A 7.5 magnitude quake struck New Zealand just after midnight on Sunday, 13th November. Since then, thousands of powerful aftershocks of varying intensity and proximity to the East of the South Island coast have continued to shake the country, with a 5.9 strength quake striking just the other day (22nd November). All these measurements are based on the Richter Scale, the most common measurement of earthquake magnitude. The fundamental point of this particular scale is that it is logarithmic, meaning that Sunday’s quake was twenty times the size of the recent aftershocks, and at least ten times the size of the disastrous 2011 Christchurch quake which measured 6.3.
So why is Christchurch still licking the wounds and remembering the 185 fatalities some 6 years on, when this week’s quakes have been exponentially stronger yet only incurred 2 deaths? Well, the focus (the exact point below the earth’s surface where the earthquake occurs) of this year’s tremor was about 90km north-east of Christchurch, out to sea, and it was by purely luck that the bulk of the energy exerted didn’t strike a densely-populated area. The focus being out to sea isn’t without its faults as a tsunami warning was prompted for much of the South Island’s East coast. Naturally, the shifting of tectonic plates and the release of extreme amounts of energy are likely to shift a bit of water (e.g. Boxing Day Tsunami, 2004).
New Zealand is part of the tectonically active and unpredictable Pacific Ring of Fire. This I’m sure is something you might have touched on in GCSE Geography; a 40,000km long seismic horseshoe shaped outline to the Pacific Ocean. It is believed that 90% of earth’s seismic activity happens along this boundary, on account of 75% of the planet’s active volcanoes being located there. It seems unfair to have all the volcanoes located at the edge of the ocean and near the land doesn’t it? But that’s precisely why and how they are there; the Pacific Ring of Fire is a result of differing tectonic plates (making up the Earth’s crust), sat on top of the earth’s mantle (a comparatively thick layer of solid and molten rock). With the mantle being partially liquid, it comes as no surprise that the tectonic plates tend to bump and grind – the results of these might be an earthquake, or the formation of a trench or volcano.
Tectonic plates are either oceanic or continental and have either convergent or divergent boundaries (clue is in the name) depending on the combination of plate types. New Zealand is home to one of the most active volcanoes in the world (Mount Ruapehu) and is subject to seismicity due to its location on the boundary between the Australasian and Pacific plates. As can be seen from the diagram, the plate boundary is convergent along the east coast of the North Island, then becomes a transform boundary further south. This means that the two plates are moving towards each other in the North Island, then move horizontally past each other. This movement is not always smooth nor is it quick: the plates sometimes get stuck, hence the expulsion of energy. Though earthquakes are commonplace in locations around the Ring of Fire, this was particularly sizeable due to the shallow depth of the focus – it is believed to only have been 10km deep.
Earthquakes are not just a one hit wonder: aftershocks are inevitable and some may even be of a similar magnitude to the initial quake. Many of the aftershocks of this quake have rivalled the 2011 Christchurch quake in size, but again, due to their location and the preparedness of the nation, the damage has not been as devastating. The shaking also provokes landslides, flooding and damage to infrastructure, and just exacerbates any further impacts on the area – the week following the earthquake, New Zealand was deluged, hindering the relief process.
Kaikoura is an infamous tourist location on the East Coast of the South Island, popular for its whale-watching and swimming with dolphins but has been the worst affected location, with access routes blocked and transport systems impacted due to ‘thousands’ of landslides and flooding. The release of energy provoked a surge of water to breach a slip dam on the Clarence River (Northwest of Kaikoura) (see image) and Marlborough District Council issued warnings to residents to flee to higher ground.
Aid to Kaikoura has been administered by helicopter and boats. This was the nearest town to the epicentre (the point on the earth’s surface directly above the focus), despite still being some 90km away. Effects were even felt some 200km away in the capital city Wellington, where power lines were damaged. It is thought that full access to the town will take ‘several months’ to be established with billions of dollars of damage caused. Much of the surrounding countryside was also destroyed; three cows managed to find themselves stranded on a patch of high ground before their farmer forged them a route to safety.
Similarly, despite widespread damage and destruction to the man-made environment, there have been serious impacts on the wildlife in the affected area. Plate movement caused a dramatic rise in the seabed of about 2m, causing a type of mollusc (paua) to be stranded. At this point in time, resources are not enough and it is still deemed too dangerous to fathom going out to sea to check the valuable whale population. Similarly, with response efforts focussed on infrastructure and human welfare, the Department of Conservation has been unable to check local wildlife, despite evidence of damage to seabird, seal and penguin habitats. Large landslides have wiped out all important breeding grounds and jeopardised the populations of the New Zealand fur seal and the already protected Hutton’s Shearwater species. It is also believed that the world’s smallest and rarest dolphin, the Hector’s dolphin is at risk as they only exist in a small area near to Kaikoura.
A fault line cut clean across some parts of the country, with a house in Kekerengu being moved off its foundations and the driveway metres away from where it originally was.
We can only hope that in the future, New Zealand and other nations that are affected by seismic activity are equipped to cope. In this case, it seemed that the nation had extremely efficient response by the Defence Force and suitable warnings issued; however, many of the poorer nations of our world are also in the danger zone of natural disasters. It is also important to recognise the wider implications of natural disasters, as often they are not just humanitarian.
(Image courtesy of BBC)