Micro-seismicity of the Whillans Ice Plain stick-slip cycle nucleation areas

Abstract:

The Whillans Ice Plain (WIP) is known for its distinctive stick-slip motion, in which steady, slow ice motion (~100m/yr equivalent) is punctuated once or twice daily by sudden slips of ~0.5 m over ~30 minutes (~5,000m/yr equivalent). These stick-slip events nucleate in one of two areas of the Ice Plain depending on Ross Ice Shelf tidal height: the Central Sticky Spot (CSS) or the Grounding Zone Sticky Spot (GZSS) (Pratt et al., 2014). Basal conditions beneath ice streams are in general poorly constrained, and the specific conditions allowing Ice-Plain-wide unstable sliding and stick-slip motion remain poorly understood. We present preliminary results from two small-aperture networks (<1 km station spacing) of borehole and surface short period seismometers installed at each nucleation area to investigate local seismicity and its relationship to the stick-slip cycle. We present new detections and locations of small basal micro-earthquakes at or near the ice/bed interface at both nucleation areas and demonstrate spatial heterogeneity in bed conditions. CSS basal seismicity occurs as hundreds of small amplitude repeating events, rupturing the same or nearly the same patches of the bed beneath the network many times during a stick-slip event, but changing between events. These events suggest that the 10’s of km wide CSS is comprised of small stick-slip patches. Micro-seismicity is detected using waveform cross correlation of manually selected template events and located using hypoDD. Time- and space-varying basal seismicity demonstrates that bed conditions vary in space, beneath the network, and in time, between slip events and observation years. The GZSS network records similar basal micro-earthquakes during slip events, though far fewer. Larger events characterized by surface waves are much more common near the GZSS. This seismicity occurs preferentially during low tide, independent of the WIP stick-slip cycle. We use beamforming to show that these events occur outside the network, towards the grounding zone. These events may be basal earthquakes providing evidence of higher grounding zone strength during low tide, or surface or basal crevassing events indicating tensional stresses at the grounding zone.