Slumps, slides and debris flows of the St. Pierre Slope: a reanalysis of slope instability due to the 1929 Grand Banks Earthquake
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The 1929 Grand Banks submarine landslide on the southwestern Grand Banks of Newfoundland was triggered by a Mw 7.2 strike-slip earthquake. It is the first studied example with an unequivocal connection between earthquake, landslide and tsunami and led to the first recognition of naturally occurring submarine turbidity currents. It ultimately caused 28 casualties and significant economic damage. The landslide has been identified as a widespread, retrogressive, shallow sediment failure (upper 20 m) in 730 m water depth (mwd). It is difficult to reconcile that this style of failure in deep water generated a large tsunami. The objective of this thesis is to investigate other potential causal mechanisms and contributing factors involved in the 1929 event. The study focusses on St. Pierre Slope, the main failure area. A comprehensive analysis of multiscale 2D seismic reflection data, multibeam echosounder data and geomechanical testing indicated that sediment failure at St. Pierre Slope is more complex than previously suggested. Results show that surficial sediment failures occurred predominately from ~25 m-high escarpments in >1700 mwd. The translational, possible retrogressive failures involved ~100 km³ of sediment material that either rapidly deposited on the slope (~60 km³) or became entrained into channelized turbidity currents (~40 km³). Numerous oblique, low angle (~17°) faults are evident underneath escarpments to ~550 m below seafloor (mbsf) with up to 100 m-high vertical displacement. The faults are interpreted as part of a massive (560 km³) complex slump with evidence of multiple décollements (250, 400-550 mbsf) and slumping in at least two directions. It is interpreted that the 1929 earthquake triggered slumping of the 550 m-thick strata of sediment. Displacement of the slump possible resulted in seafloor volume displacement of 70 to 130 km³. Instantaneous displacement of the slump, therefore, is likely more efficient for tsunami generation than translational, shallow failures. Slope stability analysis indicates that the 1929 earthquake, presence of weak layers and possible displacement of the slump caused the surficial failures. These findings indicate two failure mechanisms for the 1929 submarine landslide that both likely contributed to tsunami generation: massive slumping (~550 m thick) and widespread, surficial (<25 m) sediment failures.