Big Shakes on the Shaky Isles

The recent devastating earthquake in New Zealand was part of a series of aftershocks from an earlier event, writes Prof. John Gamble and Dr. Pat Meere from the School of BEES.

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.

Figure1: Map showing location of the Christchurch M6.3 earthquake. The green triangles give locations of permanently deployed seismic array. The blue lines are known traces of active faults in the northern South Island. The yellow and white “beach ball” gives the fault plane solution, showing that slip on the fault was predominantly ‘contractional’ in nature resulting in crustal shortening in the vicinity of the earthquake.

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 Zealand’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.

Figure 2. Location map showing the plate tectonic configuration of New Zealand. The Pacific Plate is moving west at a rate varying from ~60 mm per year in the north to ~ 30 mm per year in the south, as shown by black arrows. The red lines with barbs (in the north) delineate the plate boundary, where the Pacific Plate is subducted beneath the Australian Plate (region with barbs). Note that here the black arrows are more or less orthogonal to the red line. To the south, the direction of the arrows becomes increasingly oblique as the plate boundary comes on-land and subduction gives way to transcurrent (lateral) movement.

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  earthquakes.

Thanks to GNS http://www.gns.cri.nz/, the Auckland Herald, Dominion-Post and TVNZ http://tvnz.co.nz/

This article was previously published in the March 2011 edition of UCC News (pdf).

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