On the timing and nature of the multiple phases of slope instability on eastern Rockall Bank, Northeast Atlantic

Abstract

One of the most challenging tasks when studying large submarine landslides is determining whether the landslide was initiated as a single large event, a chain of events closely spaced in time or multiple events separated by long periods of time as all have implications in risk assessments. In this study we combine new multichannel seismic profiles and new sediment cores with bathymetric data to test whether the Rockall Bank Slide Complex, offshore western Ireland, is the composite of multiple slope collapse events and, if so, to differentiate them. We conclude that there have been at least three voluminous episodes of slope collapse separated by long periods of slope stability, a fourth, less voluminous event, and possibly a fifth more localized event. The oldest event, Slide A (200 km 3 ), is estimated to be several hundred thousand years old. The second event, Slide B (125 km 3 ), took place at the same location as slide A, reactivating the same scar, nearly 200 ka ago, possibly through retrogression of the scarp. Slide C (400 km 3 ) took place 22 ka ago and occurred further north from the other slides. Slide D was a much smaller event that happened 10 ka ago, while the most recent event, albeit very small scale, took place within the last 1,000 years. This study highlights the need to thoroughly investigate large slide complexes to evaluate event sequencing, as seismic studies may hide multiple small-scale events. This work also reveals that the same slide scarps can be reactivated and generate slides with different flow behaviors.Plain Language Summary When studying large underwater landslides, determining whether what we see in our data was created by one large landslide event or several smaller events is very difficult due to the inaccessibility of the deep sea. But, being able to distinguish between different events and their frequency allows for more accurate risk assessments. Forty years ago, a large landslide was discovered in the northeast Atlantic, on the flank of an underwater plateau offshore of western Ireland. Studies since its discovery have interpreted it as one large event. With present-day technology and a higher resolution data set, we have discovered that it is composed of several landslides. The most recent, but very small and localized event, happened in the last 1,000 years. The one before is happened 10,000 years ago, and it was the size of 680,000 Olympic-size swimming pools. Around 22,000 years ago, a landslide 250 times bigger slid down the slope. Two more similar size events happened more than 200,000 years ago, but the further back in time we go the data resolution gets poorer. We think that the sizes of large underwater landslides found in the world's oceans and lakes may have been significantly overestimated, but their frequency may have actually been underestimated.

Statistical emulation of landslide-induced tsunamis at the Rockall Bank, NE Atlantic

“…Until recently, the RBSC had been considered as one single event mass flow [ 41 ]. New sedimentary and seismic data taken from the depositional lobes of the RBSC demonstrate that the RBSC morphology is the result of multiple phases of slope failure, at least three, that were separated by long periods of slope stability [ 42 ]. Radiocarbon dating showed that the most recent event took place 21 ka, which is 3000 years post Last Glacial Maximum (LGM), when the British Irish Ice Sheet (BIIS) was still at maximum extent and was only just starting to destabilize [ 42 ].…”

“…New sedimentary and seismic data taken from the depositional lobes of the RBSC demonstrate that the RBSC morphology is the result of multiple phases of slope failure, at least three, that were separated by long periods of slope stability [ 42 ]. Radiocarbon dating showed that the most recent event took place 21 ka, which is 3000 years post Last Glacial Maximum (LGM), when the British Irish Ice Sheet (BIIS) was still at maximum extent and was only just starting to destabilize [ 42 ]. One hypothesis for the triggering mechanism suggests that sedimentation on the upper part of the slope enhanced by strong bottom current activity removed support from the base-of-slope, leading to slope instability and subsequently failure [ 35 , 41 ].…”

“…One hypothesis for the triggering mechanism suggests that sedimentation on the upper part of the slope enhanced by strong bottom current activity removed support from the base-of-slope, leading to slope instability and subsequently failure [ 35 , 41 ]. Updated research supports that bottom currents would have still been weak at the time of the slide and therefore an earthquake is considered as the most likely triggering mechanism [ 42 ].…”

“…Common values used in the numerical simulations of visco-plastic submarine flows do not exceed the value of 300 Pa s and normally range from 30 to 300 Pa s for the simulation of clay and silt-rich sediments [ 58 , 61 , 63 ]. Considering that the sediment composition in the RBSC is a mixture of clay, silt and sand material [ 42 ] a range of 10–300 Pa s was selected for the dynamic viscosity.…”

“…The estimated volume of collapse in the RBSC is 400 km 3 , the width of the failure area in the slope region is approximately 70–80 km [ 42 ]. The collapse can be considered of large dimensions and we thus attempt to provide a rough estimation of the dispersive parameter as described in equation ( 6.1 ).…”