At 4.35 am on Saturday 4th September 2010, most of the South Island of New Zealand was awakened by shaking associated with a M7.1 earthquake located about 30 km west of Christchurch, about 10 km deep, and near the village of Darfield. The break occurred on a previously unknown fault, concealed beneath the thick gravels that blanket the Canterbury Plains. There were no deaths from this earthquake, in stark comparison with a event of similar magnitude in the Caribbean island of Haiti earlier in the year, that killed at least 220,000. Now all large earthquakes are attended by a sequence of aftershocks, themselves substancial earthquakes, that decay in a roughly exponential and predictable way that has often be likened to the reshuffling of a pack of cards or stack of dominoes. Basically, the stress field is readjusted and so breaks on neighbouring faults become a possibility.
By mid February 2011, the Canterbury region had experienced hundreds of aftershocks above M3.5 and the M6.3 event on 22nd February 2011 at 12.51pm was part of the aftershock sequence, albeit a big one (Figure 1). The epicentre (the point on the land surface immediately above the focal point of the rupture) was less than 10 km from the centre of Christchurch and a mere 5 km deep. The resulting energy release produced ground accelerations greater that 200% g (two times the acceleration due to gravity), way beyond those measured in the Darfield event. This, coupled with the proximity of the event to the city, produced the intense ground shaking, building collapse and the tragic loss of life. Christchurch, after Auckland, is New Zealandâ€™s second largest city, with a population of around 400,000. Many historic buildings, severly damaged in the September 2010 event, have now been damaged beyond repair. Modern structures, despite strict construction codes in New Zealand, have suffered too, with the Canterbury Television building collapsing with floor on floor.
Why does this happen? New Zealand straddles the tectonic plate boundary where the Pacific Plate collides with the Australian Plate (Figure 2). Beneath the North Island, the Pacific Plate is being subducted, meaning that it plunges under the over-riding Australian Plate and is returned or recycled into the deep Earth. This is why there are active volcanoes in the central North Island. In the South Island the plates collide and also slide past one another, with the Pacific Plate sliding to the south as shown in the accompanying map. These events are happening at measurable rates, which have been known for many years as a result of resurveying land, but which has been greatly enhanced by the advent of GPS (Global Positioning Satellite) technology. Hence we know that the lateral slip-rate in the South Island is round 20 – 40 mm per year but is and also pushing up the Southern Alps at about 10 mm per year. These are among the fastest rates recorded on Earth and this is why New Zealand is particularly prone to Â earthquakes.
This article was previously published in the March 2011 edition of UCC News (pdf).