Monday, 14 July 2014

The triggers

68.5oN 8oE
Notes: Sun has now not set for three days, although the weather has been overcast for the last two days. Graveyard shift currently leads the daytime shift by 17 to 11 cores, not that we are counting. Alessandro is pretty poor at darts. There is a storm coming!!!!
Core Puns:
He who dares.....Cores
The Good, the Bad and the Corer
Cora! Cora! Cora!
The Italian Core
Return of the Corer
Cool Coring
The Beauty and the Corer
101 Cores
Core Story
Harry Potter and the Half Sand Core
The Core
Reservoir Cores

One of the purposes of the 2014 Pelagia Cruise is to collect information relating to the triggering of submarine landslides. Many possible triggers for submarine landslides have been identified. These include earthquakes, rapid sedimentation and gas hydrate dissociation. Working out the specific triggering factor behind individual slides is, however, extremely difficult. Which factor has preconditioned the slope to failure? Which factor has triggered the actual slope failure? Today I was covered in 4000 year old mud by certain members of the night shift (Camilla Watts), what the actual reason for this is unclear [Editorial note: it was well deserved]. Was it the fact that I was inanely putting the word core or corer in film and book titles for two hours? Was it because I thought Harry Potter deserved to be kicked in the shin or because I have never watched or am not likely to watch Game of Thrones? Or was it because I have been talking constantly about ball sports for the last year? Or was it that she randomly got bored and decided it was a good idea?

Although a submarine landslide will not be triggered by a poor pun this example indicates the complexity of the question that we are attempting to answer. Many of the submarine landslides around the Norwegian Basin are closely associated with the numerous trough mouth fans which line the continental slope. These features are produced by ice sheets. Glacial ice is a much more effective erosive agent of sediment and bedrock than rivers and is therefore able to deliver extremely large volumes of sediment to the continental margin very quickly. Rapid sediment loading from ice streams (areas of extremely fast flowing ice) is thought to lead to high pore pressures and instabilities which could lead to failure. Similarly dissociation of gas hydrates, an ice like crystalline structure, into their gaseous constituent parts can also generate high pore pressures which could lead to failure of the sediment. Dissociation of gas hydrates can be caused by a number of factors. These include pressure changes related to changes in sea level and temperature changes caused by ocean warming and cooling. Whilst these factors could trigger a submarine landslide, they could equally precondition the slope to fail.
Where rapid deposition of sediment gas hydrate dissociation has generated a slope preconditioned to fail a further trigger may be needed in order to actually achieve failure. In many cases this trigger is an earthquake. Shaking of the sediment can lead to a loss of structure and subsequent failure. Large magnitude earthquakes are currently relatively rare around the margins of the Norwegian basin. However, as we moved out of the last glacial period earthquake magnitudes in this region increased in response to glacial unloading of the crust as the ice sheets retreated. These earthquakes may therefore have represented a common trigger for many events. Despite the increase in magnitude of earthquakes associated with crustal rebound, not all earthquakes will cause slope failure to occur. Some might even lead to a strengthening of the sediment and reduced likelihood of slope failure.
I hope from this post that it is clear that isolating an individual trigger for large submarine landslides is extremely difficult and presents one of the main questions to be addressed as part of the landslide tsunami project. Only with precise dating of the landslides will we increase the possibility of linking landslides to individual triggering mechanisms, although this may in fact be impossible. Who knows?

Ed Pope

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