The magnitude 6.2 earthquake which caused considerable damage in Gisborne on the 5th of March 1966 was centred within 20 km of the city at a depth of 25 km. It struck at 11h 58m 57s on a Saturday morning.
The earthquake provided an unique opportunity for the Department of Scientific and Industrial Research (DSIR) to collaborate widely in studying the seismological aspects of the earthquake, the geology of the area, damage to services as well as damage to both public and private buildings. The department published a detailed bulletin covering these aspects of the quake, which provides the basis for this article. [see New Zealand Department of Scientific and Industrial Research, Bulletin 194, Gisborne Earthquake New Zealand March 1966, published 1969.] Contributors came from the DSIR itself, the Ministry of Works, local council, and the Earthquake and War Damage Commission.
Unlike previous strong earthquakes, the DSIR’s network of seismometers was more developed in 1966, and the existence of seismometers at Tuai power station inland from Gisborne, East Cape and Gisborne greatly aided in analysis of the many earthquakes recorded in the period February to June 1966 even though their galvanometers stuck after many of the large events. In addition, the presence of an acceleration recorder at the Gisborne Telephone Exchange provided extra information on the direction and strength of ground pulses during the earthquake.
There were 50 recognisable foreshocks that commenced at 9:30 pm on February 2nd with the last occurring 43 minutes before the main earthquake on March 5th. Most would not have been felt, but the pair of magnitude 4 events 28 minutes apart on the evening of February 5th would have been noticed by many residents.
Following the magnitude 6.2 quake, there was 25 minutes of peace before the aftershocks commenced. During the remainder of March 5th, there were 225 aftershocks; the largest that day were two magnitude 4.5 shakes, the first occurring 30 minutes after the main quake, the second at 11:40 that night. The following day, there were 83 aftershocks, including a magnitude 5 and two magnitude 4 events. Daily aftershock rates then eased back, with 466 occurring during March and a total of 513 events being recorded until the end of May.
The Ministry of Works report noted: “Generally damage was confined to an area ½ mile [800 m] wide on both sides of Turanganui and Taruheru Rivers and extending about 2 ½ miles [4 km] up from the mouths. This included the main city area on the west and the Whataupoko area on the east of the rivers.” It added that many of the older buildings had been damaged in the 1931 Hawke’s Bay and 1932 Wairoa earthquakes.
At the time of the earthquake, electric power was lost for about 10 minutes in the city, as power lines clashed and mercury tubes in transformers broke. There were 151 service-line faults to houses due to clashing of wires and pulling-off of barge boards.
There were about a dozen watermain leaks and 40 service leaks. The sewer crossing the Peel Street bridge broke, and sewerage was diverted into the Taruheru River until Sunday morning.
During the main earthquake, the acceleration recorder on the ground floor of the telephone exchange recorded a maximum acceleration of 0.28 g towards the north-north-east. The device recorded 7 acceleration pulses between 0.1 g and 0.3 g, and a further 5 or 6 pulses of about 0.1 g. Writing about the largest pulse, R.I. Skinner of the DSIR’s Physics & Engineering Laboratory stated, “This acceleration is surprisingly high in relation to the moderate damage to city buildings. The damage was limited by the short duration of severe shaking and probably also by shaking at periods long compared with the resonance period of most of the structures.” Gisborne had been lucky.
Even so, the earthquake had managed to rack up a large amount of damage in a short period of time. Approximately 3,000 chimneys required repair or replacement, and the Earthquake and War Damage Commission received insurance claims for 700 of them. The commission noted, “Most of the chimneys were cracked at roof line; many others were cracked at ceiling level or required renewal from about 18 in. [460 mm] below the roof. Damage was greater on chimneys that had been fitted with TV aerials.” Both the commission and the Ministry of Works noted that 20 newer pre-cast chimneys also sustained damage just above foundation level. Similar damage to pre-cast chimneys was noted as a result of the Seddon earthquake the following month.
The Ministry of Works report included the following amongst its conclusions: “In an age when man is close to landing on the moon we still have not designed, in New Zealand, a low-cost earthquake-resistant domestic chimney, nor a satisfactory W.C. soil-pipe connection. The bulk of the claims at Gisborne were for replacement of domestic chimneys. It seems wasteful to replace them with identical ones, which will fail in the next earthquake and possibly also cause loss of life.” About 150 toilet pans had to be replaced after the Gisborne earthquake.
The reports noted that houses had moved off their foundations, water tanks and stands had been damaged, as had rooves and wall linings. Claims made on the Earthquake and War Damage Commission (now known as EQC) totalled 1,890 costing $265,732.
Several public buildings sustained severe damage, the worst being the Chief Post Office on the corner of Customhouse Street and Gladstone Road. The first stage had been built in 1901, with additions added in 1925. Following the 1931 earthquake, the clock tower had been removed, and brick gables had been reduced in height. Cracks that had appeared in 1931 had been repaired, but they re-opened in 1966, and the wall facing Customhouse Street moved out by 2 inches (50 mm) at the roof level. Several arches were badly cracked and fresh cracks opened up on two other walls. The building was not considered economical to repair and it was demolished.
The infant block at the Mangapapa School had also been strengthened after the 1931 earthquake. A post-quake inspection detected movement of a tie-beam and additional cracks over door openings. It was recommended that the building was not used until several strengthening measures were taken. Fortunately, the education board decided that its money would be better spent on demolition and replacement with a timber structure, as part 3 of this series of articles will show.
The northern wing of Gisborne Boys’ High School suffered damage, and two classrooms were temporarily closed. However, on re-inspection it was determined that some of the damage was due to weathering, and the classrooms were able to be re-used but it was recommended that they be replaced as soon as practicable.
At the city’s Cook Hospital, plaster fell in the wards and some concrete lintels over doorways and in corridors cracked at mid-span. The elderly building had also been strengthened after the Hawke’s Bay quake of 1931, but subsequent building work had weakened some of these measures – one of the tie rods had been cut through and a concrete beam over the corridor had been seriously weakened by partial removal.
Commercial buildings suffered too. One building displayed cracks up to ¾ inch [19 mm] wide while another had cracked window arches and a gable which had moved out by 3½ inches [89 mm] to a precarious position. Another building’s roof was near collapse.
Even the monument to Captain Cook didn’t escape, with one of its massive stones moving outward.
The Post Office Tower in Gisborne was demolished after the 1.30am Wairoa quake of 16 September 1932. The record is in the GBC minute book for 1932. Gisborne was less affected by the 1931 Napier event.