Defining the geometric segmentation and Holocene slip rate of the Wellington Fault, New Zealand: The Pahiatua section

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.