Southern ocean quake biggest shallow quake since 1930s

A magnitude 7.3 earthquake that occurred in the Southern Ocean on Sunday evening did not produce a significant tsunami, despite initial concerns that it might have.

The quake, with a focal depth of 12km, struck at 6.23pm and was centred 470km southwest of Invercargill. It was felt widely in Southland and Otago as a long, rolling motion.

The earthquake was the largest shallow quake in the New Zealand region since the 1930s.

GNS Science seismologist Warwick Smith said big, shallow earthquakes that are centred a large distance away are typically felt as long, gentle rolling motions.

The main jolt was followed by dozens of aftershocks, the largest of which was a magnitude 6.8 quake at 10.47pm on Sunday 30 September. Small aftershocks are likely to continue for some weeks.

The earthquake and aftershocks occurred on the boundary of the Pacific and Australian tectonic plates at the southern end of the Puysegur Trench, a region known for being seismically active.

The movement of the ocean floor during the earthquake was not large enough to generate a damaging tsunami. Most of energy generated by the earthquake was perpendicular to the plate boundary.

This meant most of the tsunami energy was directed towards Australia. It caused a 25cm rise in water level at Dog Island near Bluff and a similar rise in water level on the Tasmanian coast.

The initial jolt caused an immediate bulletin from the Pacific Tsunami Warning Centre in Hawaii, which warned of a possible tsunami within 100km of the earthquake’s epicentre.

New Zealand authorities including the Ministry of Civil Defence and Emergency Management, GNS Science, and NIWA evaluated the threat and decided the risk of a destructive tsunami hitting New Zealand was low.

This was based on historical evidence, the magnitude of the earthquake, and tsunami modelling.