How are earthquakes generated and measured, and how do we locate the epicentre?
Earthquakes and seismic waves: the focus and epicentre; the elastic rebound mechanism; the P, S and surface waves and their properties; the difference between magnitude (the logarithmic Richter scale and its saturation, and the moment magnitude scale) and intensity (the Modified Mercalli scale); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the Eduqas Geology statement on earthquakes. Covers the focus and epicentre, the elastic rebound mechanism, P, S and surface waves and their properties, the difference between magnitude (Richter saturation and moment magnitude) and intensity (Modified Mercalli), a worked example using the P-S travel-time gap, and how triangulation from three stations locates an epicentre.
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What this dot point is asking
Eduqas wants you to define the focus and epicentre, explain the elastic rebound mechanism, describe the P, S and surface waves and their properties, distinguish magnitude (the logarithmic Richter scale, its saturation, and the moment magnitude scale) from intensity (the Modified Mercalli scale), and use P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation. The same wave physics underpins the evidence for the Earth's internal structure, so this statement ties the whole module together.
The answer
Focus, epicentre and elastic rebound
Earthquakes are generated by elastic rebound. Tectonic stress builds up across a locked fault, deforming the rock elastically (storing strain energy, like a bent ruler). When the stress exceeds the strength of the rock or the friction locking the fault, the fault suddenly slips and the rock springs back towards its undeformed shape, releasing the stored energy as seismic waves that radiate out from the focus.
Seismic waves
Three types of wave radiate from the focus:
- P waves (primary). Compressional (push-pull) body waves; the fastest, so they arrive first. They travel through solids and liquids.
- S waves (secondary). Shear (side-to-side) body waves; slower than P waves, so they arrive second. They travel through solids only, because a liquid has no rigidity and cannot resist shear.
- Surface waves (Love and Rayleigh). Travel along the surface; the slowest but with the largest amplitude, so they cause the most damage to buildings.
The fact that S waves cannot pass through liquid is the key to the shadow zones that prove the outer core is liquid (covered in the Earth-structure statement).
Magnitude versus intensity
These are two different ideas that examiners deliberately test.
- Magnitude measures the energy released at the source, from the amplitude recorded on a seismogram. It is one value per earthquake and the scales are logarithmic, so each whole-number step is about times the ground amplitude and about times the energy.
- The Richter (local magnitude) scale works well for small and moderate earthquakes but saturates for very large ones: the amplitude it measures stops growing in proportion to the true size, so it underestimates great earthquakes.
- The moment magnitude (Mw) scale 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 particular place (felt shaking, damage), recorded on the Modified Mercalli scale. It varies with location, being greatest near the epicentre and falling off with distance.
Locating an epicentre
The distance from a station to the epicentre is found from the gap between the P and S wave arrival times. Because the P wave travels faster, the longer the two waves travel the further the P wave pulls ahead, so a larger P-S gap means a greater distance. A travel-time graph converts the measured gap into a distance.
One station gives only a distance, not a direction, so the epicentre lies somewhere on a circle of that radius around the station. Drawing a circle of the appropriate radius around three stations, the single point where all three circles intersect is the epicentre. This method is called 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 from an earthquake, which is one of the central pieces of evidence that the outer core is liquid.
Example 2. Comparing two earthquakes. A magnitude earthquake beneath a remote ocean may cause little damage (low intensity where no one lives), while a magnitude beneath a city may be devastating (high intensity), which is exactly why magnitude and intensity are kept as separate measures.
Try this
Q1. Define the focus and the epicentre of an earthquake, and name the mechanism that generates earthquakes. [3 marks]
- Cue. The focus is the point at depth where the rock ruptures; the epicentre is the point on the surface directly above it; the mechanism is elastic rebound, where strain builds across a locked fault until it suddenly slips.
Q2. State which seismic wave arrives first and give two of its properties. [3 marks]
- Cue. The P wave arrives first; it is compressional (push-pull), the fastest of the waves, 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 the single point where the three circles intersect (triangulation).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20205 marksAt a seismic station the P wave arrives 70 seconds before the S wave. A travel-time graph shows that a P-S separation of 70 seconds corresponds to an epicentral distance of 630 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 from the gap, then explain triangulation.
