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New Zealand Geology and Earthquakes

Nothing is boring in NZ, even the geology is extraordinary!

The mostly submerged New Zealand continent, Zealandia, sits uneasily astride two moving segments of the planet’s surface – the Pacific and Australian plates. In the North Island, the boundary between the plates lies off the East Coast along a depression, the Hikurangi Trough, at the edge of the continental shelf. In the Marlborough region, the boundary cuts diagonally across the South Island to the West Coast. It then continues south-westward along the great Alpine Fault, and runs back out to sea near Milford Sound.

The two moving plates are colliding at a glancing angle. In the process, the sunken New Zealand continent is crumpling to form the land that now projects above sea level. In the north, ocean floor at the surface of the Pacific Plate to the east plunges beneath the continental shelf off the eastern North Island. As it does so, it pushes up the overlying rocks and sediments, creating the hilly terrain of the eastern North Island. In the South Island, the two plates are directly colliding along the Alpine Fault. This causes a much greater uplift, forming the Southern Alps.

At the same time, the country is being wrenched apart. Along the Alpine Fault in the South Island, the West Coast region west of the plate boundary is moving north-east at 2–3 metres per century, relative to the Southern Alps on the eastern side. As this movement continues in the future, the South Island will become more elongated

Large Earthquakes in New Zealand

Large Earthquakes in NZ

March 15 2011, Christchurch quake has probably claimed approx. 180 victims and billions in damages. The previous fatal earthquake in New Zealand was on the West Coast of the South Island in May 1968. Three deaths resulted. For the latest news see:gns_website

GNS Video about the Christchurch Quake

This is a technical video and a bit dry. Also see the Wellington fault video below:Wellington Fault

Earthquake Risk Areas in New Zealand

Another reason for living in Auckland, low earthquake risk

Aucklands Low Risk

From NZ Herald March 5 2011: Central Auckland would suffer only a few casualties if struck by an earthquake similar to the Lyttelton shake. A 2006 Auckland City Council report found that a 6.5 magnitude quake could cause 11 deaths, from a daytime population of 600,000, and more than $1 billion in initial damage. That's because of the hard volcanic rock under Auckland's feet and the building boom after stricter construction standards were introduced. Christchurch, on the other hand, sits on a lump of jelly. The soft sediment which washes down from the Southern Alps amplified the intensity of the Lyttelton earthquake.

Auckland's closest plate boundary is at Taupo, and the nearest faultline at 30km to 60km from the central city moves every 5000 to 10,000 years. But Auckland is not immune. The city's waterfront is built on reclaimed land, and all unreinforced masonry structures below Shortland St would suffer badly in a large tremor. About 5 per cent of commercial buildings and 2 per cent of residential structures in Auckland's CBD, worth $63 billion, are unreinforced. A council register lists 400 structures built less robustly, which don't comply with 33 per cent of current building codes. But because the seismic risk in Auckland is so low, the owners aren't encouraged to upgrade them until they renovate or make changes.

Auckland Council building control manager Bob de Leur said that unlike Christchurch, much of Auckland's CBD consisted of reinforced concrete structures that were built since the 1970s, when stricter building codes were introduced. The greatest harm could occur in older town centres such as Dominion Rd's shopping area, and liquefaction would seep into Auckland's suburbs and low-lying coastal areas.

The Wellington Faultline Video

Pacific Ring of Fire

Pacific Ring of Fire

The Pacific Ring of Fire is an area of frequent earthquakes and volcanic eruptions encircling the basin of the Pacific Ocean. In a 40,000 km horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. The Ring of Fire has 452 volcanoes and is home to over 50% of the world's active and dormant volcanoes. It is sometimes called the circum-Pacific belt or the circum-Pacific seismic belt.

Ninety percent of the world's earthquakes and 81% of the world's largest earthquakes occur along the Ring of Fire. The next most seismic region (5–6% of earthquakes and 17% of the world's largest earthquakes) is the Alpide belt which extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. The Mid-Atlantic Ridge is the third most prominent earthquake belt.

The Ring of Fire is a direct result and consequence of plate tectonics and the movement and collisions of crustal plates. The eastern section of the ring is the result of the Nazca Plate and the Cocos Plate being subducted beneath the westward moving South American Plate. A portion of the Pacific Plate along with the small Juan de Fuca Plate are being subducted beneath the North American Plate. Along the northern portion the northwestward moving Pacific plate is being subducted beneath the Aleutian Islands arc. Further west the Pacific plate is being subducted along the Kamchatka Peninsula arcs on south past Japan. The southern portion is more complex with a number of smaller tectonic plates in collision with the Pacific plate from the Mariana Islands, the Philippines, Bougainville, Tonga, and New Zealand. Indonesia lies between the Ring of Fire along the northeastern islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores, and Timor. The famous and very active San Andreas Fault zone of California is a transform fault which offsets a portion of the East Pacific Rise under southwestern United States and Mexico. The motion of the fault generates numerous small earthquakes, at multiple times a day, most of which are too small to be felt. The active Queen Charlotte Fault on the west coast of the Queen Charlotte Islands, British Columbia, Canada, has generated three large earthquakes during the 20th century: a magnitude 7 event in 1929, a magnitude 8.1 occurred in 1949 (Canada's largest recorded earthquake) and a magnitude 7.4 in 1970.

World map of earthquakes from the last 100 years

Earhquakes for the last 100 years, From NZ Herald

GNS Website

volcanos_and_faultlines/geology.txt · Last modified: 2012/07/04 13:27 by art
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