Glacier seismology is a young but rapidly evolving discipline. With the advent of robust, cheaper and more portable seismic instrumentation it is now possible to install sensors near and on glacial ice. The seismic wave field is rich at a spectrum between hundreds of seconds and hundreds of Hertz and provides insights into various ice dynamic processes (Figure 1), which are difficult or impossible to study with conventional techniques. The recently published review article by Podolskiy and Walter (2016) presents an overview about glacier seismology.
ETH's Glacier Seismology Group conducts research in several disciplines. Microseismic sources near glacier beds are an important emphasis. In certain cases, these sources are manifestations of sudden ice sliding episodes, often referred to as stick-slip motion. The glacier seismology group investigates stick-slip "icequakes" (Figure 2) to clarify their role in overall glacier motion. Although not all glacial deployments of seismometers have revealed the presence of stick-slip icequakes, there exists evidence for them in Antarctica, Greenland and various mountain glaciers around the world. Investigating the subglacial environment of glaciers and ice sheets also involves the hydraulic drainage system. In this context, seismological measurements of water tremor have been a valuable analysis tool.
Perhaps the most prominent subject of glacier seismology is that of iceberg calving. Seismologically speaking, the detachment of icebergs is a very "loud" process (Figure 3). At high frequencies (>1 Hz), iceberg calving can be detected and studied with regional seismometer networks at distances of up to 100 km. However, the largest iceberg calving events in Greenland and Antarctica also constitute large seismic events called "glacial earthquakes" (Nettles and Ekström, 2010). These events generate low frequency seismic waves (below 0.1 Hz) and can be detected at 1000's of km distances. Calving seismology offers the possibility to monitor iceberg production remotely at an unrivaled temporal resolution. It has also elucidated processes, which play key roles in hazardous unstable mountain glaciers (see project tab).
Bartholomaus, T. C., Larsen, C. F., O'Neel, S., & West, M. E. (2012). Calving seismicity from iceberg–sea surface interactions. Journal of Geophysical Research: Earth Surface, 117(F4).
Larose, E., Carrière, S., Voisin, C., Bottelin, P., Baillet, L., Guéguen, P., Walter, F., Jongman, D., Guillier, B., Garambois, S., Gimbert, F. & Massey, C. (2015). Environmental seismology: What can we learn on earth surface processes with ambient noise?. Journal of Applied Geophysics, 116, 62-74.
Nettles, M., & Ekström, G. (2010). Glacial earthquakes in Greenland and Antarctica. Annual Review of Earth and Planetary Sciences, 38(1), 467.
Podolskiy, E. A., and F. Walter (2016), Cryoseismology, Reviews of Geophysics, 54, doi:10.1002/2016RG000526.
Roeoesli, C., Helmstetter, A., Walter, F., & Kissling, E. (2016). Meltwater influences on deep stick‐slip icequakes near the base of the Greenland Ice Sheet. Journal of Geophysical Research: Earth Surface.
Unstable glaciers are a threat to high Alpine areas worldwide. Structural weaknesses of ice can lead to steep glacier rupture and resulting ice avalanches (Faillattaz et al., 2015) or to sudden failure of ice dams (Roberts et al., 2005). The hazard potential of unstable glaciers therefore demands monitoring programs and reliable early warning techniques in order to protect human settlements and infrastructure in mountainous terrain.
Up to date, unstable glaciers are typically monitored with time-lapse imagery, ground-based radar interferometry and similar measurements targeting the glacier's surface. These techniques have produced useful results detecting telltale acceleration phases prior to steep glacier ruptures or sudden decreases of glacier-dammed lake levels. Nevertheless, these approaches are dependent on weather conditions and/or may only detect a glacier failure event after its initiation. More meaningful insights could be obtained if monitoring techniques focused on processes within the glacier or near its base, where damage growth and sliding take place. Unfortunately, this poses considerable technical challenges, as a glacier's subsurface structure is difficult to image let alone monitor.
