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Earthquakes are the most widespread tectonic hazard, affecting every continent. They are responsible for more deaths than any other geological hazard: between 2000 and 2020, earthquakes and associated tsunamis killed over 780,000 people worldwide (USGS). Understanding the physical processes that generate earthquakes, the scales used to measure them, the phenomenon of tsunamis, and the factors that determine their impact is essential for A-Level Geography.
Key Definition: An earthquake is a sudden release of stored elastic energy in the Earth's lithosphere, causing seismic waves to radiate outward from the point of energy release.
The elastic rebound theory was formulated by Harry Fielding Reid (1910) following his study of the 1906 San Francisco earthquake. The theory states:
graph LR
A["Tectonic stress<br/>accumulates over<br/>years–centuries"] --> B["Elastic strain<br/>builds in rocks<br/>either side of fault"]
B --> C["Stress exceeds<br/>frictional strength<br/>of fault"]
C --> D["Sudden rupture<br/>and displacement<br/>= EARTHQUAKE"]
D --> E["Rocks rebound<br/>to undeformed<br/>shape"]
| Term | Definition |
|---|---|
| Focus (hypocentre) | The point within the Earth where the rupture begins and energy is first released |
| Epicentre | The point on the Earth's surface directly above the focus |
| Focal depth | The depth of the focus below the surface |
Earthquakes are classified by focal depth:
| Classification | Depth | Context |
|---|---|---|
| Shallow focus | 0–70 km | Most destructive; occur at all plate boundary types; ~75% of all earthquake energy is released by shallow-focus events |
| Intermediate focus | 70–300 km | Associated with subduction zones |
| Deep focus | 300–700 km | Found only at subduction zones; define the Benioff zone (an inclined plane of earthquake foci that dips beneath the overriding plate) |
When an earthquake occurs, energy is released in three main types of seismic wave:
| Wave Type | Nature | Speed | Behaviour |
|---|---|---|---|
| P-waves (Primary) | Compressional (longitudinal) — particles vibrate parallel to direction of travel | Fastest: 5–8 km/s in crust | Travel through solids, liquids and gases; first to arrive at seismograph |
| S-waves (Secondary) | Shear (transverse) — particles vibrate perpendicular to direction of travel | Slower: 3–5 km/s in crust | Travel through solids only; cannot pass through the liquid outer core — this is how we know the outer core is liquid |
| Surface waves (Love and Rayleigh) | Complex rolling and side-to-side motion at the surface | Slowest: 2–4 km/s | Cause most of the shaking damage to buildings; amplitude decreases with depth |
Exam Tip: Be precise when describing seismic waves. P-waves are compressional (not "push-pull"); S-waves are shear (not "side-to-side" — that describes Love waves specifically). Use the correct terminology to access higher mark bands.
Developed by Charles Richter at Caltech, the Richter Local Magnitude Scale (ML) measures the amplitude of seismic waves recorded on a seismograph. It is logarithmic: each whole number increase represents a 10x increase in wave amplitude and approximately a 31.6x increase in energy released.
The Moment Magnitude Scale has replaced the Richter Scale for scientific use since the 1970s (developed by Hanks and Kanamori, 1979). It is based on the seismic moment — a measure of the total energy released, calculated from:
The Mw scale has no upper limit and does not saturate for very large earthquakes (unlike the Richter scale, which becomes inaccurate above ~6.5). Both scales produce similar numbers for moderate earthquakes.
| Intensity | Description | Effects |
|---|---|---|
| I–II | Not felt to weak | Detected only by instruments or felt by few people on upper floors |
| III–IV | Weak to light | Felt indoors; hanging objects swing; similar to passing truck vibration |
| V–VI | Moderate to strong | Felt by nearly everyone; plaster cracks; unstable objects fall; slight damage |
| VII–VIII | Very strong to severe | Considerable damage to poorly built structures; chimneys fall; difficult to stand |
| IX–X | Violent to extreme | Considerable damage to well-built structures; ground cracks; landslides |
| XI–XII | Extreme to total destruction | Few structures remain standing; bridges destroyed; ground surface permanently deformed |
Key Point: The MMI measures the effects of an earthquake at a specific location, not the energy released. A single earthquake has one magnitude (Mw) but many intensities (MMI) — intensity decreases with distance from the epicentre and is influenced by local geology, building quality and soil type.
| Factor | Effect |
|---|---|
| Magnitude | Higher magnitude = more energy released = greater potential for destruction |
| Focal depth | Shallow-focus earthquakes concentrate energy near the surface, causing more damage |
| Distance from epicentre | Shaking intensity decreases with distance (though exceptions occur due to local geology) |
| Population density | Higher density = more people exposed = greater potential casualties |
| Time of day | Earthquakes at night (when people are in buildings) tend to cause more casualties |
| Building quality | Reinforced concrete and steel-framed buildings survive; unreinforced masonry and adobe collapse |
| Geology and soil type | Soft, unconsolidated sediments amplify seismic waves — liquefaction can occur when waterlogged sand behaves as a liquid during shaking |
| Secondary hazards | Fires (ruptured gas mains), landslides, tsunamis, dam failure |
| Level of preparedness | Emergency drills, building codes, early warning systems, response plans |
| Governance and wealth | Wealthier nations can enforce building codes, fund emergency services, and provide insurance |
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