KEY POINTS:
To understand the impact of an earthquake, just remember the first three rules of real estate - location, location, location.
New Zealand's last big earthquake, at Edgecumbe in 1987, caused damage because it was very shallow.
Thursday's shake was five times more powerful but caused less damage because
it was centred 40km under the sea 50km southeast of Gisborne.
Warwick Smith, a seismologist with GNS Science, said the earthquake, measuring 6.8 on the Richter scale, occurred within the Pacific plate that is bending westwards beneath the Gisborne region at a rate of about 45mm a year.
The ruptured fault probably extended into the deeper part of the Pacific plate, which runs down the centre of the North Island and across the western side of the South Island.
Seismic waves travel very efficiently along the plate, which explained the earthquake being felt as far south as Invercargill, he said.
The energy got damped out in other directions, hence fewer shocks to the northwest in places such as Auckland.
Mr Smith said deeper earthquakes often produced more high-frequency shakes than shallower events of the same magnitude. This was reflected in the strong shakes in Gisborne.
Another feature was relatively few aftershocks, with few of them big.
About 20 earthquakes followed the main jolt at 8.55pm on Thursday. Bigger aftershocks were limited to two at 6.47am and 8.33am yesterday, measuring 4.5 and 4.2 on the Richter scale, Mr Smith said.
"I would expect because there aren't that many [aftershocks] they will probably die out, perhaps within days or a week or two."
There was no tsunami because the depth of 40km was unlikely to have disrupted the sea floor.
New Zealand's most deadly earthquake occurred in Hawkes Bay on February 3, 1931, killing at least 256 people. Centred 15km north of Napier, it was of magnitude 7.8 and lasted for 2 1/2 minutes. Nearly all the buildings in central Napier and Hastings were destroyed.
The most powerful earthquake registered in New Zealand was 8.2 on the Richter scale. It occurred in the Wairarapa on January 23, 1855, rocked the southern part of the North Island and altered the landscape of Wellington.
Mr Smith said an increase of one unit on the scale increased the power of an earthquake by 30 times and about five times for half a unit.
About 200 earthquakes a year are big enough to be felt by people. Since European settlement, the country has had eight jolts of magnitude 7 and just the one of magnitude 8.
Heavy shakes
New Zealand's biggest earthquakes:
* Gisborne, December 20, 2007. Magnitude 6.8. Loss of life, 1.
* Edgecumbe, March 2, 1987. Magnitude 6.3. Loss of life, nil.
* Inangahua, May 24, 1968. Magnitude 7. Loss of life, 3.
* Wairarapa, August 2, 1942. Magnitude 7. Loss of life, 1.
* Wairarapa, June 24, 1942. Magnitude 7.2. Loss of life, nil.
* Pahiatua, March 5, 1934. Magnitude 7.6. Loss of life, 2.
* Napier, February 3, 1931. Magnitude 7.8. Loss of life, at least 256.
* Murchison, June 17, 1929. Magnitude 7.7. Loss of life, 17.
* North Canterbury, September 1, 1888. Magnitude 7.1. Loss of life, nil.
* Wairarapa, January 23, 1855. Magnitude 8.2. Loss of life, estimated 5 to 9.
* Marlborough, October 16, 1848. Magnitude 7.5. Loss of life, 3.
