The COSTA project constitutes a team of experts in sedimentary processes, gas hydrates, and margin research enabling a skilled use of front-line technologies in co-operation with industry. Detailed images of seafloor failures provided by swath mapping, side-scan sonar and 3-D seismic imaging are needed to understand the complex processes and resulting impacts. To properly analyse slope stability and submarine flow dynamics, sediments need to be sampled in and around failure-prone areas to depths of zones of weakness. This type of sampling allows detailed examination of the physical and chemical makeup of destabilised sediments and of the known or potential failure surfaces they bound. Furthermore, the effect of earthquakes on slope stability is of importance to the understanding of the variability of slope stability. We have therefore designated seafloor regions where these types of observations can best be made (Table 1). Two primary test sites have been chosen for long-term studies (up to 1 year), the first site having a high potential for both frequent seismicity and overpressurised layers, while the second has indications of gas hydrates and associated free gas. Short-term studies (up to 1 day) of geotechnical properties during cruises will be carried out in all working areas. Also, we will assess the specific conditions in each of the areas (relief, overpressurised strata, active faults, erosion, mud diapirism, gas/fluid vents).
From south to north, the working areas encompass three sectors of the European continental slope: (1) Adriatic (high seismicity and known overpressurised layers), (2) Western Mediterranean (earthquake and non-earthquake), (3) NE Atlantic (gas hydrates).
Table 1: Glacial-dominated and river-dominated margin settings with slide areas from shelf to deep-sea environments (numbers indicate the location of slides).
The Central Adriatic is a region of intense Quaternary deformation and historical seismicity. Major destructive earthquake shocks occurred in the 1223, 1627 and 1731 AD events but minor shocks are much more frequent and occur yearly. The coastal region of the Central Adriatic was also affected by large tsunamis that occurred in historical times (1627, 1646, 1731 AD were the most devastating). Failure of shallow-water deposits can be triggered by intense and recurrent seismic activity. In addition, the timing of mud diapirism, which also occurs on the Norwegian margin, may play a role. Our work will focus on long term monitoring the factors that control the variability of the observed failure geometry and the chronology of the Central Adriatic and other areas where these geometries are best developed.
The W Mediterranean target areas stretches from the Nice Bay to Gulf of Lion to the Ebro regions. The fine-grained deposits of the river systems have created large pro-delta deposits during the Holocene sea level rise, and as a result high sediment loads with excess pore pressure regimes. High sediment accumulations are prone to failure, as demonstrated by the occurrence of slump scars. Our work will focus on geotechnical measurements and seismic imaging of the architecture and geometry of slope failures including age dating.
The NE Atlantic area of the UK and the Norwegian Margin contain the Storegga Slide, one of the worlds largest submarine slides, and many indications of gas hydrates and free gas close to some of the slide scars. Gas hydrates are a solid phase composed of water and low molecular weight gases (predominantly methane) which forms under conditions of low temperature, high pressure and adequate gas concentrations. Below the gas hydrate stability zone hydrates are not stable, and methane will exist as free gas or being dissolved in the sediment pore fluid. One of the known slide events at Storegga on the Mid Norwegian Margin caused a devastating tsunami in northern lowland Europe, which has been dated to about 7400 years B.P. Recent datings of the Traenadjupet slide to the north of Storegga indicated that this slide is younger, possibly only 4000 years B.P. old. Because the causes of the slides are still unclear, minor slides are not at all documented, and exploration is going to deeper waters, it is clearly important to understand the gas hydrate distribution and shear zones in slope sediments.
The basic methodology applied to achieve the objectives of this project can be summarised as follows: