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Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.5 Hazards — seismicity: the nature of seismicity and its relationship to plate tectonics; the forms of seismic hazard; and the spatial distribution, randomness, magnitude, frequency, regularity and predictability of seismic events, together with their contrasting impacts. This depth lesson develops the physics of earthquakes — fault mechanics and elastic rebound, the four seismic-wave types, the distinction between magnitude and intensity scales, and the secondary hazards (liquefaction, tsunami, landslides) — at the rigour required to explain why apparently similar earthquakes produce wildly different outcomes. It applies the boundary processes of the plate-tectonics lesson (§3.1.5), draws on the systems idea of strain accumulation and release, and links forward to §3.2 (population/urban vulnerability) and the management lesson. Assessment objectives: AO1 (mechanism, waves, scales, secondary hazards), AO2 (applying these to a located earthquake) and AO3 (manipulating and evaluating magnitude–frequency, intensity and casualty data).
Earthquakes are the most lethal of all geophysical hazards: in most decades they kill more people than volcanoes, and the deadliest single events (Tangshan 1976, the 2004 Indian Ocean and 2010 Haiti) have each claimed hundreds of thousands of lives. Yet the physical earthquake — the release of stored elastic strain in seconds — kills almost no one directly. People are killed by collapsing buildings, tsunami, landslides and fire — that is, by the interaction of the shaking with the built environment and the physical setting. This makes seismic hazard the purest illustration of the course's organising idea, the risk equation:
Risk=CapacityHazard×Vulnerability
The grim aphorism of seismologists — "earthquakes don't kill people; buildings kill people" — is precisely a statement that the Vulnerability term (building quality, location, density) usually dominates the Hazard term (magnitude) in determining the death toll. Holding this in view turns the physics below into geography: we study fault mechanics not for its own sake but to understand the Hazard term, while never forgetting that the disaster is built as much as it is shaken.
The mechanism of earthquakes is elastic rebound, first articulated after the 1906 San Francisco earthquake. Along a fault, the two blocks of crust are driven slowly past one another by plate motion, but friction locks the fault surface. Stress accumulates and the rock on either side deforms elastically, storing strain energy — like a bent ruler or a stretched spring. When the accumulated stress finally exceeds the frictional/strength threshold of the rock, the fault ruptures suddenly: the locked surfaces slip, the strained rock "snaps back" toward its undeformed shape, and the stored elastic energy is released as seismic waves. The system then begins to re-accumulate strain for the next event — a clear instance of threshold behaviour in a system that stores and releases energy. The rupture itself is not a point but a surface: in great earthquakes the rupture propagates along hundreds of kilometres of fault. The 2011 Tōhoku rupture extended ~500 km along the Japan Trench and slipped by as much as ~50 m, which is why it released so much energy and displaced so much sea floor.
A large earthquake (the mainshock) is followed by a decaying sequence of smaller aftershocks as the crust around the rupture re-adjusts; aftershock frequency falls off roughly as the reciprocal of time (Omori's law). Aftershocks are a serious hazard in their own right — they collapse already-weakened buildings and endanger rescuers, as the Mw 7.3 Nepal aftershock of 12 May 2015 did. Occasionally smaller foreshocks precede the mainshock, but because most small earthquakes are not foreshocks, they cannot be used for reliable prediction.
Aftershock sequences are an important and under-appreciated complication for management, because they extend the period of danger for days, weeks or months after the headline event. They interrupt search-and-rescue, repeatedly re-collapse damaged structures onto survivors and responders, deter people from re-entering buildings to recover possessions or resume normal life, and prolong the psychological trauma of a disaster. In a sequence like Christchurch's — where the damaging February 2011 event was technically an aftershock of the larger September 2010 earthquake — the cumulative damage of repeated shaking exceeded what any single event would have caused. This is why response planning must assume a period of seismic danger rather than a single moment, and why the "all-clear" after a major earthquake is always provisional.
| Fault type | Block motion | Stress regime | Tectonic setting | Example |
|---|---|---|---|---|
| Normal | Hanging wall moves down | Tensional (extension) | Constructive margins, rifts | East African Rift; Basin and Range |
| Reverse / thrust | Hanging wall moves up | Compressional | Destructive & collision margins | Himalayan front; Japan Trench megathrust |
| Strike-slip | Horizontal, lateral | Shear | Conservative margins | San Andreas; North Anatolian |
The megathrust faults of subduction zones — gently dipping reverse faults where one plate is forced beneath another — are the most important for major hazards, because only they can host Mw 9+ ruptures and only they displace the sea floor vertically enough to generate ocean-crossing tsunami.
