Earthquake and volcanic hazards and impacts

The focus (also called the hypocentre) is the point within the Earth, below the surface, where the earthquake originates — where the rocks first fracture and release their strain energy.

The epicentre is the point on the Earth's surface directly above the focus.

The key difference is therefore location and depth: the focus is underground (at depth), while the epicentre is the surface point vertically above it. Damage is usually greatest at the epicentre because it is closest to where the energy is released, and a shallow focus produces more intense surface shaking than a deep one.

P-waves (primary waves) are body waves that travel through the Earth's interior. They are the fastest seismic waves, so they arrive first, and they move material by a push–pull (compressional) motion. They can pass through both solids and liquids.

S-waves (secondary waves) are also body waves but are slower, so they arrive after the P-waves. They move material from side to side (a shear motion) and, importantly, cannot pass through liquids.

The key differences are therefore speed (P faster than S), motion (P = push–pull; S = side-to-side shear) and that S-waves cannot travel through liquids whereas P-waves can.

However, the most damage at the surface is caused by the surface waves — Love waves (horizontal shaking) and Rayleigh waves (a rolling, up-and-down ground roll) — because they are concentrated near the surface where buildings and people are, producing prolonged, destructive ground motion.

Whether an eruption is explosive or effusive is controlled mainly by the viscosity (stickiness) of the magma and its dissolved gas content, which in turn depend on the magma's silica content and the plate-tectonic setting.

Effusive eruptions occur where the magma is runny (low viscosity). This is typical of basaltic, low-silica magma found at constructive boundaries (mid-ocean ridges) and hotspots such as Hawaii. Because the magma is fluid, dissolved gases can escape gently and continuously rather than building up pressure. The result is non-violent eruptions dominated by lava flows, which build broad, gently sloping shield volcanoes.

Explosive eruptions occur where the magma is sticky (high viscosity). This is typical of andesitic or rhyolitic, high-silica magma found at destructive (subduction) boundaries, where water-rich oceanic crust melts. The viscous magma traps the dissolved gases, so pressure builds up until it is released violently, shattering the magma into ash and hurling it skyward. This produces ash falls, pyroclastic flows and tall, steep composite (strato) volcanoes such as Mount St Helens (1980).

In short, runny, low-gas, low-silica magma erupts effusively, while sticky, gas-rich, high-silica magma erupts explosively — so the eruption style ultimately reflects the magma type and the tectonic setting that produced it.

Volcanic eruptions can often be predicted because they are usually preceded by measurable precursor events (warning signs) as magma rises towards the surface. Increasing seismic activity (swarms of small earthquakes) shows magma fracturing the rock; ground deformation, in which the volcano's flank bulges, can be measured by tiltmeters and GPS; rising emissions of gases such as sulphur dioxide indicate magma is degassing; and ground temperatures rise. Because these signs build up over days or weeks, scientists can issue warnings — at Pinatubo in 1991, monitoring of precursors enabled a mass evacuation that saved tens of thousands of lives.

Earthquakes, by contrast, cannot be reliably predicted because they have no consistent precursor and no useful warning time. Strain builds up silently along a fault and is released suddenly when the rock fractures, with nothing to signal the exact moment of rupture. Seismographs and historical records can map which faults are dangerous and estimate the probability of a quake over decades, but cannot forecast when one will strike. Early-warning systems can detect the first (P-wave) energy and give a few seconds' alert, but this is a reaction after rupture, not a prediction.

The key difference is therefore that a rising body of magma produces detectable, gradual warning signs, whereas the sudden release of strain in an earthquake does not — so volcanic prediction is feasible while earthquake prediction is not.

The impacts of an earthquake — the number of deaths and injuries, the scale of damage and the speed of recovery — are the product of both the physical nature of the event and the human geography of the place it strikes. Physical factors include the magnitude, the depth of the focus, the distance of population from the epicentre, and secondary hazards such as tsunami and landslides; human factors include population density, the quality of buildings and infrastructure, the level of economic development, governance and preparedness, which together make up a population's vulnerability and capacity to cope. This essay assesses their relative importance and argues that, while physical factors set the SCALE of the hazard, it is human factors that most often determine whether that hazard becomes a disaster — though physical factors can occasionally override even good preparation.

Physical factors clearly matter. A higher magnitude releases more energy, and a shallow focus concentrates that energy near the surface, intensifying ground-shaking; impacts are also worst near the epicentre and decrease with distance. Secondary physical hazards can be decisive: in the 2011 Tōhoku earthquake in Japan (~Mw 9.0), it was not the shaking but the resulting tsunami — a physical hazard — that caused the great majority of the roughly tens of thousands of deaths, overtopping sea walls and triggering the Fukushima nuclear crisis. Here a physical factor, the tsunami, dominated the impact despite Japan's excellent preparation, which shows physical factors cannot be dismissed.

