Notes · GeologyUK · A-Levels

Geohazards

This chapter applies the geology of the previous chapters to the hazards the Earth poses to people. It covers earthquakes and their measurement, volcanic hazards and their link to magma composition and plate setting, mass-movement hazards and slope stability, tsunami, and the assessment, prediction and management of geological risk.

5 sections~18 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 1

T·0888 / 13

Exam profile

AO1 · Describe seismic, volcanic, mass-movement and tsunami hazards and their causesAO2 · Apply the magnitude scale, the factor of safety and risk concepts to hazard dataAO3 · Analyse and evaluate the prediction, monitoring and management of geohazards

Operators:describeexplaincalculateanalyseevaluateassess

basic level

AS-Level expects you to describe the main geohazards, how they are measured, and basic prediction and management.

higher level

The full A-Level requires you to apply magnitude and risk relationships quantitatively and to evaluate strategies for reducing risk.

Depth

Text

Contents · 5 sections

Geohazards

Earthquake hazards#

StandardWJEC/Eduqas A level Geology (Component 3) — Geohazards

Energy and magnitude

Fig. 1The logarithm of earthquake energy is a straight line against magnitude: each unit of magnitude adds 1.5 to log energy, multiplying the energy about 32-fold.

An earthquake is the sudden release of elastic strain energy stored in rocks as they are stressed by plate movement, radiated as seismic waves from the focus. The primary hazard is ground shaking, which damages and collapses buildings; secondary hazards follow from it, including liquefaction (where shaking turns water-saturated sediment into a fluid that can no longer support foundations), landslides triggered on slopes, fires from ruptured gas and power lines, and tsunami where the sea floor is displaced. Most large earthquakes occur at plate boundaries, so their global distribution mirrors the plate map.

The size of an earthquake is described in two distinct ways that students must not confuse. Magnitude is a single number measuring the energy released at the source, determined from the amplitude of the seismic waves recorded on a seismograph; the moment magnitude scale is now used for large earthquakes. Intensity, by contrast, measures the effects and damage at a particular place, and so varies from place to place for the same earthquake, being greatest near the epicentre and on soft ground. Magnitude is one number per earthquake; intensity is a map of effects.

The magnitude scale is logarithmic, which has important quantitative consequences. Each whole-number increase in magnitude corresponds to a tenfold increase in the amplitude of ground shaking, but to roughly a thirty-two-fold (10 to the power 1.5) increase in the energy released. So an earthquake of magnitude 7 shakes the ground ten times as strongly as one of magnitude 6, but releases about thirty-two times as much energy, and a magnitude 8 releases about a thousand times the energy of a magnitude 6. Appreciating that a small step in magnitude is a large step in energy is essential to interpreting hazard.

The hazard from an earthquake depends not only on its magnitude but on its depth, the local geology and the exposure and vulnerability of the population. Shallow earthquakes are more damaging than deep ones of the same magnitude; soft, water-rich ground amplifies shaking and may liquefy; and the same earthquake causes far more death and destruction in a poor, densely built city than in a well-prepared one. This distinction between the physical hazard and the human risk it creates runs through the whole chapter and is developed in the final section.

\log_{10} E = 4.8 + 1.5\,M

Energy and magnitude

The energy E (in joules) released by an earthquake rises with magnitude M; because the relationship is logarithmic, each unit of magnitude multiplies the energy by about 10 to the power 1.5, roughly 32 times.

\frac{E_2}{E_1} = 10^{1.5\,(M_2 - M_1)}

Energy ratio between two earthquakes

The ratio of the energies of two earthquakes depends on the difference in their magnitudes; a difference of 2 gives a factor of 10 to the power 3, a thousand times.

Worked example

Comparing earthquake energies

Compare the energy released by a magnitude 7 earthquake with that of a magnitude 5 earthquake, and the ground shaking of each.

Find the magnitude difference
The difference in magnitude is 7 minus 5 = 2.
Apply the energy ratio
Energy ratio = 10 to the power (1.5 x 2) = 10 to the power 3.
$\frac{E_2}{E_1} = 10^{1.5 \times 2} = 10^{3} = 1000$
Compare the shaking
Ground-shaking amplitude increases tenfold per unit, so over two units it is 10 x 10 = 100 times greater.
Interpret
The magnitude 7 earthquake releases about 1000 times the energy and shakes the ground about 100 times as strongly as the magnitude 5.

Result: The magnitude 7 releases about 1000 times the energy and shakes about 100 times as strongly as the magnitude 5.

Typical mistakes

Confusing magnitude with intensity; magnitude is a single measure of the source, intensity varies with place and local ground.
Treating the magnitude scale as linear; each unit is ten times the shaking but about thirty-two times the energy.

