Latest earthquake rated the biggest in 73 years

Geologists have upgraded Tuesday's earthquake from 7.2 to 7.3 on the Richter scale.

"As more data comes to hand, the size and location of a quake can often change slightly - we now believe Tuesday's quake was actually deeper than 33km," said a spokesman for the Geological and Nuclear Science institute.

The new estimate makes the quake in the Puysegur Trench, 210km west of Tuatapere, Southland, the biggest in 73 years, since a 7.3 magnitude aftershock on February 13, 1931, at Napier.

The actual Napier quake - which killed 256 people on February 3,1931 - was magnitude 7.8.

But the spokesman said it was difficult to compare the impact of an undersea quake with one on land such as the 1931 Hawkes Bay aftershock.

Historical figures showed that although New Zealand could expect to average one quake of magnitude 7 to 8 every 10 years, they tended to cluster.

"Between 1929 and 1934 New Zealand was hit by five quakes of magnitude 7 or above. And in 1942, there were two magnitude 7-plus quakes within three months of each other. Then there was a long gap until the next magnitude 7 quake - in 1968 [Inangahua]".

- NZPA