Earthquake contributions to coastal cliff retreat

Modeling suggests that steep coastal regions will experience increasingly rapid erosion related to climate-change-induced sea level rise. Earthquakes can also cause intense episodes of coastal cliff retreat, but coseismic failures are rarely captured in the historical record used to calibrate most cliff retreat forecast models. Here, we disaggregate cliff-top retreat related to strong ground motion and non-seismic sources, providing a unique window into earthquake contributions to multidecadal coastal cliff retreat. Widespread landsliding and up to ca. 19 m of coastal cliff-top retreat occurred in the area of Conway Flat during the 2016 Kaikoura (New Zealand) earthquake despite relatively low (ca. 0.2 g) peak ground accelerations. While coastal cliff-top retreat has been spatially and temporally variable over the historical record, aerial imagery suggests that large earthquake-induced landslide-triggering events disproportionately contribute to an average 0.25 m yr-1 retreat at Conway Flat. The 2016 Kaikoura earthquake represents ca. 24 % of the total cliff-top retreat over 72 years and ca. 39 % of cliff-top retreat over 56 years. Additionally, we infer that significant retreat between 1950 and 1966 is the result of local seismicity. Together these two events account for ca. 57 % of cliff-top retreat over 72 years. Earthquake-related debris piles at the base of the cliffs have been rapidly eroded since the 2016 Kaikoura earthquake (more than 25 % loss of debris volume in 5 years), and there will likely be little evidence of the earthquake within the next decade. In regions with similar lithologic and coastal conditions, evidence of past widespread single-event cliff-top retreat may be limited or non-existent. The results demonstrate that cliff-top retreat projections using historical records may significantly underestimate true retreat rates in seismically active regions.