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Spec mapping (AQA 7037): Paper 1, §3.1.5 Hazards — "the nature of seismicity and its relationship to plate tectonics: forms of seismic hazard — earthquakes, shockwaves, tsunamis, liquefaction, landslides; spatial distribution, randomness, magnitude (Richter and moment magnitude scales), frequency, regularity and predictability of seismic hazards; impacts — primary/secondary, environmental, social, economic, political; short- and long-term responses — risk management, monitoring, prediction, protection, preparedness, adaptation." It applies the boundary processes from the plate-tectonics lesson and links synoptically to §3.2.x (population density, building quality and development as the vulnerability that converts shaking into deaths) and to §3.1.1 (tsunamis as a dramatic transfer of energy through the ocean system). Assessment objectives: AO1 (elastic rebound, wave types, magnitude/intensity scales), AO2 (applying these to explain contrasting impacts — Haiti vs Chile, Nepal vs Japan), and AO3 (the logarithmic magnitude–energy relationship, intensity-attenuation, deaths-vs-development data).
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. The recurring analytical thread is the decoupling of magnitude from impact: the energy released is a physical constant, but the death toll is a social variable, mediated by the risk equation.
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:
This stick–slip cycle explains both the apparent randomness of earthquake timing and the partial predictability of their location. Stress accumulates steadily but is released episodically and unpredictably, which is why short-term prediction is so difficult; yet because each fault segment must re-accumulate strain after rupturing, segments that slipped long ago are statistically more "loaded" — the basis of the seismic-gap concept. Aftershocks are the fault and surrounding crust adjusting to the new stress field after the main rupture, which is why a major event (e.g. Nepal 2015, Mw 7.8) is followed by a decaying sequence of aftershocks (including a Mw 7.3 event 17 days later) that can collapse already-weakened buildings.
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 |
The focus–epicentre geometry matters directly for hazard severity. Shaking intensity is greatest at the epicentre because that is the closest surface point to the energy release; for a given magnitude, a shallow focus concentrates that energy in a smaller surface area and produces more violent ground motion than a deep one, where energy spreads out and attenuates before reaching the surface. This is why shallow-focus earthquakes (such as Haiti 2010 at ~13 km and Nepal 2015 at ~8 km) are disproportionately destructive relative to their magnitude. The epicentre is also where rescue is concentrated, but the worst damage is not always there — local geology can shift the most intense shaking to softer ground some distance away.
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) |
The focal-depth pattern is itself diagnostic of plate setting. At conservative and constructive margins, brittle failure is confined to the cool, rigid upper crust, so earthquakes are shallow (typically < 20 km). Only at subduction zones can earthquakes occur at intermediate and deep levels, because the cold descending slab remains brittle far below the surrounding ductile mantle — tracing the inclined Benioff zone down to ~700 km. This is why the deepest earthquakes are a unique fingerprint of destructive margins, and why the most destructive megathrust events (Mw 9+ Tōhoku, Sumatra, Chile 1960) all occur where one plate is forced beneath another over a vast, locked fault interface. Distribution is therefore not random at the global scale (it follows boundaries) even though the timing of individual events is effectively random.
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.
The wave types matter for hazard impact, not just classification. Because surface waves (Love and Rayleigh) travel along the ground and have the largest amplitudes, they do most of the damage to buildings — and crucially their energy decays more slowly with distance than body waves, so a shallow-focus earthquake delivers powerful surface-wave shaking over a wide area. The P-/S-wave gap (P-waves arrive first) is also the physical basis of earthquake early-warning systems: detectors sense the fast, less-damaging P-wave and transmit an electronic alert that outruns the slower, destructive S- and surface waves, buying seconds of warning. Finally, the fact that S-waves cannot pass through the liquid outer core (creating the S-wave shadow zone) is, as noted in the plate-tectonics lesson, the key evidence for Earth's internal structure — the same waves that cause the hazard reveal the planet that generates it.
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 10× increase in wave amplitude and approximately a 31.6× increase in energy released. The energy–magnitude relationship is captured by the Gutenberg–Richter energy formula:
log10E=1.5M+4.8(E in joules)
From this, the energy ratio between two earthquakes differing by ΔM in magnitude is:
E1E2=101.5ΔM
So a one-unit increase gives 101.5≈31.6× the energy, and a two-unit increase gives 103=1000×. This is why the Chile 2010 (Mw 8.8) earthquake released roughly 101.5×1.8≈500 times the energy of Haiti 2010 (Mw 7.0) — the single most important quantitative fact in the seismic-hazards unit, because it sets up the magnitude-versus-impact paradox.
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. The reason Richter "saturates" is that it measures the amplitude of a single wave type, which stops growing in proportion to true energy for the largest ruptures; the moment magnitude, by contrast, is tied to the physical size of the rupture (fault area × slip × rigidity), so it remains accurate for the giant megathrust earthquakes (Mw 9+) that dominate global seismic-energy release and generate the deadliest tsunamis. For exam purposes you should refer to moment magnitude (Mw) as the modern standard while recognising "Richter" as the historical scale the public still uses.
| 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.
This distinction underpins a common AO3 resource: an isoseismal map, on which contours (isoseismals) join places of equal Mercalli intensity. To interpret one, describe how intensity is highest near the epicentre and decreases outward (attenuation), then look for anomalies — pockets of high intensity far from the epicentre usually mark soft sediments or reclaimed land that amplified the shaking (e.g. the 1985 Mexico City earthquake, where the old lake-bed sediments beneath the city amplified surface waves and caused disproportionate damage ~350 km from the epicentre). Because MMI also folds in building quality, two places equidistant from the focus can record different intensities — making the isoseismal map a neat visual demonstration that impact is a product of physical attenuation, ground conditions and vulnerability together, not magnitude alone.
| 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 — these often cause more deaths and damage than the shaking itself |
| 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 |
Worked AO3 skills exemplar — a deaths-versus-magnitude scatter. Suppose a resource plots earthquake deaths (y, log scale) against magnitude (x) for ten recent events, colour-coded by national income. (i) Describe: there is no clear positive correlation between magnitude and deaths; the highest death tolls cluster among low-income events (Haiti Mw 7.0, Nepal Mw 7.8), while several high-magnitude, high-income events (Chile Mw 8.8, Japan Mw 9.1 shaking deaths) plot low. (ii) Manipulate: compute an energy-normalised death rate — deaths per unit of released energy E=101.5M+4.8 — and the contrast widens further, because the high-death events also released less energy. You might calculate that Haiti's deaths-per-joule exceeded Chile's by several orders of magnitude. (iii) Explain: invoke the risk equation — development (a proxy for capacity: enforced codes, emergency services, insurance) divides risk down, so income, not magnitude, structures the scatter. (iv) Evaluate: caution that the sample is small and that confounders (focal depth, time of day, distance of epicentre to a major city) also matter — Haiti's shallow focus directly beneath Port-au-Prince amplified its toll independently of income. A robust judgement notes that the data strongly supports development as the dominant control while acknowledging physical co-variates.
Key Definition: Liquefaction occurs when saturated, loosely packed, fine-grained sediment (typically sand) temporarily loses its strength and behaves as a liquid during prolonged seismic shaking. The water pressure between grains increases until it equals the overburden pressure, and the soil effectively flows.
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