SHAKING UP THE SYSTEM: MAPPING POST-EARTHQUAKE EVOLUTION OF THE WAIAU TOA/CLARENCE RIVER VALLEY, KAIKOURA, NEW ZEALAND

Earthquakes can significantly alter the geomorphology of a river system if the fault surface rupture occurs in the river channel. Fault rupture through rivers can result in the formation of pools and lakes, the abandonment of channels, and the reactivation of pre-existing channels in the area. River systems can continue to change for months to years after the initial earthquake. Research concerning faults that intersect rivers is sparse. Here we show how the 2016 Mw 7.8 Kaikoura earthquake in New Zealand altered the Waiau Toa/Clarence River valley through the creation of a new, preferred channel and a fault-dammed pool. We map geomorphic units from aerial images taken pre-earthquake and at 2-month intervals post-earthquake. The diversity of landforms increased immediately post-earthquake and sporadically decreased in response to large rainfall events. However, landform diversity remained higher than pre-earthquake conditions over the course of the study. The most significant areas of lateral migration occurred closer in time to the earthquake events and were more prominent in areas with lower topographic relief. Our results demonstrate that fault rupture within a system can drastically shape the geomorphology of the river and can create long lasting changes that resist typical phenomena such as fluctuations in rainfall and discharge. We anticipate that these results can inform models relating to fault-river intersections and serve as an example of how to study future impacts so landowners can be better informed and prepared.