Evidence of tsunami forces shaping clifftop boulder fields from integrated morphometric and numerical modeling approaches

Abstract

Abstract Cliff-top boulder deposits are rare but striking geomorphic features found along high-energy coastlines. Their formation mechanisms—whether from storm waves or tsunamis–remain a subject of scientific debate. This study examines tsunami-induced boulder transport on the steep southern coast of Mallorca Island (Western Mediterranean), integrating detailed morphometric analysis with high-resolution simulations using a non-linear numerical model. Boulder characteristics, including volume, elevation, distance from cliff edges, and orientation, were mapped and used to compute mobilization thresholds under subaerial, submerged, and joint-bounded block (JBB) conditions. Our simulations tested 360 tsunami scenarios varying in wave height, period, and direction. Results show that wave height alone is insufficient to explain boulder transport; instead, velocity thresholds–particularly for saltation and JBB lifting—provide more accurate indicators. The model successfully replicates tsunami propagation, cliff impact, and inland flooding, revealing that only long-period, high-magnitude waves from southern directions exceed the thresholds needed to displace the observed boulders. Boulder deposits differ across the study area: smaller, higher-elevation blocks in the western sector are only mobilized by a narrow range of extreme tsunami conditions, while larger, lower-elevation eastern boulders respond to a broader spectrum. Storm wave simulations failed to reach or displace the boulders, strengthening the tsunami hypothesis. These findings highlight the critical role of numerical modeling in tsunami hazard assessment and call for re-evaluation of other cliff-top deposits globally. The methodology presented here demonstrates a robust, multidisciplinary approach to interpreting boulder emplacement and contributes to refining coastal hazard mapping in tsunami-prone regions.