Did a slump source cause the 1929 Grand Banks tsunami?

On November 18, 1929, a Mw 7.2 earthquake occurred beneath the upper Laurentian Fan, south of Newfoundland. The earthquake displaced about 100 km3 of sediment volume that rapidly evolved into a turbidity current revealed by a series of successive telecommunication cable breaks. A tsunami with fatal consequences along the south coast of Newfoundland also resulted. This tsunami is attributed to sediment mass failure as no seafloor displacement due to the earthquake is observed or expected. Although sidescan sonar, sub-bottom profiler and modern multibeam data show surficial sediment slumping and translational slide activity in the upper part of the slope, no major headscarp, single evacuation area or large mass transport deposit are observed. Sediment mass failure has been interpreted as broadly distributed and shallow, likely occurring in a retrogressive fashion. The question remained, therefore, as to how such complex failure kinematics could generate a tsunami. The Grand Banks tsunami is the only landslide tsunami for which traces are found at transoceanic distances. Despite being a landmark event, only a couple of attempts to model the tsunami exist. None of these have been able to match tsunami observations. Recently acquired seismic reflection data suggest that rotational slumping of a thick sediment mass ( 500 m) on the St. Pierre Slope may have occurred, causing seafloor displacements (fault traces) up to 100 m in height. The previously mapped surficial failures were a consequence of slumping of the thicker mass. Here, we simulate tsunami generation using the new geophysical information to construct different tsunamigenic slump sources. In addition, we undertake simulations assuming a flowing surficial landslide. The numerical simulations shows that its large and rapid vertical displacements render the slump source more tsunamigenic than the alternative surficial landslide. The simulations using the slump source roughly complies with observations of large run-ups on the Burin Peninsula along the south coast of Newfoundland, in contrast to previous modelling attempts. As the source extensions complies with new observations of rotational failures at the slope, the simulations suggest that a slump source is the most likely explanation for the large tsunami observations due to the Grand Banks event.