In collaboration with the Swiss Seismological Service (SED), this project employs seismic monitoring as a new approach to study the processes leading to glacier failure. Harnessing recent advances in crustal seismology, the project will emphasize the analysis of seismic background noise (e.g. Campillo and Roux, 2014). The driving idea is that brittle processes within glacier ice do not only lead discrete "icequake" events, but also have an influence on the velocity of seismic waves traveling within the ice. Moreover, extended tremor signals of englacial water flow will be scrutinized, because water is known to play a central role in glacier sliding. As part of this project, new seismic data sets from high-altitude hanging glaciers, steep glacier tongues (see Figure 1 ''Weisshorn'') and unstable ice dams will be collected. At the same time, existing multi-seasonal icequake catalogs will be included as a means to evaluate the sensitivity of englacial fracture growth to external forcings.
Ongoing climate change affects glacier stability, as previously cold glaciers, which are frozen to their bed, may transition to temperate thermal regimes. This process takes place within and under the glacier, and cannot be observed from the surface. The interior of a glacier and its bed, however, are also not readily accessible, as drilling on such glaciers is prohibitively expensive and provides point measurements, only. Therefore, we infer the processes at the glacier bed and within the glacier from passive seismic measurements. We study icequakes, caused by the opening and closing of crevasses, by break-off events of the glacier front, and possibly by the sliding of the glacier over its bed. We also explore the use of state-of-the-art seismological techniques such as ambient noise monitoring and spectral methods on glaciers.
A recent deployment targeted Bisgletscher in the Matter valley (Switzerland), where we acquired around 2 months of data from an array of seismometers in the accumulation zone, just behind the unstable front (Fig. 2). We have also experimented with installing seismometers directly on the unstable part of hazardous glaciers, for example on the hanging glacier in the Weissmies northwest face (Preiswerk et al., 2016).
Surface meltwater which reaches and pressurizes the subglacial environment of glaciers or ice sheets promotes basal motion. Currently, the impact of increased available meltwater due to climate warming on recently observed ice stream acceleration is debated. But also in the light of possibly highly destructive glacial floods associated with the sudden release of huge amounts of water, the glacial drainage system plays a critical role. However, mainly due to the spatially and temporally limited inaccessibility of the subglacial environment, our understanding of the subglacial drainage system and its evolution over time is still incomplete.
To gain further insights into the drainage process, we use passive seismic recordings acquired at the glacier surface: Turbulent water flow in sub- and englacial channels generates sustained seismic tremors whereas fracturing events (and thus the opening of new water pathways) excite transient signals. We continuously record these naturally occurring seismic emissions on time scales of several weeks to months with the goal to remotely monitor the evolution of the subglacial drainage system. In this context we employ seismic methods such as beamforming techniques, seismic tremor analysis, seismic anisotropy measurements and seismic interferometry. The seismic recordings are supplemented by measurements of the glacier surface velocity and the monitoring of glacial lake levels.
- Campillo, M., & Roux, P. (2014). Seismic imaging and monitoring with ambient noise correlations. B. Romanowicz and A. Dziewonski (Elsevier, Amsterdam), 1, 256-271.
- Faillettaz, J., Funk, M., & Vincent, C. (2015). Avalanching glacier instabilities: review on processes and early warning perspectives. Reviews of Geophysics.
Preiswerk, L.E., Walter, F., Anandakrishnan, S., Barfucci, G., Beutel, J., Burkett, P.G., Dalban Canassy, P., Funk, M., Limpach, P., Marchetti, E., Meier, L., Neyer, F. (2016). Monitoring unstable parts in the ice-covered Weissmies northwest face. Proceedings of the 13th Interpraevent Congress, Lucerne (full paper). Article
- Roberts, M. J., Russell, A. J., Tweed, F. S., & Knudsen. (2000). Ice fracturing during jökulhlaups: implications for englacial floodwater routing and outlet development. Earth Surface Processes and Landforms, 25 (13), 1429-1446.
The study of small earthquakes related to glacier flow, so-called icequakes, can reveal important insights into the dynamics and hydraulics of glaciers. Icequakes near the glacier bed are particularly interesting, because they may be a manifestation of microseismic stick-slip sliding. This phenomenon is not fully understood and has yet to be captured in ice flow models. At the same time, basal seismicity can be a manifestation of hydraulic fracturing as melt water opens up new water channels within and beneath the glacier, a process, which is analogous to the hydraulic stimulation within geothermal reservoirs.
