Monitoring Greenland ice sheet buoyancy-driven calving discharge using glacial earthquakes

Abstract

Since the 2000s, Greenland ice sheet mass loss has been accelerating, followed by increasing numbers of glacial earthquakes (GEs) at near-grounded glaciers. GEs are caused by calving of km-scale icebergs which capsize against the terminus. Seismic record inversion allows a reconstruction of the history of GE sources which captures capsize dynamics through iceberg-to-terminus contact. When compared with a catalog of contact forces from an iceberg capsize model, seismic force history accurately computes calving volumes while the earthquake magnitude fails to uniquely characterize iceberg size, giving errors up to 1 km3. Calving determined from GEs recorded ateight glaciers in 1993–2013 accounts for up to 21% of the associated discharge and 6% of the Greenland mass loss. The proportion of discharge attributed to capsizing calving may be underestimated by at least 10% as numerous events could not be identified by standard seismic detections (Olsen and Nettles, 2018). While calving production tends to stabilize in East Greenland, Western glaciers have released more and larger icebergs since 2010 and have become major contributors to Greenland dynamic discharge. Production of GEs and calving behavior are controlled by glacier geometry with bigger icebergs being produced when the terminus advances in deepening water. We illustrate how GEs can help in partitioning and monitoring Greenland mass loss and characterizing capsize dynamics.

Contribution of calving to frontal ablation quantified from seismic and hydroacoustic observations calibrated with lidar volume measurements

“…The reasons are underwater noise level being in general higher and calvingrelated signals tending to be less well-defined and localized in time at higher frequencies. We find that hydroacoustic calving signals often consist of multiple arrivals and weak signals emitted in the aftermath of the ice-water impact, possibly caused by ice avalanches at the terminus, ice breakup at the calved ice block, and air bubble noise from melting freshly calved ice (Urick, 1971;Tegowski et al, 2014;Pettit et al, 2015). Figure 2 presents signals of two typical calving events on KRBN2, KRBS3, ACB, and KBS with signal envelopes shown as black curves.…”

“…However, inferring ice volumes from those signals using physical models is challenging (Podolskiy and Walter, 2016;Aster and Winberry, 2017). For example, it requires different approaches dependent on the calving style, i.e., for glacial earthquake signals from buoyancydriven nontabular iceberg calving such as observed in Greenland (e.g., Murray et al, 2015;Sergeant et al, 2019) or for seismic calving signals generated during iceberg-sea or lake surface interactions (Bartholomaus et al, 2012). For the first calving style, a physical model has been recently presented by Sergeant et al (2019), who used a seismo-mechanical coupling approach to infer calving volumes from glacial earthquakes based on a catalog of contact forces computed for an iceberg capsize numerical model.…”

“…For example, it requires different approaches dependent on the calving style, i.e., for glacial earthquake signals from buoyancydriven nontabular iceberg calving such as observed in Greenland (e.g., Murray et al, 2015;Sergeant et al, 2019) or for seismic calving signals generated during iceberg-sea or lake surface interactions (Bartholomaus et al, 2012). For the first calving style, a physical model has been recently presented by Sergeant et al (2019), who used a seismo-mechanical coupling approach to infer calving volumes from glacial earthquakes based on a catalog of contact forces computed for an iceberg capsize numerical model. The only successful approach so far for the second type of calving (sea or lake surface impacts) is calibrating seismic or water-wave records to directly inferred calving sizes using empirical models valid for a particular glacier and recording site.…”

Constraints on Terminus Dynamics at Greenland Glaciers From Small Glacial Earthquakes

“…The relationship agrees broadly with a simple model relating iceberg surface area and the glacial‐earthquake CSF amplitude. This result, along with progress in understanding seismic signals using numerical and analog models of calving (e.g., Murray, Nettles, et al, ; Sergeant et al, , ), increases the utility of the seismic data as a remote‐sensing tool for interrogating calving processes and glacier dynamics at large tidewater glaciers.…”

“…Direct observations of individual calving events generating glacial earthquakes at Jakobshavn Isbræ and Helheim Glacier have yielded iceberg‐volume estimates between 0.32 and 1.2 km 3 (Amundson et al, ; James et al, ; Murray, Nettles, et al, ; Walter et al, ). In recent years, 30–50 glacial earthquakes have been detected in Greenland annually (Olsen & Nettles, ), suggesting that roughly 25–50 Gt of Greenland's annual dynamic ice loss may be through buoyancy‐driven calving events; Sergeant et al () calculate a similar annual discharge rate (15–50 Gt). However, the size distribution of icebergs lost through buoyancy‐driven calving is not known, hindering assessment of the importance of this calving mechanism within the total calving budget.…”