New Zealand Journal of Geology and Geophysics abstracts
Holocene paleoearthquakes on the strike-slip Porters Pass Fault,
Canterbury, New Zealand
Matthew Howard1*
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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