Tsunami of March 26, 1947 swept in after quake

The tsunami of March 26, 1947 flooded the Tatapōuri Hotel up to the windowsills. Minutes later the receding tide sucked small outbuildings out to sea. Tairāwhiti Musuem picture. Reference 811:10

Last Tuesday was the 77th anniversary of the first of two severe earthquakes that occurred off the coast of Tairāwhiti Gisborne in 1947.

This earthquake generated one of the largest tsunamis in New Zealand’s historical records. It was observed along 115km of coastline and caused damage to beachside cottages and buildings, bridges, fences, and roads, according to a GeoNet statement marking the anniversary.

“The first earthquake, on March 26, 1947, was a magnitude 7.0 and occurred 70km south-east of Gisborne. Although the ensuing tsunami was large, residents at the time did not report extreme shaking from the earthquake. Scientists think the lack of strongly felt shaking was likely due to a slow rupture.

“Slow rupture speed is observed globally for some earthquakes that cause tsunami. As the earthquake occurs it ruptures its fault more slowly than usual (although still much faster than slow slip earthquakes). The tsunami that these earthquakes produce could be much larger than we would expect from their magnitudes. These types of earthquakes are often called “tsunami earthquakes” and the March 26 event is one of them.

“That is one of the reasons for the importance of the ‘or’ in the advice of ‘Long OR Strong, Get Gone!’ safety messaging. An earthquake that lasts more than a minute OR makes it hard to stand up is a natural tsunami warning.

“At Pouawa and the south side of Turihaua Point, the tsunami was most evident with large breaking waves said to be 10-13m high observed offshore. At the northern end of Pouawa Beach, where SH35 crosses the Pouawa River, seaweed was found 12m above sea level in telegraph wires well inland from the beach. The decking and superstructure of a 16m span wooden bridge across the Pouawa River was swept about 800m inland.

“At the southern end of the beach, the Hall household was hit by the tsunami and the building swept from its foundation. Three people inside the house were trapped in the kitchen which filled to head height with water. The kitchen was the only section of house not demolished. It was moved 3m and twisted during the first wave, and then during the second wave the rest of the house was smashed and swept out to sea. Two people, who had escaped to the road above the house before the first wave hit, were then washed up on to the road during the second wave.

“Four people at the Tatapōuri Hotel, 13 kilometres by road north of Gisborne, saw the tsunami coming and rushed up a hill. Two waves swept through the ground floor of the hotel up to windowsill height, and retreating water then washed small buildings out to sea.”

Just a few weeks later, on May 17,  1947, another offshore earthquake generated a tsunami that hit the coast between Gisborne and Tolaga Bay. This quake, a magnitude 6.9, occurred about 30km further north, and the greatest damage was at Waihau Beach, where logs piled ready to repair the bridge damaged in the tsunami two months previously, were washed away.

“The tsunami from this second earthquake was not well observed, as it occurred on a stormy winter night about half an hour after the earthquake (about 1.5 hours before low tide). Nevertheless, its effects were noticeable the next day from Wainui Beach, near Gisborne, to at least Tolaga Bay, and possibly as far as Tokomaru Bay, spanning 50-80km of coastline.

“Evidence showed that the tsunami travelled further inland and to a higher level than in the March tsunami. Water swept 50m inland at Tolaga Bay and the water height was estimated to be 1.8-2.4m higher than in the March 26 event (likely due to the May earthquake being closer to Tolaga Bay). Taking the tide level into account, this suggests the waves may have reached 4-5m above sea level at Tolaga Bay.

“These two historic events occurred along the Hikurangi subduction zone, New Zealand’s largest plate-boundary fault. The Hikurangi Subduction Zone and the Tairāwhiti region experience a large range of earthquake sizes and behaviour. In more recent times strong shaking has been generated by the 2007 M6.7 Gisborne earthquake, which did not have a tsunami, and the 2016 M7.1 Te Araroa earthquake, which generated a small tsunami (wave height more than one metre) in some places.”

Only a handful of the more than 700 earthquakes over magnitude 4 in the region in the last two decades have caused damage through shaking or tsunami.

The message to the community from Ben Green, Tairāwhiti Civil Defence Emergency Group manager, is, “Understand the hazard, don’t fear it; however, be aware of it and have a plan that will focus on you and your whānau getting to safety.”