New Zealand Journal of Geology and Geophysics abstracts

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Holocene paleoearthquakes on the strike-slip Porters Pass Fault, Canterbury, New Zealand

Matthew Howard^1*
Andrew Nicol²
Jocelyn Campbell¹
Jarg R. Pettinga¹

¹Natural Hazards Research Centre
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
²Institute of Geological & Nuclear Sciences Ltd
P.O. Box 30 368
Lower Hutt, New Zealand

^*Present address: URS NZ Ltd, P.O. Box 4479, Christchurch, New Zealand.

Abstract  The Porters Pass Fault comprises a series of discontinuous Holocene active traces which extend for c. 40 km between the Rakaia and Waimakariri Rivers in the foothills of the Southern Alps. There have been no historical earthquakes on the Porters Pass Fault (i.e., within the last 150 yr), and the purpose of this paper is to establish the timing and magnitudes of displacements on the fault at the ground surface during Holocene paleoearthquakes. Displaced geomorphic features (e.g., relict streams, stream channels, and ridge crests), measured using either tape measure (n = 20) or surveying equipment (n = 5), range from 5.5 to 33 m right lateral strike slip and are consistent with six earthquakes characterised by slip per event of c. 5–7 m. The timing of these earthquakes is constrained by radiocarbon dates from four trenches excavated across the fault and two auger sites from within swamps produced by ponding of drainage along the fault scarp. These data indicate markedly different Holocene earthquake histories along the fault length separated by a behavioural segment boundary near Lake Coleridge. On the eastern segment at least six Holocene earthquakes were identified at 8400–9000, 5700–6700, 4500–6000, 2300–2500, 800–1100, and 500–600 yr BP, producing an average recurrence interval of c. 1500 yr. On the western segment of the fault in the Rakaia River valley, a single surface-rupturing earthquake displaced Acheron Advance glacial deposits (c. 10 000–14 000 yr in age) and may represent the southward continuation of the 2300–2500 yr event identified on the eastern segment. These data suggest Holocene slip rates of 3.2–4.1 mm/yr and 0.3–0.9 mm/yr on the eastern and western sections of the fault, respectively. Displacement and timing data suggest that earthquakes ruptured the western segment of the fault in no more than one-sixth of cases and that for a sample period of 10 000 yr the recurrence intervals were not characteristic.

Keywords  Porters Pass Fault; paleoearthquakes; recurrence interval; displacement; Holocene; trench; radiocarbon dates

G02031; Received 20 June 2002; accepted 10 March 2004; Online publication date 23 March 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48: 59–74
0028–8306/05/4801–0059 © The Royal Society of New Zealand 2005

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