Not All Iceberg-Generated Tsunamis Are Alike. Here’s How They Differ

Many people are familiar with ocean tsunamis caused by earthquakes, such as the devastating Japan 2011 tsunami, but fewer know they can also be caused by iceberg calving. As glaciers and ice sheets undergo intensified melting, we can expect to see more frequent tsunamis triggered by icebergs dropping off the face of the world’s glaciers. These events threaten the lives of people in nearby coastal settlements, whether residents or tourists, and infrastructure as well.

In a recent study published in the journal Scientific Reports, lead researcher Valentin Heller and colleagues investigate the potential for five different calving mechanisms in producing tsunami waves. They knew that iceberg calving, also known as glacier calving, accounted for most of the mass loss from the Antarctic Ice Sheet and about a third for the Greenland Ice Sheet between 2009-2012. Their research could not only contribute to science but have practical effects. Identifying the impacts of  different calving scenarios is beneficial for implementing disaster management strategies and strengthening disaster resilience in coastal regions.

Scientists observed that iceberg calving events in polar regions interact differently with the surrounding waters through distinct calving mechanisms. They investigated five types of calving events: capsizing, gravity-dominated fall, buoyancy-dominated fall, gravity-dominated overturning, and buoyancy-dominated overturning.

To test the tsunami energy potential of each type of calving event, large-scale experiments were conducted in a 50 by 50 meter wave basin at Deltares in Delft, Netherlands. Sixty-six experiments were conducted , at depths of 1 or 0.75 meters. The researchers used PPH blocks, a thermoplastic material with similar density to ice, as a proxy for icebergs.

The researchers implemented various methods of control to simulate the five types of calving events. To represent capsizing, for example, the researchers  fed a wooden rod through the centers of the blocks in order to control the rotation. They simulated buoyancy-dominated fall by pulling the blocks underwater with rope and stabilizing them with a steel beam from above.

Falling and overturning icebergs, and sketches of calving mechanisms (Source: Heller et al.)

They then quantified the maximum heights and energies of the iceberg-tsunamis and found the relative energy releases of the iceberg calvings. They then  analyzed and compared the results with the predictive methods of landslide-tsunamis. By doing this, researchers aimed to transfer knowledge from a well-established research field to the relatively new field of iceberg-tsunamis.

The team found large differences in tsunami height between the mechanisms. The two gravity-dominated mechanisms were found to be better predicted by landslide-tsunami models than the others. These results are significant in understanding the relative impact and prediction capabilities of specific calving events, which is vital to disaster management. Yet the results will be of most use for cases of gravity-dominated calving events. More research will need to be done to better analyze the other calving mechanisms.

One thing not considered in the comparison was the movement of icebergs along coastal locations such as harbours. Researchers noted that even significantly smaller iceberg-tsunamis from capsizing can cause large destruction. They team also scrutinized the existing landslide-tsunami models for failing to capture the physics of the capsizing and buoyancy-driven mechanisms of A, C, and E, which are important iceberg events.

Calving at the Hubbard glacier in eastern Alaska (Source: Navin Rajagopalan/Flickr)

Lead author Valentin Heller, who’s an assistant professor of hydraulics at the University of Nottingham, said the experiments showed that icebergs falling into water were about 10 times larger than those breaking off underwater and moving to the surface, as well as capsizing icebergs. He said the researchers were surprised that this large difference has never been quantified before.

“The overall aim of the study is to be able to predict the tsunami magnitude in function of the size of the iceberg, its initial position relative to the water surface, and on how it interacts with the surrounding water,” Heller said. “This helps to predict the iceberg-tsunami height at any location in front of the glacier front to provide guidelines for tourist boats on how close they can safely approach a glacier front.”  

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Earthquakes Rattling Glaciers, Boosting Sea Level Rise

An iceberg from the Helheim Glacier in calm waters, Sermilik fjord, East Greenland. ©  Mads & Trine
An iceberg from the Helheim Glacier in calm waters, Sermilik fjord, East Greenland.
© Mads & Trine

Talk of earthquakes likely calls to mind giant fissures opening up along the earth’s crust, the trembling of rock, buildings crumbling to their knees and, depending on your age and cast of mind, the love of Superman for Lois Lane. But it does not likely conjure up images of giant tongues of sliding ice or the splash of calving icebergs. And yet it should.

Most earthquakes are generated by the friction produced by two bodies of rock rapidly sliding past each other on a fault in the Earth’s crust, but a different breed of earthquakes was discovered in 2003: glacier earthquakes.

