article: Dec 21 2007 - East Coast damaged by offshore quake

A magnitude 6.8 earthquake struck last night at 8:55 pm on Thursday, December 20. Damage to chimneys and buildings has occurred at several places along the East Coast, particularly Gisborne. Experience with this type of earthquake suggests only a weak aftershock sequence will follow.

ShakeNZ displaying shaking intensity across New Zealand in the first few minutes after the main shock.

Mainshock

The earthquake occurred within the Pacific plate, which is being thrust westwards beneath the Gisborne region at about 45 mm/year in a process known as subduction. As this plate subducts, it is subject to bending stress and stresses arising from the plate heating up. It is these stresses which caused the earthquake. The fault which ruptured most likely extends into the deeper part of the Pacific plate, where seismic waves travel very efficiently. This explains why the earthquake was felt as far south as Dunedin.

A feature of such deeper earthquakes in the subducted plate is that they often produce more high-frequency shaking than shallower events of the same magnitude. This is reflected in the strong shaking experienced in Gisborne.

Thursday, December 20 at 8:55 pm, magnitude 6.8

Major aftershocks to date

Only a weak aftershock sequence has followed the main earthquake, with about 20 earthquakes in the first 4 hours. These earthquakes tend to have relatively short aftershock sequences, with few large aftershocks.

Friday, December 21 at 6:47 am, magnitude 4.5
Friday, December 21 at 8:33 am, magnitude 4.2
Saturday, December 22 at 11:35 am, magnitude 4.8
Sunday, December 23 at 11:49 am, magnitude 3.9

No tsunami expected

The depth of the earthquake at approximately 40 km places it within the subducting Pacific plate, the depth to the plate interface being around 10 km near the earthquake location. An earthquake of this size at this depth is unlikely to disrupt the sea floor and cause a tsunami.

Reported damage

The Ministry of Civil Defence and Emergency Management report that 3 buildings have collapsed in the central business district of Gisborne, and severe structural damage has occurred to others.

GeoNet felt reports to date indicate the mainshock was felt from Auckland to Dunedin and on the Chatham Islands. Aside from the severe damage in Gisborne noted above, small items (eg. glassware, ornaments, books) toppled and fell off shelves in many locations including Gisborne, Hawke's Bay, Bay of Plenty, Palmerston North and Wellington. Contents were ejected from cupboards in Gisborne, Hawke's Bay, Bay of Plenty and Wellington. Small appliances and furniture (eg. TVs) slid a lot or fell over in Gisborne, Hawke's Bay, Bay of Plenty, Paraparaumu and Wellington, and large appliances and furniture fell over in Gisborne. Electricity was affected in Gisborne and Napier, and other infrastructure such as gas, telephone, internet, water and sewage were also affected in Gisborne. Broken crockery and glassware were reported in Gisborne and Wairoa. Building damage such as damage to chimneys, broken windows and cracks in exterior walls was reported from Gisborne and Wairoa, and severely distorted buildings and totally collapsed walls were reported in Gisborne.

Past earthquakes affecting the Gisborne region

Since 1960, the following earthquakes have caused minor damage or worse:

• Saturday, March 5 1966, magnitude 6.0, centred just offshore Poverty Bay.
• Sunday, March 21 1976, magnitude 5.4, in Hawke Bay just south of Wairoa.
• Monday April 23 1979, magnitude 5.1, offshore in central Hawke Bay.
• Thursday, March 8 1984, magnitude 6.4, deep under the Raukumara Peninsula.
• Thursday, November 30, 1989, magnitude 5.0, 30 km directly under Gisborne.
  
• Tuesday, August 10 1993, magnitude 6.3, centred near Ormond.

You can visit our list of Historic Earthquakes to read about other significant shakes that have affected New Zealand.

This report will be updated as more information comes to hand. Last updated December 24, 2:45 pm.