The magnitude-frequency relationship of sea cliff failures in strong, low retreat rate cliffs, was studied using systematic historical inventories carried out in the coasts of Portugal and Morocco, in different geological and geomorphological settings, covering a wide size scale, from small to comparatively large rockslides, topples and rockfalls, at different time and spatial scales. The magnitude-frequency expressed in terms of volume displaced and of horizontal area lost at the cliff top showed good fit by inverse power laws of the type p=a.x^−b, with a values from 0.2 to 0.3, and exponents b close to 1.0, similar to those proposed for rockfall inventories. The proposed power laws address the magnitude-frequency for sea cliff failures, which is an important component of hazard assessment, to be completed with adequate models for space and time hazard components. Maximum local retreat at the cliff top provided acceptable fitting to inverse power laws only for failures wider than 2m, with a = 4.0, and exponent b = 2.3, which may be useful to assess the cliff retreat hazard for the use of areas located near the cliff top.

Sea cliff evolution is dominated by the occurrence of slope mass movements of different types and sizes, which are a considerable source of natural hazard, making their assessment a relevant issue in terms of human loss prevention and land use regulations. To address the assessment of the spatial component of sea cliff hazards, i.e. the susceptibility, a statistically based study was made to assess the capacity of a set of conditioning factors to express the occurrence of sea cliff failures affecting areas located along their top. The study was based on the application of the bivariate information value and multivariate logistic regression statistical methods, using a set of predisposing factors for cliff failures, mainly related to geology (lithology, bedding dip, faults) and geomorphology (maximum and mean slope, height, aspect, plan curvature, toe protection), which were correlated with a photogrammetry-based inventory of cliff failures that occurred in a 60 yr period (1947–2007). The susceptibility models were validated against the inventory data using standard success rate and ROC curves, and provided encouraging results, indicating that the proposed approaches are effective for susceptibility assessment. The results obtained also stress the need for improvement of the predisposing factors to be used in this type of study and the need for detailed and systematic cliff failure inventories.