Edexcel Specification focus:
‘P, S and L waves cause crustal fracturing, ground shaking and secondary hazards such as liquefaction and landslides.’
This subsubtopic explores how different types of seismic waves generate primary earthquake hazards and trigger a range of damaging secondary effects.
Primary Earthquake Hazards
Earthquakes release seismic energy from a point called the focus, which radiates in the form of seismic waves. These waves travel through and along the surface of the Earth, causing varying degrees of shaking and destruction.
Seismic Waves
There are three main types of seismic waves, each playing a distinct role in how earthquake energy affects the ground and built environment.
P waves (Primary waves): The fastest seismic waves, travelling through solids and liquids, causing particles to move in the same direction as the wave (compression and expansion).
S waves (Secondary waves): Slower than P waves and only travel through solids. They move particles perpendicular to the direction of wave travel, producing shear stress in rocks.
L waves (Love waves): Surface seismic waves that cause the most ground movement. They travel along the Earth's crust and produce horizontal shearing, often resulting in the greatest structural damage.
P and S waves contribute to crustal fracturing, particularly near the earthquake’s epicentre. L waves, which arrive last but with the most energy concentrated at the surface, lead to severe ground shaking, collapsing buildings and rupturing roads and pipelines.
Crustal Fracturing and Ground Shaking
Crustal fracturing occurs when stress accumulated along faults is suddenly released. This breaks rock apart, often leaving visible fault lines and displacing land surfaces.
Ground shaking intensity varies depending on the earthquake’s magnitude, focal depth, distance from the epicentre, and local geology.
Amplification of shaking can occur in areas with unconsolidated soils or reclaimed land, worsening damage to infrastructure.
Secondary Earthquake Hazards
The effects of an earthquake extend beyond the initial ground motion. Secondary hazards often cause widespread damage and can be even more deadly.
Liquefaction
Liquefaction: A process in which saturated, unconsolidated sediments temporarily lose their strength and behave like a liquid due to ground shaking.
Occurs in areas with loose, water-saturated soils (e.g. river valleys, coastal plains).
Vibrations increase pore water pressure, reducing effective stress between soil particles, effectively removing their structural integrity.
Buildings and infrastructure can tilt, sink or collapse as the ground can no longer support weight.
Underground tanks, pipelines, and cables may rise to the surface or rupture.
Landslides
Landslide: The downslope movement of rock, earth, or debris under the influence of gravity, often triggered by ground shaking or saturation.
Common in mountainous or hilly terrain, particularly where slopes have already been weakened by rainfall or deforestation.
Earthquakes destabilise slopes by shaking and fracturing material, reducing shear strength and increasing shear stress.
Earthquake-induced landslides can:
Bury settlements and roads.
Block rivers and create temporary quake lakes, which may later cause flash floods.
Hinder emergency responses by cutting off access routes.
Aftershocks and Fire Hazards
Aftershocks are smaller tremors that follow the main earthquake. They can:
Collapse structures already weakened.
Trigger additional landslides or liquefaction.
Fires often break out after earthquakes due to:
Ruptured gas mains and damaged power lines.
Disruption of water supplies impeding firefighting efforts.
Examples include the 1995 Kobe earthquake, where fires caused substantial additional damage.
Factors Influencing Hazard Severity
Several physical and human factors influence how devastating earthquake hazards and their secondary effects can be:
Physical Factors
Magnitude and depth:
Shallow-focus earthquakes (<70 km depth) cause more intense surface shaking.
Higher magnitude releases more energy, affecting a larger area.
Distance from epicentre:
Closer proximity typically experiences more severe ground motion.
Local geology:
Soft or water-saturated sediments amplify shaking and increase liquefaction risk.
Bedrock reduces the amplitude of seismic waves.
Human Factors
Population density and building design:
Densely populated urban areas are at higher risk due to greater human exposure and structural vulnerability.
Poorly engineered buildings collapse more readily during ground shaking or after liquefaction.
