Cascadia Earthquake-Triggered Rockslide Burial of Beeswax Galleon Wreck Timbers in a Sea Cliff Wave-Cut Platform Site, North Smuggler Cove, Oregon, USA

Abstract

Shipwreck timbers (n=27) from the Spanish galleon Santo Cristo de Burgos, also known as the Beeswax Wreck (lost in 1693), are examined for origins of multiple, simultaneous rockslide burials that protected and preserved the timbers on an intertidal wave-cut platform in the small Smuggler Cove in the Northern Oregon coast. The rockslides (n=3–4) that buried the shipwreck timbers are compared to nearby historic rockslide analogs to better establish the mechanisms of boulder distributions on the wave-cut platform. Large boulders (n=20), generally ≥1.0 m intermediate diameter, in the North Smuggler Cove (NSC) timber burial site were measured for estimated mass (1–41 t) and alongshore distribution below small gullies that routed the boulders to the wave-cut platform from overlying short (~100 m distance) but steep hillslopes (40–60% gradient). The multiple independent rockslides in the NSC site, dated to ~300 years by the buried Beeswax Wreck timbers, showed catastrophic, simultaneous activation by a widespread trigger. Anomalous rainfall and/or storm wave attack are unlikely mechanisms for these rockslide activations due to 1) hillslope protections from extended-upslope surface water discharge and 2) sea cliff protection from direct storm wave impacts. The most likely widespread trigger for rockslide activation in the NSC site is seismic ground shaking from an earthquake in the Central Cascadia subduction zone. Modern storm wind velocities (>10 s-1 sustained velocity and 170–225° bearing) and storm surge generated rip currents (≥0.5 km offshore distance) indicate that Beeswax Wreck timbers could have been transported north (1–10 km) to Smuggler Cove, within several winter seasons after the breakup of the Santo Cristo de Burgos. The most likely seismic trigger that could have activated the multiple rockslides in the NSC site, shortly after the brief accumulation of shipwreck timbers on the narrow, intertidal wave-cut platform in Smuggler Cove, is the 1700 Cascadia great earthquake (Mw~9.0). Additional sea cliff rockslide sites (n=8) in the central Cascadia margin are suggested for further investigations of seismically activated slope failures by the 1700 Cascadia great earthquake.