Quick questions on Earthquakes and plate tectonics: HSC Earth and Environmental Science Module 6

The elastic rebound theory explains the release. Rock on either side of a locked fault bends elastically as the plates move, storing strain energy like a compressed spring. When stress exceeds the rock's strength, the fault ruptures, the rock snaps back toward its original shape, and the stored energy radiates outward as seismic waves. The point of rupture at depth is the focus (or hypocentre); the point on the surface directly above it is the epicentre.

Seismic energy travels as several wave types. Primary (P) waves are the fastest; they are compressional, push-and-pull waves that travel through solids and liquids and arrive first. Secondary (S) waves are slower shear waves that move rock side to side and cannot pass through liquids, which is why the liquid outer core casts an S-wave shadow. Surface waves travel along the ground surface, move more slowly still, and cause most of the destructive shaking.

Australia lies near the centre of the Indo-Australian Plate, so it does not experience boundary earthquakes. It does, however, have intraplate earthquakes caused by stresses transmitted across the plate from distant boundaries. These are less frequent but can still be damaging because Australian cities are not always built to high seismic standards. The 1989 Newcastle earthquake, magnitude 5.6, killed 13 people and caused major damage to unreinforced masonry buildings, prompting changes to Australian building codes.

Explain how the difference in arrival times of P and S waves at a seismograph is used to locate an earthquake. [3 marks]

Account for the high damage caused by the 1989 Newcastle earthquake despite its moderate magnitude. [4 marks]