A Massive Slump on the St. Pierre Slope, A New Perspective on the 1929 Grand Banks Submarine Landslide

The 1929 Grand Banks submarine landslide on the southwestern Grand Banks of Newfoundland was triggered by a M-w 7.2 strike-slip earthquake. It is the first studied example of a submarine mass movement known to have caused a turbidity current and tsunami. The event resulted in 28 casualties and caused severe economic damage. The St. Pierre Slope is the main source area for the sediment failure. It contains translational and probable retrogressive surficial failures (<25 m); the majority of which lie in >1,700-m water depth. These observations contradict what might be expected for a tsunamigenic event; thus, the objective of this study is to look for other potential causal mechanisms. A comprehensive analysis of 2-D seismic reflection data of various resolutions and multibeam bathymetry allowed mapping of new stratigraphic and structural features. Numerous, low-angle (similar to 17 degrees) faults are present throughout the Quaternary section of the St. Pierre Slope that are associated with seafloor escarpments (750- to 2,000-m water depth). These faults have up to 100-m high displacement and are interpreted as part of a massive (similar to 560 km(3)), complex slump. There are multiple decollements (250- and 400- to 550-m below seafloor) within this slump and there is indication for slumping in at least two directions. Evidence suggests slumping as a result of the 1929 earthquake occurred along these faults, with similar to 100-m seafloor displacement in places. The 1929 submarine landslide therefore involved two failure mechanisms: massive slumping (similar to 500-m thick) and consequent widespread, surficial (<25 m) sediment failure. Both failure mechanisms likely contributed to tsunami generation.