Explain how earthquakes generate seismic waves and distinguish magnitude from intensity

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.

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.