This project focuses on the occurrence of sliding-related stick-slip icequakes beneath Aletschgletscher. Whereas these events have been confirmed beneath the polar ice sheets (e.g. Smith et al., 2015; Roeoesli et al., 2016), it is to date not clear if they also exist beneath relatively flat Alpine glaciers. To this end, the SED conducted a two-step seismometer deployment on the glacier tongue. In a first step, three shallow borehole seismometers were installed and maintained for 1.5 years (January 2015-July 2016). These data identified an icequake cluster at the glacier bed, which was monitored in near-real time with the help of state-of-the-art real-time data communication. In June 2016, an additional network with six seismometers was installed above the cluster to confirm that the recorded icequakes are indeed stick-slip events and to provide better hypocenter locations.
The seismic Aletschgletscher record is unique in that it contains stick-slip icequakes recorded at an unrivaled quality over more than a year. This will provide an unprecedented look at seismogenic stick-slip sliding and its changes over the course of a full year. As a result of harsh weather conditions in high-melt areas, on-ice seismometer installations are extremely difficult on Alpine glaciers. Consequently, the Aletschgletscher data set offers a first-of-its-kind view on how stick-slip seismicity changes as the glacier reacts to climatic changes.
Our project is embedded in cross-disciplinary collaborative research between the Glaciology division at the Laboratory of Hydraulics, Hydrology, and Glaciology (VAW) at ETH Zurich and the Exploration and Environmental Geophysics group at the Institute of Geophysics at ETH Zurich. In the past, these groups have collected milestone records in the field of glacier seismology (Podolskiy and Walter, 2016). The joint interpretation of the Aletschgletscher data will help to improve our understanding of glacier flow and its reaction to climate change and glacier retreat, also affecting the Swiss Alps.
E.A. Podolskiy and F. Walter (2016). Cryo-seismology. Reviews of Geophysics, 54.
C. Roeoesli, F. Walter, J.-P. Ampuero, E. Kissling (2016). Seismic Moulin Tremor. Journal of Geophysical Research - Solid Earth, 121(8), 5838-5858.
E. A. Podolskiy, S. Sugiyama, M. Funk, F. Walter, R. Genco, S. Tsutaki, M. Minowa, M. Ripepe (2016). Tide-modulated ice flow variations drive seismicity near the calving front of Bowdoin Glacier, Greenland. Geophysical Research Letters, 43(5), 2036-2044.
L. E. Preiswerk, F. Walter, S. Anandakrishnan, G. Barfucci, J. Beutel, P. G. Burkett, P. Dalban Canassy, M. Funk, P. Limpach, E. Marchetti, L. Meier and F. Neyer (2016). Monitoring unstable parts in the ice-covered Weissmies northwest face. 13th Congress INTERPRAEVENT 2016.
C. Roeoesli, A. Helmstetter, F. Walter, E. Kissling (2016). Meltwater influences on deep stick-slip icequakes near the base of the Greenland Ice Sheet. Journal of Geophysical Research - Earth Surface, 121, 223–240, doi:10.1002/2015JF003601.
A. Sergeant, A. Mangeney, E. Stutzmann, J.-P. Montagner, F. Walter, L. Morretti, and O. Castelnau (2016). Complex force history of a calving-generated glacial earthquake derived from broadband seismic inversion. Geophysical Research Letters, 43, 1055–1065, doi:10.1002/2015GL066785.
P. Dalban Canassy, C. Roeoesli and F. Walter (2016). Seasonal variations of glacier seismicity at the tongue of Rhonegletscher (Switzerland) with a focus on basal icequakes. Journal of Glaciology, 62(231), 18-30.
E. Larose, S. Carrière, C. Voisin, P. Bottelin, L. Baillet, P. Guéguen, F. Walter, D. Jongmans, B. Guillier, S. Garambois (2015). Environmental Seismology: what can we learn on Earth surface processes with ambient noise? Journal of Applied Geophysics, 116, 62-74.
F. Walter, P. Roux, C. Röösli, A. LeCointre, D. Kilb, P.-F. Roux (2015). Using Glacier Seismicity for Phase Velocity Measurements and Green's Function Retrieval. Geophysical Journal International, 201(3), 1722-1737.
D. S. Heeszel, F. Walter, D. L. Kilb (2015). Humming Glaciers. Geology, 42(12), 1099- 1102.