Alluvial fan response to Alpine Fault earthquakes on the Westland piedmont, Whataroa, Aotearoa‐New Zealand

We examined the stratigraphy of alluvial fans formed at steep range front Southern Alps Te Taho, on north bank Whataroa River in central West Coast, South Island, New Zealand. The coincides with Alpine Fault, an Australian-Pacific plate boundary fault, which produces regular earthquakes. Our study revealed aggradation 100- to 300-year intervals. Radiocarbon ages and soil residence times (SRTs) estimated by a quantitative profile development index allowed us elucidate characteristics four episodes since 1000 CE. postulate repeating mode fan behaviour (fan response cycle [FRC]) linked earthquake cycles via earthquake-triggered landslides. FRCs are characterised short time (aggradation followed incision) long phase when channels entrenched surfaces stable (persistence time). Currently, Taho latter phase. building we determined from OxCal sequence model correlate Fault earthquakes (or other subsidiary events) support prior landscape evolution studies indicating ≥M7.5 as main driver episodic sedimentation. findings consistent historic non-earthquake events Coast but indicate faster responses than sites Zealand elsewhere where rainfall stream gradients (the basis for power) lower. Judging thickness deposits times, conclude that pastoral farming (current land-use) probably across much would be impossible several decades after major earthquake. sustainability regional tourism agriculture is risk, more so because vulnerability single through road region (State Highway 6).