New Understanding of Iceberg Calving, Mass Loss, and Glacier Dynamics in Greenland Through Analysis of Glacial Earthquakes

• Main Author: Olsen, Kira
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: Geophysics; Geology; Ice calving; Glaciers > Measurement; Seismology; Greenland; glacier; Ice Sheet; Jakobshavn
• Online Access: https://doi.org/10.7916/d8-gycj-3h54

I apply a suite of seismic techniques to investigate iceberg calving at large glaciers around Greenland. Iceberg calving accounts for up to half of the Greenland Ice Sheet's annual mass loss, which makes understanding the physics of the calving process vital to gaining a clear picture of current behavior and future evolution of the Greenland Ice Sheet. However, the varied and complex modes of calving behavior at individual glaciers, paired with the challenges to data collection presented by an actively calving glacier, mean that much remains unknown about the dynamics of calving at marine-terminating glaciers. Seismic data offer a unique opportunity to study this active phenomenon, by allowing remote observation of calving events and quantification of the forces active during calving. Using seismic data collected during the most productive three years of buoyancy-driven calving on record, I estimate the forces active during iceberg calving at 13 glaciers around Greenland. My waveform-modeling results highlight the large number of buoyancy-driven calving events currently occurring at Jakobshavn Isbrae and other glaciers in west Greenland. I demonstrate that a glacier's grounded state exerts control on the production or cessation of rotational calving events and investigate the dynamics of calving at individual glaciers. I pair seismic results with terminus imagery to identify the location of individual calving events within calving sequences that occur over days to weeks at a single glacier terminus. By applying a new cross-correlation technique to seismic data collected within 100 km of three of Greenland's largest glaciers, I identify the occurrence of buoyancy-driven calving events with iceberg volumes up to two orders of magnitude smaller than previously observed. These small calving events frequently occur within ~30 minutes of a larger calving event. In between calving sequences, a glacier terminus changes little, suggesting that the majority of ice lost from marine-terminating glaciers occurs through these sequences. I estimate that these small events may contribute up to 30% more to dynamic mass loss than previously thought (up to 15 Gt/yr). I find no evidence of the cliff failure predicted by the marine-ice-cliff-instability hypothesis, in which catastrophic failure occurs when an ice cliff reaches a theoretical maximum-height limit, despite the three glaciers I investigate in detail having some of the tallest ice cliffs in the world. I use independent constraints on iceberg size from high-quality terminus imagery to present the first demonstration of an empirical relationship between glacial-earthquake magnitude and iceberg size. I investigate this relationship further by considering additional metrics of glacial-earthquake magnitude, and find advantages to using maximum force, rather than the more commonly employed mass-distance product Mcsf, as a measure of glacial-earthquake size. Through a detailed investigation into the character of the glacial-earthquake source, I identify key characteristics of the source function that generates the glacial-earthquake signal. I use experiments on both synthetic and observed waveforms to demonstrate that more-accurate estimates of glacial-earthquake size can be retrieved using source models constructed using a representation of the force history that is more sophisticated than that captured by the simple boxcar model. I confirm the presence of a correlation between iceberg volume and glacial-earthquake size, which moves us closer to having the ability to use remotely recorded seismic signals to quantify mass loss at Greenland glaciers. This work presents testable hypotheses for future model development.