Abstract Sediment slumps are known to have generated important tsunamis such as the 1998 Papua New Guinea (PNG) and the 1929 Grand Banks events. Tsunami modellers commonly use solid blocks with short run-out distances to simulate these slumps. While such methods have the obvious advantage of being simple to use, they offer little or no insight into physical processes that drive the events. The importance of rotational slump motion to tsunamigenic potential is demonstrated in this study by employing a viscoplastic landslide model with Herschel–Bulkley rheology. A large number of simulations for different material properties and landslide configurations are carried out to link the slump's deformation, rheology, its translational and rotational kinematics, to its tsunami genesis. The yield strength of the slump is shown to be the primary material property that determines the tsunami genesis. This viscoplastic model is further employed to simulate the 1929 Grand Banks tsunami using updated geological source information. The results of this case study suggest that the viscoplastic model can be used to simulate complex slump-induced tsunami. The simulations of the 1929 Grand Banks event also indicate that a pure slump mechanism is more tsunamigenic than a corresponding translational landslide mechanism.

Abstract On 18 November 1929, an M w 7.2 earthquake occurred south of Newfoundland, displacing >100 km 3 of sediment volume that evolved into a turbidity current. The resulting tsunami was recorded across the Atlantic and caused fatalities in Newfoundland. This tsunami is attributed to sediment mass failure because no seafloor displacement due to the earthquake has been observed. No major headscarp, single evacuation area nor large mass transport deposit has been observed and it is still unclear how the tsunami was generated. There have been few previous attempts to model the tsunami and none of these match the observations. Recently acquired seismic reflection data suggest that rotational slumping of a thick sediment mass may have occurred, causing seafloor displacements up to 100 m in height. We used this new information to construct a tsunamigenic slump source and also carried out simulations assuming a translational landslide. The slump source produced sufficiently large waves to explain the high tsunami run-ups observed in Newfoundland and the translational landslide was needed to explain the long waves observed in the far field. However, more analysis is needed to derive a coherent model that more closely combines geological and geophysical observations with landslide and tsunami modelling.

Abstract A M w 7.2 earthquake centred beneath the upper Laurentian Fan of the SW Newfoundland continental slope triggered a damaging turbidity current and tsunami on 18 November 1929. The turbidity current broke telecommunication cables, and the tsunami killed 28 people and caused major infrastructure damage along the south coast of Newfoundland. Both events are believed to have been derived from sediment mass failure as a result of the earthquake. This study aims to identify the volume and kinematics of the 1929 slope failure in order to understand the geohazard potential of this style of sediment failure. Ultra-high-resolution seismic reflection and multibeam swath bathymetry data are used to determine: (1) the dimension of the failure area; (2) the thickness and volume of failed sediment; (3) fault patterns and displacements; and (4) styles of sediment failure. The total failure area at St Pierre Slope is estimated to be 5200 km 2 , recognized by escarpments, debris fields and eroded zones on the seafloor. Escarpments are typically 20–100 m high, suggesting failed sediment consisted of this uppermost portion of the sediment column. Landslide deposits consist mostly of debris flows with evidence of translational, retrogressive sliding in deeper water (>1700 m) and evidence of instantaneous sediment failure along fault scarps in shallower water (730–1300 m). Two failure mechanisms therefore seem to be involved in the 1929 submarine landslide: faulting and translation. The main surficial sediment failure concentrated along the deep-water escarpments consisted of widely distributed, translational, retrogressive failure that liquefied to become a debris flow and rapidly evolved into a massive channelized turbidity current. Although most of the surficial failures occurred at these deeper head scarps, their deep-water location and retrogressive nature make them an unlikely main contributor to the tsunami generation. The localized fault scarps in shallower water are a more likely candidate for the generation of the tsunami, but further research is needed in order to address the characteristics of these fault scarps.

The epicenter of the 1929 “Grand Banks” earthquake (M s = 7.2) was on the continental slope above the Laurentian Fan. The zone in which cables broke instantaneously due to the earthquake is characterized by surface slumping up to 100 km from the epicenter as shown by sidescan sonographs and seismic reflection profiles. The uppermost continental slope, however, is almost undisturbed and is underlain by till deposited from grounded ice. The Eastern Valley of the Laurentian Fan contains surficial gravels molded into large sediment waves, believed to have formed during the passage of the 1929 turbidity current. Sand sheets and ribbons overlie gravel waves in the lower reaches of Eastern Valley. Cable-break times indicate a maximum flow velocity of 67 km/hr (19 m/s). The occurrence of erosional lineations and gravel on valley walls and low intravalley ridges suggest that the turbidity current was several hundred meters thick. The current deposited at least 175 km 3 of sediment, primarily in a vast lobe on the northern Sohm Abyssal Plain where a bed more than 1 m thick contains material ranging in size from gravel to coarse silt. There is no apparent source for so much coarse sediment on the slumped areas of the muddy continental slope. We therefore infer that there was a large volume of sand and gravel available in the upper fan valley deposits before the earthquake. This coarse sediment was discharged from sub-glacial meltwater streams when the major ice outlet through the Laurentian Channel was grounded on the upper slope during middle Wisconsinan time. This sediment liquefied during the 1929 event, and the resulting flow was augmented by slumping of proglacial silts and gas-charged Holocene mud on the slope. Although earthquakes of this magnitude probably have a recurrence interval of a few hundred years on the eastern Canadian margin, we know of no other deposits of the size of the 1929 turbidite off eastern Canada. For such convulsive events, both a large-magnitude earthquake and a sufficient accumulation of sediment are required.