Landslides and ground damage caused by the Mw 7.1 Inangahua earthquake of May 1968 in northwest South Island, New Zealand

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Hancox, G.T.; Ries, W.F.; Lukovic, B.; Parker, R.N. 2014 Landslides and ground damage caused by the Mw 7.1 Inangahua earthquake of May 1968 in northwest South Island, New Zealand. Lower Hutt, N.Z.: GNS Science GNS Science report 2014/06 89 p. + folded map

Abstract: This report presents the results of a retrospective study of landsliding and ground damage caused by the MW 7.1 Inangahua earthquake of 24 May 1968 in northwest South Island, New Zealand. A comprehensive database of 1400 coseismic landslides has been compiled using 1968 and 1974 aerial photos, geotechnically evaluated, and analysed in ArcGIS. This new landslide dataset comprises 1400 landslides larger than 2500 m2 (~50 x 50 m), of which 1154 are first-time failures, and 246 (18%) are reactivations or enlargements of landslides caused by the 1929 Murchison earthquake. The total area affected by landslides was ~3500 km2, extending from Murchison to Westport and north to Little Wanganui. Most (83%) of the landslides occurred within an area of about 1250 km2 within 20¨C30 km of the epicentre near Inangahua and the mountains north and south of the Buller River. Within this main landslide-affected area Modified Mercalli shaking intensities ranged from MM9¨CMM10, with peak ground acceleration (pga) of 0.58 g recorded at Reefton, and ¡Ý1.0 g estimated at Inangahua. Environmental criteria (mainly the size and density of landslides) were used to revise the extent of the MM8¨C10 isoseismals in isolated hill country where there are few buildings or inhabitants. Landslide occurrence shows strong correlation with the main area of aftershocks and the 30 km-long seismogenic fault plane, which dips 45¡ã northwest and had at least 4 m reverse fault slip. The landslides were clearly concentrated on the hanging wall of this fault, mainly to the west and north of Inangahua. There is statistical evidence to show that some slopes that did not fail in 1929 but failed in 1968, were to some extent preconditioned by the 1929 earthquake, and this could have influenced the distribution of landslides in 1968. (auth)