Ice shelf rift propagation and the mechanics of wave-induced fracture

Abstract

Distant storms, tsunamis, and earthquakes generate waves on floating ice shelves. Previous studies, however, have disagreed about whether the resulting wave-induced stresses may cause ice shelf rift propagation. Most ice shelf rifts show long periods of dormancy suggesting that they have low background stress concentrations and may therefore be susceptible to wave-induced stresses. Here, I quantify wave-induced stresses on the Ross Ice Shelf Nascent Rift and the Amery Ice Shelf Loose Tooth T2 Rift using passive seismology. I then relate these stresses to a fracture mechanical model of rift propagation that accounts for rift cohesive strength due to refrozen melange, ice inertia, and spatial heterogeneity in fracture toughness due to the presence of high toughness suture zones. I infer wave-induced stresses using the wave impedance tensor, a rank three tensor that relates seismically observable particle velocities to components of the stress tensor. I find that wave-induced stresses are an order of magnitude larger on the Ross Ice Shelf as compared to the Amery Ice Shelf. In the absence of additional rift strength, my model predicts that the Nascent Rift should have experienced extensive rift propagation. The observation that no such propagation occurred during this time therefore suggests that the Nascent Rift experiences cohesive strengthening from either refrozen melange or rift tip processes zone dynamics. This study illustrates one way in which passive seismology may illuminate glacier calving physics.

Ross Ice Shelf Icequakes Associated With Ocean Gravity Wave Activity

“…The extensional strain perturbation Σ induced by gravity wave impacts may be associated with ice fracturing. Assuming A SV is primarily due to flexural waves, Σ is proportional to k 2 A SV , where k is the wave number of the flexural waves (Lipovsky, ). The peak in the strain response is in the IG band (Figures d and S6).…”

“…Although internal glaciological stresses may be the dominant driving force of crevasse formation and expansion (Bassis et al, ), surface and basal melting can facilitate this process by thinning the ice and providing additional pressure in fissures (Scambos et al, ). Experiments on the Ross Ice Shelf (RIS) suggest that stress perturbations induced by ocean gravity waves (Lipovsky, ) and tides (Olinger et al, ) may also contribute to ice fracture and rift propagation, further reducing ice shelf integrity. Persistent gravity wave forcing with intermittent strong impacts, including swell, infragravity (IG) waves, and tsunami (Bromirski et al, , ; Chen et al, ), may fatigue ice shelves, promote iceberg calving (Brunt et al, ), and potentially trigger the collapse of weakened ice shelves (Massom et al, ).…”

“…Classical studies model an ice shelf response to gravity wave impacts as a floating plate of uniform thickness (Fox & Squire, ; Papathanasiou et al, ; Press & Ewing, ). Recent studies include two‐dimensional (Ilyas et al, ; Sergienko, ) and three‐dimensional (Sergienko, ) finite‐element simulations and estimates of extensional stress induced by flexural and extensional waves from in situ seismic data (Lipovsky, ). Such simulations indicate that induced dynamic stress strongly depends on the system geometry, including ice shelf thickness and physical properties, subshelf cavity thickness, and bathymetry, each having significant uncertainty and spatial variability not adequately accounted for.…”

Seasonal and spatial variations in the ocean-coupled ambient wavefield of the Ross Ice Shelf

“…Nascent iceberg (NIB) is a semi-detached spur at the RIS ice front partially bounded by a 46 km long rift. Expansion of the rift – and eventual calving of NIB – has apparently been arrested by propagation of the rift into a suture zone with a higher fracture toughness than the surrounding ice shelf (Borstad and others, 2017; LeDoux and others, 2017; Lipovsky, 2018) and has been in near-steady-state since at least 2004 (Okal and MacAyeal, 2006; Lipovsky, 2018). By design, three stations located at the northern terminus of the array transected NIB for the purpose of studying ice front mechanics and cryoseismic signals.…”

A High Resolution, Three‐Dimensional View of the D‐28 Calving Event From Amery Ice Shelf With ICESat‐2 and Satellite Imagery

“…Studies of the calving process have focused on understanding the factors that control the propagation of rifts (e.g., Bassis et al., 2005, 2008; Benn et al., 2007; Fricker et al., 2005; Heeszel et al., 2014; Hulbe et al., 2010; Larour et al., 2004; MacGregor et al., 2012; Walker et al., 2013, 2015), some of which have suggested that surface environmental factors such as temperature and wind speed are not necessarily drivers (Humbert & Steinhage, 2011; Larour et al., 2004; Joughin & MacAyeal, 2005). Other studies have argued that ocean stresses, such as storm‐induced swell, infra‐gravity waves and tsunamis might drive rift propagation (Bromirski et al., 2010; K. Brunt et al., 2011; Lipovsky, 2018; MacAyeal et al., 2006, 2017; Sergienko, 2010). Walker et al.…”