Inangahua Quake Jolted NZ Out of Complacency
This week marks the 50th anniversary of the magnitude 7.1 Inangahua earthquake and scientists are among those reflecting
on how much progress has been made in the world of earthquake science since 1968.
Inangahua-quake: Geologist John Foster surveys damage to State Highway 7 near Reefton Saddle, about 30km south of
Inangahua, a couple of days after the magnitude 7.1 earthquake in 1968. Photo – Lloyd Homer, GNS Science
It would be unthinkable today, but it took about six hours before the epicentre of the Inangahua quake was known with
any confidence. And its magnitude took even longer. Today that all happens within a few minutes of an earthquake and the
information is made available publicly straight away.
When Inangahua happened there had not been a large damaging earthquake in New Zealand for 26 years and there was an
element of complacency about natural hazards.
The pre-dawn jolt at 5.24am on 24 May 1968 resulted in three deaths and 14 injuries. Two people were killed in one of
the many landslides and the other death occurred when a motorist hit a bridge abutment in the dark. A further three
people died when a rescue helicopter crashed.
The earthquake left 70 percent of the houses in the township of Inangahua – population 260 – uninhabitable. It damaged
or destroyed 50 bridges and twisted railway tracks so badly that 100km of railway line had to be relayed.
Roads were cut off in all directions, phones were out of action, and there was no electricity. So locals had no way of
communicating with the outside world. Eventually a truck driver in Inangahua managed to contact a colleague in Gisborne
on his vehicle radio.
The first radio news bulletins reported that tremors were felt throughout New Zealand. There was initially no mention of
the South Island’s West Coast.
Former chief seismologist with GNS Science Warwick Smith, now retired, recalls that there were just two paper drum
seismographs operating in New Zealand in 1968. One in the Seismological Observatory in Wellington and the other at
Canterbury Museum.
When staff arrived at work to check the instrument in Wellington, they found the force of the main shock had shot the
pen right off the paper and splattered black ink on the surrounding area.
“From the gap between the P and S waves on the paper, scientists could tell the epicentre was about 200 kilometres from
Wellington but they weren’t sure in which direction. It soon became apparent that it wasn’t to the north,” Dr Smith
said.
As reports began to come in, attention focussed on the Buller region on the West Coast. This was confirmed by aerial
reconnaissance by an electricity company reporting that the most severe damage in the landscape appeared to be around
the Inangahua Valley.
It was nearly half a day before scientists knew the epicentre of the earthquake with any confidence.
In addition to the two paper drum seismic recorders, there were about 20 photographic recorders around the country
located at places such as power stations and light houses. One sheet of photo paper was equivalent to 24 hours of
recording.
An operator would change the paper every day and at the end of the week all seven papers were developed in a dark room
and posted to Wellington for analysis by scientists.
“It was extremely tedious work. You had to manually measure all the seismic traces and then work out magnitudes and
locations. At best, the information about individual earthquakes was about eight days old by the time it became
available,” Dr Smith said.
“It’s important to note that the purpose of the network in the 1960s and 1970s was not to produce rapid information. It
was for obtaining data for scientific research. If any rapid information became available, that was a bonus.”
Fast forward to 2018 and the GeoNet project operates about 650 instruments in its nationwide monitoring network. About
half are seismometers sending their data in real time to GeoNet headquarters continuously.
GeoNet, which started in 2001, is operated by GNS Science and is funded largely by the Earthquake Commission. Its
nationwide network enables the quick location of earthquakes and the information is broadcast to all New Zealanders via
the GeoNet website, app, and social media.
There is huge uptake for this information, particularly during big events. For example, traffic to the GeoNet website
following the November 2016 magnitude 7.8 Kaikoura earthquake peaked at 35,000 hits second.
Geologist Simon Nathan, who was a fresh graduate in 1968 and one of the first scientists on the scene after the quake,
made a priority of getting an extensive photographic record of the damage. It was his first job out of university
working for the Geological Survey of NZ, now GNS Science.
“When the earthquake struck, I was astonished by its force. I had to hold on to the side of my bed to stop myself
falling out.
“There hadn’t been a big earthquake in New Zealand since 1942 and it was an exciting experience for a young geologist to
witness such a massive upheaval of the landscape.”
The experience made a lasting impression on Dr Nathan and to this day he remains cautious about buying a house that
could be prone to earthquake damage.
He said the quake initiated significant improvements to building standards, the planning a siting of infrastructure, and
to the way civil defence operates. These improvements have continued incrementally ever since.
Earthquake geology specialist and principal scientist at GNS Science, Kelvin Berryman, said the earthquake had a major
positive impact on geological studies of past big earthquakes and for understanding earthquake hazards in New Zealand.
“It boosted efforts to map active faults throughout New Zealand and to set up instrument monitoring networks across
active faults to investigate the nature of strain build-up and release,” Dr Berryman said.
“The surface fault movements associated with the Inangahua earthquake were very complex, much more complicated than had
been observed in previous earthquakes in places such as Marlborough and Wairarapa.
“So in 1968 it was very difficult to answer the question as to which fault was responsible for the earthquake.”
The main fault rupture that cased the earthquake did not break the surface and it took many years of work by scientists
to understand the deep geological structure and to explain the nature and mechanism of the main shock at Inangahua.
Inangahua-quake2: A car is partly swallowed by a large pothole caused by the collapse of a drainage tunnel in Inangahua.
Photo – Lloyd Homer, GNS Science
In this YouTube video, retired geologist Simon Nathan recalls his experience at the scene of the earthquake in 1968: https://youtu.be/q6ak1krtiXA
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