallel to the , as opposed to the more chaotic, disintegrative dynamics in broader types. Regarding scale and , slumps are generally slower and more localized, with movement rates typically ranging from slow (13 mm/year to 1.5 m/year) to moderate (1.5 m/year to 13 m/year), affecting volumes on the order of thousands to millions of cubic meters in confined areas. Rock avalanches, another subtype, by comparison, are extremely rapid (>3 m/minute, often exceeding 100 km/h) and expansive, mobilizing tens to hundreds of millions of cubic meters over kilometers, as seen in events like the 1962 avalanche in . These differences highlight slumps' more contained, rotational progression versus the high-mobility, fragmenting surges characteristic of many .

Differences from Flows

Flows in mass wasting, such as earthflows or debris flows, involve the downslope movement of saturated or partially saturated soil, rock, and debris as a fluid-like , where the material behaves with high fluidity and extensive mixing of particles. Unlike slumps, which maintain coherent blocks, flows lack significant internal cohesion and spread laterally due to their matrix-supported structure, often forming tongue-shaped or lobate deposits. A primary distinction lies in the preservation of material integrity: slumps move as relatively rigid, intact masses along a defined slip surface with minimal grain-to-grain interaction or internal deformation, whereas flows exhibit chaotic mixing and flowage without a discrete slip plane, allowing the material to advance as a continuous, deforming body. This results in slumps producing stepped or blocky morphology, in contrast to the smooth, ridged surfaces typical of flows. Rheologically, slumps operate under frictional resistance as rigid bodies sliding along a failure plane, while flows display viscous behavior, often modeled as with a yield strength that enables flow once a critical stress is exceeded. , in particular, incorporate 20-40% to achieve this slurry-like , facilitating rapid downslope propagation. Although slumps and flows represent fundamentally different initial movement mechanisms, disrupted slump blocks can transition into flows if further liquefied by water saturation or fragmentation, altering the rigid motion into fluid-dominated transport.

Notable Examples

Terrestrial Examples

One prominent example of a terrestrial landslide (with slump-like features) is the 1983 Thistle landslide in Utah, USA, which exemplifies a slide triggered by saturation from heavy precipitation and rapid snowmelt in weak sedimentary rocks. The event involved the downslope movement of a large mass of detritus from the North Horn and Ankareh Formations, displacing the small town of Thistle and damming the Spanish Fork River, which created a temporary lake and disrupted transportation routes including U.S. Highway 6/89 and a major railroad line. The landslide had an estimated volume of approximately 12 million cubic meters, measured roughly 300 meters wide, 60 meters thick, and 1.6 kilometers long, resulting in the town's abandonment as a ghost town. Another significant case is the 1964 Turnagain Heights slump in , USA, a rotational slump initiated by the magnitude 9.2 Great Earthquake in sensitive, overconsolidated clay deposits along the Bootlegger Cove Formation. The earthquake-induced caused rapid rotation and flow-like movement of the slumped material, which traveled up to 600 meters toward , destroying over 75 homes across 130 acres and contributing to four fatalities. This event highlighted the role of seismic shaking in mobilizing cohesive soils, transforming a slump into a highly mobile with block rotations and lateral spreading. The Swift Creek landslide in , USA, represents an ongoing slow-moving earthflow that reactivated in the early 1940s due to prolonged heavy rainfall on slippery on Sumas Mountain. Spanning about 1.6 kilometers in length and 0.4 kilometers in width, this complex landslide-earthflow complex continues to erode and deposit sediment into Swift Creek, posing a persistent threat to nearby urban areas like Everson through increased flood risk and water contamination from naturally occurring . Long-term monitoring by state agencies has tracked its movement rates, which vary seasonally but average 4–5 meters per year, underscoring the challenges of managing chronic terrestrial landslides in populated regions. In tectonically active regions like the , slumps and related slope failures occur frequently due to steep , fractured , and seismic activity, with studies documenting a pronounced increase in events since the early linked to wetter conditions from . For instance, in the French and , the frequency of shallow landslides, including slumps, has risen significantly, with post-2000 occurrences more than doubling in some areas compared to the late , driven by intensified spring rainfall and reduced cover leading to earlier saturation. This trend illustrates how changing precipitation patterns exacerbate slump initiation in alpine environments.