Map showing 252 glacial earthquakes in Greenland for the period 1993–2008, detected and located using the surface-wave detection algorithm. (b) Map showing the improved locations of 184 glacial earthquakes for the period 1993–2005 analyzed in detail by Tsai Ekström (2007). ©  Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University
Map showing 252 glacial earthquakes in Greenland for the period 1993–2008, detected and located using the surface-wave detection algorithm. (b) Map showing the improved locations of 184 glacial earthquakes for the period 1993–2005 analyzed in detail by Tsai Ekström (2007). © Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University

These newly documented earthquakes are occurring in glaciated areas of Alaska, Antarctica and Greenland and are caused by the dumping of giant icebergs–equal in size to, say, 400,000 Olympic swimming pools–into the sea. They produce seismic signals equivalent to those found in magnitude 5 earthquakes, which can be felt thousands of kilometers away. And there are many more of them today than there were just a couple of decades ago: six to eight times more than in the early 1990s have been recorded at outlet glaciers along the coast of Greenland.

This sudden surge in glacier earthquakes is expected to set off a series of events that will result in faster sea level rise over the coming century than had previously been estimated, according to research conducted there by Dr. Meredith Nettles, Associate Professor of Earth and Environmental Sciences at Columbia University, and some of her colleagues, as a part of Project SERMI. In 2013, the Intergovernmental Panel on Climate Change (IPCC) revised estimates for the next century dramatically upward (from 11-17 inches by 2100 to 10-39 inches) when taking Dr. Nettles and her colleagues’ earthquake research into account for the first time. This upward revision reflects the fact that the earthquakes change the internal dynamics of the glaciers, causing them to flow more rapidly, and to shed more ice into the ocean.

Monitoring station on Helheim glacier. © SERMI
Monitoring station on Helheim glacier. © SERMI

Nettles gave a talk on glacier earthquakes last November at the American Museum of Natural History. In the summer of 2006, she and 11 other scientists from six institutions in the U.S., Denmark and Spain traveled to a small town in East Greenland to take seismic, GPS and time-lapse photography measurements of the Helheim Glacier. They wanted to examine the location, dynamics and frequency of glacier earthquakes and to develop a method for using seismic data to map changes in the ice. They also wanted to learn how these earthquakes shape the behavior of outlet glaciers, which cluster around coastlines and deposit ice and meltwater into the oceans.

After setting up camp in town, the scientists flew a helicopter out to the glacier, drilled holes 6 feet deep in the ice, and drove 9-foot poles into those holes to anchor their GPS, time-lapse and seismic equipment. From the data they collected, they learned that short-term acceleration of glacier ice flows—up to 25% increases in velocity—coincided with the earthquakes. They also found that the increase in glacier earthquakes corresponded to net retreat of the ice front in Greenland. In particular, the section of the Greenland coast with earthquake-producing glaciers expanded northward. And whereas in the 1990s, a few glaciers were causing earthquakes; by 2005, those glaciers were associated with more frequent earthquakes, and other glaciers began to have seismic activity as well.

Map showing locations of GPS stations (blue and yellow dots). Arrows show average velocities over this time period. Red dots represent locations of rock-based GPS reference sites. Dashed lines show the location of the calving front at the beginning (eastern line) and end (western line) of the network operation period. Inset shows location of Helheim glacier in southeast Greenland (black arrow) and locations of glacial earthquakes (white dots). © Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University
Map showing locations of GPS stations (blue and yellow dots). Arrows show average velocities over this time period. Red dots represent locations of rock-based GPS reference sites. Dashed lines show the location of the calving front at the beginning (eastern line) and end (western line) of the network operation period. Inset shows location of Helheim glacier in southeast Greenland (black arrow) and locations of glacial earthquakes (white dots).
© Glacial Earthquakes in Greenland and Antarctica, Meredith Nettles and Göran Ekström, Lamont-Doherty Earth Observatory of Columbia University

Future research should focus on ice-ocean interactions that promote or reduce glacier calving, said Nettles. And scientists still need to better understand the specific mechanisms of loss of ice at the calving front and the effects of loss of ice on flow speeds. Nettles’ current research examines the impact of tides on glacier calving. Preliminary analysis of the data suggests that glacier earthquakes are more likely to occur at low tide.

Nettles and her colleagues collected most of their seismic data and GPS observations of the glacial earthquakes through facilities run jointly by IRIS (Incorporated Research Institutions for Seismology) and the USGS (U.S. Geological Survey). Thanks to grants from the USGS and the National Science Foundation, that data is open sourced and available to the public.

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