Earthquake Processes and Hazards

Earthquakes are among the most destructive natural hazards, responsible for the greatest loss of life from tectonic events in the 21st century. This lesson examines the physical processes that generate earthquakes, the types of seismic waves they produce, how they are measured, and the primary and secondary hazards they create. This content supports Edexcel A-Level Geography Enquiry Question 1 (EQ1) and provides essential knowledge for the case studies in EQ2.

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What Causes an Earthquake?

An earthquake is a sudden release of energy stored in the Earth's crust or upper mantle, producing seismic waves that radiate outward from the point of rupture. The vast majority of earthquakes are caused by the sudden slip of rock along a fault — a fracture in the Earth's crust where blocks of rock have moved relative to each other.

The Elastic Rebound Theory (Reid, 1910)

Following the 1906 San Francisco earthquake, geologist H.F. Reid proposed the elastic rebound theory:

1. Tectonic stress is applied to rocks adjacent to a fault over years, decades or centuries
2. The rocks deform elastically (like stretching a rubber band), storing strain energy
3. When the accumulated stress exceeds the frictional strength of the fault (the point of failure), the fault ruptures
4. The deformed rock rebounds to its original shape, releasing stored energy as seismic waves
5. The process begins again — this is the earthquake cycle

The point within the Earth where rupture initiates is the focus (or hypocentre). The point on the Earth's surface directly above the focus is the epicentre. The plane along which rupture occurs is the fault plane.

Earthquake Depth Classification

Category	Depth	Typical Setting	Character
Shallow	0–70 km	All boundary types; intra-plate	Most destructive; ~75% of all seismic energy release
Intermediate	70–300 km	Subduction zones	Moderate damage at surface
Deep	300–700 km	Subduction zones only	Usually little surface damage; important for understanding slab geometry

Shallow earthquakes are the most destructive because seismic energy has less distance to travel to the surface and less attenuation occurs. The 2010 Haiti earthquake had a depth of only ~13 km, which contributed significantly to the devastation.

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Seismic Waves

An earthquake generates several types of seismic wave, each with distinct properties that determine the nature and severity of ground shaking.

Body Waves

Body waves travel through the Earth's interior:

Primary (P) waves:

• Compressional (longitudinal) waves — particles vibrate in the same direction as wave travel
• Fastest seismic waves (~6–8 km/s in the crust; up to 13 km/s in the mantle)
• Travel through solids, liquids and gases
• Cause back-and-forth ground motion
• Lower amplitude — cause less damage than S waves
• First to arrive at seismograph stations (hence "Primary")

Secondary (S) waves:

• Shear (transverse) waves — particles vibrate perpendicular to the direction of wave travel
• Slower than P waves (~3.5–4.5 km/s in the crust)
• Travel through solids only — cannot pass through the liquid outer core (this is how we know the outer core is liquid)
• Cause up-and-down and side-to-side motion
• Higher amplitude than P waves — more destructive
• Arrive second at seismograph stations (hence "Secondary")

Surface Waves

Surface waves travel along the Earth's surface and are generally more destructive than body waves because their energy is concentrated near the surface where people and buildings are located:

Love waves (L waves):

• Named after mathematician A.E.H. Love
• Produce horizontal side-to-side motion perpendicular to the direction of wave travel
• Cause severe damage to building foundations
• Faster than Rayleigh waves

Rayleigh waves (R waves):

• Named after Lord Rayleigh
• Produce elliptical rolling motion (like ocean waves) — both vertical and horizontal movement
• Slowest seismic waves but often the most destructive
• Responsible for the characteristic "ground rolling" felt during major earthquakes

graph TD
    A["Earthquake<br/>(Rupture at Focus)"] --> B["Body Waves"]
    A --> C["Surface Waves"]
    B --> D["P-waves<br/>Compressional<br/>Fastest<br/>Through all media"]
    B --> E["S-waves<br/>Shear<br/>Through solids only<br/>More destructive"]
    C --> F["Love waves<br/>Horizontal shearing<br/>Very destructive"]
    C --> G["Rayleigh waves<br/>Elliptical rolling<br/>Slowest<br/>Most destructive"]

Shadow Zones

The behaviour of seismic waves within the Earth reveals its internal structure:

• P-wave shadow zone: Between 103° and 142° from the epicentre, P waves are refracted (bent) by the liquid outer core, creating a zone where no direct P waves are detected. Weak, refracted P waves arrive within this zone but at lower amplitudes.
• S-wave shadow zone: Beyond 103° from the epicentre, S waves are completely absent because they cannot travel through the liquid outer core. This extends from 103° to 180° on both sides of the epicentre.

These shadow zones provided the first evidence that the Earth has a liquid outer core — one of the most important discoveries in Earth science.

Exam Tip: When explaining shadow zones, always link your answer back to the properties of P and S waves. The fact that S waves cannot travel through the outer core (because S waves require a rigid medium) is direct evidence for its liquid state. This type of linking between theory and evidence is expected at A-Level.

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Measuring Earthquakes

The Richter Scale (1935)

Developed by Charles Richter, this was the first widely used quantitative measure of earthquake magnitude. It measures the maximum amplitude of seismic waves recorded on a seismograph and is a logarithmic scale — each whole number increase represents a 10× increase in amplitude and approximately a 31.6× increase in energy released.

Magnitude	Description	Approximate Frequency
< 2.0	Micro — not felt	~8,000/day
2.0–3.9	Minor — rarely felt; no damage	~55,000/year
4.0–4.9	Light — felt; minor damage	~6,200/year
5.0–5.9	Moderate — damage to weak structures	~800/year
6.0–6.9	Strong — significant damage in populated areas	~120/year
7.0–7.9	Major — serious damage over large areas	~18/year
8.0–8.9	Great — devastating over hundreds of km	~1/year
9.0+	Exceptional — catastrophic over thousands of km	~1/20 years

The Moment Magnitude Scale (Mw)

The moment magnitude scale has largely replaced the Richter scale for scientific purposes because it is:

• Accurate across all magnitudes (the Richter scale saturates above ~Mw 7)
• Based on the physical properties of the earthquake source (seismic moment = rigidity × fault area × slip distance)
• Applicable to all types of seismograph data, including those from very distant stations