The physical and cultural traces of a 19th century volcano-collapse tsunami: New constraints on the magnitude and impacts of the 1888 Ritter Island tsunami, Papua New Guinea

Abstract. The lateral collapse of Ritter Island volcano, Papua New Guinea, on 13^th March 1888, generated a devastating tsunami with observed effects at distances of up to 500 km. As the largest recorded volcano lateral collapse, the event is significant in improving our general understanding of volcano instability and associated hazards. Proximal records of the tsunami are limited to post-event reports of extensive coastal damage of nearby islands. Here, we present new field observations of tsunami deposits on these coastlines, supplemented by and interpreted alongside oral accounts from coastal communities (from 2004) and a reanalysis of contemporary reports. Our data reveal the widespread presence of massive and chaotic conglomerates deposited by the tsunami at sites up to 30 km from Ritter, with thicknesses of up to 2.0 m and extending up to 400 m from the shoreline. Such deposits may be indicative of large-magnitude landslide-generated tsunamis, and are generally structureless, coarsest and thickest near the coast, and contain mixed terrigenous and marine clasts, including corals, shells and benthic foraminifera. Isolated large coral blocks are also widespread tsunami inundation markers, reaching >200 m from the shore. The deposits extend to approximately half the maximum inundation distance indicated by oral accounts; the latter preserve precise detail consistent with the timing, spatial characteristics and impacts of the Ritter tsunami interpreted from prior geological surveys, distal accounts and simulations. Collectively, this study indicates that the impacts of the 1888 tsunami on proximal shorelines were larger than has been previously inferred from post-event reports. The maximum run-up height was likely many tens of metres on Sakar and Umboi and extensively exceeded 20 m on western New Britain. From the collated descriptions, we also estimate that ~2000–3000 deaths were caused directly by the tsunami. These new observations add to previous submarine surveys of the event, allowing the coupled process of landslide motion and tsunami generation at Ritter Island to be investigated more comprehensively.