Darfield Earthquake and Aftershocks

The most disruptive earthquake to strike New Zealand since the 1931 Hawke’s Bay earthquake struck the Canterbury region of the South Island this morning.

Saturday 4th September 2010

This morning’s magnitude 7.1 earthquake, which struck close to the town of Darfield, is the most disruptive earthquake to strike New Zealand since the earthquake which struck Hawke’s Bay in 1931.

The Hawke’s Bay quake of the 3rd of February 1931 was magnitude 7.8 with an associated quake of magnitude 7.3 ten days later.

Since then, New Zealand territory had experienced fifteen earthquakes of seventh magnitude prior to this morning’s event. The quakes were:

Pahiatua, March 1934, magnitude 7.6
Wairarapa, June 1942, magnitude 7.2
Wairarapa, August 1942, magnitude 7.0
south of the South Island, February 1950, magnitude 7
south of the South Island, August 1950, magnitude 7.3
Bay of Plenty, September 1953, magnitude 7.1
Fiordland, May 1960, magnitude 7.0
Inangahua May 1968, magnitude 7.1
southern Kermadec Islands, September 1985, magnitude 7.0
East Cape, February 1995, magnitude 7.0
Fiordland, August 2003, magnitude 7.1
Puysegur Trench, November 2004, magnitude 7.2
Kermadec Islands, May 2006, magnitude 7.4
Auckland Islands, September 2007, magnitude 7.3
Dusky Sound, July 2009, magnitude 7.8

Seven of the events were located offshore at a distance from New Zealand’s main islands. The largest of the eight onshore events were the magnitude 7.6 Pahiatua event of 1934 and the magnitude 7.8 Dusky Sound event of 2009. Both quakes were not close to major centres of population.

The magnitude 7.1 Inangahua quake of 1968 was the most significant of modern times in terms of damage to property and deaths – it killed three people.

This morning’s earthquake was magnitude 7.1 at the shallow depth of 10 km. It was centred 10 km west of the Canterbury Plains town of Darfield, 40 km west of Christchurch. The quake occurred on a previously unknown fault, buried beneath the gravels and sediments deposited over millennia on the Canterbury Plains.

Whilst an earthquake was not anticipated at this particular epicentre, the damage to Christchurch infrastructure that has occurred was expected from a major earthquake on the Alpine Fault further inland to the west.

Twenty-one aftershocks since the main quake had been analysed by GeoNet by four o’clock this afternoon, and there have been several others. Three quakes were magnitude 5, fifteen were magnitude 4 and three were magnitude 3.

The three largest events were a magnitude 5.3 quake at 4:56 a.m., a magnitude 5.2 event at 7:56 a.m. and a magnitude 5.3 shock at 11:12 a.m.

It is possible that an aftershock of about 6th magnitude will occur if the aftershock sequence follows the trend of other shallow major quakes in similar locations.

As reports from the wider region begin to come in, liquefaction of soft soils after a wet winter seems to be a region-wide problem. The Canterbury town of Kaiapoi has experienced significant liquefaction, the soils acting like soft mud during the intense shaking caused by the main earthquake. A report of a building sinking a metre into liquefied soil at Kaiapoi has yet to be confirmed.

Christchurch CBD has been closed to all but residents while building damage is assessed. Plans are underway to move the 8,000 residents out of the inner city until fresh and waste water services and electricity supplies can be re-established..

Earthquakes of magnitude seven have caused damage in Canterbury before today’s event. On the 1st of September 1888 a magnitude 7.1 quake in North Canterbury caused damage in the city and damaged the cathedral spire. The Arthur’s Pass quake of the 9th of March 1929, which caused 17 deaths, was also magnitude 7.1 and caused damage throughout the region.

Two smaller quakes of magnitude 6.7 and 5.9 in June 1994 were centred about 40 km north-west of this morning’s event, but did not cause significant damage in the area.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST; and the US Geological Survey and its contributing agencies.]

7 Responses to “Darfield Earthquake and Aftershocks”

  1. Lizzie from Gizzie says:

    So pleased I wasn’t there, feel sorry for the people that were. Also so pleased to read your reports.

  2. Denise Andrews says:

    Good morning – this report and links are the most informative I’ve found on this subject. I live in d’Entrecausteaux Channel in Tasmania and, on 3rd Sept., was chatting to my sister on Skype when I made the following observation
    ‘[3/09/2010 3:59:23 PM] Denise Andrews: Just looking outside – the sea is a bit messy, although there’s no wind, and the waves breaking on the beach are huge. Wonder if there’s some ‘message’ there…..
    [3/09/2010 3:59:46 PM] Denise Andrews: Tsunami??
    [3/09/2010 3:59:59 PM] Denise Andrews: Thank goodness I’m very high up on a cliff.
    Since moving here, I’ve always been interested in the Puysegur trench and the subjects of earthquakes in New Zealand, looking at the proximity to our area. I’ve looked on Google Earth and Darfield is almost parallel.
    Do you think my observation could have had any relation to the occurence in NZ?

