A steep slope is a geological feature where the inclination approaches or exceeds the material’s angle of repose. This angle is the steepest angle at which unconsolidated material can remain stable without sliding, typically ranging from 25 to 45 degrees for dry materials. Slopes steeper than this are held in place by the internal friction and cohesion of the rock or soil mass. While gravity constantly exerts a driving force, an earthquake provides the sudden, intense trigger necessary to overcome the material’s resisting forces. Seismic activity introduces dynamic forces that push the slope past its breaking point, leading to catastrophic mass movement and diverse hazards.
The Destabilizing Force of Seismic Shaking
Earthquake shaking directly attacks the stability of a slope by introducing ground acceleration, which acts as an additional, temporary force pulling the slope material downhill. This horizontal body force increases the shear stress acting on potential failure surfaces within the rock or soil mass. The repeated stress from seismic waves, known as cyclic loading, temporarily reduces the material’s effective strength and cohesion. This reduction in shear strength means the slope becomes less resistant to the constant pull of gravity.
When ground acceleration is high enough, it can cause the soil or rock mass to yield, resulting in accumulated displacement. This process is often analyzed by comparing the seismic demand to the slope’s capacity to resist movement. Even a minor earthquake can cause a significant reduction in the slope’s overall safety factor, increasing the likelihood of failure. The duration and frequency content of the ground motion also affect the final displacement, with longer shaking periods inducing greater movement.
Deep-Seated Slope Failures
Deep-seated failures involve massive volumes of earth where the movement surface is located far beneath the ground surface, often deeper than ten meters. These large-scale movements are characterized by a failure surface that extends into the underlying bedrock or deep soil layers. Two common forms are slumps and translational slides, both involving coherent masses of material.
Slumps, also known as rotational slides, move along a distinctly curved or concave-upward failure surface. This rotational movement often results in the displaced mass exhibiting a backward tilt at the top, creating a prominent crescent-shaped rupture at the crest of the slope. These failures can mobilize millions of cubic meters of material, leading to the widespread destruction of infrastructure built on or below the affected area.
Translational slides, in contrast, move along a relatively flat or planar failure surface. This plane is frequently a pre-existing weakness, such as a bedding plane, joint, or fault in the rock strata. These slides can be some of the largest and most damaging forms of mass movement, as the entire block of material slides as a relatively intact unit. The massive scale of these events means they can dramatically alter the landscape and pose a significant long-term hazard.
Surface Hazards
Earthquakes also trigger rapid, localized movements of loose material or fragmented rock near the surface, posing immediate danger. Rockfalls are one of the most common and fastest types of surface failure, where individual blocks or chunks of bedrock detach from a cliff face or steep slope. The dislodged material descends primarily through free fall, bouncing, or rolling down the slope, often traveling at high speeds.
These events are prevalent in areas with steep road cuts or fractured, jointed bedrock, as the seismic vibration easily shakes loose material. The resulting collection of fallen rock accumulates at the slope’s base, forming a talus cone. Debris slides involve the rapid movement of unconsolidated material, such as soil, loose rock, and vegetation, along a shallow failure surface. These slides are highly chaotic and often follow existing drainage paths, quickly turning into destructive debris flows that can cover significant distances.
The Danger of Saturated Ground
The presence of water in the ground introduces a specific and highly destructive failure mechanism during seismic shaking. This occurs in saturated, loose, granular soils, such as sand and silt, which are often found in coastal and river areas. The cyclic stress from the earthquake causes the pore water pressure to increase rapidly. This phenomenon, known as liquefaction, temporarily reduces the contact forces between the soil grains, effectively turning the solid ground into a heavy liquid.
When liquefaction occurs beneath a steep slope, the fluidized material can move rapidly downhill as a flow slide. These flow slides are among the most dangerous types of mass movement due to their extreme speed and ability to travel long distances. Notably, this catastrophic failure can even happen on gentle slopes of just a few percent, provided there are specific geological conditions. The presence of low-permeability layers, such as silt or clay, can trap the rising pore water, concentrating the pressure and further reducing the shear strength at the base of the liquefied layer. This then facilitates the devastating flow.