Recurring, year-round, icequakes at a Greenland tidewater glacier

Abstract:

Processes occurring at the interface between glaciers and their crystalline or sedimentary beds allow fast glacier flow and control short-term velocity variations. However, the inaccessibility of this interface limits our understanding of subglacial processes. Passive seismology allows for continuous, non-invasive monitoring of subglacial processes. At Kangerlussuup Sermia
in the Uummannaq region of central west Greenland (71.5° N, 51.4° W), two years of passive seismic records reveal consistently recurring seismic events. The seismometer was buried in shallow sediment adjacent to the terminus of the 4.2 km-wide tidewater glacier, which flows 1.8 km/yr at its front. These recurring seismic events occur on average once per 55±15 s and last approximately 20 s, year-round. Events have peak frequencies between 1.0 and 1.5 Hz with characteristic, gamma-distributed amplitudes. Events are emergent, without clear P- or S-wave arrivals. Several of these event properties are shared with calving icequakes, such as emergent arrivals and frequencies near 1.5 Hz, however, the regular recurrence time and characteristic amplitudes are inconsistent with a calving source. These seismic event properties vary by several tens of percent during our record, on both monthly and daily timescales. Comparison between the properties of these seismic events and other time series, such as runoff and ice flow speed, allow us to assess the hypothesis that these seismic signals are icequakes produced by basal slip. We explore connections between these signatures of brittle/elastic failure of the ice-bed interface and components of the glacier force balance.