Observations at high spatial and temporal resolution could be key for improving our understanding of the
physical processes that govern outlet-glacier flow variations. We collected simultaneous high-rate GPS observations
at several locations distributed along and across Helheim Glacier, East Greenland, during the four Arctic summers of
2006-2009, along with other geophysical observations, to study glacial earthquakes and glacier dynamics. GPSderived
position estimates of centimeter-level precision reveal the surface expression of glaciological signals,
occurring from sub-hourly to daily time-scales and beyond.
We find that the flow velocity of Helheim Glacier is modulated by ocean tides in a region including both sides of the
grounding line. An admittance analysis of the tidal signal shows an exponential decrease in amplitude with distance
from the calving front, along with increasing time delays in both the vertical and horizontal components. Moreover, the
along-flow admittance amplitude and time delay estimates are highly time dependent, both with a periodicity of ~15
days, thus suggesting a response to a fortnightly tidal frequency. In addition, we observe transitory increases in the
amplitude of the tidally-driven flow response coinciding with glacial earthquakes events, which are associated to large
losses of mass at the calving front and step-like increases in flow speed. We will present an analysis of the tidal
modulation signals observed at Helheim and discuss how those signals may place new constraints on models for
glacier flow variations.