Earthquakes, seismic waves, magnitude and intensity: WACE Year 12 Earth and Environmental Science
“Explain how earthquakes generate seismic waves and distinguish magnitude from intensity”
Earthquakes happen when stress stored in rock along a fault is suddenly released as the rock slips, sending out seismic waves. P waves travel fastest and arrive first, S waves follow, and surface waves cause the most damage. Magnitude measures the energy released at the source and has one value per earthquake; intensity measures the shaking felt at a place and varies with distance, depth and ground conditions.
What this dot point is asking
SCSA wants you to explain how earthquakes generate waves and to clearly distinguish magnitude from intensity, a distinction examiners test repeatedly. Get the focus and epicentre, the wave types, and the two measurement scales straight.
How earthquakes happen
Tectonic stress builds up in rock along a fault as plates move. The rock deforms elastically until the stress exceeds its strength, then it suddenly slips and snaps back, releasing energy. This is elastic rebound.
- The focus (hypocentre) is the point underground where rupture begins.
- The epicentre is the point on the surface directly above the focus.
- The released energy radiates outward as seismic waves.
Types of seismic wave
- P waves (primary) are compressional, travel fastest, arrive first, and pass through solids and liquids.
- S waves (secondary) are slower shear waves that arrive second and travel only through solids.
- Surface waves travel along the ground, arrive last, and cause most of the damage because of their large, rolling motion.
Because P waves outrun S waves, the time gap between their arrivals at a station shows how far away the earthquake was. Combining distances from three or more stations pinpoints the epicentre, the basis of how seismologists locate earthquakes.
Magnitude versus intensity
This is the key distinction.
- Magnitude measures the energy released at the source. An earthquake has a single magnitude. The modern moment magnitude scale is preferred over the older Richter scale for large quakes. It is logarithmic, so each whole step is about ten times more ground shaking and roughly thirty times more energy.
- Intensity measures how strongly shaking is felt at a particular place, based on observed effects. It is described by the Modified Mercalli scale and has many values for one earthquake, decreasing with distance from the epicentre.
Focus: the underground point where rupture begins. Epicentre: the point on the surface above the focus. Magnitude: energy released at the source, one value per quake. Intensity: strength of shaking felt at a place, many values. Moment magnitude: the modern magnitude scale. Modified Mercalli scale: the intensity scale based on observed effects.
What controls the shaking felt
Even at equal magnitude, the damage at a site depends on several factors.
- Distance from the epicentre: shaking weakens with distance.
- Focal depth: shallow earthquakes shake the surface harder than deep ones.
- Ground conditions: soft sediment amplifies shaking and can liquefy, while solid bedrock shakes less.
- Building construction: poorly built structures fail at lower intensity.
Australia, in a plate interior, has fewer earthquakes than plate boundaries, but it does experience them; the 1968 Meckering earthquake in WA caused significant damage despite the continent's relative stability.
If asked why a smaller-magnitude earthquake caused more damage than a larger one, separate magnitude from intensity. The smaller quake may have been shallower, closer to a populated area, on soft ground that amplified shaking, or affected weaker buildings, all of which raise intensity even though magnitude (energy at the source) was lower.
Liquefaction and the role of ground conditions
One of the most important controls on earthquake damage is the ground a building sits on, and liquefaction is the dramatic case. When loose, water-saturated sediment is shaken, the grains lose contact with one another and the water pressure between them rises, so the soil temporarily behaves like a liquid. Buildings can tilt or sink, underground pipes float upward, and roads buckle, even though the structures themselves were never directly broken by the wave. This explains why two identical buildings the same distance from an epicentre can fare very differently: one on solid bedrock may shake mildly while one on reclaimed land or river sediment suffers severe damage. The lesson SCSA draws out is that intensity, and therefore risk, is as much about site geology and engineering as about the earthquake's magnitude, which is why hazard mapping and building codes target ground conditions specifically.
- Confusing magnitude and intensity
- Magnitude is energy at the source (one value); intensity is felt shaking (many values).
- Treating the scales as linear
- Magnitude scales are logarithmic; each step is about tenfold more shaking.
- Mislabelling waves
- P waves are fast compressional, S waves slower shear, surface waves most damaging.
- Mixing focus and epicentre
- The focus is underground; the epicentre is on the surface above it.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksAt a seismic station the P wave arrived 35 seconds before the S wave. Given P waves travel at about 8 km/s and S waves at about 4.5 km/s through the crust, outline how this time gap is used to find the distance to the earthquake, and explain why readings from at least three stations are needed to locate the epicentre.Show worked answer →
A 6 mark answer rewards the distance logic and the need for triangulation.
Using the gap. P waves outrun S waves, so the longer the gap between their arrivals, the further away the earthquake. The 35 second gap arises because the slower S wave falls progressively behind over distance; the size of the gap can be read off a travel-time graph to give the distance from that station to the earthquake (a single distance, not a direction).
Why three stations. One station gives only a distance, which places the earthquake somewhere on a circle of that radius around the station. Two circles intersect at two points; a third station's circle resolves which point is correct, pinpointing the epicentre. This is triangulation.
Markers reward the larger-gap-means-greater-distance reasoning and the explanation that a single station gives distance but not direction, so three circles are needed to fix the epicentre.
WACE 2020-style7 marksDistinguish between the magnitude and intensity of an earthquake, and explain why a magnitude 5.5 earthquake might cause more damage than a magnitude 6.5 earthquake.Show worked answer →
A 7 mark answer needs a precise distinction plus a multi-factor explanation.
Distinction. Magnitude measures the energy released at the source and has a single value per earthquake (moment magnitude scale, logarithmic, so each whole step is about ten times more shaking). Intensity measures the strength of shaking felt at a particular place (Modified Mercalli scale) and has many values for one earthquake, decreasing with distance.
Why the smaller quake can do more damage. Damage depends on intensity at the site, not just source energy. A magnitude 5.5 quake could cause more damage if it was shallower (less energy lost before reaching the surface), closer to a populated area, on soft sediment that amplifies shaking or liquefies, or affected poorly constructed buildings. A magnitude 6.5 quake that is deep, remote, on bedrock and away from people may shake populated areas only weakly.
Markers reward the energy-at-source versus felt-shaking distinction and at least two factors (depth, distance, ground conditions, construction) raising intensity for the smaller quake.