Pre-Christchurch earthquake sequence rockfalls in the Port Hills, Christchurch : Wakefield Avenue trench

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Litchfield, N.J.; Van Dissen, R.J.; Massey, C.I. 2016 Pre-Christchurch earthquake sequence rockfalls in the Port Hills, Christchurch : Wakefield Avenue trench. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/25. iii, 32 p.

Abstract: The 2010–2011 Christchurch Earthquake Sequence (CES) triggered widespread rockfalls in the Port Hills, Christchurch, but to date there has only been one study constraining the timing of past rockfall events. Understanding the magnitude and frequency of past rockfall events is important to place the CES rockfall events in the longer-term context, and also to provide geomorphic constraints on the seismic hazard. We undertook a pilot study to investigate the timing and magnitude of past rockfall events through excavation of a trench through CES-rockfall and older talus. In this report we present the stratigraphy, surveying and radiocarbon dating results from the trench excavated below a fossil coastal cliff at 48 Wakefield Avenue, Sumner. The Wakefield Avenue Trench exposed a ~5 m thick sequence of beach sands, dune sands, and colluvium with variable amounts of gravel and buried soils. One prominent, clast-rich, wedge-shaped, colluvial layer is interpreted as the probable toe of a rockfall deposit and the young charcoal ages from above, below and within this layer suggest a maximum age for the rockfall event is 300 cal. yr BP (AD 1650). Shells from beach sands near the base of the trench have a maximum age of 1,530–1,300 cal. yr BP, and show that the entire sequence is relatively young. The upper dune sand is historical, as it contains nails, glass, and pieces of metal. The large amount of dune and beach sand exposed in the trench suggests that the previous estimates of pre-CES rockfall volumes calculated for the talus slopes at this site may be overestimated, however it is noted that only the distal limits of the talus slope were trenched. The probable rockfall event identified in the Wakefield Avenue trench is younger than the event dated at Rapaki (8,000–6,000 years old), and it could potentially be a historical event possibly recorded in archaeological or newspaper records. (auth)