ABSTRACT. Glacial earthquakes are caused by large iceberg calving events, which are an important mechanism for mass loss from the Greenland ice sheet. The number of glacial earthquakes in Greenland has increased sixfold over the past two decades. We use teleseismic surface waves to analyze the 145 glacial earthquakes that occurred in Greenland from 2011 through 2013, and successfully determine source parameters for 139 events at 13 marine-terminating glaciers. Our analysis increases the number of events in the glacial-earthquake catalog by nearly 50% and extends it to 21 years. The period 2011-13 was the most prolific 3-year period of glacial earthquakes on record, with most of the increase over earlier years occurring at glaciers on Greenland's west coast. We investigate changes in earthquake productivity and geometry at several individual glaciers and link patterns in glacial-earthquake production and cessation to the absence or presence of a floating ice tongue. We attribute changes in earthquake force orientations to changes in calving-front geometry, some of which occur on timescales of days to months. Our results illustrate the utility of glacial earthquakes as a remote-sensing tool to identify the type of calving event, the grounded state of a glacier, and the orientation of an active calving front.

Dissipation of tidal energy is expected to generate seismicity on icy‐ocean worlds; however, the distribution and timing of this seismic activity throughout an orbital cycle is not known. We used new observations from an icy‐ocean‐world analog environment on Earth to examine the relationship between tidally driven tensile stress and seismic activity within an ice shell. We investigated a pair of rifts within Antarctica's Ross Ice Shelf which are tidally stressed in a manner analogous to the orbital cycle of tidal stress experienced by Enceladus' Tiger Stripe Fractures. We found that seismic activity at the Antarctic rifts is sensitive to both the amplitude and the rate of tensile stress across the rifts. We combined these findings with calculated stress values along Enceladus' Tiger Stripe Fractures to predict seismic‐activity levels expected along the ice‐shell fractures. We predict a peak in seismicity along the four Tiger Stripe Fractures when Enceladus is 90°–120° past pericenter in its orbit around Saturn, at which point tensile stresses would reach ∼2/3 of their maximum value. We also used the magnitude distribution of icequakes along Antarctic rifts to investigate implications for the likely size of stick‐slip rupture patches along icy faults on Enceladus. Our findings predict that the Tiger Stripe Fractures should produce sustained, low‐magnitude seismic events that involve rupture along discrete portions of each fracture's total length. We predict that seismicity would fall to 50% of peak levels when stresses across the Tiger Stripe Fractures are dominantly compressional.

Many large calving events at Greenland's marine-terminating glaciers generate globally detectable glacial earthquakes. We perform a cross-correlation analysis using regional seismic data to identify events below the teleseismic detection threshold, focusing on the 24 hr surrounding known glacial earthquakes at Greenland's three largest glaciers. We detect additional seismic events in the minutes prior to more than half of the glacial earthquakes we study and following one third of them. Waveform modeling shows source mechanisms like those of previously known glacial earthquakes, a result consistent with available imagery. The seismic events thus do not represent a failure of the high subaerial ice cliff like that expected to trigger large-scale calving and a marine ice-cliff instability but, rather, rotational, buoyancy-driven calving events, likely of the full glacier thickness. A limited investigation of the prevalence of smaller seismic events at times outside glacial-earthquake windows identifies several additional events. However, we find that calving at the three glaciers we study-Jakobshavn Isbrae, Helheim Glacier, and Kangerdlugssuaq Glacier-often occurs as sequences of discrete buoyancy-driven events in which multiple icebergs ranging in size over as much as three orders of magnitude are all lost within ∼30 min. We demonstrate a correlation between glacial-earthquake magnitude and iceberg size for events with well-constrained iceberg-area estimates. Our results suggest that at least 10-30% more dynamic mass loss occurs through buoyancy-driven calving at Greenland's glaciers than previously appreciated.

The Gutenberg-Richter (G-R) relationship describes a relationship between the number of earthquakes in a region greater than a certain magnitude and that magnitude (Gutenberg & Richter, 1956). This relationship states:where a represents the number of earthquakes when the moment magnitude (M W ) = 0, and b is the slope of the scaling relationship. For instance, when b = 1, there are 10 1 times more seismic events at a given magnitude than at the next lower magnitude value. On average, the b-value of global earthquakes is ∼1 (Lay & Wallace, 1995), but for slow slip events and active volcanic regions, the b-value is close to 1.

The number of gigaton‐sized iceberg‐calving events occurring annually at Greenland glaciers is increasing, part of a larger trend of accelerating mass loss from the Greenland Ice Sheet. Though visual observation of large calving events is rare, ∼60 glacial earthquakes generated by these calving events are currently recorded each year by regional and global seismic stations. An empirical relationship between iceberg size and MCSF, a summary measure of glacial‐earthquake size, was recently demonstrated by Olsen and Nettles (2019), https://doi.org/10.1029/2019JF005054. However, MCSF is known to be sensitive to choices made in modeling the seismic source. We incorporate constraints on the seismic source from laboratory studies of calving and test multiple source time functions using synthetic and observed glacial‐earthquake waveforms. We find that a simple, fixed time function with a shape informed by laboratory results greatly improves estimates of earthquake size. The average ratio of estimated to true peak force values is 1.03 for experiments using our preferred source model, compared with an average of 0.3 for models used in previous studies. We find that maximum‐force values estimated from waveform modeling depend far less on model choices than does MCSF, and therefore prefer maximum force as a measure of glacial‐earthquake size. Using both synthetic and real data, we confirm a correlation between maximum force and iceberg mass. Our results support the possibility of developing useful scaling relationships between seismic observables and physical parameters controlling glacier calving.

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Fractures within ice shelves are zones of weakness, which can deform on timescales from seconds to decades. Icequakes produced during the fracturing process show a higher b-value in the Gutenberg-Richter scaling relationship than continental earthquakes. We investigate icequakes on the east side of rift WR4 in the Ross Ice Shelf, Antarctica. Our model suggests a maximum icequake slip depth that is ˜7.8 m below rift surface, where the slip area can only grow laterally along the fracture planes. We propose ductile deformation below this depth, potentially due to saturation of unfrozen water. We use remote sensing and geodetic tools to quantify surface movement on different time scales and find that the majority of icequakes occurred during falling tides. The total seismic moment is < 1% of the estimated geodetic moment during a tidal cycle. This study demonstrates the feasibility of using seismology and geodesy to investigate ice rift zone rheology.