The nature of earthquakes, volcanic eruptions and tsunamis; their measurement; and their primary and secondary, social, economic and environmental impacts.

What this dot point is asking

Eduqas wants you to explain the nature of earthquakes, volcanic eruptions and tsunamis, how they are measured, and their primary and secondary social, economic and environmental impacts, and why those impacts vary.

The answer

Earthquakes and their measurement

Earthquakes occur where accumulated strain at a fault is released suddenly. The moment magnitude scale is logarithmic, so each whole number is about $32$ times more energy: a magnitude $8$ quake releases roughly $1{,}000$ times the energy of a magnitude $6$. Shallow earthquakes (a focus near the surface) are far more destructive than deep ones because the energy reaches the surface with less attenuation. The intensity of shaking, and so the damage, depends on magnitude, depth, distance, and crucially the local geology (soft sediments amplify shaking) and building quality.

Volcanoes and tsunamis

Volcanic style depends on magma chemistry: runny basaltic magma produces gentle effusive eruptions, while viscous, gas-rich andesitic magma traps pressure and erupts explosively, generating fast, lethal pyroclastic flows and widespread ash. Tsunamis are generated when a megathrust earthquake (or landslide) suddenly displaces a column of water; the waves travel across the ocean at jet speed and rear up on reaching shallow coasts, causing catastrophic coastal flooding, as in 2004 and 2011.

Primary and secondary impacts

Impacts are classified as primary (the immediate, direct effects of the event: building collapse, lava and pyroclastic flows, the deaths and injuries they cause) and secondary (the indirect effects that follow: tsunamis, landslides, liquefaction, fires from ruptured gas mains, disease from broken sanitation, homelessness and long-term economic disruption). Each is also social (deaths, injury, displacement), economic (damage, lost output, cost of recovery) and environmental (landscape change, habitat loss). A central Eduqas point is that secondary impacts frequently cause more deaths and longer-term harm than the primary event, as the Tohoku tsunami did, and that the scale of impact depends on the interaction of physical magnitude with human vulnerability.

Examples in context

Example 1. The Tohoku earthquake and tsunami, Japan (2011). A magnitude $9.0$ megathrust earthquake off north-east Japan, at a destructive boundary, generated a tsunami over $10$ m high that overwhelmed coastal defences, killed around $18{,}000$ people and triggered the Fukushima nuclear accident. The primary impact (shaking) caused relatively few deaths because Japan's buildings are well engineered; the overwhelming majority of deaths were a secondary impact, the tsunami. Economic losses exceeded $200$ billion US dollars. Tohoku is the standard Eduqas case showing that secondary impacts can dominate and that even a high-income, well-prepared country can be overwhelmed.

Example 2. Mount Pinatubo eruption, Philippines (1991). The 1991 eruption of Mount Pinatubo, a subduction-zone (destructive boundary) volcano, was one of the largest of the twentieth century (VEI 6), producing huge pyroclastic flows, vast ash falls and, in the following years, deadly lahars when monsoon rain mobilised the ash. Successful monitoring and evacuation beforehand saved many thousands of lives, though lahars continued to cause damage and displacement for years afterwards. Pinatubo illustrates explosive volcanic hazards, the importance of secondary hazards (lahars) and the value of monitoring, a versatile Eduqas example.

Try this

Q1. Distinguish between the focus and the epicentre of an earthquake. [2 marks]

• Cue. The focus is the point at depth where the fault ruptures and energy is released; the epicentre is the point on the surface directly above the focus.

Q2. Explain why the secondary impacts of a tectonic hazard can be greater than the primary impacts. [3 marks]

• Cue. Secondary impacts (tsunamis, landslides, liquefaction, fire, disease) can affect far larger areas and populations and persist for years, often killing more people than the immediate shaking or eruption, as the Tohoku tsunami did.