Underwater avalanches are powerful natural events that occur constantly beneath the ocean surface. They cannot be seen and are extremely difficult to measure, so we know very little about how they work.
But these phenomena pose a threat to our global communications networks. The spread of the Internet required an ever-expanding network of fiber-optic cables on the seabed, through which virtually all global Internet traffic is carried.
My new study of an ancient underwater avalanche challenges our understanding of how underwater avalanches develop and could change the way geologists assess their hazard potential.
It is estimated that there are currently over 550 active seabed cables worldwide with a total length of 1.4 million kilometers – enough to wrap around the Earth 35 times.
When an underwater avalanche ruptures seafloor cables, the consequences can be far-reaching and costly. The 2006 Pingtung earthquake in Taiwan triggered underwater avalanches that ruptured many seafloor cables connecting Southeast Asia to the rest of the world.
China’s largest internet operator reported a 90% loss of traffic to the US at the height of the event, and Taiwan saw internet traffic to neighboring islands drop by 74 to 100%.
This damaged global markets as the number of possible financial transactions was drastically reduced. Restoring the network to full capacity took 39 days and cost millions of dollars in transit time.
The underwater avalanche that ripped through these cables was moving rapidly, with a maximum speed of 72 km/h. But compared to the huge underwater avalanches I have studied in the Atlantic, it was relatively small.
The good news is that there are so many cables on the sea floor that it is extremely unlikely that an underwater avalanche could bring down the internet worldwide. The Pingtung earthquake is an example of how even if the main routes are disrupted, at least some traffic can flow along an alternative route.
In new research, my colleagues and I have mapped the destructive power of a massive underwater avalanche that occurred 60,000 years ago from its source off the coast of Morocco.
It travelled 400 km through the largest underwater canyon in the world and a further 1,600 km across the Atlantic seabed. It is the second largest underwater avalanche ever documented.
We mapped the avalanche using a combination of detailed topographic mapping of the seafloor and hundreds of sediment cores that penetrated the avalanche deposits over a huge area. In each core, we analyzed the deposits for fossils, which allowed us to determine the age of the event to be 60,000 years ago. This also meant we could correlate the individual avalanche layers across thousands of kilometers.
The avalanche contained enough sediment to fill 140,000 Wembley stadiums (162 km³). It was as high as a skyscraper (more than 200 meters), moved at a speed of at least 54 km/h and tore a trench 30 meters deep and 15 km wide over a distance of 400 km (the distance from London to Liverpool), destroying everything in its path.
It then spread over an area the size of Germany and buried it under a layer of sand and mud about a meter thick.
However, we show that the avalanche actually started as a small landslide that then grew in size by more than 100 times along its path. This extreme growth in size is much larger than land-based avalanches, which typically grow between four and eight times larger and are tiny in comparison. This refutes the scientists’ belief that large avalanches are caused by large slope collapses.
Instead, we now know that underwater avalanches can start small and grow into disasters of extraordinary force as they travel. These findings may therefore change our assessment of the geohazard potential of these phenomena and lead us to focus more on the avalanche path than on the initial landslide zone.
How often these events happen depends on where you are. Seafloor canyons that start relatively close to river mouths with rainy catchments may experience several small avalanches per year. In other systems far from river mouths, such as the Agadir Canyon in northwest Morocco, a huge avalanche occurs only once every 10,000 years.
There are a variety of potential triggers for underwater avalanches, including earthquakes, tides, typhoons, river floods, and even volcanic eruptions. Climate change will make some of these triggers more frequent and intense.
However, triggers are no guarantee that an avalanche will occur, nor do they have any relation to the size of the event. For example, in 1755 a large earthquake struck the coast of Portugal, destroying large parts of Lisbon and killing tens of thousands of people. However, it only triggered a tiny underwater avalanche.
By comparison, in 1929, a major earthquake off the coast of Newfoundland, Canada, triggered the largest underwater avalanche ever documented.
My colleagues and I have reconstructed the characteristics of this event using detailed seafloor and sediment core studies. The event moved at 68 km/h, carrying a concentrated mix of rock, sand and mud, breaking 11 seafloor cables on its journey down the slope.
The avalanche was so large that it triggered a tsunami that killed 28 people along the coast. It is the first and only giant underwater avalanche that could be measured directly from cable breaks.
Our understanding of underwater avalanches is still in its infancy, but research continues to provide new insights into where they occur, how they work, and how powerful and destructive they can be. These fascinating events remind us of the many wonders that still lie hidden in the deep sea.
Christopher Stevenson, Lecturer in Quantitative Sedimentology, University of Liverpool
This article is republished from The Conversation under a Creative Commons license. Read the original article.
