Glacial quake

False-color satellite image of the calving area of ​​the Helheim Glacier ( Terra , 2003), one of the largest outlet glaciers of the Greenland Ice Sheet and one of several in Greenland where quakes have been registered.

Glacier quakes or glacial earthquakes ( English glacial earthquake or glacialquake ) are seismic events with large Kalbungsereignissen of marine glaciers are related. The phenomenon, which was first described in 2003 on the outlet glaciers of the Greenland Ice Sheet and has meanwhile also been proven in Antarctica, forms an interdisciplinary research field from glaciology , seismology and physics and is considered an indicator of global warming .

discovery

The scientific discovery of glacier quakes happened by chance in 2003. Göran Ekström and Meredith Nettles, at the time both working at the Department of Earth and Planetary Sciences at Harvard , filtered teleseismic data from the Global Seismographic Network (GSN) from 1999 to 2001 in search of them "Slow earthquake ". The of them developed and two years earlier on a geophysical conference presented algorithm should seismic sources track that mostly down -frequency , elastic waves with periods from 35 to 150 seconds radiate and find as earthquake unconventional production patterns. In doing so, they came across a series of previously unregistered events in the usually aseismic Greenland . As further investigations of the data showed, the researchers had discovered a new type of tremor.

description

Glaciological and climatological relationships

Calving front of Jakobshavn Isbræ in West Greenland (2012)

By analyzing seismic data series from 1993 to 2013 using the method developed by Ekström and Nettles, it was possible to show that seismic activity in Greenland has steadily increased since 1993. Twice as many glacier earthquakes were recorded in 2005 as in the years before 2002 and between 2011 and 2013 around a third of all events identified in the entire 21 years took place (139 of 444).

The increase in seismic activity is accompanied by changes in the dynamics of numerous outlet glaciers. An increase in the flow rate and a continuous retreat of the calving front of many of these glaciers has been observed, which is why the earthquakes are causally related to climate change . The three largest glaciers in Greenland recorded a speed increase of around 50% between 2002 and 2005. All glacier quakes recorded to date have occurred on glaciers with flow speeds of at least one kilometer per year. The reason for the increase in flow speed is that ice increasingly melts on the upper sides of the glaciers during the summer months, with the corresponding meltwater reaching the glacier bottom via crevices in the ice, where it acts as a “lubricant”.

Furthermore, due to their landward retreat, the calving fronts of the glaciers end up in increasingly shallow water. This so-called near grounding is seen as favoring the breaking off of narrow, high icebergs with an unstable swimming position, which is the main mechanism for generating the seismic signal (see below ). On the other hand, glacier tongues that extend relatively far out to sea and float on deeper water tend to break off broad (tabular) icebergs with a stable swimming position. In addition, the close proximity of the calving front enables better transmission of the seismic signal to the solid rock substrate.

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Temporal and rough spatial distribution of the 444 glacial earthquakes identified in Greenland from 1993 to 2013. Note, among other things, the general increase in the number and density of events over time.

Differences from earthquakes

Normal earthquakes occur in the earth's crust : When mechanical tensions , which have built up in the course of plate tectonics , for example , are suddenly discharged, energy is released in the form of high- frequency, briefly periodic waves. Glacier quakes, on the other hand, form on the surface of the earth. After initially assuming that the earthquakes occurred at the bottom of the glacier as a result of the jerky advance of the ice mass, the calving , especially the detachment of very large icebergs (with volumes in the range of cubic kilometers) from the glacier front, could later be identified as the trigger (see below ) . This creates slow, long-period waves that are transmitted to the solid rock substrate and propagate on its surface. This low-frequency wave type has significantly less potential for danger to the environment and can not be detected with conventional seismometers . The measured magnitudes of the glacial earthquakes are between 4.6 and 5.1 ( M _w ). While a normal earthquake of magnitude 5 (M _w ) lasts just two seconds, a glacier quake of the same magnitude takes 30 to 60 seconds, i.e. 15 to 30 times the time.

Around a third of the glacier quakes in Greenland that were clearly identified between January 1993 and October 2005 took place in the summer months of July and August when the glaciers were retreating. Such a seasonal variance cannot be determined in ordinary earthquakes. Over the entire summer half-year, the glacial seismic activity in this period was on average five times higher than in the winter months.

Emergence

By comparing seismic data with satellite images and the water pressure measured in the vicinity of the Helheim glacier , it was possible to determine for the first time that glacier quakes occur during large calving events. The analysis of data collected since 2006 on other large outlet glaciers in Greenland (including Jakobshavn Isbræ ) also showed a simultaneous occurrence of calving and glacial quakes. It is now widely accepted that large calving events are a cause of glacier quakes. Only the exact mechanisms and circumstances are still the subject of intensive research.

