Bed Conditions Inferred from Basal Earthquakes Beneath the Whillans Ice Plain, West Antarctica

Seismicity near the bed of fast-moving ice streams informs our understanding of basal controls on fast ice flow and the nature of small scale sources of basal resistance to sliding (sticky spots). Small basal earthquakes (BEQs) occurring at or near the base of ice streams express the current dominant basal stress state and allow observation of bed heterogeneities on spatial scales of 10s to 100s m that are difficult to observe otherwise. Temporal changes in the source mechanisms of these BEQs indicate changing basal conditions, and comparison of basal seismicity with GPS-determined ice velocity allows insight into the interplay between seismically active small basal sticky spots and fast ice motion. We present unique highly local observations of BEQs occurring beneath the Whillans Ice Plain, in West Antarctica, from a network of 13 surface and borehole seismometers overlying the WIP stick-slip cycle high tide initiation area. We record seismicity only 100s-1000s of m from basal seismic sources. We compare the occurrence of these BEQs with co-located GPS observations of ice surface velocity. We detect BEQs by cross correlation, using a catalog of hand-picked events with seismic wave arrivals at multiple sites. We then locate each BEQ and determine source parameters by fitting the S wave spectra: moment magnitude, stress drop, and rupture area. The basal earthquakes occur in families of remarkably repeatable events. The time interval between subsequent events within a BEQ family typically depends on ice velocity, but there is a complex interplay between ice velocity and source parameters. We also search for temporal changes in BEQ source parameters and seek to relate these changes to ice velocity measurements, thereby inferring changing bed conditions. Our preferred interpretation is that BEQs are rupture at or near the surface of an over-consolidated till package, suggesting that changes in basal seismicity may directly indicate changing subglacial till conditions.