Insights into temporal earthquake clustering from the Settlement Fault, southeastern Otago, Aotearoa New Zealand

Abstract

We combine previous studies, fieldwork, lidar data, and trenching to examine late Quaternary activity of the SE-dipping ≥23 km long Settlement Fault in the southeastern South Island. Trenching of a scarp crossing a small alluvial fan exposed a > 3 m thick sequence of folded sandy to silty fan alluvium. The uniformity of folding and four Optically Stimulated Luminescence dates indicate that these sediments were deposited relatively rapidly at ∼20 ka and subsequently accommodated 2.5 m of throw following ≥1 ruptures on an underlying buried reverse fault. Other key constraints on late Quaternary Settlement Fault activity are: (1) stranding of a shore platform at ∼3.7 ka, due to 1.5–2 m of fault uplift, and (2) a flat bedrock erosion surface overlain by loess and correlated to the last interglacial maximum (∼125 ka), indicating 0–4 m of fault uplift. From incorporating this information into a stochastic model of fault displacement accumulation we derive a slip rate of ∼0.02 mm/yr since 125 ka, and ∼0.27 mm/yr since 20 ka. We link the Settlement Fault’s current phase of heightened activity to a similarly timed increase in Akatore Fault activity, despite the tips of these faults being separated by between 9 and 25 km.