Imaging transient stick-slip processes near a subglacial conduit using a nodal seismic array

Abstract:

The choice of basal sliding law implemented in glacier flow models controls ice dynamics, and by extension, projections of global sea-level rise. Experimental and field studies document rate-weakening frictional behavior that is generally unaccounted for in hard-bedded numerical sliding laws. We investigated these transient behaviors at Saskatchewan Glacier in Banff National Park, Canada, over 18 days during the 2019 melt season. Data acquired includes passive seismic recordings from a dense geophone array that revealed abundant basal seismicity, and a suite of hydrologic, geodetic, and meteorologic observations. Rates of basal seismicity varied by 2-3 orders of magnitude in response to diurnal surface water supply, accompanied by variations in ice-surface velocity. Hypocentral solutions for high SNR events were estimated using a Bayesian inverse approach, and are highly consistent with the estimated ice-bed interface derived using the HV spectral ratio method. These events illuminate a curvilinear feature that is oriented primarily across hydropotential gradients, and down gradient of active moulins, which we interpret as regions of the ice-bed interface bordering a subglacial conduit. Peak diurnal seismicity follows peak stage in a subglacial outlet stream, indicating that seismicity initiates as water pressure falls in the conduit. We hypothesize that regions near the conduit experience large transients in basal water pressure, which influence the frictional resistance between the bedrock and clasts entrained in the basal ice. Increased debris-bed friction during the period of falling conduit pressure – in conjunction with increasing sliding velocity – triggers observed periods of elevated seismicity. These observations extend laboratory investigations of basal seismicity under transient conditions, helping to constrain basal friction and sliding laws. Furthermore, such behaviors are not currently represented in models projecting future sea-level rise.