The influence of small stresses on the dynamics of glaciers and subduction zones
The fast flow of glaciers and rupture of earthquakes on subduction zones both occur at interfaces notoriously difficult to observe. When slip events occur on such interfaces, elastic energy is radiated in the form of seismic waves. Here, we analyze these and other signals, such as continuous GPS, for two glacier systems and a subduction zone. We find that the slip events at an Antarctic ice stream, a Greenland outlet glacier, and a subduction zone in Costa Rica are all modulated by the ocean tide, however, the manifestations of the modulations vary considerably.
In Antarctica, we directly measure the bidaily rupture process of the Whillans Ice Plain using broadband seismometers. The average rupture speed of these events varies by a factor of 2, which is dependent upon the recurrence interval. Based on variations of rupture near the source region, we observe that rupture speed depends on loading conditions.
In Greenland, the breakup of the sea ice and iceberg mixture that forms during the winter causes changes in the flow velocity of Store Glacier, West Greenland. We measure velocity using continuous GPS 16 km away from the terminus and time-lapse photography near the terminus. We observe a step-change in velocity near the terminus in response to the ice melange breakup, corresponding to a 30-60 kPa loss of buttressing stress. Further, we observe semi-diurnal periodicity in GPS speed perturbations 16 km from the terminus, likely due to ocean tides.
At the subduction zone, we identify and locate tremor events, distinct from earthquakes in their low amplitude, which occur as shear failure on the plate interface. We locate the events on portions of the plate believed to be undergoing stable sliding, adjacent to locked portions of the plate. Furthermore, we provide multiple forms of geodetic and pressure evidence of an offshore event that occurred in 2008.
Observations of various behaviors that include relatively small stresses may provide insight into the unique dynamics of glaciers and subduction zones. Small stresses that drive failure suggests either a weak basal interface or a system near its critical stress state.