Active revision

Explain why an earthquake of magnitude 7 can be far more than 'a bit worse' than one of magnitude 5, referring to both amplitude and energy.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — earthquakes (British Geological Survey)

Volcanic hazards#

StandardWJEC/Eduqas A level Geology (Component 3) — Geohazards

The internal structure of a composite volcano

Fig. 2A composite (strato-) volcano: viscous, gas-rich magma rises from the chamber through the conduit to erupt explosively from the vent as pyroclastic flows, ash and gas.

The hazards a volcano poses depend above all on how explosive its eruptions are, and that is controlled by the composition of its magma, in a chain of cause and effect worth learning precisely. Silica-rich (acidic) magma is highly viscous and holds dissolved gases which cannot escape easily, so pressure builds until the magma erupts violently and explosively; silica-poor (basic) magma is runny (low viscosity) and lets gases bubble out gently, so it erupts effusively as lava flows. Because magma composition is tied to plate setting, explosive volcanoes cluster at subduction zones (andesitic and rhyolitic magma) and gentle, effusive volcanoes at mid-ocean ridges and hot spots (basaltic magma).

Explosive eruptions produce the most lethal hazards. Pyroclastic flows (or nuees ardentes) are fast-moving, ground-hugging clouds of hot gas and ash that race down the volcano's flanks at high speed and incinerate everything in their path; they are the greatest killer at explosive volcanoes. Ash falls collapse roofs, ruin crops, contaminate water and endanger aircraft; volcanic gases such as sulfur dioxide and carbon dioxide can poison and can alter climate; and lahars — mudflows of volcanic ash mixed with water from rain or melted snow — sweep down river valleys with great destructive power, sometimes long after the eruption.

Effusive eruptions, typical of basaltic volcanoes, are far less immediately lethal because their runny lava flows are usually slow enough to escape on foot, though they destroy property and land. The contrast is captured by the shape of the volcano: explosive, viscous magma builds steep-sided composite (strato-) volcanoes of alternating ash and lava, whereas effusive, runny basalt builds broad, gently sloping shield volcanoes. So the form of a volcano is itself a guide to the hazards it presents, linking the igneous petrology of an earlier chapter directly to risk.

Assessing volcanic hazard therefore begins with identifying the type of volcano and its magma, and reading its past eruptions from the deposits around it. A stratovolcano at a subduction zone with a history of pyroclastic flows demands a very different response — exclusion zones and rapid evacuation — from a Hawaiian-type shield producing predictable lava flows. Matching the hazard to the volcano, and understanding the plate-tectonic reason for the difference, is the core AO2 and AO3 skill of this section.

Worked example

Predicting eruption style from magma

A volcano at a destructive plate margin erupts andesitic, silica-rich magma. Predict its eruption style and two hazards, and explain the link.

Read the magma
Silica-rich (andesitic) magma is highly viscous and traps dissolved gases, so pressure builds until it erupts explosively.
Predict the style
The eruption will be explosive, building a steep composite volcano.
Name the hazards
Explosive eruptions produce pyroclastic flows and heavy ash falls (and lahars if there is water), the main killers.

Result: Viscous, gas-rich andesitic magma erupts explosively, producing pyroclastic flows and ash falls — the characteristic hazards of a subduction-zone stratovolcano.

Typical mistakes

Saying lava flows are the main killer at explosive volcanoes; pyroclastic flows, ash and lahars cause most deaths, not slow lava.
Reversing the viscosity link; silica-rich magma is viscous and explosive, silica-poor magma is runny and effusive.

Active revision

Explain why a subduction-zone stratovolcano is more dangerous than a mid-ocean-ridge or hot-spot shield volcano, referring to magma composition, viscosity and gas.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — volcanoes (British Geological Survey)

Mass-movement hazards and slope stability#

StandardWJEC/Eduqas A level Geology (Component 3) — Geohazards

A rotational slump

Fig. 3A rotational slump: the mass slides along a curved failure surface and rotates back; water raises pore pressure and reduces the friction resisting movement.

Mass movement is the downslope movement of rock and soil under gravity, ranging from slow creep to catastrophic slides and flows. The main types are rockfalls (free fall of rock from a cliff), slides (movement of a coherent mass along a planar surface, such as a bedding plane dipping out of a slope), slumps (rotational slides along a curved surface, which tilt the moving block back), and flows (debris flows and mudflows in which water-saturated material moves as a fluid). The type of movement depends on the material, the water content and the geometry of the slope.

Slope stability is governed by the balance between the driving force, which is the down-slope component of the weight of the material, and the resisting force, which is the friction and cohesion along the potential failure surface. This balance is expressed as the factor of safety, the ratio of resisting to driving forces: when it is greater than one the slope is stable, and when it falls below one the slope fails. Anything that increases the driving force or reduces the resisting force lowers the factor of safety and brings the slope closer to failure.

