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Outlet glaciers in Greenland have retreated and lost mass over the past decade. Understanding the dynamics of tidewater glaciers is crucial for forecasting sea-level rise and for understanding the future of the Greenland Ice Sheet, given the buttressing support that tidewater glaciers provide to inland ice. However, the mechanisms controlling glacier-front location and the role played by external forcings (e.g., meltwater input and tidal oscillation) in basal motion and fracture formation leading to iceberg calving are poorly understood. Today it is known that glaciers generate seismic and infrasound signals that are detectable at local and teleseismic distances and can be used to monitor glacier dynamics. Here, we present examples of data recorded by a temporary network of seismic and infrasound instruments deployed at a tidewater glacier (Bowdoin Glacier, Greenland) in July 2015. Some stations were installed on ice at distances as close as ～ 250 m from the calving front, representing the closest deployments to the calving front that have been made to date. Multiple seismic and infrasound events were recorded by five seismic and six infrasound sensors, and linked to surface crevassing, calving, and ice-cliff collapses, and presumably also hydrofracturing, iceberg rotations, teleseismic earthquakes, and helicopter-induced tremors. Using classic seismological and array analysis approaches (e.g., “short-term averaging/long-term averaging” and “f-k” analysis), as well as image processing techniques, we explore this unique dataset to understand the glacial response to external forcings. Our observations, supported by GPS measurements of ice velocity, local weatherstation records, and time-lapse photography, provide a valuable resource for studying seismogenic glacial processes and their dependence on ocean tides and other environmental factors.

The Greenland ice sheet and peripheral ice caps are rapidly losing mass. This mass change has been captured by satellite remote sensing, but more detailed investigations are necessary to understand the spatiotemporal variations and mechanism of the ice loss. It has increased particularly in northwestern Greenland, but in-situ data for northern Greenland are generally sparse. To better understand the ice mass loss in northwestern Greenland, we studied the ice sheet, ice caps and calving glaciers in the Qaanaaq region, as a part of the Green Network of Excellence (GRENE) Arctic Climate Change Research Project. Field and satellite observations were performed to measure the mass loss of the ice caps and calving glaciers in the region. Detailed processes were investigated based on field measurements to understand mechanisms driving the ice loss. The field activities include mass balance monitoring on Qaanaaq Ice Cap since 2012, integrated field observations near the front of Bowdoin Glacier since 2013 and ocean measurements near the calving glaciers. In this contribution, we summarize the results of the GRENE Greenland project, and introduce an overview of the next project to be carried out under the framework of the Arctic Challenge for Sustainability Project (ArCS).

Glacier microseismicity is a promising tool to study glacier dynamics. However, physical processes connecting seismic signals and ice dynamics are not clearly understood at present. Particularly, the relationship between tide-modulated seismicity and dynamics of calving glaciers remains elusive. Here we analyze records from an on-ice seismometer placed 250m from the calving front of Bowdoin Glacier, Greenland. Using high-frequency glacier flow speed measurements, we show that the microseismic activity is related to strain rate variations. The seismic activity correlates with longitudinal stretching measured at the glacier surface. Both higher melt rates and falling tides accelerate glacier motion and increase longitudinal stretching. Long-term microseismic monitoring could therefore provide insights on how a calving glacier's force balance and flow regime react to changes at the ice-ocean interface.

第6回極域科学シンポジウム分野横断セッション：[IA] 急変する北極気候システム及びその全球的な影響の総合的解明―GRENE北極気候変動研究事業研究成果報告2015―11月19日（木） 国立極地研究所 2階 大会議室

Explosive activity often generates visible flashing arcs in the volcanic plume considered as the evidence of the shock front propagation induced by supersonic dynamics. High speed image processing is used to visualize the pressure wavefield associated with flashing arcs observed in strombolian explosions. Image luminance is converted in virtual acoustic signal compatible with the signal recorded by pressure transducer. Luminance variations are moving with a spherical front at a 344.7 m/s velocity. Flashing arcs travel at the sound speed already 14m above the vent and are not necessarily the evidence of a supersonic explosive dynamics. However seconds later the velocity of small fragments increases and the spherical acousto luminance wavefront becomes planar recalling the Mach wave radiation generated by large scale turbulence in high speed jet. This planar wavefront forms a Mach angle of 55° with the explosive jet axis suggesting an explosive dynamics moving at Mo = 1.22 Mach number.