However, the central evidence points to human factors as the usual determinant of the human impact. The clearest proof is the contrast between Japan and Haiti. The 2010 Haiti earthquake was far SMALLER (~Mw 7.0) than Tōhoku, yet it killed vastly more people. This was because Haiti, a low-income country, had no enforced building codes, so densely packed, poorly built and unreinforced housing collapsed on its occupants; the shallow focus lay close to the densely populated capital, Port-au-Prince; and the weak state could not mount an effective rescue or recovery, so many of the injured died and reconstruction stalled for years. Japan, a high-income country, applied strict aseismic building codes, early-warning systems and routine drills, so its far larger quake caused relatively few shaking deaths. The same physical hazard would have produced utterly different outcomes in the two countries — and a smaller hazard in the more vulnerable country was far more deadly. This is powerful evidence that human factors, above all economic development and building quality, govern the impact.

The most accurate assessment is that the two sets of factors interact rather than compete, which the relationship Risk = Hazard × Vulnerability ÷ Capacity to cope captures well. Physical factors determine the size and type of the hazard — the input — but human factors determine the vulnerability and the capacity to cope, which convert that input into an impact. The worst disasters occur when a severe physical hazard (a shallow, high-magnitude quake) coincides with high human vulnerability (a dense, poor, poorly built city with weak governance), as in Haiti. Where vulnerability is low, even a great physical hazard produces a manageable impact, as the shaking in Japan largely did.

In conclusion, I agree, on balance, with the view: the impacts of earthquakes depend MORE on human factors than physical ones, because vulnerability and the capacity to cope decide whether a given hazard becomes a catastrophe — demonstrated by the smaller Haiti quake killing far more than the larger Tōhoku quake. The important qualification is that physical factors can occasionally override human preparation, as the Tōhoku tsunami did, overwhelming even one of the best-prepared nations on Earth. The fairest judgement, therefore, is that human factors usually dominate the human impact, but physical factors set the limits within which human management operates and can, in extreme cases, prevail.

Management of tectonic hazards aims to reduce loss of life and damage through prediction, preparation (resistant design, hazard mapping, land-use planning and education), emergency response and long-term recovery. The effectiveness of these strategies differs between volcanoes and earthquakes, largely because the two hazards differ in how far they can be predicted. This essay assesses whether management is genuinely more effective for volcanoes, and argues that it usually is — because volcanic eruptions can be predicted and so people can be evacuated — but that the effectiveness of management for BOTH hazards ultimately depends on a country's level of economic development, so the contrast is not absolute.

The strongest argument for the statement is that volcanic eruptions can often be predicted, whereas earthquakes cannot. Volcanoes give measurable precursor events as magma rises — increasing seismicity, ground deformation (bulging measured by tiltmeters and GPS), rising sulphur-dioxide emissions and higher ground temperatures — over days or weeks. This allows authorities to evacuate the population before the eruption: at Pinatubo in the Philippines in 1991, monitoring of precursors enabled a mass evacuation that saved tens of thousands of lives, despite a very large (VEI ~6) eruption. Because the deadliest volcanic hazards — pyroclastic flows and lahars — are fast and unsurvivable once they arrive, prediction-led evacuation is by far the most effective response, and it works. Earthquakes, by contrast, strike without warning: they have no reliable precursor and no useful warning time, so populations cannot be evacuated in advance. In this crucial sense, management is indeed more effective for volcanoes.

However, the argument is weakened by several points. First, not all volcanic hazards can be managed by evacuation: ash clouds can ground aviation across continents, as the relatively small Eyjafjallajökull eruption in Iceland in 2010 did, causing huge economic disruption that no evacuation could prevent. Lava flows can be diverted only with difficulty. Second, earthquake management, although it cannot predict, can be highly effective through PREPARATION. Japan's strict aseismic building codes, retrofitting, early-warning systems and routine drills meant that the enormous 2011 Tōhoku earthquake caused relatively few deaths from shaking — a clear management success. So earthquakes can be managed well even without prediction, by reducing vulnerability.

The decisive consideration is that, for BOTH hazards, the effectiveness of management depends overwhelmingly on economic development. Volcanic monitoring requires expensive equipment and scientific expertise, and evacuation requires organisation and somewhere for people to go; the Pinatubo success relied on international scientific support. Earthquake-resistant construction is costly, which is why Japan could afford it but Haiti could not — the 2010 Haiti earthquake, a smaller event, killed vastly more people precisely because the low-income country lacked codes, monitoring and response capacity. So management effectiveness is gated by wealth for both hazards, and a poor country may manage neither well.

In conclusion, I agree to a large extent that management is more effective for volcanic than earthquake hazards, because the predictability of eruptions allows life-saving evacuation that is simply impossible for earthquakes — the contrast between the Pinatubo evacuation and the no-warning Haiti quake makes this clear. The important qualifications are that some volcanic impacts (ash clouds, economic disruption) resist management, that earthquake preparation can be very effective even without prediction (Tōhoku), and that, above all, the effectiveness of management for either hazard depends on a country's wealth and governance. The fairest judgement is that management is more effective for volcanoes mainly because they can be predicted, but that in the poorest countries — which face the highest risk — management of BOTH hazards remains dangerously limited.