Science. Communication. Community.
New Zealand’s fascinating geology exposes it to destructive earthquakes. On July 19 and 21 the country was hit by a 5.7 and 6.5 magnitude earthquake. Kiwis must rebuild once again.
Less than a month after the Earthquake Commission lifted the 2011 cordon around the Christchurch central business district, another powerful set of shakers hit New Zealand––magnitudes 5.7 and 6.5 300km north of Christchurch at Cook Strait. Though it is too early to predict the total cost, it is likely to be in the billions. And the government is already burdened with NZ$ 30 billion in estimated repairs for the 2011 quakes––the 7.1 Darfield mainshock and the February 6.3 Christchurch aftershock.
Until you see it in person, the scope of the devastation is hard to grasp. Before I visited Christchurch in 2012, I thought most of the damage would be repaired. What I saw was a different story. Entire neighborhoods were deserted. Slumped street lamps showed the impact of the quake in liquefaction zones. Shattered coastal roads remained closed. Otherwise accessible streets were lined with shipping containers to barricade houses against crumbing cliffs.
Though the Cook Strait quake was larger than Christchurch’s, the destruction was not as severe because the ground accelerations were much smaller––.2g compared to 2g––and the epicenter was farther away from the nearest city center. But the risk of damage from aftershocks continues: as of July 29th the probability of another magnitude 6 earthquake or greater in the next week is 6%. The probability of a magnitude 5 is over 40%. And the chance of a magnitude 4 is almost guaranteed.
New Zealand’s fascinating geology is at fault for all this violent activity. It sits in the deformation zone between the Australian and Pacific tectonic plates. On the southeast section of the north island, the dense Pacific Plate dives beneath the lighter Australian Plate. Within the south island, the plates slide past each other for 600 km along the Alpine fault––historically generating magnitude 8 ruptures. At the south island’s southern tip, the plate interaction changes again, and there the Australian Plate subducts beneath the Pacific Plate.
In its initial analysis, GeoNet––New Zealand’s official source for geo-hazard news–– stated that scientists think the first set of earthquakes leading up to the recent magnitude 5.7 quake occurred through a thrusting mechanism––where the Pacific Plate subducts beneath the Australian Plate––but that the 6.5 shock occurred through a transverse mechanism––two plates sliding past each other. With time, more analysis–and seismicity–will come.
Mother Nature is a powerful force in New Zealand. In the past year alone, there have been 2,959 earthquakes greater than magnitude 3, of which twenty-seven of these were greater than magnitude 5. This tectonic unrest is the price of having such a beautiful and unique landscape––where glacial mountains border beaches.