It is worth dwelling on why different boundaries produce different earthquake characteristics, because this is exactly the kind of explanatory link between lesson 1 and this lesson that AQA rewards. At a constructive margin, the lithosphere is thin, hot and weak, so it cannot store much elastic strain before it fails; earthquakes are therefore frequent but shallow and small. At a conservative margin, two thick, cool, strong plates are locked along a near-vertical fault and can store strain for centuries before a sudden, shallow, high-magnitude rupture (San Andreas, North Anatolian) — these are the events that flatten cities without any tsunami. At a collision margin, the over-thickened continental crust fails along shallow thrust faults, again producing powerful shallow earthquakes but no tsunami (Nepal 2015). And at an oceanic destructive margin, the locked megathrust stores the most strain of all and, when it ruptures, produces both the largest earthquakes and — because the sea floor is thrust vertically upward — the great tsunami (Tōhoku, Sumatra). So the type of boundary predicts not just whether there will be earthquakes, but their depth, maximum magnitude and tsunami potential — which in turn shapes how they must be managed.
The energy released at the focus radiates outward as seismic waves of four types, each producing different ground motion and damage, and arriving in a characteristic order that is itself useful for warning systems.
flowchart LR
F["Focus: rupture, energy release"] --> P["P-waves arrive first (fastest)"]
P --> S["S-waves arrive second"]
S --> L["Surface waves (Love + Rayleigh) arrive last"]
L --> D["Greatest damage: large amplitude, long duration"]
Surface waves are slower than body waves but have the largest amplitude and longest duration, so they typically inflict the greatest damage — especially to taller structures whose natural sway period can resonate with the longer-period surface waves.
A practical use of the wave arrival sequence is the basis of earthquake early warning. Because P-waves travel ~1.7 times faster than S-waves, a network of seismometers near the epicentre can detect the first P-wave arrivals, rapidly estimate the earthquake's location and size, and transmit an alert (at the speed of light) that outruns the slower, more destructive S and surface waves to more distant cities. The warning is short — seconds to a few tens of seconds, longer the further a place is from the epicentre — but enough to halt high-speed trains, stop surgery, open fire-station doors and trigger automatic shut-offs of gas and industrial processes. It is crucial to understand that this is not prediction: rupture has already begun, so the system simply buys a head start on waves already travelling. Even so, it is one of the most effective interventions available for an otherwise unforecastable hazard, and Japan's system performed exactly this role during Tōhoku.
Identical seismic waves do not produce identical shaking everywhere. Soft, water-saturated sediments amplify ground motion (slowing the waves increases their amplitude) and prolong shaking, while solid bedrock transmits it with less amplification. Mexico City's catastrophic damage in 1985, despite being ~350 km from the epicentre, occurred because the city sits on a former lake bed whose soft clays amplified the long-period waves and resonated with mid-rise buildings — a classic demonstration that local geology is part of the hazard. The phenomenon of resonance deserves emphasis because it is frequently misunderstood: every building has a natural period of sway (roughly proportional to its height), and when the ground shaking contains energy at that same period the building is driven into ever-larger oscillations and may fail, while a taller or shorter building nearby survives. In Mexico City the soft basin amplified long-period waves that matched the sway of 6–15-storey buildings, so it was precisely the mid-rise structures that collapsed while low-rise and very tall ones often survived. This is why two requirements — not building on amplifying ground where it can be avoided, and designing buildings whose natural period does not match the expected shaking — are central to seismic engineering, and why microzonation maps (showing how shaking will vary across a city according to its sub-surface geology) are a key planning tool.
A central conceptual distinction — and a favourite exam discriminator — is between scales that measure the energy released at the source (magnitude) and scales that measure the effects experienced at a place (intensity).
The energy–magnitude relationship can be written:
E∝101.5M
so the energy ratio between two earthquakes is E1/E2=101.5(M1−M2). This is the equation behind the AO3 exemplar below and is well worth memorising.
| Feature | Moment Magnitude (Mw) | Modified Mercalli Intensity (MMI) |
|---|---|---|
| Measures | Energy released at the source | Effects/shaking experienced at a place |
| Values per event | One | Many (varies by location) |
| Basis | Instrumental, physics-based | Observational |
| Objectivity | Objective | Subjective |
| Best use | Comparing earthquake size globally | Mapping damage and shaking; historical events |
The key analytical point is that magnitude is not the same as impact. A high-Mw earthquake in a remote area with sound buildings may produce low MMI and few deaths, while a moderate-Mw shallow earthquake beneath a poorly-built city produces high MMI and catastrophic loss. Confusing the two is the single most common error in the topic.
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