Water is the single most important trigger of mass movement, and it acts in several ways at once. It adds weight to the slope, increasing the driving force; it raises the water pressure in the pores, which pushes the grains apart and reduces the friction (the resisting force); and it can lubricate and weaken clay layers. This is why landslides so often follow heavy or prolonged rain, and why undercutting a slope (by a river, the sea or an excavation), steepening it, loading its top, or removing the vegetation that binds it, all make failure more likely by tipping the force balance.

Managing mass-movement hazard therefore means shifting the balance back toward stability: draining water from the slope to restore friction, reducing the slope angle or removing weight from the top, building retaining walls or rock bolts to add resisting force, planting vegetation to bind the soil, and avoiding building on or below unstable slopes. Recognising an unstable slope from its geology (weak or clay-rich rocks, bedding or joints dipping out of the slope, evidence of past movement) and recommending appropriate measures is a practical, examinable engineering-geology skill developed further in the map chapter.

\text{factor of safety} = \frac{\text{resisting forces}}{\text{driving forces}}

Factor of safety

A slope is stable when the factor of safety exceeds 1 (resisting forces greater than driving forces) and fails when it falls below 1; water typically lowers it by reducing friction and adding weight.

Worked example

Applying the factor of safety

On a slope the driving force is 800 kN and the resisting force is 1000 kN. Calculate the factor of safety and state whether the slope is stable. Then heavy rain reduces the resisting force to 700 kN; recalculate and comment.

Dry factor of safety
Factor of safety = resisting / driving = 1000 / 800 = 1.25, which is greater than 1, so the slope is stable.
After rain
With the resisting force reduced to 700 kN, factor of safety = 700 / 800 = 0.875.
$\text{FoS} = \frac{700}{800} = 0.875$
Interpret
The factor of safety has fallen below 1, so the slope now fails; the rain reduced friction (by raising pore pressure) enough to tip the balance.

Result: Dry FoS = 1.25 (stable); after rain FoS = 0.875 (< 1), so the slope fails because water reduced the resisting force below the driving force.

Typical mistakes

Thinking water helps only by adding weight; it also raises pore pressure and reduces the friction resisting movement.
Confusing a slide (movement along a planar surface) with a slump (rotation along a curved surface).

Active revision

A slope has a factor of safety of 1.2 in dry weather. Explain how prolonged heavy rain could cause it to fail, referring to the driving and resisting forces.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — landslides (British Geological Survey)

Tsunami and multiple hazards#

FoundationWJEC/Eduqas A level Geology (Component 3) — Geohazards

How a tsunami is generated

Fig. 4Tsunami generation: an earthquake abruptly displaces the sea floor and the water column; the waves travel fast and low across the ocean, then shoal into a high, destructive wave at the coast.

A tsunami is a series of very long-wavelength waves generated by the sudden displacement of a large volume of water, most commonly by a submarine earthquake that abruptly moves the sea floor up or down at a subduction zone, but also by submarine landslides, volcanic collapses or eruptions, and even meteorite impacts. Because a whole column of water is displaced, a tsunami carries enormous energy, and it is quite different from a wind-driven wave, which disturbs only the surface.

In the deep ocean a tsunami travels very fast, at hundreds of kilometres per hour, but has a low height and a very long wavelength, so it passes ships almost unnoticed. As it enters shallow water near a coast it slows down, and its energy is compressed into a shorter, higher wave that can rise to many metres and surge far inland, often preceded by a dramatic withdrawal of the sea as the trough arrives first. The result is catastrophic inundation, drowning, and destruction of everything in the wave's path, as in the great Indian Ocean tsunami of 2004 and the Tohoku tsunami of 2011.

Geohazards frequently occur in linked chains rather than in isolation, and a single triggering event can set off several hazards in sequence. A large earthquake can trigger landslides on land and submarine slides offshore, cause liquefaction, rupture services and start fires, and, if it displaces the sea floor, generate a tsunami; a volcanic eruption can produce pyroclastic flows, ash, lahars and, through flank collapse, a tsunami. Recognising these cascades is important because the secondary hazards sometimes cause more casualties than the primary event.

This linkage shapes how hazards are managed. Because a tsunami travels more slowly than the seismic waves that warn of the earthquake, a warning system can detect the earthquake and sea-level change and issue an alert with enough time to evacuate distant coasts, which has saved many lives around the Pacific. Understanding both the physical generation of a tsunami and the way hazards cascade from a single event is the foundation for the risk assessment and management that conclude the chapter.

Worked example

Explaining a hazard cascade

A large subduction-zone earthquake strikes a densely populated coast. Describe the cascade of hazards that could follow and identify which might cause the most casualties.

