The 1904 Cape Turnagain earthquake struck north of Cape Turnagain on the morning of 9 August with a magnitude estimated at 6.8 and 7.0–7.2 It was felt throughout the North Island and upper South Island, with many communities within a radius reporting noticeable ground shaking. Heavy damage occurred to the landscape and personal property and one man died. It was the largest to strike New Zealand since the 7.0 North Canterbury earthquake in 1888.
Tectonic setting
New Zealand lies along the boundary between the Australian and Pacific plates.
In the South Island, most of the relative displacement between these plates is taken up along a single dextral (right lateral) strike-slip fault with a major reverse component, the Alpine Fault. In the North Island, displacement is mainly taken up along the Kermadec-Tonga subduction zone, although the remaining dextral strike-slip component is accommodated by the North Island Fault System. Subduction off the coast takes place at the Hikurangi Trench, which runs parallel to the East Coast of the North Island and is the southern extension from the Kermadec Trench, however earthquakes here occur less frequently. The 1904 earthquake struck approximately to the west of the Hikurangi Trench, on the shallow plate interface between the Pacific and Australian plates, approximately 16 km beneath the surface.
Epicentre
Initially when the earthquake struck, the given epicentre being Cape Turnagain was only an estimation. The earthquake was recorded on the worldwide Milne seismograph network and on other seismographs in Europe, but very few of these still exist to this day. Seismograph data of the earthquake was held at stations in Melbourne and Honolulu, as well as various sites across New Zealand. It was with those recordings that seismologists could confirm that the true epicentre of the earthquake was indeed Cape Turnagain, only being about 10 km off from the estimation.
Damage
The earthquake caused massive amounts of damage to the surrounding landscape, with landslides being reported in an area of over 3,500 km2 between Napier and Featherston. Liquefaction and sand fountains were also reported in several towns, including Napier, Porangahau and Carterton. Property damage occurred on a mass scale, with cracked walls, broken windows and fallen chimneys being the most common structural problems. In Gladstone, a hillside house was completely destroyed, and multiple buildings across the lower North Island burnt down. Personal items such as cutlery, glassware and furniture were reportedly damaged from as far afield as Wellington and Wairoa, both about from the epicentre.
The ground damage recorded in the Cape Turnagain earthquake is noted for being distinctly different from other earthquakes in the area, such as Hawke's Bay in 1931, Pahiatua in 1934 and Masterton in June and August 1942. For example, there were more widespread landsliding and a larger amount of liquefaction. The distribution of landscape deformation such as landsliding was very extensive (about 3,500 km2) and significant landslides were triggered at considerable distances.
Landslides
Heavy rockfall was recorded along the Hawke's Bay and Wairarapa coastline, where the Mercalli intensity of the earthquake was VII and VIII. The cliffs from Cape Kidnappers to Bluff Hill, at Blackhead, Cape Turnagain as well as the inland McLaughlin cliffs and hills in the Pohangina Valley had severe rockfall occur. There was also a reported rockfall from the Hatepe pumice cliffs in Lake Taupō, 215 km away, where estimated shaking from the event was only III.
Numerous landslides were reported in and around Herbertville, and on the summit of Puketoi Range near Masterton. Tauweru, also near Masterton, reported rocks and boulders of up to 1 metre in diameter tumble down hill with some small landslides occurring. In Napier, several tonnes of rock fell from Bluff Hill during the earthquake and smaller slips continually occurred for at least three days with each passing aftershock. Many landslides also occurred near Wimbledon, with the largest landslide being near the top of Sergeant's Hill, about a 40–50-meter wide slump gave way, taking down hundreds of trees with it.