Sediment gravity flows triggered by remotely generated earthquake waves

Recent great earthquakes and tsunamis around the world have heightened awareness of the inevitability of similar events occurring within the Cascadia Subduction Zone of the Pacific Northwest. We analyzed seafloor temperature, pressure, and seismic signals, and video stills of sediment-enveloped instruments recorded during the 2011-2015 Cascadia Initiative experiment, and seafloor morphology. Our results led us to suggest that thick accretionary prism sediments amplified and extended seismic wave durations from the 11 April 2012 M(w)8.6 Indian Ocean earthquake, located more than 13,500km away. These waves triggered a sequence of small slope failures on the Cascadia margin that led to sediment gravity flows culminating in turbidity currents. Previous studies have related the triggering of sediment-laden gravity flows and turbidite deposition to local earthquakes, but this is the first study in which the originating seismic event is extremely distant (> 10,000km). The possibility of remotely triggered slope failures that generate sediment-laden gravity flows should be considered in inferences of recurrence intervals of past great Cascadia earthquakes from turbidite sequences. Future similar studies may provide new understanding of submarine slope failures and turbidity currents and the hazards they pose to seafloor infrastructure and tsunami generation in regions both with and without local earthquakes. Plain Language Summary Turbidites are marine landslides on continental margins that are known to be triggered by local earthquakes. When triggered over a wide area by a nearby earthquake, these mud slides can in turn cause local tsunamis to inundate nearby coastal areas. Recent data from 3years of ocean bottom seismometer deployments on the Cascadia margin in the Pacific Northwest have demonstrated that turbidites can also be triggered by remote earthquakes, including a distant large magnitude earthquake on the Sumatra margin, over 13,000km distant from the eastern Pacific Ocean.