We investigated seismic signals generated during a large-scale, multiple iceberg calving event that occurred at Jakobshavn Isbræ, Greenland, on 21 August 2009.We investigated seismic signals generated during a large-scale, multiple iceberg calving event that occurred at Jakobshavn Isbræ, Greenland, on 21 August 2009. The event was recorded by a high-rate time-lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus. During the event two full-glacier-thickness icebergs calved from the grounded (or nearly grounded) terminus and immediately capsized; the second iceberg to calve was two to three times smaller than the first. The individual calving and capsize events were well-correlated with the radiation of low-frequency seismic signals (<0.1 Hz) dominated by Love and Rayleigh waves. In agreement with regional records from previously published ‘glacial earthquakes’, these low-frequency seismic signals had maximum power and/or signal-to-noise ratios in the 0.05–0.1 Hz band. Similarly, full waveform inversions indicate that these signals were also generated by horizontal single forces acting at the glacier terminus. The signals therefore appear to be local manifestations of glacial earthquakes, although the magnitudes of the signals (twice-time integrated force histories) were considerably smaller than previously reported glacial earthquakes. We thus speculate that such earthquakes may be a common, if not pervasive, feature of all full-glacier-thickness calving events from grounded termini. Finally, a key result from our study is that waveform inversions performed on low-frequency, calving-generated seismic signals may have only limited ability to quantitatively estimate mass losses from calving. In particular, the choice of source time function has little impact on the inversion but dramatically changes the earthquake magnitude. Accordingly, in our analysis, it is unclear whether the smaller or larger of the two calving icebergs generated a larger seismic signal.Acknowledgments. We acknowledge the contributions of Bryn Hubbard, Göran Ekström, Trine Dahl-Jensen, Jake Walter, Matt Haney, and an anonymous reviewer to the improvement of this manuscript. This work also benefited from discussions with Victor Tsai, John Clinton, and Martin Lüthi. Gabi Laske assisted with the crust2.0 model. The inversion scheme was programmed in MATLAB®, which was also used to generate most figures. Figure 1 was prepared with the Global Mapping Tool (GMT). Seismograms were processed with MATLAB® and the Seismic Analysis Code (SAC) [Goldstein et al., 2003]. Funding was provided by NASA’s Cryospheric Sciences Program (NNG06GB49G and NNX08AN74G) and the U.S. National Science Foundation (AEC0909333, ANT0739769, ANT0944193, and IPY0732726). The USGS Climate and Land Use Change program supported this work by paying the salary of S. O’Neel. Seismic stations ILULI, KULLO, and NUUG are operated by ETH Zurich; station SFJD is operated by IRIS, GFZ, and GEUS. The data were collected and distributed by the Greenland Ice Sheet Monitoring Network (GLISN) federation and its members.Ye
