Seismic Evidence for Pervasive Surface Fracturing Near the Grounding Line of the Ross Ice Shelf

Data collected from a dense seismic array near the grounding line of the Whillans Ice Plain (WIP) reveals an abundance of tidally-modulated seismicity that is temporally correlated with falling tide. Our analysis indicates that the source of this seismicity is surface fracturing due to ice shelf flexure under tidal forcing. We suggest that passive seismic monitoring and characterization near ice shelf grounding lines may be a practical strategy for tracking ice shelf strength and damage evolution.

We examined the grounding zone (GZ) events over five consecutive field seasons from 2012-2016 to investigate their source and tidal dependence. The seismograms are strongly dominated by surface waves, and show very little body wave energy. Because picking P and S waves was impractical given their low amplitudes, we located event sources using beamforming techniques. The resulting event locations were then used to compute synthetic seismograms for the specific geometry and velocity structure of the WIP. Event source depth was then constrained by comparing the surface-to-body wave ratios of the computed seismograms with that of the recorded seismograms.

The GZ events were located near the grounding line, and constrained to shallow depths of no more than 30 meters. The observed seismicity's temporal correlation with low tide and location characterization strongly indicate that the source is likely tensile fracturing at the surface in the form of strand cracks or buried crevasses. Our result implies that despite the odd ice flow geometry at this location (i.e. ice flow being parallel to the grounding line), and the lack of surface expression, the changing ocean height is sufficient to produce fracturing on diurnal timescales.

Additionally, we note the distinct importance of not only low tidal height to induce seismicity, but also the speed at which the tide is falling. The observed GZ seismicity begins at peak falling tide velocity and ceases at maximum low tide. This observation is consistent with the findings of similar studies conducted elsewhere on the Ross Ice Shelf, and laboratory experiments indicating the rate-dependent fracture toughness of ice.