Geology and geological hazards of the Auckland urban area, New Zealand

Auckland, in the north of New Zealand's North Island, is the largest urban area in the country, accommodating over a quarter of its population. The Auckland landscape is one of subdued rolling hills formed on weathered Mesozoic and Early Miocene sedimentary rocks, with many distinctive small volcanic cones rising abruptly from low hills in the central isthmus area, between the Hauraki Gulf on the east coast and Manukau Harbour in the west. Auckland's geology consists of basement Mesozoic greywackes overlain by Late Eocene to Early Miocene, and locally Pliocene, mainly marine sediments and volcaniclastics. Allochthonous Cretaceous and Paleogene rocks are present in the north. Thin, mainly terrestrial, Quaternary sediments cover older rocks in lowland areas and locally interfinger with and are overlain by Quaternary basalts. Geological hazards facing the urban area include slope instability, the threat of renewed eruption within the Auckland Volcanic Field, and the growing recognition of the potential effects of ash deposits from distant volcanic centres (central North Island). Less well appreciated are hazards related to seismic activity including ground shaking amplification, fault rupture, and tsunami. Recent recognition of at least one active fault close to the urban area has changed the perception of seismic risk. (C) 2002 Elsevier Science Ltd and INQUA. All rights reserved.