1926 Tsunami: Tolaga Bay, North Island, New Zealand

Reference #5324:

Source and exact year unknown but between 1927-28.

Sometime after the beginning of construction of the Tolaga Bay wharf, a long structure reaching hundreds of metres into Tolaga Bay, three large waves, referred to at the time as tidal waves, severely damaged the structure when just five piles had been driven and were in the process of being secured. The waves were noted to be of unusual size, their crests reaching to the deck level, which was 3.3 m above the high water springs mark. The waves were only noted to have damaged the wharf and nothing is noted of their effect on the nearby shore, or anywhere else along the coast.

No source is known for the large waves that damaged the wharf or the exact date on which the waves occurred, so association with distant/offshore meteorological conditions or with a local or distant earthquake is unknown. Locally, it was noted that there was a moderate easterly breeze, which only slightly roughened the sea. A large local earthquake can be excluded, and as the waves seem to have impacted at Tolaga Bay only and there are no events on the NGDC site that are large enough, a distance source can also be excluded. De Lange & Fraser (1999) suggest a similar source to the March and May 1947 tsunamis (q.v.), which de Lange (1997) proposed were caused by marl diapirism. De Lange & Moon (2004) have since proposed a landslide mechanism. However, the 1947 earthquakes have now been shown to exhibit many source and tsunami characteristics associated with ‘tsunami earthquakes' (Downes et al., 2001; Doser & Webb, 2003).

Considering the possibility that the source of the waves in 1926-28 may be an earthquake, including mildly felt earthquakes like the 1947 events, the Poverty Bay Herald has been searched for a few days following known earthquakes in 1926-27, without finding any accounts of the damage to the wharf. However, the earthquake file is probably incomplete for this era. Although a day-to-day check of the newspapers is probably warranted, it is problematic that the waves that damaged the Tolaga Bay wharf do not seem to have impacted elsewhere on the coast, noting the March 1947 event was observed along 120 km of coastline and the May 1947 tsunami along 80 km of coastline. Given this, and until such time as more evidence of a tectonically generated tsunami can be found, a few rogue waves or a localised meteo-tsunami seem to be the most probable causes of the waves, although one would expect a longer series of waves in the latter case and greater impact on the nearby shore.

In contrast to Fraser (1998), who assigns a reliability of 1 (=certain tsunami) a validity of 2 (questionable, unlikely) following the NGDC classification is assigned.