Sediment Gravity Flows Triggered by Remotely Generated Earthquake Waves on the Cascadia Margin

The tremendous impacts of mega-thrust earthquakes has been exemplified by recent great earthquakes at Sumatra, Chile, and Japan and by their associated tsunamis. Assessing potential triggers of turbidity events helps our understanding of how accretionary margins are remolded, sediments transported, and paleo-turbidites are interpreted. We studied temperature data recorded using Paroscientific sensors deployed on the seafloor of the Cascadia subduction zone offshore Washington and Oregon, as a part of the Cascade Initiative experiment. Our analysis of data from the first three years of the four year experiment revealed anomalous temperature signals (> 3σ of the background noise) at six of the instrument sites. Based on the combination of pressure and seismometer signals, seafloor morphology, ROV video, published examples from elsewhere and other remote sensing data, we infer that temperature anomalies are the result of sediment gravity flows fed by slope failures. These temperature anomalies could be cause by shallower, warmer, sediment-laden water moving to greater depths and colder ambient temperatures. Three adjacent temperature anomalies straddling the deformation front appear to result from slope failures triggered by seismic waves generated by the 2012 Mw8.6 Indian Ocean Earthquake, which were amplified and prolonged by the Cascadia accretionary wedge. If correct, the Cascadia accretionary wedge may host slope failures and sediment-laden gravity flows between major Cascadia subduction zone earthquakes, possibility remotely triggered by waves from distant earthquakes, that should be considered in inferences of recurrence intervals of past great Cascadia earthquakes from turbidite sequences (although the flows we observe are too small to leave a geologic record), models of sediment transport, and the hazards they pose to seafloor infrastructure in regions both with and without local earthquakes.