1831 Tsunami: Poverty Bay, North Island, New Zealand

1831-1834. Burns (1859) records the following: "One morning I was strolling along the beautiful beach in Poverty Bay, admiring the rising sun ... I had not proceeded far before I felt a powerful shock of the earthquake which nearly took me off my feet, rocking the trees to and fro and causing an immense swell in the sea. I knew perfectly well, from experience, that it was not an earthquake, but an electric shock caused by the eruption of a volcano or burning mountain, about 40 miles in the interior of the country; an occurrence not infrequent in those parts ...".

Despite Burns associating the shock with an eruption, his account clearly describes an earthquake, with an intensity of at least MM6-MM7. His association of the earthquake with the "immense swell of the sea" suggests the two events occurred within short space of time, indicating a local source or seiching caused by surface waves from a more distant large (M>7.5) earthquake. However, seiching is unlikely to occur on the open coast, except within in a confined harbour or estuary, unlikely to be more than a metre and unlikely to initiate the comment "immense swell". This suggests a local earthquake source within a short tsunami travel time (< 30 minutes), perhaps 50 km from Gisborne.

Given that "tsunami earthquakes" like the 1947 March and May events (q.v.), which caused only mild shaking, have occurred on this part of the coast, determining an appropriate magnitude for an earthquake large enough to cause a tsunami and close enough for it to arrive within a short time, but only large enough to cause moderate intensities is problematic. However, a large strong shaking plate interface earthquake (magnitude greater than 8 with maximum shaking intensities of MM8 and larger) is not indicated.

According to the Dictionary of New Zealand Biography Vol 1, Burns first came to NZ about 1830. A short time after February 1831 he returned to NZ as a trader at or near Mahia. Early in 1832 he moved to inland Poverty Bay, remained a few months only, and then moved to Tolaga Bay where he remained until October 1834, when he permanently left NZ. This indicates the earthquake occurred between 1831 and 1834, not 1832-38 as suggested by Grouden (1966) and later publications based on this data. The date range is not restricted to the time Burns was in Poverty Bay (a few months in 1832) as he may have travelled on occasions to Poverty Bay from Mahia or Tolaga Bay.

A validity of 3 is assigned [tsunami]. (reference #5324)