Remembering the Inangahua quake - 50 years ago this week

Geologist John Foster surveys damage to State Highway 7 near Reefton Saddle, about 30km south of Inangahua. Photo/ Lloyd Homer.

This week marks 50 years since the deadly quake that rocked Inangahua residents out of bed, destroying the rural West Coast township.

Three people died in the pre-dawn jolt at 5.24am on May 24, 1968, and another three were killed when a rescue helicopter crashed. The remaining residents escaped hastily with little warning.

A seismologist takes a closer look at the earth science then, compared to now.

Today, working out the magnitude of an earthquake takes just a few minutes. Back then, it took more than six hours.

Former chief seismologist with GNS Science Warwick Smith recalled that there were just two paper drum seismographs operating in New Zealand back then.

"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," Smith said.

A car partly swallowed by a large pothole caused by the collapse of a drainage tunnel in Inangahua. Photo/ Lloyd Homer

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.

The shake left 70 per cent of Inangahua's township in ruins. Fifty bridges were destroyed and 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.

Locals had no way of communicating until 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.

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 lighthouses. One sheet of photo paper was equivalent to 24 hours of recording.

All that remained of a house destroyed by a landslide during the Inangahua earthquake. Photo/ file

Smith said 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.

"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," Smith said.

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.

Geologist Simon Nathan was a fresh graduate in 1968 and one of the first scientists on the scene after the quake.

He said he 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."

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," Berryman said.