New Zealand Journal of Geology and Geophysics abstracts
Defining the geometric segmentation and Holocene slip rate of the
Wellington Fault, New Zealand: the Pahiatua section
R. M. Langridge
K. R. Berryman
R. J. Van Dissen
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
r.langridge@gns.cri.nz
Abstract The Wellington Fault is a major active, right
lateral, strike-slip fault in southern North Island that can be divided
into three distinct geometric sections based on changes in neotectonic
character and structural complexities. These are, from south to north:
the Wellington-Hutt Valley segment; Tararua section; and Pahiatua
section. The Pahiatua section is a 42 km long, straight, NNE-striking
fault section defined between two geometric endpoints near Putara in
the south and Woodville. This section has been mapped in detail and
exhibits classic strike-slip tectonic geomorphology and late Quaternary
dextral displacements of up to 125 m. Three trenches excavated at sites
along the Pahiatua section are used to define the dextral slip rate for
this section. At Bennett trench site, a stream is dextrally deflected
50 ± 6 m. Peaty silts underlying “deflected” channel
deposits in
the trench yield an age of 8390–8700 cal. yr BP, providing a
minimum
dextral slip rate of 5.1–6.7 mm/yr. At Hughes 1 site, a stream is
deflected 60 ± 5 m. Peaty material found stratigraphically above
the deflection there was dated (10 500–11 160 cal. yr BP),
yielding a
maximum slip rate of 4.9–6.2 mm/yr for this displacement. Ebbett
1 trench was excavated across the fault zone where a smaller
displacement
(18 ± 2 m) yielded a slip rate range of 3.2–5.2 mm/yr. Our
preferred dextral slip rate (5.1–6.2 mm/yr) comes from the
combination
of the minimum and maximum rates from Bennett and Hughes 1 sites. This
range is generally lower than (but slightly overlaps) that for the
Wellington-Hutt Valley segment (6–7.6 mm/yr). The single-event
displacement range for the Pahiatua section determined from field
measurements is 4.5 ± 1 m. From this data we calculate a
recurrence interval for surface-rupturing earthquakes of 564–1080
yr. The overlap of results from both the Pahiatua section and the
Wellington-Hutt Valley segment allow us to assess the likely slip rate,
single-event displacement, recurrence interval range, and
characteristics of the 53 km long, bush-covered Tararua section of the
Wellington Fault. The data show that both the Pahiatua and Tararua
sections: (1) are moderate slip rate, strike-slip fault sections; (2)
produce multi-metre single-event dextral displacements; (3) have short
recurrence intervals; and (4) individually have the capability to
generate surface-rupturing earthquakes of Mw > 7.
Keywords Wellington Fault; fault segmentation;
Pahiatua section; Tararua section; neotectonics; seismic hazard; slip
rate; recurrence; NZMS 260 sheets S25, T24, T25
G04021; Received 3 May 2004; accepted 2 June 2005; Online
publication date 29 September 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
591–607
0028–8306/05/4804–0591
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