How are earthquakes generated and measured, and how do we locate an epicentre?
Earthquakes: focus and epicentre; the elastic rebound mechanism; the types of seismic wave (P, S and surface waves) and their properties; magnitude (the logarithmic Richter and moment magnitude scales) versus intensity (Modified Mercalli); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the OCR H414 dot point on earthquakes. Covers focus and epicentre, elastic rebound, P, S and surface waves, the difference between magnitude (Richter and moment magnitude) and intensity (Modified Mercalli), and how P and S wave travel times and triangulation locate an epicentre.
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What this dot point is asking
OCR wants you to define focus and epicentre, explain the elastic rebound mechanism, describe the P, S and surface waves and their properties, distinguish magnitude (Richter and moment magnitude, both logarithmic) from intensity (Modified Mercalli), and use P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
The answer
Focus, epicentre and elastic rebound
Earthquakes are generated by elastic rebound: tectonic stress builds up across a fault, deforming the rock elastically; when the stress exceeds the strength of the rock (or the friction locking the fault), the rock suddenly slips back, releasing the stored energy as seismic waves.
Seismic waves
Three wave types radiate from the focus:
- P waves (primary). Compressional (push-pull); the fastest, so they arrive first; travel through solids and liquids.
- S waves (secondary). Shear (side-to-side); slower, so they arrive second; travel through solids only (liquids cannot resist shear).
- Surface waves (Love and Rayleigh). Travel along the surface; slowest, largest amplitude, and cause the most damage.
Magnitude versus intensity
Two different ideas that students often confuse:
- Magnitude measures the energy released at the source, from the amplitude on a seismogram. It is one value per earthquake and the scales are logarithmic (each whole number is about times the ground amplitude and about times the energy).
- Richter (local magnitude) works for small to moderate quakes but saturates (underestimates) very large ones.
- Moment magnitude (Mw) is based on the seismic moment (fault area, slip and rock rigidity), so it does not saturate and is preferred for large earthquakes.
- Intensity measures the effects at a place (felt shaking, damage) on the Modified Mercalli scale; it varies with distance, greatest near the epicentre.
Locating an epicentre
The distance to an epicentre is found from the gap between the P and S wave arrivals: because the P wave is faster, the longer they travel the further it pulls ahead, so a larger P-S gap means a greater distance. A travel-time graph converts the gap into a distance.
One station gives only a distance (a circle of possible locations), not a direction. Drawing a circle of that radius around three stations, the single point where all three circles intersect is the epicentre. This is triangulation.
Examples in context
Example 1. The shadow zones. Because S waves cannot pass through the liquid outer core, an S wave shadow zone forms on the far side of the Earth, which is one of the main pieces of evidence that the outer core is liquid.
Example 2. Comparing two quakes. A magnitude earthquake under a remote ocean may cause little damage (low intensity where no one lives), while a magnitude under a city may be devastating (high intensity), showing why magnitude and intensity are different measures.
Try this
Q1. Define the focus and the epicentre of an earthquake. [2 marks]
- Cue. The focus is the point at depth where the rock ruptures; the epicentre is the point on the surface directly above the focus.
Q2. State which seismic wave arrives first and one of its properties. [2 marks]
- Cue. The P wave arrives first; it is compressional (push-pull), the fastest, and travels through both solids and liquids.
Q3. Explain why three seismic stations are needed to locate an epicentre. [2 marks]
- Cue. Each station gives only a distance (a circle of possible positions), not a direction; the epicentre is where the three circles intersect (triangulation).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/03 20204 marksAt a seismic station, the P wave arrives 50 s before the S wave. Using a travel-time graph on which this P-S separation corresponds to a distance of 450 km, state the distance to the epicentre and explain how readings from three stations are used to find the epicentre exactly.Show worked answer →
Read the distance, then explain triangulation.
Distance from this station. The time gap between the P and S wave arrivals increases with distance, because the faster P wave pulls further ahead of the slower S wave the longer they travel. A gap corresponds, from the travel-time graph, to a distance of , so the epicentre lies from this station.
Triangulation. One station gives only a distance, not a direction, so the epicentre lies somewhere on a circle of radius around the station. Drawing such a circle for each of three stations, the single point where all three circles intersect is the epicentre.
Markers reward using the P-S gap to read the distance and the use of three circles intersecting at one point (triangulation) to locate the epicentre.
OCR H414/01 20194 marksExplain the difference between the magnitude and the intensity of an earthquake, and state why the moment magnitude scale is preferred to the Richter scale for large earthquakes.Show worked answer →
Define each measure, then justify moment magnitude.
Magnitude is a measure of the energy released at the source, calculated from the amplitude of the seismic waves on a seismogram. It has a single value for an earthquake.
Intensity is a measure of the effects of the earthquake at a particular place (damage to buildings, what people feel), measured on the Modified Mercalli scale. It varies from place to place, being greatest near the epicentre and decreasing with distance.
Why moment magnitude is preferred for large quakes. The Richter scale saturates: for very large earthquakes the amplitude it measures stops increasing in proportion to the true size, so it underestimates them. Moment magnitude is based on the seismic moment (the fault area, the slip and the rigidity of the rock), so it more accurately reflects the energy of the largest earthquakes.
Markers reward magnitude as energy at source (one value) versus intensity as effects (varying with place), and moment magnitude not saturating for large events.
Related dot points
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
- Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the contrasts between oceanic and continental crust; the seismic evidence for the layering (changes in wave velocity at boundaries such as the Moho); the P and S wave shadow zones as evidence for a liquid outer core; the link between the core and the Earth's magnetic field.
A focused answer to the OCR H414 dot point on Earth structure. Covers the crust, mantle, outer core and inner core, oceanic versus continental crust, the seismic evidence for the layering (velocity changes and the Moho), the P and S wave shadow zones as evidence for a liquid outer core, and the link between the core and the magnetic field.
- Plate tectonics: the development of the theory from continental drift (Wegener's evidence) through sea-floor spreading to plate tectonics; the evidence of palaeomagnetism and symmetrical magnetic striping at mid-ocean ridges; the increasing age of oceanic crust away from ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the OCR H414 dot point on the development of plate tectonics. Covers Wegener's continental drift evidence, sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, and the driving mechanisms of mantle convection, ridge push and slab pull.
- Earthquake hazards: the primary and secondary hazards (ground shaking, surface rupture, liquefaction, landslides and tsunamis); the distinction between hazard, vulnerability, exposure and risk; the factors that determine the impact of an earthquake (magnitude, depth, ground conditions, population density, building design and preparedness); monitoring and mitigation (building codes, land-use planning, early-warning systems and education); the limits of earthquake prediction.
A focused answer to the OCR H414 dot point on earthquake hazards. Covers primary and secondary hazards (shaking, surface rupture, liquefaction, landslides, tsunamis), the distinction between hazard, vulnerability, exposure and risk, the factors controlling impact, monitoring and mitigation, and the limits of earthquake prediction.
- Volcanism: the control of magma composition (silica content), viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions and andesitic or rhyolitic explosive eruptions; volcanic products (lava, tephra, pyroclastic flows and gases); volcanic landforms (shield volcanoes, stratovolcanoes, calderas and fissures); the link between volcanism and plate setting.
A focused answer to the OCR H414 dot point on volcanism. Covers how silica content, viscosity and gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, volcanic products (lava, tephra, pyroclastic flows, gases), the landforms (shield, stratovolcano, caldera, fissure), and the link to plate setting.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)