  3. Ken says:

    Thanks for your comments, Denise.

    The Darfield quake wasn’t expected to generate a sea disturbance because it was a good distance inland. To have done so, it would have had to trigger an undersea landslide nearby.

    I kept a close eye on GeoNet’s tsunami gauges after the quake just in case a landslide had occurred, and no appreciable sea level variation was seen. (There’s a link to the tsunami gauges on the side panel of my website).

    There has been a succession of high and low pressure systems traversing the Tasman during the past ten days (we certainly experienced the resulting weather!), and I suspect that you were seeing the effects of the varying pressure in the larger than normal waves even though your immediate area was calm at the time.

    Thanks for your observations, though. A big event makes us focus on our immediate environment and that’s how we pick up on the unexpected.

  4. Peter Dorn says:

    A couple of days have now passed and I have noted similarities to the Gisborne ’66 quake. I used to live in Chch back in the 50s and 60s. Dad moved to Gisborne with the Lands and Survey Dept in 1963. I spent 2 years at a boarding school in Central Hawkes Bay, wandering around the school farm, strictly against the rules. There was a nice escarpment to scale and explore. I found plenty of sea shells and “funny” rocks but missed the fossils that the area is now famous for. Back to Gisborne…

    I was outside, chasing the lawn mower around the section, trying to get finished before lunch. Just as I was doing the edges of the garden out the front; where you hold the mower on 2 wheels and balance it over the garden, the quake “hit”. The mower bit into the sandy dirt and stalled. I stood in the middle of a dust storm. I glanced around and took off into the middle of the road, to watch the power poles swing to and fro. I was standing right under the power lines to our house! They were getting pulled tight and stretching as the power pole swayed back and forth. A few flashes and a couple of bangs down the street and no more problems… No more power either!

    Dad was promoted and transferred to Gisborne as “2IC” (second in command) and while I was away at school, fell in love with a local widow whom he married in 1965. We moved into her house and I went to a new local high school in 1966. Nice new “Nelson block” styled school. Timber framing on concrete base, rigid but flexible. No damage… no holiday either. I left school at the end of the first term to start an electrical apprenticeship. Our workshop had to be rebuilt as a result of the quake. Our retail shop in Peel street had every large pane of glass replaced. The Government Buildings where Dad worked were brand new and survived without any structural damage. I got to remove some of the wiring and old fittings from the old Post Office building as part of my first job. Tenter-hooks and tip-toes are words that come to mind.

    One thing that I remember that my Dad said about Poverty Bay was the similarity of the ground to cold porridge. I guess it was his way of explaining the liquifaction even though it was between a couple of impervious layers. I put down a bore, many years later, to water the garden. “We” were always short of water. The well point passed through two layers of compressed clay and bought up bits of driftwood; wet, soggy driftwood that didn’t float in the backwash.

    In Chch we hardly watered the garden and always had great vegetables. The water table was always very high. I hope the people of Christchurch learn from the past and do not try to save every building that has historical significance. Some are going to be too expensive to save and if there is another quake, there could be loss of life as well as property. Remember that Gisborne was hit by a 6.8 quake in December 2007 and several buildings that were repaired in ’66 needed to be demolished. They originally had their parapets and sticky-outy bits removed and a bit of strengthening done. They should have been demolished and replaced by quake resistant designs. Luckily no one was in them when the quake hit.

    “Lesson learned?”

  5. Ken says:

    Peter,

    Thanks for your account of the ’66 Gisborne Quake. It certainly was a doozy. A psychologist was saying the other day that the experience of the Darfield earthquake will stay with the local children for life – this was so in our case for the ’66 quake.

    The damage to Christchurch’s heritage buildings is heartbreaking. In March I had a guided tour of the old Provincial Govt Chambers (Canterbury was the only province with enough sense to keep theirs for posterity) and they are such a treasure. There are so many other buildings in this class.

    I am in two minds about this issue. The safety view would insist that the unstable old brick and masonry buildings should go. But the stunning old buildings are so much of what Christchurch such a lovely city and so unique.

    I hope a middle path can be found. Some will have to go because they have sustained serious damage and are a threat to life and limb as the city tries to get back to normal. But, some might be able to be shored-up and left unoccupied until some of the new restoration techniques can be applied.

    If ever there was an illustration of “Hobson’s choice” this is it.

  6. Denise says:

    Ken, hello. Thank you so much for your response. It’s great learning more about this beautiful place, and I appreciate your explanation. Warm thoughts to our close neighbours who have suffered the earthquake – what a trauma. Kind wishes, Denise

  7. Denise says:

    Ken, hello. Thank you so much for your response. It’s great learning more about this beautiful place, and I appreciate your explanation. Warm thoughts to our close neighbours who have suffered the earthquake – what a trauma.

    Peter, I read your intriguing recollection of the earthquake you experienced, and read Ken’s response. The heartbreak in Christchurch must be great, deciding on the fate of the beautiful heritage buildings. I’ve never been there but, looking at the pictures that have appeared in our local newspaper, I really feel the pain….

    Kind wishes, Denise

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