Map of quake shaking intensity in Christchurch
THE EARTHQUAKE THAT HIT Christchurch on 22 February was not classed as a massive earthquake (6.3 on the Richter scale), but its effects were devastating. At least one-third of the city centre was reduced to rubble and while the death toll remains officially at 166, officials expect it to reach 200.
This earthquake’s effects were much worse than the September 7.1-maginitude quake because its epicentre was so close to the centre of Christchurch, and because it was so shallow – just 5 km underground. This meant that so much of the energy from the quake was unleashed on Christchurch, and not much dampened by the limited amount of Earth’s crust it travelled through.
This natural-colour image (above) from NASA’s Earth Observatory, taken by a satellite on 4 March, is overlaid with geological data that shows which parts of Christchurch experienced the worst ground shaking. The darker the red colour for the circles, the worse the ground-shaking. As anecdotally reported, the city centre was one of the areas to shake the most.
When an earthquake hits, two main types of energy are unleashed: horizonal and vertical energy waves. Like a ripple in a pond, these waves travel out from the epicentre in all directions:
* ‘P waves’, known as ‘primary waves’ arrive first and cause a vertial jolt. This is often announced with what sounds like a loud explosion – much like a sonic boom – as the upward-travelling energy reaches the surface.
* ‘S waves’, or ‘secondary waves’ arrive afterwards (they travel about twice as slow as P waves) and cause the damaging side-to-side shaking that makes buildings sway, roads crack and liquifaction to occur.
The further away a place is from the epicentre, the greater the time lag between these waves, and scientists use this time lag to work backwards and pinpoint the location of the epicentre.
In the case of Christchurch, the time lag between P and S waves was merely one second.