Isaac Davison writes in the New Zealand Herald about how hard rock near the epicentre of Tuesday’s Christchurch earthquake may have made the situation worse, by reflecting more seismic activity towards the city.
While earthquake waves can be dispersed or absorbed by some types of material, they are also deflected by hard surfaces, such as the the rock close to Lyttelton. Such effects could also help explain why the destruction in the city is so uneven.
An excerpt: (read in full here)
“Geologist Hamish Campbell of GNS Science said seismic activity travels in waves, so it could be compressed, refracted or reflected like an optical waveform.
“While the waves could be dispersed or absorbed by some properties, they could also rebound off hard surfaces.
“”We suspect that the epicentre was probably on the north side of the Port Hills, where a huge amount of energy would have been literally ‘pinged’ off the basalt rock, almost behaving like a mirror,” said Dr Campbell.
“”You can imagine an explosion going off and energy going out towards Christchurch city, but a lot of energy also hitting the hard rock at depth, then being reflected, bounced back and compounding the effect.”
“This event, called seismic lensing, could explain the hotch-potch damage to the city and suburbs.”