MICRO-EARTHQUAKES BENEATH ICE STREAM-B AND ICE STREAM-C, WEST ANTARCTICA - OBSERVATIONS AND IMPLICATIONS

Micro-earthquakes have been monitored at two locations on Ice Stream B and one on Ice Stream C using a seismographic array built specifically for that purpose. Subglacial micro-earthquakes are 20 times more abundant beneath Ice Stream C than beneath Ice Stream B, despite the 100 times more rapid movement of Ice Stream B. Triangulation shows the foci beneath Ice Stream C, like those beneath Ice Stream B, to be within a few meters of the base of the ice, presumably within the uppermost part of the bed, and fault-plane analysis indicates slips on horizontal planes at about a 30-degrees angle to the presumed direction of formerly active flow. Source parameters, computed from spectra of the arrivals, confirmed that the speed of slip is three orders of magnitude faster beneath Ice Stream C than beneath Ice Stream B which means that a five orders-of-magnitude greater fraction of the velocity of Ice Stream C is contributed by the faulting, although that fraction is still small. We attribute the difference in activity beneath the two ice streams to the loss of dilatancy in the till beneath Ice Stream C in the process that led to its stagnation.