Coastal earthquake induced landslide susceptibility during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand

Abstract. Coastal hillslopes often host higher concentrations of earthquake induced landslides than those further inland, but few studies have investigated the reasons for this occurrence. As a result, it remains largely unclear if regional earthquake induced landslide susceptibility models trained primarily on inland hillslopes are effective predictors of coastal susceptibility. The 2016 Mw 7.8 Kaikōura earthquake on the northeast South Island of New Zealand resulted in c. 1,600 landslides > 50 m2 on slopes > 15° within 1 km of the coast. This forms an order of magnitude greater landslide source area density than inland hillslopes within 1 to 3 km of the coast. In this study, the distribution of regionally predictive landslide susceptibility variables, or features, and logistic regression modelling are used to investigate how landslide susceptibility differs between coastal and inland hillslopes and determine the factors that drive the distribution of coastal landslides initiated by the 2016 Kaikōura earthquake. Strong model performance (Area under the Receiver Operator Characteristic Curve or AUC of c. 0.80 to 0.92) was observed across eight models, which adopt four simplified geology types. The same landslide susceptibility factors, primarily geology, steep slopes, and ground motion are strong model predictors for both inland and coastal landslide susceptibility in the Kaikōura region. In three geology types (which account for more than 90 % of landslides source areas) a 0.03 or less drop in model AUC is observed when predicting coastal landslides using inland trained models. This suggests little difference between the features driving inland and coastal landslide susceptibility in the Kaikōura region. Geology is similarly distributed between inland and coastal hillslopes and PGA is generally lower in coastal hillslopes. Slope angle, however, is significantly higher in coastal hillslopes and provides the best explanation for the high density of coastal landslides during the 2016 Kaikōura earthquake. Existing regional earthquake induced landslide susceptibility models trained on inland hillslopes using common predictive features are likely to capture this signal. Interestingly, in the Kaikōura region, most coastal hillslopes are isolated from the ocean by uplifted shore platforms. Enhanced coastal landslide susceptibility from this event appears to be a legacy effect of past active erosion, which preferentially steepened these coastal hillslopes.