News from another country
Watching from the comfort of a European city, the images streamed from Japan of muddy, debris filled seawater flooding towns during the Sendai tsunami seemed entirely foreign. Think back though and we come to the events at Lisbon in 1755, when earthquake, fire and wave killed thousands. More recently, tsunamis caused 27 fatalities in Newfoundland during 1929, and significant damage in Nice during 1979.
In the comfort of Europe we have short memories when it comes to hazard risk. The risk of North Atlantic tsunamis is there: will it increase with a warming climate? This is the question addressed by this snack.
To do this, the causes of tsunamis will be outlined and placed in a North Atlantic context. Climate change will then be discussed with respect to the triggers of tsunamis, and submarine landslides in particular.
Causes of (North Atlantic) tsunamis
Fundamentally a tsunami is caused by rapid displacement of water. This displacement is produced by a range of events, with a range of frequencies: asteroid or comet (bolide) impact; explosive volcanism; submarine landslides; and subaerial landslides impacting water. Globally, frequency ranges from hundreds of thousands of years for bolide impact to decadal for earthquakes. Different regions experience different triggers operating at different frequencies.
The North Atlantic consists of rifted margins and is much less seismically active than the Pacific, which is bounded by the so-called ‘ring of fire’ where subduction plate boundaries are marked by deep ocean trenches, large earthquakes and volcanoes. Margins of the sort observed within the North Atlantic are termed passive.
However, this does not mean that they do not display seismic activity. A 1,100 year return period is estimated for a magnitude 7 (moment magnitude) earthquake in Norway. The Charleston earthquake of 1886, which caused massive damage to the southeast coast of North America, did not occur at a tectonic plate boundary but is estimated to have had a magnitude of 7.3-7.4.
Within the overall pattern of the rifted margin there are other fault zones, for example the Azores-Gibraltar transform fault at the boundary between the European and African plates that was responsible for the 1755 Lisbon earthquake, with an estimated magnitude of 8.7. This event sent shock waves through 18th century Europe and left a cultural imprint that can be seen in works such as Voltaire’s Candide.
The Canary Islands, off the coast of North Africa are volcanic and pose a significant risk of tsunamis via island flank collapse. One day, the island of La Palma will split in two and trigger a regionally destructive tsunami. Further risk comes from explosive volcanism in the Caribbean, where recent eruptions at Montserrat, and Martinique last century caused devastation.
It seems unlikely that these tectonic and volcanic triggers will be influenced by climate change (at least not on a centennial timescale), but the North Atlantic has been, and is, subject to large submarine landslides. As demonstrated by events at Sissano (Papua New Guinea) and Newfoundland during the 20th century, submarine landslides do trigger lethal tsunamis.
In the more distant past, the Norwegian Storegga submarine landslide is known to have triggered an event that devastated neolithic Europe 8,100 years ago. This is recorded in peat deposits that reveal wave run-ups of up to 25 m: comparable to the Sendai and Indian Ocean tsunamis.
So, what causes submarine landslides in the North Atlantic and will a warming climate make them more frequent? At the most simple level the stability of a slope is governed by the balance between gravity, which encourages failure, and friction, which prevents movement. The driving, gravitational force is called shear stress and the resisting, friction force shear strength. A slope is more likely to fail when stress is increased or strength is decreased.
The more obvious methods to increase stress are: increased slope angle from ocean basin subsidence or continental uplift; increased weight of sediments via deposition; and motion of sediments via ground shaking. Reduced strength is caused by changes within the sedimentary column.
One of these potential mechanisms is dissociation of gas hydrates. These deposits consist of a cage-like molecular structure that traps a gas molecule (commonly methane) within an ice-like lattice. They exist in environments of high pressure and/or low temperature, on the Earth these conditions are found beneath the oceans or within permafrost.
Influence of climate change on marine slope stability
As gas hydrates are only stable in cold and/or high pressure environments, if the temperature increases, or pressure decreases they may dissociate: releasing gas and weakening the sediments. Recent work suggests that present day changes in the Gulf Stream may be causing destabilisation of 2.5 gigatonnes of methane hydrates in the northwest Atlantic. Striking evidence of this mechanism is visible near Svalbard, where plumes of methane are observed in the water column.
Warmer temperatures are melting ice in Greenland and this process may increase the likelihood of submarine landslides in a number of ways. For example, increased risk of earthquake following the removal of the massive weight of ice and increased rates of sediment deposition from melting glaciers. This process may represent a modern analogue for the role played by sedimentation from melting ice in the triggering of the Storegga landslide offshore Norway. This neolithic age event is believed, in part, to have been caused by sediment loading pushing water laterally within the sedimentary column, causing solid clay hundreds of metres below the seabed to turn to liquid or plastic causing the failure of overlying sediments.
There is an important caveat though. Due to the effect of diffusive heat flow through sediments and the length of time required to increase sediment pore water pressure a submarine slide may occur a considerable time after the climatic change that drives the instability. So the increased risk related to climate change may not be realised for a century or more.
Increased risk of North Atlantic tsunamis?
Based on past events and mechanisms associated with a warming North Atlantic region, climate change may increase the risk of tsunamis, triggered via submarine landslides.
In the meantime, as we watch images of devastation from Thailand, Indonesia and Japan, we should remind ourselves of Lisbon, Storegga and Caribbean and mid-Atlantic volcanism, and be aware that the current level of North Atlantic tsunami risk may be greater than we first think.
References are provided via hyperlinks in the text.