Frequency and triggering mechanisms of submarine mass movements and their geohazard implications

Abstract

Submarine mass movements are one of the most important processes for moving sediment across our planet. They represent the dominant process for moving sediment in many parts of the world’s oceans, freshwater lakes and reservoirs. These flows also represent a significant geohazard. They can generate damaging tsunami and have the potential to damage strategically important seafloor infrastructure. It is therefore important to understand the frequency and triggering mechanisms of these events. This thesis aims to further our understanding using a variety of different data types (artificial data, deposits found in cores, seismic stratigraphy and submarine cable breaks) across different spatial scales.

First, artificial data is used to analyse the impacts of large age uncertainties on identifying a triggering mechanism for large (>1 km3) landslides in a global database. It is shown that the size of age uncertainties, the small number of landslides within the database and the combination of multiple different settings into one dataset will likely result in landslides appearing to occur randomly. As a result it is suggested that it is prudent to focus on well-dated landslides from one setting with similar triggers rather than having a poorly calibrated understanding of landslide ages in multiple settings which may prevent a trigger being identified.

Second, a global database of subsea fibre optic cable breaks is used to investigate the triggering of submarine mass movements by earthquakes and tropical cyclones. Globally earthquakes between Mw 3 and Mw 9.2 are shown to trigger mass movements. However, in contrast to previous assertions it is shown that there is not a specific earthquake magnitude that will systematically trigger mass movements capable of breaking a cable. The susceptibility of slopes to fail as a consequence of large and small earthquakes is dependent on the average seismicity of the region and the volume of sediment supplied annually to the continental shelf.

The frequency of damaging tropical cyclone triggered submarine mass movements is lower than earthquake triggered mass movements. Analysis of the cable break database reveals three mechanisms by which mass movements are triggered. First, tropical cyclones trigger flows directly, synchronous to their passage due to dynamic loading of the seabed. Second, flows are triggered indirectly, as a consequence of peak flood discharges delivering large volumes of sediment to the continental shelf. Third, flows are triggered indirectly following a delay as a consequence of the large volumes of rapidly deposited sediment that occurs after the passage of a tropical cyclone. No clear global relationship between future climate change and flow frequency is shown, however, changes to cyclone activity in specific regions appears likely to increase damaging flow frequency.

Third, using a new piston core dataset, the timing and frequency of glacigenic debris-flows on the Bear Island Trough-Mouth Fan is investigated. The timing of glacigenic debris-flows over the last 140,000 years is shown to be controlled by the presence of an ice stream close to the shelf edge. Moreover, it is shown that the frequency and volumes of these flows is controlled by the overall dynamics of the Barents Sea Ice Sheet which vary significantly over the 140,000 year time period.

Last, a review of the relationship between ice sheets and submarine mass movements around the Nordic Seas over the Quaternary is presented using published seismic and sediment core datasets. From these data sources, the growth and decay histories of the Greenland, Barents Sea and Scandinavian Ice Sheets are tracked relative to the different types of submarine mass movements identified on their margins. The type and frequency of submarine mass movement is shown to be highly variable as a consequence of variable ice sheet extent, rates of sediment transport and meltwater export of sediment. These records have allowed the identification of first order controls on sediment delivery to continental margins at ice sheet scales. It has also enabled updated conceptual models of trough-mouth fan processes, glaciated margin development and submarine landslide occurrence to be developed.