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.

A Mw 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 km2, 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 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 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 (~17°) 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 (~560 km3), complex slump. There are multiple décollements (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 ~100‐m seafloor displacement in places. The 1929 submarine landslide therefore involved two failure mechanisms: massive slumping (~500‐m thick) and consequent widespread, surficial (<25 m) sediment failure. Both failure mechanisms likely contributed to tsunami generation.

&lt;p&gt;The 1929 Grand Banks submarine landslide was triggered by a M&lt;sub&gt;w&lt;/sub&gt; 7.2 strike slip earthquake on the southwestern Grand Banks of Newfoundland. Studies following the event by several decades were the first to recognize that slope failure can cause tsunamis. These studies identified St. Pierre Slope as the main failure area and showed widespread, shallow (&lt;25 m-thick), translational and possibly retrogressive sediment failures occurred predominately in &gt;1700 m water depth (mwd). It seems unlikely this style of failure in deep water generated a tsunami that had &gt;13 m of run-up along the coast of Newfoundland. The objective of this study is to identify possible alternative tsunami source mechanisms and pre-conditioning factors that may have led to sediment instability. These objectives are addressed using a comprehensive data set of multiscale 2D seismic reflection, multibeam swath bathymetry and laboratory geomechanical test data. Results show numerous reflection offsets within the Quaternary section of the slope underneath modern seafloor escarpments (750-2300 mwd).&amp;#160; These offsets appear down to 550 m below seafloor (mbsf) and are interpreted as low angle (~17&amp;#176;), planar-normal faults of &lt;100 m-high vertical and ~330 m of horizontal displacement. The faults are interpreted as part of a massive (~560 km&amp;#179;) complex slump with evidence for multiple d&amp;#233;collements (250 mbsf &amp; 400-550 mbsf) and slumping in at least two directions. Infinite slope stability analysis using peak ground acceleration (PGA) indicates that a combination of earthquake loading and the presence of geomechanical weak layers are needed to explain the slope failure. At St. Pierre Slope, the analysis of sediment cores shows that geomechanical weak layers form as a consequence of underconsolidation in connection with excess pore pressures that are related to: 1) high sedimentation rates, 2) instantaneous deposition of mass transport deposits (MTD&amp;#8217;s) and sandy turbidites, and 3) the presence of gas. The d&amp;#233;collements of the slump are associated with MTD&amp;#8217;s and sediment waves that likely form weak layers. The layers of sediment waves are assumed to consist of sorted silts or fine sands and are therefore likely to be susceptible to excess pore pressure development during earthquake loading. Excess pore pressure development results in reduced effective stress and higher potential for instability. It is interpreted, therefore, that the 1929 earthquake triggered displacement of a 550 m-thick slump with ~100 m of vertical seafloor displacement. This instantaneous displacement of the slump in 750 mwd with a seafloor volume displacement of 70 to 130 km&amp;#178; is likely a more effective source for tsunami generation than the translational, shallow (&lt;25 m) failures in deeper water.&lt;/p&gt;

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