Earthquake in Auckland?
FRIDAY SEPTEMBER 17, 2010
Could an Earthquake happen in Auckland?
NZ is actually about half the size of Australia, but most is under the sea with only the high land - North Island, South Island, Chathams and Campbells Island jutting above water. 25 million years ago Auckland was under the sea. About 20 million years ago NZ was cleaved by the boundary betyween the Pacific and Indian plates, and movements between these plates have caused earthquakes to the present time. Because of movements in this plate the South Island crumpled to form the Southern Alps and the North Island stretched to form the Taupo Volcanic Zone.
The belt of volcanic activity has been moving southwards through the North Island for millions of years as the east of the NI slips to the south, relative to the west. 20 million years ago East Cape lay alongside Northland, and as the land slipped south, younger volcanoes sprung up in its wake. 15 million years ago Great Barrier Island was in eruption, and 5 million years ago the Coromandel peninsula was volcanically created. As it moved south the eastern half of the North island rotated clockwise, creating a rift about 2 million years ago which produced volcanic events from Taupo to Mt Edgecumbe. Lake Taupo has erupted many times and is still considered an active volcano.
There were a chain of volcanoes along the west coast. The main eruption forming Auckland was a huge volcano at Muriwai about 17 million years ago. This Waitakere volcano has mainly been eroded, but conglomerate from it formed the Waitakere Ranges. The foothill area is Auckland today, and ash deposit can be found in Parnell and on the eastern foreshore. The Waitemata Harbour was once the Waitemata River, a valley draining water away from the Waitakere hills with headwaters in the north which included the Kaipara harbour area. The Waitakere Ranges also contain many dikes, volcanic plugs, etc., that formed part of this volcano. Streams from the Waitakere Ranges still flow down narrow gorges surrounded by conglomerate rocks formed from debris from the Waitakere volcano. The Piha Gorge, Pararaha Gorge and Karamatura Valley are good examples. Maori Bay, just south of Muriwai, the home of a gannet colony, has good examples of pillow lavas that would have come from the volcano that formed Auckland. If you stand on the beach where the road is at Maori Bay and look back at the cliffs you will see pillow lava that looks like big logs.
After the rising up of Auckland there was a period of general geological quiet. About 7 or 8 million years ago there was a renewal of earth movements that still periodically shake NZ. Auckland is not considered vulnerable. Strong earth tremors felt in Auckland are usually centred to the south, with the most violent in recorded history being off the Waikato Heads in 1891 and shocks at Pakiri in 1956. An area that gets only two in 120 years can hardly be considered high-risk. There have been no earthquakes in the Auckland area, the most we can expect are reports of shakes "felt as far away as Auckland," for instance the 1934 Horoeka earthquake.
The active fault lines in NZ mostly go from Fiordland under the Alps to the Marlborough and then from Wellington to the right of Taupo and under White Island, effectively a south-north fault line. East/west fault lines appear to be only in the N Canterbury/Marlborough region. Auckland has no known active surface faults, although the Hunua Ranges in the southeast shows where the ground has been pushed upwards as the result of fault ruptures several tens of thousands to millions of years ago. Even so, and although rare, there has been one, a shake that registered 4.5 mag, in 2007, that occurred in the Gulf, centered 30 miles offshore from Orewa. I attach for reference a link to article. It was felt it at the time by anyone living in Panmure. It's location was distant from the city itself, and could be argued to be part of the fault passing through White Island to the southeast and curving north towards the Kermadecs. It appears to be the only one in recent times to be reported in the Greater Auckland region.
Rather than earthquakes, it is more likely Auckland's dormant volcanoes, which began 100,000 years ago, would spring to life first, but residents of Mt Eden, Mt Roskill, Mt Albert and Mt Wellington do not seem to be overly concerned, nor need they be. On seismic grounds it is believed that the whole of NZ north of the Manukau Harbour has been an essentially stable area since long before the beginning of the Ice Age about a million years ago.
Aucklanders can relax.
(photo: Chris Gin)