Hva førte til megastore undersjøiske ras utenfor norskekysten?

For 8000 år siden skapte et stort undersjøisk ras utenfor kysten av Trøndelag en 20 meter høy tsunamibølge langs Atlanterhavskysten. Hvorfor skjedde et slikt stort undersjøisk ras, og kan det skje på nytt?

Didier Leynaud

Storeggaraset er antakelig det største undersjøiske raset vi kjenner til på kloden. Didier Laynauds mål i hans doktorgradsarbeid har vært å finne årsaken til at 2500 km3 med sedimenter raste nesten 800 km ut i dypvannsbassenget.

Landhevingen etter istiden førte sannsynligvis til en kraftig økning i jordskjelvaktiviteten (jordskjelv større enn 7,0 på Richters skala) for mellom 10 000 og 8000 år siden. Dynamiske modelleringer viser at jordskjelv alene ikke kan forårsake undersjøiske ras hvor vinkel av kontinentalskråningen bare er èn grad. Men forandringer i det sedimentære avsetningsmiljøet fra istiden til den varme perioden bidrar til å utvikle ustabile sedimentlag i undergrunnen. Poretrykket økte i interglasiale sedimenter flere hundre meter under havbunnen, og utviklet glideplan for store, undersjøiske ras. Hele prosessen er ikke fullstendig forstått, men Leynauds forskning i samarbeid med oljeindustrien har betydelig økt kunnskapene om dette.

Didier Laynaud disputerte for sin doktorgrad ved Institutt for geologi ved Universitetet i Tromsø. Tittelen på avhandlingen er “Slope stability assessment on continental margins of the Norwegian and Mediterranean Seas” .

Didier Laynaud har mesteparten av sin utdanning i geologi og geofysikk fra Université Montpelier og mastergrad i seismologi fra L'Université à Grenoble i Frankrike. Fra 1998 til 2000 jobbet han som forskningsassistent ved Université de Liège i Belgia. Hans doktorgradsavhandling ved Universitetet i Tromsø var finansiert av EU COSTA (COntinental slope STAbility).

Kontaktinformasjon:
Didier Leynaud,
leynaud.didier@wanadoo.fr

Se en mer utfyllende presentasjon av denne doktorgraden med illustrasjon i den engelske versjonen nedenfor.




Why did mega-scale underwater slides occur on the Norwegian margin over the last 10 000 years ?

8000 years ago, a large underwater slide off the coast of Trøndelag created a huge 20-meter high tsunami wave that hit the Atlantic coastlines. Why did this mega-scale slide happen and could it happen again?

The Storegga Slide is probably the largest recent submarine landslide in the world (Figure) and happened approximately 8000 years ago. The sudden removal of c. 2500 km3 of sediments from the seabed generated a devastating 20 m high tsunami that hit the lowlands surrounding the northern Atlantic Ocean. This is the subject of a new Ph.D thesis by Didier Leynaud from the department of Geology at the University of Tromsø.

Location of the large-scale slides on the Norwegian continental margin.

Evidence of such a catastrophical event inevitably led to another important question “Is the present-day seabed stable or can another large submarine slide occur these days ?”. Another slide and subsequent tsunami would have severe implications, considering the high population along the coastlines as well as the offshore oil and gas activities and infrastructure.

The most straightforward way to explain a slide with such dimensions is to involve a large earthquake (Magnitude > 7.0). We know from onshore fault scarps that such seismic events have occurred in northern Scandinavia about 9000 years ago, and that the seismicity was probably related with the continental uplift after retreat of the ice cap. However, dynamic modelling using seismic acceleration cannot explain this slide on a very gently dipping continental margin (1° in the area of the slide for the reconstructed slope which corresponds to a difference in altitude of 17 m every 1 km).

The dissociation of gas hydrates could form an alternative explanation. Hydrates are an ice-like mixture of methane and water that may occur in the sediment’s pore space. These structures are metastable, and could dissociate with pressure fall (e.g. sea level drop) or temperature increase (e.g. inflow of warmer water masses). Hydrate dissociation would lead to excess pore pressure and therefore facilitate slope failure especially in relatively shallow settings. However, the Storegga Slide was probably initiated in relatively deep waters, suggesting that hydrate dissociation is probably a secondary process in slope instability rather than the primal course.

Today, we know that a specific sediment, fine-grained sediment deposited by contour currents on the seafloor, is very often associated with the slip planes of the Storegga slide as well as older slides in the vicinity like Tampen, Møre or further north like the Trænadjupet slide which occurred 4000 years ago. Secondly, we know that a huge amount of sediment (glacial clay) was delivered at the end of each glacial period (the last one about 15000 years ago) at a rapid rate, generating high overpressures in the underlying sediments.

As the largest submarine events offshore the Norwegian margin (Storegga, Trænadjupet and Andøya slides) happened during the Holocene (from 10000 years ago to present), a combination of modelling and observation suggests that the key parameters of the large-scale sliding could be the association of the sediment type and overpressure developing in the sediment, resulting in a dramatic change of sediment behaviour.

The failure process is still not very well understood, and the former question still unresolved, but we are moving closer. The culprit(s) of this natural crime is still running since 8000 years, but for how long?