Preparedness and infrastructure:
Early warning systems, emergency services, and public education reduce risk and response time.
Infrastructure resilience (e.g. flexible pipelines, base-isolated buildings) mitigates damage.
Summary of Key Hazards
Primary hazards:
Crustal fracturing
Ground shaking
Secondary hazards:
Liquefaction
Landslides
Aftershocks
Fires
Understanding the mechanics behind seismic waves and their secondary effects is essential for predicting damage patterns and improving hazard management in earthquake-prone areas.
FAQ
Higher magnitude earthquakes release more energy, resulting in stronger and more prolonged ground shaking. This intensifies the likelihood and impact of secondary hazards.
Liquefaction is more likely to occur during high-magnitude quakes because sustained shaking increases pore water pressure in saturated soils. Similarly, steeper slopes subjected to intense shaking are more prone to large and deeper landslides.
However, local conditions — such as soil type, water saturation, and slope angle — also influence severity, so magnitude is not the only determining factor.
Yes, landslides can be delayed, sometimes occurring hours or even days after the main earthquake.
This happens because:
Ground materials may be loosened during shaking but not immediately collapse.
Aftershocks can trigger delayed failures.
Rainfall after an earthquake can saturate weakened slopes, reducing friction and triggering slides.
This delayed response complicates rescue and recovery efforts in mountainous or hilly terrain.
Several factors increase a location’s vulnerability to liquefaction:
Soil type: Loose, sandy, or silty soils are most prone.
Water saturation: Areas with a high water table or recent rainfall increase the risk.
Land use: Reclaimed land and river deltas often contain loose, uncompacted sediments.
Lack of drainage: Poor drainage keeps soils saturated, maintaining liquefaction potential.
Urban areas built on floodplains or reclaimed coastal zones are particularly at risk during seismic events.
Liquefaction can severely damage underground and surface structures, with long-term consequences including:
Tilted or sunken buildings requiring demolition or costly repairs.
Damaged roads, pipelines, and sewage systems, leading to service disruptions.
Contaminated groundwater if pipelines rupture.
Difficulty in reconstruction due to unstable or permanently deformed ground layers.
Reinforcing the ground or improving drainage may be required before rebuilding, delaying recovery.
Love waves travel along the Earth’s surface, where buildings and infrastructure are located, making their effects directly felt at ground level.
Their side-to-side horizontal motion:
Produces large amplitude vibrations.
Places extreme lateral stress on buildings, bridges, and railways.
Tends to resonate with the foundations of structures, causing collapse.
Unlike P and S waves, Love waves do not lose energy quickly, allowing them to cause widespread damage over a broader area.
Practice Questions
Question 1 (2 marks)
Identify two secondary hazards that can result from ground shaking during an earthquake.
Mark Scheme:
One mark for each correct secondary hazard identified (maximum 2 marks).
Accept:
Liquefaction (1)
Landslides (1)
Do not accept “ground shaking” as it is a primary hazard.
Question 2 (6 marks)
Explain how the different types of seismic waves contribute to earthquake hazards.
Mark Scheme:
1–2 marks: Basic description of seismic waves with limited reference to their role in earthquake hazards. May name one wave type correctly.
3–4 marks: Sound explanation of how P, S or L waves contribute to ground shaking or damage, with at least two types of waves described.
5–6 marks: Clear and developed explanation covering all three wave types (P, S and L), accurately linked to their speed, movement, and associated hazards.
Creditworthy points may include:
P waves are the fastest, travel through solids and liquids, and cause compression/expansion of rock — minor damage. (1)
S waves travel through solids, are slower than P waves, and move particles perpendicular to wave travel — more destructive than P waves. (1)
L waves travel along the surface, have the highest amplitude, and cause intense ground shaking and structural damage. (1)
L waves are typically responsible for the greatest destruction during an earthquake. (1)
Clear comparison of wave behaviour and their effects. (1–2)
