Earth Hazards: Earthquakes, Eruptions, Landslides and Tsunamis
What Makes a Geological Event a Hazard?
Not every earthquake or volcanic eruption causes widespread harm. A geological event only becomes a hazard when people or infrastructure lie in its path. The same magnitude earthquake that passes unnoticed beneath an uninhabited ocean floor can devastate a densely populated city. Understanding this distinction is central to the way geologists study risk.
Two factors determine the level of risk: the hazard probability and the vulnerability of the population. A sparsely populated, well-prepared nation facing a moderate eruption may suffer far fewer casualties than a densely populated, under-resourced community facing a smaller event. Geologists use the term risk to describe this combination: risk = hazard probability × vulnerability.
The level of risk from a geological hazard is shaped by population density, level of economic development (LEDC versus MEDC), quality of education, and the effectiveness of communication systems and early warning schemes.
Earthquakes
Earthquakes occur when stress built up along faults is suddenly released, sending seismic waves radiating outward from the focus. Shaking at the surface is strongest directly above the focus, at a point called the epicentre. Earthquake intensity is described using the Modified Mercalli Intensity Scale (I to XII), which measures observed effects — from barely felt tremors at intensity I, through structural damage at V–VII, to total devastation at XII. The Richter scale (and its modern replacement, the moment magnitude scale) measures the energy released: it is logarithmic, so a magnitude 7 earthquake releases roughly 32 times more energy than a magnitude 6.
Secondary hazards from earthquakes include liquefaction — where waterlogged sediment temporarily behaves like a liquid — and landslides triggered by ground shaking on steep slopes. In coastal areas or under the ocean, earthquakes can displace the sea bed and generate tsunamis. The 2011 Tōhoku earthquake (magnitude 9.0) off Japan showed how a single event can combine ground shaking, structural collapse, tsunami run-up and a nuclear emergency into an overlapping cascade of hazards.
The Mercalli scale is qualitative — it records what people experienced and what damage occurred, so it varies from place to place for the same earthquake. The Richter (or moment magnitude) scale is quantitative — it gives a single number representing the total energy released at the source. You need to be able to explain both. Remember: Mercalli describes effects; moment magnitude describes energy. A useful check — can a location 200 km from the epicentre have a different Mercalli intensity than a location 20 km away? Yes. Can it have a different Richter magnitude? No.
Volcanic Eruptions
Volcanic hazards depend heavily on the composition of the magma. Low-viscosity basaltic magma (common at divergent boundaries and hot spots) allows gases to escape gradually, producing effusive eruptions with fast-moving lava flows. High-viscosity rhyolitic or andesitic magma (common at subduction zones) traps gases, building pressure until explosive eruptions blast out ash clouds and pyroclastic flows — fast-moving mixtures of hot gas, ash and rock fragments that travel at speeds exceeding 100 km per hour and reach temperatures above 700°C. Pyroclastic flows are among the most lethal volcanic hazards and cannot be outrun.
Additional volcanic hazards include lahars (volcanic mud flows formed when ash mixes with water from ice melt or rainfall), tephra fall (solid material ejected into the atmosphere), and volcanic gases such as sulfur dioxide, which can cause acid rain and respiratory problems far downwind. The Volcanic Explosivity Index (VEI) measures eruption size on a logarithmic scale from 0 (non-explosive) to 8 (super-eruption). The 1980 Mount St Helens eruption was VEI 5; the 1991 Pinatubo eruption was VEI 6.
Pyroclastic flows are the most dangerous volcanic hazard. They travel faster than 100 km per hour, reach temperatures above 700°C, and can travel downhill in any direction. Lava flows are slower and more predictable, allowing evacuation time.
Landslides and Subsidence
A landslide occurs when the driving force of gravity on a slope exceeds the resistance provided by the strength of the material and friction. Triggers include earthquake shaking, volcanic activity loading slopes with ash and lava, heavy rainfall saturating soils, and undercutting by rivers or coastal erosion. Landslides can bury settlements, block rivers to form lakes that burst, and trigger secondary tsunamis if they enter the sea. Subsidence — the gradual sinking of the ground surface — can result from the withdrawal of groundwater or hydrocarbons, from dissolving limestone (creating sinkholes), or from mining.
Less economically developed countries (LEDCs) typically have weaker building regulations, fewer resources to enforce them, less access to early warning technology, lower levels of public education about hazards, and less developed emergency response infrastructure. The 2010 Haiti earthquake (magnitude 7.0) killed around 230,000 people, while a similarly sized earthquake in the United States or Japan would likely kill far fewer because of stricter building codes and better preparedness. MEDCs can invest in seismic engineering, real-time monitoring networks, and public evacuation drills — all of which reduce vulnerability even when the hazard probability stays the same.
Tsunamis
Tsunamis are generated when a large volume of water is suddenly displaced — most commonly by a submarine earthquake along a subduction zone, but also by submarine landslides or volcanic caldera collapse. In the open ocean, tsunami waves are long (hundreds of kilometres) and low (less than 1 metre high), travelling at speeds approaching 800 km per hour. As the wave enters shallow coastal water it slows, causing its height to increase dramatically — a process called shoaling. The 2004 Indian Ocean tsunami, triggered by a magnitude 9.1 earthquake off Sumatra, generated waves exceeding 30 metres in parts of Aceh province, Indonesia, killing over 230,000 people across 14 countries. The geological context is key: the tsunami was caused by sudden vertical movement of the sea floor at the convergent boundary between the Indo-Australian and Eurasian plates.