Submarine Examples

One of the largest known submarine slumps is the Agulhas Slump off the southeastern coast of , extending approximately 750 km in length and 106 km in width with an estimated volume of over 20,000 km³. This Pleistocene-age feature, likely post-Pliocene in origin, consists of a composite mass of deformed detached from the continental , as revealed by seismic reflection profiles showing chaotic internal structures and a thin overlying drape of younger . Its scale underscores the potential for massive sediment mobilization in tectonically sheared margins influenced by strong bottom currents. The , located on the mid-Norwegian , represents another monumental submarine slump event that occurred around 8,200 years ago, involving a volume of approximately 3,000 km³ across an area spanning over 100 km wide and up to 800 km in runout length. This failure was primarily linked to glacial unloading following the last , which reduced and increased slope instability in glaciated margins. High-resolution seismic data illustrate its retrogressive nature, with headwall scarps and blocky debris flows indicative of slump mechanics transitioning to more fluidal movement downslope. In the Hawaiian Islands, ongoing submarine slumps are exemplified by the on the south flank of volcano, where coherent blocks of and volcanic edifice slide seaward at background rates of 8–10 cm per year, accelerating to several meters during episodic events driven by dike intrusions. Multibeam and seismic surveys reveal a broad, active slump zone up to 100 km wide and extending over 50 km offshore, with rotational failures and faulting that accommodate the volcano's rapid growth and flank instability. These dynamic processes highlight how volcanic loading and intrusive activity sustain persistent submarine in intraplate settings. Recent mapping along the U.S. West Coast, particularly off , has identified nearly 1,500 features, including numerous slumps, through high-resolution multibeam covering extensive continental slope areas. These features range from small rotational slumps to larger translational ones.

Hazards and Mitigation

Associated Risks

Slumps present direct hazards primarily through abrupt downslope movement that damages such as roads, buildings, and utilities, often burying affected areas under large volumes of displaced material. While human fatalities from slumps are uncommon due to their typically localized nature, they can occur in populated regions where slopes are developed, as seen in cases of rapid during storms or earthquakes. Secondary risks amplify the dangers of slumps beyond the initial failure zone. On land, slumps may block river channels, forming temporary dams that lead to upstream inundation and catastrophic downstream flooding upon breaching, endangering communities far from the original site. In submarine environments, slumps pose severe threats by displacing water to generate tsunamis; for instance, the 1998 event off involved a slump that produced waves up to 15 meters high, contributing to over 2,200 deaths. Environmentally, slumps disrupt ecosystems by scarifying slopes, destroying vegetation, and fragmenting habitats, which hinders and recovery for years. They also mobilize substantial sediment loads into rivers and coastal waters, elevating , smothering aquatic habitats, and degrading , which impacts fisheries and downstream ecosystems. Globally, slumps contribute to the broader impacts of mass-wasting events, with landslides causing an estimated $20 billion in annual economic damages and thousands of fatalities, particularly in regions with steep terrain. Vulnerability is elevated in developing countries, where inadequate and dense settlement on unstable slopes exacerbate exposure to these hazards. projections for the forecast heightened risks from slumps due to intensified extreme rainfall events driven by global warming, potentially increasing failure frequency by 40% or more in susceptible areas.

Prevention and Monitoring

Engineering measures for preventing slumps primarily focus on stabilizing slopes through structural , , and biological interventions. Retaining walls, such as timber , steel bin walls, and structures, provide lateral support to prevent downslope movement by resisting shear forces in unstable masses; these are particularly effective for small-scale slumps where the wall volume constitutes 10-15% of the stabilized . Drainage systems, including surface ditches with a minimum 2% and horizontal drainpipes intersecting potential failure surfaces, reduce and , thereby increasing ; in clay , full effectiveness may require up to five years. Vegetation-based approaches, such as soil bioengineering with live cuttings of or dogwood (0.75-3 inches in diameter, 2-5 feet long), reinforce through development and immediate structural support, suitable for shallow slumps up to 4 feet deep; techniques like brush layering and branch packing enhance long-term stability by binding particles. Monitoring techniques enable early detection of slump initiation by tracking ground deformation and subsurface changes. Inclinometers installed in boreholes measure lateral displacements with 1 cm accuracy at 10 cm vertical resolution, identifying potential sliding surfaces in real-time. GPS systems provide continuous three-dimensional surface displacement monitoring with sub-5 mm precision, deployed at multiple sites to capture episodic movements. Satellite and airborne (InSAR), including NASA's UAVSAR, has advanced since 2015 for of slow-moving slumps, achieving <1 cm accuracy over areas up to 10 m resolution and enabling wide-area deformation mapping in regions like northern California. Predictive modeling for slumps utilizes Geographic Information Systems (GIS) to delineate hazard zones and forecast risks. GIS-based deterministic models, such as the infinite slope or Bishop simplified methods, compute factors of safety (FS) across raster grids (10-30 m resolution) by integrating slope angle, soil cohesion (6-120 kPa), friction angle (30-42°), and groundwater saturation; zones are classified from very high risk (FS <1) to very low (FS >1.5). Probabilistic approaches, including simulations, incorporate parameter uncertainties to estimate failure probabilities, enhancing regional zonation. Climate data integration, via hydroclimatological models, predicts shallow slump initiation by linking thresholds and antecedent to spatial hazard maps, improving temporal . Policy aspects emphasize land-use regulations to mitigate slump hazards in vulnerable areas. In , post-1980s frameworks include zoning restrictions, such as San Mateo County's limits on development in slide-prone zones (1 dwelling per 40 acres) and Pacifica City's prohibitions on slopes exceeding 35%; these integrate geologic disclosures and subdivision ordinances requiring slide delineation on plats. Grading ordinances in , implemented after 1952 and refined in the 1980s, have reduced losses by 97% through enforced standards. Successes like San Mateo County's mapping have informed , demonstrating effective integration of scientific into regulatory decisions for sustained risk reduction.

References

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