At 5:24 am on the 24th of May 1968 the central part of New Zealand was rocked by a magnitude
7.1 earthquake. It was centered near Inangahua, a tiny rural community on the west coast of the
South Island. The earthquake was a big one, near the epicenter people were thrown out of bed and
had to hastily escape from their homes amidst the chaos of toppled furniture, opened cupboards and
falling roof tiles. The earthquake woke up people all over New Zealand but it was half a day before
anyone realized where the epicentre was. In Wellington the recorder at the seismological
observatories was thrown off scale splattering ink in the surrounding area, as you can see in
the bottom right. As a young geologist fresh out of university I was astonished at the force of
the earthquake which was larger than anything I've ever felt before. I was staying in an
old wooden house in Westport about 50 kilometres away from Inangahua, and I can remember grabbing
the side of the bed so that I didn't fall out. The shaking seemed to last for several minutes
then there was a tremendous crash above my head which turned out to be the chimney collapsing.
In the weeks that followed I was involved in earthquake investigations, and gradually came
to realise how widespread the impact of the Inangahua earthquake had been. Inangahua is
surrounded by forest covered hills, the shaking caused huge landslides on the steeper slopes.
This image shows the road through the upper Buller Gorge in the centre, parts of which were wiped out
by the landslide. It was many months before the road was open again for traffic. Of immediate
concern was a huge landslide in the Buller Gorge a few kilometres upstream from Inangahua which
temporarily dammed the Buller River. The danger of a dam burst flooding the area downstream led
to the evacuation of everyone at Inangahua. The next morning it was clear that the danger had passed.
The River over-topped the dam and cut down through the toe of the landslide without causing a flood,
but the landslide can still be seen 50 years later and there are rapids in the river at this point.
There was only a single power line passing through Inangahua, bringing power to industries in
westport such as the mines and the cement works. Despite the widespread landslides and rock falls
the lines were unbroken and power was quickly restored. The hills around Inangahua includes
steep limestone cliffs which were the source of a huge number of rock falls and landslides,
some of which extended hundreds of metres away from the source area. Two of the three deaths
directly caused by the earthquake occurred near White Cliffs where a house beneath the
limestone cliff was destroyed by a rock fall. The inhabitants of the house were unlucky because the
rock fall destroyed the house but the garage a few metres away to the right was undamaged,
the remains of the car parked outside the house can be seen in the foreground. A train was passing
near Inangahua at the time of the earthquake and several of the carriages were derailed.
Fortunately the railway from Reefton to Westport was not badly damaged and was reopened within
a matter of weeks. We soon discovered a small fault offsetting the railway line near Inangahua
station. It could be traced for several hundred meters and seemed to join up with the fault that
had moved many times in the past. Mapping over the next few months showed that several small
faults had moved during the earthquake, but we finally concluded that the major rupture that
caused the earthquake had not broken the surface. Roads had subsided in many places around Inangahua
but mainly in areas of unconsolidated fill. Some sections of the road had little damage, but others
were pockmarked with holes and collapses. It was clear that there needed to be better quality control and road construction.
Liquefaction was widespread on some of the flat terraces around Inangahua
it wasn't a major hazard at the time because there were few houses built on this land,
but it was recognized by engineers and geologists that it could be a problem in urban areas built on unconsolidated sediment.
Almost all the buildings around Inangahua were single-story wooden houses
with corrugated iron roofs. These houses generally survived the earthquake with little damage the
garage on the left appears to have collapsed because it had no internal bracing.
This little wooden church jumped off its piles moving about a meter to the left but was otherwise undamaged.
These railway houses all had heavy tiles on the roof many of which were dislodged during the earthquake.
Two of the most modern houses in Inangahua had brick veneer over timber
framing. The brick veneer cracked badly and the houses were eventually reclad with weatherboard.
Note the contrast to the wooden house on the left with a corrugated iron roof which was practically undamaged.
The Inangahua earthquake came as a terrible shock. There hadn't been a big earthquake in New
Zealand for more than two decades and people had become complacent about the cost and disruption
such a disaster causes. Because it occurred in a sparsely populated area casualties were light, but
it acted as a warning to improve our preparation and response to disasters. A lot has changed over
the last 50 years, and today GeoNet gives instant information on the location of any earthquake. But
although we've learned a lot more about earthquake science we still can't predict when or where they
will occur. The Inangahua earthquake reminded us about some of the earthquake hazards we face
in New Zealand including landslides, rock falls, liquefaction and the damage that shaking can cause
to buildings. This is why we need to look back 50 years and remember the Inangahua earthquake.