Distance saves Gisborne

Fifty kilometres of sea were the difference between life and death for Gisborne residents shaken up by Thursday night's earthquake.

Stresses and strains that had been building in the Pacific plate just over the horizon south-east from Gisborne suddenly shifted rocks about 40km below the sea bed about 8.55pm, tearing a gash in the Earth's crust.

Six seconds later, the fastest vibrations from the magnitude 6.8 on the Richter scale earthquake raced onshore, causing alarm, damaging buildings and cutting electricity and communications.

The huge forces resonated around the country, reaching Christchurch some 90 seconds later and rolling into Dunedin about two minutes after the initial shock in Gisborne.

GNS Science Geonet data centre manager Kevin Fenaughty said it was fortunate the rip in the crust did not reach up to the sea bed.

If it had, the displacement of sea it caused would have generated a tsunami that would have struck Gisborne with little or no warning within 15 minutes.

It was fortunate too that the earthquake was centred offshore in part of the Hikurangi Trough. Had its epicentre been onshore and any closer to Gisborne, deaths from falling building materials would have been almost certain.

On the GNS Science website, more than 2600 people reported feeling the earthquake, with four locals classifying it at eight - defined as "heavily damaging" -- on the Modified Mercalli (MM) scale of felt intensity.

An MM8 quake is described as one in which "alarm may approach panic", the steering of vehicles is affected, some buildings collapse, houses not secured to foundations may move, rockslides may occur and cracks appear in wet ground.

Comparisons are already being made with the great 7.8 Napier earthquake of February 1931, which killed 256 people and injured thousands, but that was in a different league.

This week's major event comes after a run of quakes. More than 20 earthquakes rocked the South Island in September alone, with a magnitude 7.3 on September 30 the largest earthquake in the New Zealand region for 74 years.

Seismologists say sudden increases in activity are normal.

Fenaughty said there was no method for accurately predicting earthquakes.

"Meteorologists can't tell you exactly where lightning is going to strike, but they know where thunderstorms will be."

It was possible Canterbury could be affected by a large earthquake generated in Pegasus Bay but the cause of it would be different to Gisborne's quake.

"The big worry for us is that the Alpine Fault slips. If you get a slip along that, it will damage Christchurch, but at least you are some distance away," Fenaughty said.

- © Fairfax NZ News