Seismic Hazards in Detail

Earthquakes are the most widespread and frequently lethal tectonic hazard. This lesson covers the mechanics of earthquake generation, seismic wave behaviour, measurement scales, and the secondary hazards that earthquakes produce.

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Earthquake Mechanics

Focus and Epicentre

• The focus (hypocentre) is the point within the Earth where the earthquake rupture originates
• The epicentre is the point on the surface directly above the focus
• Shallow-focus earthquakes (0–70 km depth) are the most destructive; intermediate (70–300 km) and deep-focus (300–700 km) events are typically less damaging at the surface

Elastic Rebound Theory

Tectonic stress builds up along a fault over time. When the stress exceeds the frictional strength of the rock, sudden rupture occurs and the rock snaps back to its original shape, releasing energy as seismic waves. The fault plane may extend for hundreds of kilometres (e.g., the Tōhoku rupture in 2011 extended ~500 km along the Japan Trench).

Types of Faulting

Fault Type	Motion	Tectonic Setting	Example
Normal	Hanging wall moves down	Extensional (constructive boundaries, rifts)	Basin and Range, USA
Reverse (thrust)	Hanging wall moves up	Compressional (destructive boundaries)	Himalayas; Japan Trench
Strike-slip (transform)	Lateral horizontal movement	Conservative boundaries	San Andreas Fault; North Anatolian Fault

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Seismic Waves

Body Waves

• P-waves (Primary) — compressional waves; fastest (6–8 km/s in the crust); travel through solids, liquids, and gases; cause back-and-forth motion in the direction of travel
• S-waves (Secondary) — shear waves; slower (3–5 km/s); travel through solids only; cause perpendicular motion; responsible for much of the structural damage

The fact that S-waves do not pass through the outer core (liquid iron-nickel) provided key evidence for the Earth's internal structure.

Surface Waves

• Love waves — horizontal shearing motion; cause buildings to sway side to side; often the most destructive wave type
• Rayleigh waves — elliptical rolling motion (like ocean waves); cause the ground to ripple; slower than Love waves but can be highly damaging

Surface waves travel more slowly than body waves but typically have larger amplitude and longer duration, causing the most destruction.

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Measuring Earthquakes

Moment Magnitude Scale (Mw)

• The standard scientific measure used today, replacing the older Richter scale
• Based on the seismic moment — a function of fault area, average slip, and rock rigidity
• Logarithmic: each whole number increase represents approximately 32 times more energy released
• No upper limit, but the largest recorded earthquake was the 1960 Chile earthquake (Mw 9.5)

Modified Mercalli Intensity Scale (MMI)