Computer models , with the help of which the possible physical processes during a glacier quake were simulated, provided the best agreement with the real observed data, if a so-called nonequilibrium calving event was assumed as the cause of the quake . One such event is that a very voluminous iceberg , the length of which (measured in the direction of flow of the glacier) is less than its height (corresponds to the thickness of the glacier), detaches itself from the glacier front. Due to its proportions and the resulting relatively high position of its center of gravity as well as its buoyancy , it is in an “imbalance” ( nonequilibrium ) at this moment . Such an iceberg can tip 90 degrees within 10 to 15 minutes. The horizontal acceleration experienced by the ends of the rotating iceberg thereby produces a horizontally directed "Kalbungskontaktkraft" ( calving-contact force , F _C ) exerted by the one of the ends of the glaciers front. As the iceberg gradually reaches its stable floating position, the horizontal movement of the ends slows and F _C decreases. This increase and decrease in F _C during a calving event is at least responsible for the horizontal component of the seismic signal. A particularly good agreement with the observed data was found when the model was expanded to include a counterforce exerted by the so-called “proglacial ice melange” on the upper end of the tipping iceberg, rotating away from the glacier front. This mélange is a mixture of sea ​​ice and smaller icebergs that is often found in front of the glacier front in the fjords.

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Scheme for the theoretical model of the formation of glacial earthquakes through “ nonequilibrium calving ”: (a) Simple model. The blue dotted arrow shows the direction of rotation of the iceberg (here so-called top-out calving , i.e. the upper end of the iceberg rotates away from the calving front, see also web links ), the black double arrow shows the horizontally acting “calving contact force” F _C , which leads to the seismic signal leads, the other forces are shown by simple black arrows, x _i and y _i are the coordinates of the center of mass of the iceberg, θ is the angle of rotation. (b) Force vectors acting on the rotating iceberg (including weight, buoyancy, flow resistance). (c) Sketch for the same model, which has been expanded to include resistant "ice melange" in front of the calving front.

The results of this theoretical approach from 2008 were in principle confirmed and extended by direct observations on the Helheim Glacier in summer 2013. In the case of at least ten major calving events recorded, which were always accompanied by glacial earthquakes, a brief reversal of the flow direction of the “end piece” (terminus) of the glacier could be measured. In so-called bottom-out calvings , in which the lower end of the iceberg rotates away from the glacier front, a brief downward bend of the terminus that does not touch the bottom of up to ten centimeters was registered at the same time. The cause of this bending is probably a strong reduction in the water pressure between the glacier front and the rotating iceberg. The seismic data suggest that, for the same reason, the solid bedrock below the calving front was briefly raised slightly. These movements of the bedrock are responsible for the vertical component in the seismic signals, which is not explained by the original nonequilibrium Calving model. Due to the strong ice drift in the Helheim Fjord, no pressure measurements could be made in the water, but the mechanism of action derived from the other field data could be verified with the help of a laboratory replica (see web links ).

Despite the high dynamics of the process, the earthquakes contribute little to the glacier movement.

Occurrence

Northwest- Greenland Rinks Isbræ Jakobshavn Isbræ Daugaard Jensen Glacier Kangerlugssuaq Helheim Glacier Southeast Greenland

Distribution of the 182 glacial earthquakes in Greenland between 1993 and 2005

Glacier quakes occur on massive, dynamic sea or outlet glaciers with corresponding flow velocities. The main distribution area is the edge of the Greenland Ice Sheet , where a total of 444 events could be located between January 1993 and December 2013. The geologist Ekström pointed out that some of the glaciers as large as Manhattan and as tall as the Empire State Building were, so it was not unusual that in the case of relatively abrupt disturbance in the normal sequence of movements of such ice in the order of 10 centimeters per minute (the mean flow rate is a maximum of 3 centimeters per minute) seismic waves would arise. The three largest Greenland outlet glaciers - Kangerlussuaq , Jakobshavn Isbræ and Helheimgletscher - recorded 57% of all glacier quakes on the island. 103 earthquakes were detected on the Helheim glacier alone.

While 182 glacial quakes were recorded in Greenland from 1993 to 2005, there were only 14 at the edge of the Antarctic ice sheet in the same period. Vincennes Bay (7), the coast of the Antarctic peninsula (3), the Georg V coast (2) as well as the edge of the Filchner-Ronne ice shelf and the Banzare coast (1 quake each) were determined as the starting points for the seismic waves. Due to the respective position, the connection of glacier quakes to calving events is also obvious for Antarctica, but the small number of quakes makes it difficult, unlike in Greenland, to recognize a seasonal pattern. Research assumes that the greater extent of the tongues, the mouths of the glaciers into the ice shelf and, above all, the lower melting rates are responsible for the significantly smaller number than in Greenland.

