Auckland regional faults
Auckland regional faults | |
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New Zealand geology database (includes faults) |
The Auckland regional geologic faults have low seismic activity,[6][1] compared to much of New Zealand, but do result in an earthquake risk to the Auckland metropolitan area, New Zealand's largest city. There is also evidence of past tectonic, volcanic associations in a city located within what is, at best, a very recently dormant Auckland volcanic field.
The only definite active fault in the Auckland Region, as defined geopolitically, is the Wairoa North Fault.[2] However, there could be other possibly active faults within the Auckland region. These include the Drury Fault[7] and the Firth of Thames Fault.[8] Further, the adjacency of the Hauraki Rift to Auckland means that the active Kerepehi Fault with its risk of magnitude 7 or above events is relevant to seismic risk.[3]
General context
As much of Auckland is located upon two volcanic fields which have been active relatively recently in the geological timescale context, evidence of active faulting could be buried under volcanic deposits. A similar situation existed with recent sedimentary deposits in the Canterbury region of New Zealand, and meant the 2010 Canterbury earthquake and its significant 2011 Christchurch aftershock were unexpected. Accordingly, there have been recent attempts to better understand the multiple inactive faults which are associated with the landforms of the Auckland region.[4]
Geology
The basement rock structures in the Auckland region were deposited in the late
North Auckland
The predominant inactive North-South faults in the west are the Muriwai-Helensville Fault, which leads into the faults that underlie the extinct volcanic remnants of the
Central Auckland
Waitākere Ranges
The Waitākere Ranges have a large number of known inactive faults. From their west, there are the North-South faults of the postulated Westcoast linement matching volcanoes as already noted, extending with a postulated fault line through the North Manakau Heads. The proven Hiui fault extends North – South up the valley of the dam and the ridge line as seen from the city, is related to the East Scenic Drive Fault, which has been characterised for over 23 miles.[4] This last fault may continue to the south as the inferred Awaiti Fault.[4]
Inferred Central Auckland Faults
The Cornwallis fault is largely inferred as the southern limit of the Waitākere Ranges being in the middle of the outlet of Manukau Harbour to the sea. Under the city proper, there is a large number of inferred faults with at least moderate confidence from the sampling data.[4] Essentially, the volcanic rock overlay of the Auckland volcanic field hides surface confirmation, and few of the volcanic vents are directly over such postulated faults, although many are within 500 m (1,600 ft) of these.[4] The first proven faults to the east are on Motutapu Island and the short Bucklands Fault which extend then into the proven faults of the Hunua Ranges as you go south.
Hunua Ranges
The Hunua Ranges have significant North-South and East-West faults and extend from the Drury Fault to the Firth of Thames Fault which defines the eastern border of the Hauraki Rift. Much of the recent seismic activity in the Auckland Region has been under the Hunua Ranges.[6]
Hauraki Plains
The largest amount of recent seismic activity in the Auckland Region is associated with the western borders of the Hauraki Rift extending into the Hauraki Gulf where presumably faults exist.[6] In the middle of the land portion of the rift, but outside the Auckland Region, is the Kerepehi Fault in the Hauraki Plains. Because it has the potential to have earthquakes greater than magnitude 7 its presence dictates current building codes in the Auckland region.[3][7]
South Auckland
The predominant fault is the Drury Fault which trends NNW along the base of the Hunua Range foothills. It is associated with a range of old volcanoes to the west of Drury. Assessment in 2005 identified that the last rupture was about 45,000 years ago, with current slip rates in the range of 0.01 mm (0.00039 in) to 0.03 mm (0.0012 in) per year. Some of the recent seismic activity in Auckland has been close to the line of the Drury Fault and to its east.[6] Accordingly there may be some seismic hazard.[7] Moving towards the west from the Drury Fault in the north there are a fair number of mainly East – West orientated inferred subsurface faults. To the south in the area of the Bombay Hills and the Waikato River are a number of defined faults which do appear to have relationships with southern South Auckland volcanic field volcanoes. These include the Waikato Fault which is the most southern fault in the region, and to its north the Pukekoke , Aka Aka, St Stephens and Pokeno faults.[4] There have been faults identified by seismic surveys off the west coast as part of oil exploration work.[5]
Tectonic volcanism
A line of extinct