DESARROLLO DEL PANEL

Through the integrated use of passive seismic observations, terrestrial time–lapse photography, weather data, and qualitative observational records of calving events, substantial advances in the understanding of calving–generated icequakes has occurred in recent years (e.g., O'Neel et al., 2007; Amundson et al., 2008; Tsai et al., 2008; Bartholomaus et al., 2012). However, there is currently no consensus as to what mechanism during the calving process is associated with the release of seismic energy (Bartholomaus et al., 2012). This is unsurprising, as a complex arrangement of mechanisms occurs simultaneously throughout the calving process. Potential sources described in the literature include ice fracture (e.g., Neave and Savage, 1970; Cichowicz, 1983; Roux et al., 2008; Walter et al., 2009; West et al., 2010); fluid–driven crack resonance and other hydraulic transients (e.g., Métaxian et al., 2003; O'Neel et al., 2007); the overturning and scraping of icebergs on the glacier and proglacial foreland (e.g., Amundson et al., 2008; Tsai et al., 2008); grinding and fracturing within an ice mélange that occurs during the detachment of icebergs (e.g., Amundson et al., 2010); and the interaction between icebergs and the surface of the proglacial water body post–calving (e.g., Bartholomaus et al., 2012). The following sections highlight the main characteristics of source mechanisms associated with calving–generated seismic signals.

2.6.2.1Ice fracture and crevassing

The fracturing of ice and crevasse development is an energetic process which releases seismic energy across a range of frequencies between 10 and 100 Hz (Neave and Savage, 1970; Cichowicz, 1983; Métaxian et al., 2003; O'Neel et al., 2007; Dalban Canassy et al., 2012). Icequakes associated with ice fracture are generally impulsive, have short durations (< 1 second), and are localised within the upper 20 to 30 m of a glacier (Neave and Savage, 1970; Métaxian et al., 2003; Roux et al., 2008; Dalban Canassy et al., 2012), with swarms of such icequakes observed in densities of 2–10 events/s for durations of 1–15 s (Neave and Savage, 1970; Walter et al., 2008). Such swarms were associated with the sequential ‘cracking’ and connection of pre–existing flaws in the ice, generated by an increase in tensile stress within the crevasse zone, forming crevasses (Neave and Savage, 1970; Roux et al., 2008). Additional icequakes may also be associated with the deepening and lengthening of original cracks as shear stress increases. The typical waveform characteristics of ice fracture rules it out as the sole source mechanism for the bulk of energy released during calving events. However, they are thought to play a part during calving onset (Bartholomaus et al., 2012).

2.6.2.2Resonance within a fluid filled crack

The narrow frequency band (1–3 Hz) of calving–generated icequakes has led authors to suggest an alternative source mechanism that involves the resonance of water within a crack instead of

hydraulic transient. Within this process cracks or fractures in the ice are enlarged due to the presence of fluid. As the crack grows, resonance of the crack walls develops and propagates (Métaxian et al., 2003; Stuart et al., 2005; O'Neel and Pfeffer, 2007), creating the observed frequencies for a given crack geometry. O’Neel and Pfeffer (2007) indicate that a variety (infinite) of crack geometries are able to reproduce the narrow band of frequencies (1–3 Hz) observed for calving events. Furthermore, as the spectral composition is uniform regardless of calving event size, the fault rupture size may be similar across all events, indicating that larger events are made up of a series of small ruptures rather than a single large rupture. These smaller ruptures occur after a critical (but small) rupture destabilises a larger section of ice. Similar ruptures are thought to also occur within the basal zone of glaciers (e.g., Métaxian et al., 2003).

2.6.2.3Iceberg detachment and rotation

Seismograms of calving events have been associated with the rotation of full–thickness (up to kilometre–sized) icebergs and the resulting interaction (scraping) between the iceberg and the glacier terminus or sea or lake floor (Amundson et al., 2008; Joughin et al., 2008a; Tsai et al., 2008; Amundson et al., 2012). Icequakes (or glacial earthquakes at larger scales) are generated as icebergs overturn and rotate during calving due to hydrostatic imbalances (Figure 2.19). The rotation and break–up of icebergs post–calving contributes to the release of energy and the generation of icequakes through the transfer of momentum to the surrounding water–body. However, it is the scraping of the rotating iceberg when they come into contact with either the terminus or the former glacier bed and surrounding margins that has been hypothesised to account for the bulk of energy released (Amundson et al., 2008; Tsai et al., 2008; Amundson et al., 2010).

