Using displaced river terraces to determine Late Quaternary slip rate for the central Wairarapa Fault at Waiohine River, New Zealand

Abstract

We analyse a flight of post-Last Glacial Maximum terraces at Waiohine River, New Zealand that are progressively displaced by the dextral-slip Wairarapa Fault. The Waiohine River is shown to have smoothed its faulted river banks after each earthquake so that riser displacements are only recorded after abandonment of their lower bounding terrace tread. Digital elevation models produced from newly collected high-precision topographic data allowed us to precisely measure the cumulative dextral displacement of five risers and two palaeochannels, and the vertical displacement of six treads. The ratio of horizontal to vertical slip at the terraces ranges from 5.0 to 10.2 (average of 6.993.5 (2s)). Combining our new displacements for the 'Waiohine' aggradation terrace and next younger terrace with new and previous optically stimulated luminescence (OSL) ages for Waiohine terrace silts, we calculate Late Quaternary dextral slip rates for this central part of the Wairarapa Fault of 11.992.9 mm a (1 (2s) and !9.291.3 mm a(1 (2s). Based on the smallest and next smallest observed offsets, the magnitudes of the inferred 1855 (smallest) and penultimate (next-smallest) single-event displacements are 12.490.8 m (2s) and 9.791.7 m (2s), respectively.

Conditional probability of rupture of the Wairarapa and Ōhariu faults, New Zealand

“…At Waiohine River there is a flight of alluvial terraces that are progressively displaced, both vertically and dextrally, by the Wairarapa Fault (Lensen & Vella 1971;Grapes & Wellman 1988;Carne et al 2011). This is the only site along the Wairarapa Fault, south of its intersection with the Carterton and Masterton faults, where well preserved and dated landforms having an age of 10 ka are cut and offset by the fault in a place where it consists of a single strand.…”

“…The Waiohine terrace is regarded as equivalent to the Ohakean terrace, both surface and fill, elsewhere in the North Island (Palmer 1984). The age of aggradation, cessation of aggradation and subsequent abandonment of the Ohakean (0Waiohine) terrace has been the subject of a good deal of recent investigation (Formento-Trigilio et al 2003;Litchfield & Berryman 2005;Wang & Grapes 2008;Carne et al 2011; see also references cited therein). Litchfield & Berryman (2005) interpret the timing of abandonment of the Ohakean terrace in the three largest catchments in southeastern North Island to be 15Á20 ka.…”

“…As described in Carne et al (2011), there is no direct measurement of dextral displacement of the Waiohine surface at Waiohine River, but one can be approximated by applying the horizontal to vertical displacement ratio derived from younger offset terraces in the Waiohine sequence to the known vertical displacement of the Waiohine surface. Wellman (1955), and more recently Carne et al (2011), have done just this, and Carne et al estimate a dextral displacement of 129918 m for the Waiohine surface which encompasses the c. 120 m estimate of Wellman (1955).…”

“…Wellman (1955), and more recently Carne et al (2011), have done just this, and Carne et al estimate a dextral displacement of 129918 m for the Waiohine surface which encompasses the c. 120 m estimate of Wellman (1955). For the purposes of our slip-rate calculations, we adopt a triangular probability distribution for the estimated dextral displacement of the Waiohine surface with maximum probability at 129 m, and minimum and maximum bounds at 111 and 147 m, respectively.…”

“…At Waiohine River the next terrace below the Waiohine surface is Terrace B (adopting the nomenclature of Lensen &Vella 1971 andCarne et al 2011 (2008) samples collected from the higher Waiohine terrace. We suspect that these three samples of Carne et al (2011) were incompletely bleached, and have retuned ages that are older than the true age of deposition.…”