Due to the unusual characteristics of the quake focus and its geographical location near the Dall Glacier , a single seismic event that took place on September 4, 1999 in the Alaska range was initially classified as a glacier quake. However, seismic data subsequently obtained in Alaska indicate a landslide as the trigger.

literature

• Göran Ekström, Meredith Nettles & Victor C. Tsai: Seasonality and Increasing Frequency of Greenland Glacial Earthquakes. In: Science Vol. 311 (March 24, 2006), pp. 1756-1758 online PDF (English).
• T. Murray, M. Nettles, N. Selmes, LM Cathles, JC Burton, TD James, S. Edwards, I. Martin, T. O'Farrell, R. Aspey, I. Rutt & T. Bangé: Reverse glacier motion during iceberg calvings and the cause of glacial earthquakes. In: Science Vol. 349 (July 17, 2015), pp. 305–308 online PDF (English).
• Victor C. Tsai, James R. Rice & Mark Fahnenstock: Possible mechanisms for glacial earthquakes. In: Journal of Geophysical Research Vol. 113 (August 5, 2008), F03014 doi: 10.1029 / 2007JF000944 (Open Access, English).

Remarks

1. ↑ The mean flow velocity of the fastest outlet glacier of the two large ice sheets (Antarctica and Greenland), the seismically active Jakobshavn Isbræ, was determined in 2012 to be 17,100 meters per year.

Web links

Commons : Glacier Quake  - Collection of images, videos and audio files

• When the glaciers dance (heise.de)
• Model Iceberg Capsize - model of a "nonequilibrium iceberg" (bottom-out) tipping off the glacier front in the laboratory tank (YouTube channel of the Burton Lab, Department of Physics, Emory College of Arts and Science)
• Model Iceberg Capsize - Top Out - Model of a "nonequilibrium iceberg" (Top-Out) tipping off the glacier front in the laboratory tank (YouTube channel of the Burton Lab, Department of Physics, Emory College of Arts and Science)

Individual evidence

1. ↑ ^{a b c d} Göran Ekström, Meredith Nettles & Geoffrey A. Abers: Glacial earthquakes . In: Science Vol. 302 (October 24, 2003), pp. 622-624 online PDF (English).
2. ^ ^{A b c d} Ian Joughin: Greenland Rumbles Louder as Glaciers Accelerate. In: Science Vol. 311 (March 24, 2006), pp. 1719–1720 online PDF (English).
3. ↑ Göran Ekström & Meredith Nettles: Detection and location of slow seismic sources using surface waves. In: Eos, Transactions, American Geophysical Union Vol. 83, No. 47 (November 19, 2002), Fall Meeting Supplement, Abstract S72E-06 Online (English)
4. ↑ ^{a b c d e f} Meredith Nettles & Göran Ekström: Glacial Earthquakes in Greenland and Antarctica. In: Annual Review of Earth and Planetary Sciences Vol. 38 (2010), pp. 467-491 (English).
5. ↑ ^{a b c d e f} Göran Ekström, Meredith Nettles & Victor C. Tsai: Seasonality and Increasing Frequency of Greenland Glacial Earthquakes. In: Science Vol. 311 (March 24, 2006), pp. 1756-1758 online PDF (English).
6. ↑ ^{a b c d} Stephen A. Veitch, Meredith Nettles: Spatial and temporal variations in Greenland glacial-earthquake activity, 1993-2010. In: Journal of Geophysical Research: Earth Surface Vol. 117, F04007 (December 2012), doi: 10.1017 / jog.2017.78 (Open Access, English).
7. ↑ ^{a b c d} Kira G. Olsen, Meredith Nettles: Patterns in glacial-earthquake activity around Greenland, 2011–13. In: Journal of Glaciology Vol. 63 (December 2017), pp. 1077-1089, doi: 10.1029 / 2012JF002412 (Open Access, English).
8. ↑ ^{a b} Katja Seefeldt: When the glaciers dance. Telepolis , March 24, 2006, accessed March 31, 2018 . 
9. ↑ ^{a b c d} T. Murray, M. Nettles, N. Selmes, LM Cathles, JC Burton, TD James, S. Edwards, I. Martin, T. O'Farrell, R. Aspey, I. Rutt & T. Bangé: Reverse glacier motion during iceberg calvings and the cause of glacial earthquakes. In: Science Vol. 349 (July 17, 2015), pp. 305–308 online PDF (English).
10. ↑ ^{a b c d e} Victor C. Tsai, James R. Rice & Mark Fahnenstock: Possible mechanisms for glacial earthquakes. In: Journal of Geophysical Research: Earth Surface Vol. 113 (August 5, 2008), F03014 doi: 10.1029 / 2007JF000944 (Open Access, English).
11. ↑ Carol Clark: Calving icebergs fall back, spring forward, causing glacial earthquakes. Emory University , June 25, 2015, accessed March 31, 2018 . 
12. ^ I. Joughin, BE Smith, DE Shean & D. Floricioiu: Further summer speedup of Jakobshavn Isbræ. In: The Cryosphere Vol. 8 (2014), pp. 209–214 doi: 10.5194 / tc-8-209-2014 (Open Access, English)