Such calving events have been observed at numerous Greenland outlet glaciers that are retreating into deeper water (Amundson et al., 2008; Joughin et al., 2008a; Joughin et al., 2008b; Amundson et al., 2010) and to a lesser extent the smaller tidewater glaciers of Alaska (Walter et al., 2010). Such large calving events are preceded by the thinning of glaciers to the point of flotation, allowing for the development of rifts in the glacier surface and basal crevasses to penetrate the full thickness of the glacier. It is this process of iceberg calving and rotation that has been hypothesised (Amundson et al., 2008; Tsai et al., 2008) to account for large–scale (e.g., kilometre–scale) events that have been recorded on global seismic networks up to earthquake magnitudes of ML 5 (e.g., Ekström et al., 2003; Tsai and Ekström, 2007).

Figure 2.19: Schematic diagram showing the bottom (A and C) and (B) top out rotation and scrapping of calved icebergs at floating (A and B) and grounded (C) termini (adapted from Amundson et al., 2010).

2.6.2.4Fracturing of ice melange and wave action

The presence of an ice mélange bounding a glacier terminus has been shown to have an effect on not only the calving of icebergs from termini, but also the seismic response of calving events (Amundson et al., 2010). The transfer of momentum due to the rotation of calved icebergs causes the development of waves within the ice mélange that can exceed 1 metre in height. It is the combination of the transfer of waves through the ice mélange and fracturing of ice within it that contributes to the overall seismicity of calving events. For example, Amundson et al. (2010) attributed the long (> 10 minutes) protracted coda of calving events at Jakobshavn Isbræ to the movement and breakup of ice mélange, with seismicity not returning to background levels until the ice mélange stiffens and stops moving. At glaciers without an ice mélange, the development of waves in the proglacial water–body can have the same effect of transferring momentum and seismic energy as the mélange, although they will decay at a faster rate as they will reach equilibrium faster (Amundson et al., 2010). It is this wave action that has also been hypothesised to account for the emergent onset of large calving events (Amundson et al., 2008).

2.6.2.5Ice–water interaction

At smaller scales, recent work by Batholomaus et al. (2012) has indicated that the dominant 1–5 Hz frequencies generated by calving events are associated with the interaction between the calved section of ice and the proglacial water body. Utilising synchronised video of the terminus and seismograms (Welty et al., 2013) the authors identified that a major ‘step’ in seismic amplitude within the 1–5 Hz frequency range occurs when an ice block that is calved from the subaerial ice cliff impacts with the water (Figure 2.20). Peak seismic amplitudes following this step coincided with the emergence of high velocity jets after the submergence of the entire ice block. As a result, Bartholomaus et al. (2012) has suggested that seismicity associated with calving events is generated at some point during this process. Four possible mechanisms may account for observed seismicity. They are: (1) the deceleration of the ice block as it enters the proglacial water–body and the resulting transfer momentum; (2) the formation of an air cavity above the descending ice block (Figure 2.20B); (3) cavity wall collapse (Figure 2.20C); or (4), the eventual pinching off of the cavity (Figure 2.20D). The result of this cavity closure is the formation of Worthington jets (Gekle and Gordillo, 2010) that are emitted from the centre of the collapsing cavity (Figure 2.20E). Modelling of processes 2 and 4 suggest that they are able to generate forces over time–scales expected for the generation of 1–5 Hz seismic signals (Bartholomaus et al., 2012). In contrast, processes 1 and 3 are also able to generate significant forces but over millisecond time–scales. As a result, all processes may play a role in generating seismic signals associated with calving, but the deceleration and cavity pinch off represent more realistic source mechanisms.

Figure 2.20: Schematic diagram of a calved ice block entering a proglacial water–body and potential the potential seismogenic mechanisms. After the iceberg ‘slams’ (A) in the proglacial water body it transfers its momentum to the water, decelerating the iceberg (B) and causing a cavity to form above it. This cavity begins to collapse (C), eventually pinching off (D). A Worthington jet (E) is often formed after the cavity pinch off and complete submergence of the iceberg. Illustration adapted from Gekle and Gordillo (2010), and Bartholomaus et al. (2012).