The Seddon Earthquake of 1966

On average, New Zealand experiences an earthquake of magnitude 6 or greater once a year. However, just seven weeks after Gisborne experienced a damaging magnitude 6 earthquake in 1966 New Zealand was, against the odds, to experience another strong shake. What’s more, the second magnitude 6 quake was to occur near two sizeable towns and the country’s second biggest city, causing minor damage.

The Seddon earthquake of 1966 damaged commercial buildings and dwellings on both sides of Cook Strait.

For movie-goers at Blenheim’s picture theatre, the evening screening was to be foreshortened. The 60s were the days when the advertising patter for picture theatres announced screenings at “eleven, two, five and eight” and the “five o’clock flicks” were in session.

At 6:49 p.m. on April 23rd 1966, a magnitude 6.1 earthquake about 22 km deep in Cook Strait shook Wellington, Blenheim, Ward, Seddon and the nearby coastal settlement of Seaview. Best estimates put the epicentre off Cape Campbell, about 35 km from Seddon.

According to the Department of Scientific and Industrial Research (DSIR) Bulletin 199 “Seddon Earthquake, New Zealand April 1966“, the earthquake was felt as far south as Hokitika and Banks Peninsula and at Taranaki in the north. Isolated felt reports came in from Gisborne and Cambridge. The earthquake’s intensity reached MM VII at Seddon and MM VI at Blenheim and Wellington. Wellington had not experienced MM VI shaking since the strong Wairarapa earthquakes of 1942.

The Modified Mercalli (MM) Intensity Scale measures the destructiveness of an earthquake’s ground shaking as opposed to Richter Magnitude which is a measure of energy released. At MM VI an earthquake is felt by everyone, furniture moves and minor damage to chimneys and plaster finishes occurs. At MM VII there is general alarm and people have trouble standing. Weak masonry buildings sustain damage, windows crack and small landslides and rock-falls occur.

The earthquake cut power to most of Marlborough and the picture theatre at Blenheim was plunged into darkness as the building shook. The western parapet of the building facing Market Street cracked badly but fortunately did not collapse. Quick action by the theatre operator restored power from an emergency source, and the patrons were able to evacuate without panic.

Elsewhere in Blenheim, 14 other buildings sustained damage ranging from damaged foundations to cracked walls. The fire engines, which were unwisely housed in a condemned building, were swiftly moved out to the street.

In Wellington, electricity supplies failed in some areas for up to 20 minutes, and telephone exchanges overloaded. Buildings up to eight storeys high showed earthquake damage. The DSIR report documented damage to 23 buildings in Thorndon, Lambton Quay, Featherston Street, Victoria Street, Taranaki Street and further afield near The Basin Reserve and on Mount Victoria. It was noted that most of the affected buildings were on reclaimed land and stood mainly on piles founded below the old beach level.

At Seddon, nearer the epicentre, the Ministry of Works reported, “Practically all houses, new and old, had chimney damage, in some cases complete disintegration and collapse, in others loss of upper sections, particularly on unsupported exterior chimneys. Minimum damage usually was horizontal cracking at roof, fireplace, gathering, or foundation level.”

Water mains burst, and telephone service was lost for a time owing to batteries overturning and sliding. One government and several commercial buildings showed cracking damage as did several churches. The main trunk railway line 1 km south of Seddon was distorted for about 60 metres of its length where it crossed the Hogg Swamp fault.

Nineteen kilometres away, at the Cape Campbell Lighthouse, the driving mechanism for the heavy mirror and light had shifted. During the quake a guide roller for the prism sheared off, hit the 2½ ton prism and bounced through an outside window.

There were 41 quakes in the Seddon sequence – one foreshock (not felt) about 6 hours before the main shock, and 39 aftershocks. It was unusual in that the epicentres of the various quakes were spread across 20-odd km of the seabed in Cook Strait.

Seven years earlier, during the latter half of 1961 a swarm of at least 20 earthquakes had shaken the coastal settlement of Seaview, 9 km from Seddon. G. J. Lensen of the DSIR reported, “Although most of the shocks were felt in the Seaview area only, they were big enough to alarm the local residents (lights swinging, doors and windows rattling, etc.). Some bigger shocks were also felt in Seddon and the lower Awatere Valley in general. … Considering the low magnitude of the earthquakes and the small extent of the areas in which they were felt [a narrow elongated zone centred around Seaview], the hypocentres are likely to have been at very shallow crustal depths. That sequence of earthquakes was unusual in a region that is normally shaken at intervals of a few years by larger shocks.”

The detailed DSIR bulletin, which was published in 1970 even explored the relationship of the earthquakes to weather and tides. It noted, “Hayes (1952) has made a study of a sequence of earthquakes in 1950 centred [near the Marlborough Sounds] about 60 km to the north of the shocks of the Seddon series. He states that ‘there was a marked tendency for increased activity when both the [atmospheric] pressure and the tide in the epicentral region were below normal and falling.’ This was not the case for the Seddon series, in which the main shock occurred within half an hour of high water at Cape Campbell, and the two main aftershocks occurred at a similar stage of the tide, two cycles later.”

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