Yesterday’s Seddon 6.5 magnitude earthquake

Map showing the epicentres of the recent quakes (Source: Geonet)

Map showing the epicentres of the recent quakes (Source: Geonet)

Yesterday, at 5.09 in the afternoon, a magnitude 6.5 earthquake struck in Cook Strait, between the North and South islands. It was felt strongly in central New Zealand, sending people under doorways and tables, but fortunately only four people have been reported injured. There has been some damage, especially to buildings in Wellington’s central business district and around Wellington’s wharves. Workers have been advised to stay out of Wellington’s CBD today, a building in Blenheim has been evacuated, and residents of Seddon are advised to boil their water.

This earthquake – which was centred 20 kilometres east of Seddon in Marlborough (south-east of Blenheim), at a depth of 17 kilometres – was the largest of a swarm of quakes that are being referred to as the ‘Seddon sequences‘. The earthquakes are still continuing, but the probability of another large aftershock decreases with time. The earthquakes are mainly centred on an area where the Australian and Pacific plates meet. Even though under the sea, the earthquake wasn’t big enough to trigger a tsunami – they usually have to be a magnitude 7.0 or greater. However, it was certainly felt by the people on the inter-island ferry that was in the area at the time, with the captain at first thinking they might have hit something.

The ‘Seddon sequence’ began with a magnitude 5.7 just after 9 on Friday morning. Te Ara staff experienced their first earthquake on the 13th floor of their new building and, like other high-rise office workers, found the swaying rather disconcerting. I was quite pleased to still be at home in my wooden Edwardian house on the hill, comforted by the fact that wooden houses tend to stand up very well in earthquakes.

1848 Marlborough earthquake

This was a fact first noted by Wellington’s early settlers in 1848, when they experienced an earthquake estimated at magnitude 7.5, centred on the Awatere Valley in Marlborough (which is also where Seddon is). When the citizens of Wellington rebuilt, noting that most of the brick and stone buildings had suffered damage, many rebuilt with timber, leading to the distinctive wooden architecture the city has today.

1855 Wairarapa earthquake

Probably the best-known earthquake to have shaken Wellington is the 1855 Wairarapa earthquake, which was centred on a fault in Palliser Bay. At magnitude 8.2, it is the largest recorded earthquake in New Zealand. It caused considerable damage in Wellington and elsewhere in central New Zealand, and between five and nine people died. It also changed the geography of Wellington considerably, lifting up the land around Wellington’s waterfront. Much of Wellington’s CBD is now on land that was previously under the harbour. The advantage was more land, but a disadvantage is that the land is less stable, and much of the reported damage from yesterday’s quake seems to be in that area.

1942 Wairarapa earthquakes

The most recent large quakes to affect central New Zealand were in 1942, during the Second World War. Two powerful (7.2 and 6.8) earthquakes, centred near Masterton, shook the lower North Island. Masterton had the most damage and, similar to yesterday’s quake, some buildings in Wellington’s CBD were also damaged. Also similar to yesterday, both were outside of usual business hours, which likely meant there were fewer injuries, and perhaps deaths, than might otherwise have been the case. There was only one fatality – a man in Wellington was killed by coal gas escaping from a fractured pipe.

Further information

For information about what causes earthquakes, how they are measured and  seismic activity in New Zealand, read Te Ara’s entry on Earthquakes.

For more about past earthquakes in New Zealand, check out our Historic earthquakes entry.

If you’re visually inclined, check out our fantastic infographic: The shaky isles: Canterbury & other quakes.

GeoNet and GNS are the go-to people for information about the current earthquakes. This blog post gives good information about what’s going on: A weekend of Earthquakes - what is going on???; Preliminary science from the Seddon sequences also looks at the science of these quakes, and the New Zealand Herald’s live chat with GNS scientist John Ristau will answer a lot of your questions.

Civil Defence’s Get Ready, Get Thru website has good information about what to do in an earthquake.

Wellington City Council is posting updates on its website http://wellington.govt.nz/, and also on its Wellington Region Emergency Management Office Facebook page.

Feel free to add other useful links below. And, if you’re living anywhere in an earthquake risk zone, it’s a good time to get your emergency kit ready. Be safe.

Update: Looks like GNS is now calling the earthquakes the ‘Cook Strait earthquakes’, and have set up an information page, which they’ll be updating regularly: http://info.geonet.org.nz/display/home/Cook+Strait+Earthquakes

One comment added so far

  1. Comment made by Johanna Knox || July 24th, 2013

    Great round up, Helen!

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