Seismic investigation of moulin tremor and basal icequakes of the Greenland Ice Sheet

The Greenland Ice Sheet is, with currently 0.7-1.1mm per year, a major contributor to the current global sea level rise. Its contribution has been increasing over the past decades and is divided between surface melt and dynamic discharge (also referred to as calving). Changes in dynamic discharge are linked to rising ocean temperatures, which compromise the stability of marine-terminating ice streams. At the same time, the increase in melt runoff may affect the flow dynamics of the Greenland Ice Sheet by altering basal motion. The extent to which this mechanism plays a role in the mass loss of the ice sheet is debated and is the motivation for this thesis. The analysis presented here used passive seismic measurements to explore the subglacial environment of a slow-flowing region (peak velocities of approximately 180ma−1) near the western margin of the Greenland Ice Sheet. Between 2nd July and 16th August 2011, seismic data were acquired with a network of 17 seismometers. The instruments were installed at the ice surface and in boreholes of up to 300m depth with a total network aperture of about 1.5 km. The network recorded a broad variety of seismic signals in different frequency ranges and amplitudes. In particular, short-term icequakes (duration <1 s) related to fracturing, and long-term tremors (duration minutes to hours) related to resonance of water were observed. Icequake sources are located at all depths within the ice sheet and include the well-known shallow icequakes (surface crevasses) and intermediate icequakes (hydrofracturing) as well as the lesser known basal icequakes. More than 10,000 basal icequakes were located in the vicinity of the ice sheet bed. They were grouped into 110 clusters, whose members shared high waveform similarity indicating repeated rupture of the same seismic fault plane. Polarization analysis of Pwave arrivals for individual icequake clusters revealed a double-couple source mechanism interpreted as stick-slip motion. As these sources were distributed beneath the entire study area, basal stick-slip motion was considered a main characteristic of the ice sheet flow. Cluster locations and arrival time modeling revealed a complex ice sheet bed topography consisting of a subglacial till layer of variable thickness. Several activity patterns of clusters were correlated in icequake magnitudes and inter-event times with the water level measured inside a moulin in the center of the seismometer network. This water level represented subglacial water pressure within regions connected to the efficient subglacial drainage system. Locally, this subglacial water pressure affected the pore pressure within the subglacial till layer. As the till’s frictional strength depends on its pore pressure, variations in subglacial water pressure can thus weaken or strengthen the ice sheet bed and, with this, influence the inter-event time and magnitude of the basal icequakes within one cluster. Different cluster activity patterns, therefore, reveal the connectivity of ice sheet bed with the efficient drainage system connected to moulins. Seismic tremor signals with a duration of 4-16 h are another focus of this thesis. With a frequency content of 3-11Hz, the tremors were recorded in correlation to high water level inside the central moulin. The tremor source was found to be located at this moulin by applying amplitude attenuation techniques on signals recorded at all seismic stations. These ’moulin tremors‘ changed amplitude, frequency content and particle motion over the duration of one episode. Within this thesis was shown that the surface stream impinging the water surface inside the moulin triggered the seismic signal, and that the amount of inflowing water controlled tremor amplitude. Frequency content was attributed to resonance following the model of a semi-open organ pipe. The changes in frequency content for one tremor episode corresponded to a resonance body with changing height between 20-75m. Accordingly, it is interpreted that at a depth of about 180m a kink in the englacial conduit forms the bottom of the resonating section of the moulin. The particle motion was successfully modeled with two interfering source types: (1) A cylindric source caused by the interaction of the water column with the conduit walls, and (2) a single source caused by the interaction of the water column with the resonator bottom. In addition, other details in frequency signature such as different absorption bands were attributed to site effects in the vicinity of the moulin shaft. The present thesis introduced the moulin tremors and stick-slip icequakes as new seismic signals of the Greenland Ice Sheet. Since the ice sheet’s subsurface is difficult to access by applying traditional glaciological techniques, this work provides a new perspective on ice sheet dynamics. Accordingly, the results show that passive seismic observations are a powerful technique for characterizing ice sheet flow and for monitoring englacial and subglacial changes over timescales of milliseconds to several weeks.