Question

How do earthquakes cause landslides?

Short answer

Earthquakes cause landslides by shaking slopes, reducing cohesion and increasing shear stress, leading to instability.

Step-by-step solution

Understanding Earthquakes

Earthquakes are caused by the movement of tectonic plates beneath the Earth's surface. This movement releases energy in the form of seismic waves, which can shake the ground violently.

Nature of Landslides

Landslides occur when the stability of a slope changes from a stable to an unstable condition. This instability can be triggered by various factors, such as saturation from heavy rain, rapid snow melt, or physical disturbances like earthquakes.

Link Between Earthquakes and Landslides

When an earthquake shakes the earth, the ground movement acts as a significant destabilizing force, particularly on slopes. The shaking can weaken the slope materials (soil, rock) by lowering cohesion and increasing shear stress.

Trigger of a Landslide

The crucial factor here is the reduction of friction and cohesion within the slope material due to the earthquake-induced shaking. This process can trigger a landslide if the slope was already on the verge of being unstable.

Consequences and Examples

Post-earthquake, landslides can cause destruction by covering roads, buildings, and possibly blocking rivers to create reservoirs, leading to further secondary disasters like floods. Historical instances, such as those seen after the 1994 Northridge Earthquake in California, demonstrate how common landslides are following significant seismic activity.

Key concepts

Tectonic Plates

Tectonic plates are huge slabs of Earth's crust that fit together like a puzzle covering the Earth's surface. These plates are constantly moving, albeit very slowly, upon the more fluid asthenosphere beneath. The boundaries where these plates meet are sites of intense geological activity. Sometimes, these movements are smooth and gradual, but often they lead to powerful shifts causing earthquakes.

Picture the Earth as a cracked eggshell with slightly shifting pieces. When these pieces move suddenly, massive energy is released in the form of earthquakes. Understanding tectonic plates helps us see how their movements can affect structures on the surface, influencing natural events like earthquakes and landslides.

Seismic Waves

Seismic waves are the energy waves that travel through the Earth due to the sudden release of energy during an earthquake. They can be classified into two main types: body waves and surface waves. Body waves travel through the interior of the Earth and are further divided into P-waves (primary waves) and S-waves (secondary waves).

P-waves are faster and travel through solids, liquids, and gases, whereas S-waves only move through solids, making them slower. On the other hand, surface waves move along the Earth's surface and result in the shaking felt during an earthquake. These waves cause the most damage, particularly in areas with slopes.

Seismic waves, especially surface waves, can disrupt and destabilize the slopes of hills and mountains, setting the stage for landslides.

Slope Stability

Slope stability refers to how likely a slope is to withstand movement and stay in place without collapsing. Several factors determine how stable or unstable a slope is: the type of materials making up the slope, the slope's angle, water content, vegetation, and external influences like earthquakes and human activity.

A stable slope means that the forces holding the materials in place are greater than the forces trying to move them. However, if any destabilizing factors increase these forces, the slope can become unstable. Earthquakes significantly impact slope stability because they shake the ground, reducing the cohesion and increasing shear stress within slope materials.

This shaking can lead to a reduced frictional resistance, pushing the slope toward instability and making landslides more likely to occur.

Landslide Triggers

Landslide triggers are factors that initiate the sudden movement of slope material. While many potential triggers exist, including natural and anthropogenic causes, earthquakes are one of the most significant. Earthquake-induced landslides often occur because the shaking reduces friction and loosens the slope material.

Other common triggers include:

• Heavy rainfall, leading to the saturation of slope material.
• Removal of vegetation, which stabilizes the soil through root structures.
• Overloading, such as construction or accumulation of heavy materials on slopes.
• Rapid snowmelt, which increases water flow and stability issues.

Earthquake-induced landslides are particularly dangerous as they can occur with little warning and cause widespread destruction, blocking roads and infrastructure, damming rivers, and posing significant risks to human safety.