Monday, November 14, 2016

Earthquakes in New Zealand: Some questions

A powerful 7.8-magnitude earthquake killed two people and caused massive infrastructure damage in New Zealand on Monday. Here are some facts about the seismically active nation known as the Shaky Isles:
Why so few casualties?

The key was distance from major population areas. In a devastating 2011 quake in nearby Christchurch, which left 185 dead, the shallow epicentre was very close to the city centre, causing large buildings to collapse which accounted for many of those who died. In contrast, Monday’s quake was centred in a rural area around halfway between Christchurch and Wellington. Seismologists said the 2011 disaster also had more powerful high-frequency energy, lasting barely 10 seconds, while Monday’s quake, despite having a higher magnitude, was deeper and of a lower frequency. This saw the energy released more slowly over a longer period – up to two minutes – as it travelled in a “rolling” motion along an estimated 200km  fault line.

So where does this quake rank?
The quake, near Kaikoura, was measured at 7.8 by the US Geological Survey and 7.5 by New Zealand’s official GeoNet service. Regardless of which figure is used, it still ranks as one of the largest ever recorded in the South Pacific nation. The biggest was an 8.2 monster that hit Wellington in 1855. The deadliest was a 7.8 tremor in Hawke’s Bay in 1931 that claimed 256 lives. Using USGS’s 7.8 figure, Monday’s quake was the joint second strongest in New Zealand. GeoNet’s 7.5 magnitude ranks it jointly as the sixth most powerful on record.

How does NZ cope with so many quakes?

New Zealand has strict building compliance codes that aim to limit quake damage and keep casualties to a minimum. Modern buildings such as “The Beehive” parliamentary complex in Wellington have complex foundation systems incorporating huge rubber blocks that allow it to shake without sustaining structural damage. Residential houses are generally built of timber, which flexes and bends in a quake, rather than collapsing like masonry. The system failed in the 2011 Christchurch quake simply because the fault line which caused the tremor was unknown before it ruptured, so authorities thought the city was relatively stable and did not impose the strict conditions used elsewhere.

Can New Zealand expect ‘the big one’?
New Zealand sits on the boundary of the Australian and Pacific tectonic plates, which form part of the so-called “Ring of Fire”, and experiences up to 15,000 tremors a year. The most powerful to date was the 8.2-magnitude disaster in Wellington in 1855 that caused four deaths and changed the city’s entire geography, pushing the shoreline out 200 metres  as it thrust the harbour floor upwards. New Zealand’s government-run GNS Science says on its website that the Alpine Fault, which runs for about 600 kilometres up the spine of the South Island, has a high probability of rupturing in the next 50 years. This will produce one of the biggest earthquakes since European settlement. It has only ruptured four times in the past 900 years, each time producing an earthquake of about magnitude 8.0.
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