ROLE-PLAY

As with many other central South Island reverse faults (e.g. SCFS), the geologic slip rates presented for faults in this study fall well below those predicted by geodetic modelling (Berryman et al. 2002; Wallace et al. 2007; Amos et al. 2007). Even discounting the predicted strike-slip component across the FPF and FCF, for which there is no evidence in the field, and accounting for conversion of surface slip to subsurface slip, geologically derived rates are 1-4 mm yr-1 slower than geodetic rates. Three possible reasons for this are the (i) the under-prediction of actual slip rates because the FPF and FCF are late in their seismic cycles, (ii) unmeasured distributed deformation on the hanging walls of the faults, or (iii) partitioning onto unrecognised faults in the eastern Southern Alps. Taking into

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account paleoseismic data from both faults (i.e. over a hanging wall deformation zone of c. 5-10 km for the FPF) paleoseismically-determined slip rates are still significantly smaller than geodetic rates (Fig. 3.33).

Figure 3.33: Best estimates of combined slip rates for the Fox Peak and Forest Creek Faults. Net slip PDF is derived by adding a FCF SED of 1 m to each event in Fig. 3.29. Age probability distributions are based off simplified and combined PDFs of event ages for the FPF and FCF (Appendix 5). The places where the two PDFs intersect form a line with a slope equal to the combined slip rate that is separate from the surface slip rates derived earlier for the FPF. The best-fitting line is for a slip rate of c. 1.2 mm yr-1. Slip rates of 0.7, 2.0 and 5.0 mm yr-1 are shown for reference, with the latter two representing the limits of dip-slip rates from Wallace et al. (2007).

It is likely that the ‘missing’ dip-slip and strike-slip components are taken up on either unidentified faults in the Southern Alps (e.g. Cox et al. 2012) or many small faults and folds surrounding the geodetic block boundary defined by Wallace et al. (2007). Structures like the SCA and EFPF, while clearly not as active as the FPF and FCF, may accommodate some of the shortening over time. The contribution of NW-striking reverse and left-lateral oblique faults south of the FPF may also account for some of the ‘missing’ shortening.

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3.10 Conclusions

Determining the rupture segmentation, and thus seismic hazard, of active reverse faults requires an examination of fault behaviour over many temporal scales. The cumulative net slip of reverse faults is reflected in the topographic expression of the hanging wall anticlines (mountain ranges) they produce. For the Fox Peak and Forest Creek Faults (FPF and FCF), this has resulted in the uplift of the Sherwood and Two Thumb Ranges, respectively. Modern surface expression of the faults and structural mapping were used to establish the endpoints of geometrically-defined segments along the rangefronts. The locations of spatially abrupt slip rate gradients on the FPF correlate well with the structure inferred from long-term topography (e.g. Fig 3.18), indicating that the segments accumulated displacement without significant lengthening (e.g. Amos et al. 2010). Despite large uncertainties in paleoseismic event ages, clustering of events and large single-event displacements suggest that geometrically and slip rate delineated segment boundaries are most likely breached by earthquakes, or that segment ruptures occur in close temporal succession.

The major findings of the present study are as follows:

(i) The Cloudy Peaks Segment of the FPF is an imbricate thrust wedge that has formed in response to the foreland propagating FCF; it has been assimilated into the FPF. This is reflected in the overlap of the Two Thumb and Sherwood Ranges.

(ii) The Eastern FPF (EFPF) and Stony Creek Anticline (SCA) are not as active as the FPF and FCF, and should be considered separate seismic sources.

(iii) The MRE on the FPF and FCF occurred less than c. 3 ka ago, and probably around 2.5 ky ago. The recurrence interval determined from surface slip rates and single event displacements, as well as through grouping of paleoseismic event ages, is 2-3 ka. The faults are thus likely to be late in their seismic cycles.

(iv) Trenches across bending moment faults provide important, but incomplete, archives of surface ruptures on the underlying principal fault.

(v) As the crestal graben and zone of hanging wall extension expands more normal faults will be created or activated. Therefore, multi-event scarps will lie closest to the thrust front and may fail simultaneously with young, single event scarps near the anticlinal axis.

(vi) The MW derived from segment-specific SEDs are much larger than those derived from

the segment length. This, along with the age overlap of paleoseismic events and likelihood of off-fault, distributed deformation on the Ribbonwood Segment, points towards full-length, multi-segment FPF earthquakes.

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(vii) The best estimate MW for full-length rupture of the FPF is 7.2 ± 0.3, though full

characterisation of magnitude potential requires delineation of fault segments as well as consideration of fault-to-fault rupture probabilities.

(viii) Geologically-derived slip rates for the Fox Peak and Forest Creek Faults fall between 1-1.5 mm yr-1, collectively.

(ix) Future studies of segmented reverse faults should focus on defining segment boundaries as barriers to earthquake propagation.

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3.11 Appendix 1

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3.12 Appendix 2

Figure 3.35: Rapidly eroding silts and clays from Pliocene Kowai Formation gravels in Butlers Creek. (A) Butlers Creek in flood; (B) The source of the suspended sediment is from the Kowai outcrop (right branch), and not upstream (left branch). Thus, the inferred age earthquake shaking, as deduced by the presence of prominent

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3.13 Appendix 3

Figure 3.36: Microtopographic (Total Station) survey of Trench 1 site. White footprint is approximate location of the trench. The trench location is within a graben crossing a paleochannel. The local topographic low increases the likelihood of ‘trapping’ suitable sediments and charcoal for dating the stratigraphy. Trench 2 is

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3.14 Appendix 4

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3.15 Appendix 5

Figure 3.41: Probability density functions of event ages from paleoseismic trenching. The shape of some distributions were specified a priori based on geologic observations and constraining ages. (i) MRE in Trench 5 (Forest Creek Fault) as an exponential function decreasing from c. 500 years BP to a maximum value of 3500 years BP; (ii) MRE in Trench 3 (Fox Peak Fault at Cloudy Peaks) as an exponential function decreasing from a maximum value of 2500 years BP; (iii) MRE in Trench 4 (Fox Peak Fault at Fox Peak ski field road, Bray Segment) as an exponential function decreasing from a maximum age of 3500 years BP; (iv) Penultimate event at the South Opuha River terraces, inferred from terrace ages as a normal distribution with 2σ constrained by upper and lower 95th percentiles for bounding terrace ages; (v) Combined uniform distribution for the penultimate and antepenultimate events in Trench 5 – a simplification to remove large uncertainties in the age of the antepenultimate event; (vi) Inferred penultimate event age at Cloudy Peaks from on-fault recurrence interval and bounding ages of MRE/antepenultimate events; (vii) Preferred age of the antepenultimate event at Cloudy Peaks (Trenches 1 and 2); (viii) Age of antepenultimate event inferred from abandonment (SHD) age of T6. The combined curve was derived by concatenating the probability distributions for each event.

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