Distance from this station. The time gap between the P and S arrivals grows with distance, because the faster P wave pulls further ahead of the slower S wave the longer they both travel. From the travel-time graph, a gap corresponds to , 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 drawn 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 off the distance () and the point that three circles drawn around three stations intersect at one point (triangulation) to fix the epicentre.
Eduqas 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 very large earthquakes.Show worked answer →
Define each measure clearly, then justify the moment magnitude scale.
Magnitude is a measure of the energy released at the source of the earthquake, calculated from the amplitude of the seismic waves on a seismogram. It has a single value for a given earthquake.
Intensity is a measure of the effects of the earthquake at a particular place (the shaking felt, the damage to buildings), recorded 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 wave amplitude it measures stops increasing in proportion to the true size, so it underestimates them. The moment magnitude scale is based on the seismic moment (the fault rupture area, the amount of slip and the rigidity of the rock), so it reflects the energy of even the largest earthquakes accurately and does not saturate.
Markers reward magnitude as energy at the source (one value) versus intensity as effects (varying with place), and moment magnitude not saturating for the largest events.
Related dot points
- Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the mechanical layers (lithosphere and asthenosphere); the seismic evidence for the layering from changes in P and S wave velocity at boundaries such as the Moho; the P and S wave shadow zones as evidence for a liquid outer core; the use of meteorites and density as evidence for the composition of the core and mantle.
A focused answer to the Eduqas Geology statement on Earth structure. Covers the crust, mantle, outer and inner core, the lithosphere and asthenosphere, the seismic evidence from P and S wave velocity changes and the Moho, the shadow zones proving a liquid outer core, the meteorite analogy for the core, and locating an earthquake epicentre from the P-S travel-time gap.
- Plate margins and their features: the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-ocean island arcs, ocean-continent margins and continent-continent collision); the Benioff zone, subduction and decompression melting; the diagnostic rocks, structures, earthquakes and volcanoes of each margin type.
A focused answer to the Eduqas Geology statement on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-ocean, ocean-continent and continent-continent sub-types, the Benioff zone, subduction and decompression melting, and the diagnostic rocks, structures, earthquakes and volcanoes that identify each margin in the exam.
- Plate tectonics theory and evidence: the development of the theory from Wegener's continental drift, through Hess's sea-floor spreading, to plate tectonics; the evidence of palaeomagnetism and the symmetrical magnetic striping of the ocean floor; the increasing age of oceanic crust away from mid-ocean ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the Eduqas Geology statement on the development of plate tectonics. Covers Wegener's continental drift evidence and why it was rejected, Hess's sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, a worked spreading-rate calculation, and the driving forces of mantle convection, ridge push and slab pull.
- Volcanic activity and eruption styles: the control of silica content, viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions (shield volcanoes and fissures) and andesitic or rhyolitic explosive eruptions (stratovolcanoes, pyroclastic flows and calderas); the volcanic products (lava, tephra and pyroclastic material); the link between eruption style and plate setting.
A focused answer to the Eduqas Geology statement on volcanic activity. Covers how silica content, viscosity and dissolved gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, the products (lava, tephra and pyroclastic material), the landforms (shield, stratovolcano, caldera and fissure), and the link to plate setting.
- The lithosphere, mantle plumes and hotspots: the structure and composition of oceanic and continental lithosphere; the principle of isostasy and isostatic adjustment; mantle plumes and hotspots as a cause of intraplate volcanism; hotspot tracks and the age progression of volcanic island chains (for example Hawaii); the basis of the Component 3 geology of the lithosphere option.
A focused answer to the Eduqas Geology statement on the lithosphere, mantle plumes and hotspots. Covers oceanic and continental lithosphere, the principle of isostasy and isostatic rebound, a worked density calculation, mantle plumes and hotspots as a cause of intraplate volcanism, hotspot tracks and the age progression of island chains such as Hawaii, and the Component 3 lithosphere option.
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)