How do earthquakes generate seismic waves, and how do magnitude and intensity differ?
Explain how earthquakes generate seismic waves and distinguish magnitude from intensity
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on earthquakes. Covers elastic rebound, focus and epicentre, P, S and surface waves, the moment magnitude scale, the Mercalli intensity scale, and the factors that affect shaking, with Australian context.
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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.
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.
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.