Primary hazard
The earthquake causes ground shaking, damaging or collapsing buildings.
Secondary hazards
Shaking can liquefy soft ground, trigger landslides, and rupture gas and power lines causing fires; if the sea floor is displaced, a tsunami is generated.
Worst hazard
For a coastal subduction earthquake the tsunami often causes the most casualties, as it inundates low-lying, densely populated coasts with little warning.

Result: The earthquake triggers shaking, liquefaction, landslides, fires and a tsunami; the tsunami is often the deadliest for a coastal population.

Typical mistakes

Describing a tsunami as a single giant wind wave; it is a series of long-wavelength waves displacing the whole water column.
Forgetting that the sea often withdraws before a tsunami arrives, as the trough of the wave reaches the coast first.

Active revision

Explain why a tsunami is barely noticeable in the open ocean but devastating at the coast, referring to its speed, wavelength and height.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — tsunamis (British Geological Survey)

Risk, prediction and management#

AdvancedWJEC/Eduqas A level Geology (Component 3) — Geohazards

Hazard, exposure, vulnerability and risk

Fig. 5Risk arises from the combination of hazard, exposure and vulnerability; because the hazard can rarely be changed, management usually reduces exposure and vulnerability.

A crucial distinction underlies all hazard management: a hazard is a natural event with the potential to cause harm, whereas risk is the likelihood of harm actually occurring, which depends on people and property being in harm's way. Risk is often expressed as the product of the hazard (its probability and severity), the exposure (the people and assets in the area) and the vulnerability (how susceptible they are to harm). This is why an identical earthquake can be a minor event in an empty, well-built area and a catastrophe in a poor, crowded city, and why risk can be reduced even where the hazard cannot be changed.

Prediction and forecasting aim to reduce risk by giving warning. Some hazards can be forecast statistically — the long-term probability of an earthquake on a given fault, or of an eruption at a given volcano — from the geological record of past events, and volcanoes in particular can often be predicted in the short term by monitoring the precursors of eruption: swarms of small earthquakes, ground deformation as magma rises, changes in gas emissions and heat. Earthquakes remain very difficult to predict in the short term, so for them the emphasis falls on preparedness rather than prediction.

Management strategies fall into reducing the hazard, the exposure or the vulnerability. The hazard itself can rarely be reduced, though lava flows have occasionally been diverted and slopes stabilised; more often risk is reduced by controlling exposure — hazard mapping and land-use planning to keep people and vital buildings away from the most dangerous zones, and evacuation when a warning is given — and by reducing vulnerability — earthquake-resistant building design and codes, tsunami warning systems and sea walls, education and emergency planning. A combination of measures, tuned to the hazard and to the resources available, is always needed.

Evaluating hazard management is a classic AO3 task that rewards balance. Every strategy has costs, limitations and trade-offs: prediction can give false alarms that erode trust, engineering solutions are expensive and can fail, land-use planning conflicts with economic pressure to build in dangerous places, and the poorest communities, most exposed and least able to afford protection, are often at greatest risk. A good answer weighs the effectiveness, cost and practicality of the options for a particular hazard and setting, and recognises that reducing exposure and vulnerability is usually more achievable than reducing the hazard itself.

Worked example

Reducing risk where the hazard is fixed

A city lies near an active fault, so the earthquake hazard cannot be changed. Suggest and justify three measures to reduce the risk, referring to exposure and vulnerability.

Reduce vulnerability
Enforce earthquake-resistant building codes and retrofit vulnerable buildings, so structures survive shaking and fewer collapse.
Reduce exposure
Use hazard mapping and land-use planning to keep critical facilities and dense housing off the worst ground (for example soft, liquefiable soils near the fault).
Improve preparedness
Educate the population, run drills and install early-warning and emergency systems, so people respond effectively and casualties fall.

Result: Since the hazard is fixed, risk is cut by reducing vulnerability (building codes), exposure (planning) and by preparedness (education and warning).

Typical mistakes

Using 'hazard' and 'risk' interchangeably; risk depends on people and property being exposed and vulnerable, not just on the natural event.
Claiming earthquakes can be reliably predicted in the short term; they cannot, so preparedness matters more than prediction.

Active revision

Explain why two cities exposed to the same earthquake hazard can face very different levels of risk, and suggest how the higher-risk city could reduce its risk.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — earth hazards (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Geohazards

5 sections~18 min reading time3 competenciesLevel Foundation 1 · Standard 3 · Advanced 1

Geohazards

Key points

Comparing earthquake energies

Key points

Predicting eruption style from magma

Key points

Applying the factor of safety

Key points

Explaining a hazard cascade

Key points

Reducing risk where the hazard is fixed

Sources

Geohazards

Key points

Comparing earthquake energies

Key points

Predicting eruption style from magma

Key points

Applying the factor of safety

Key points

Explaining a hazard cascade

Key points

Reducing risk where the hazard is fixed

Sources