Spatial and Temporal Variability in Cryoseismicity Rate Associated with Supraglacial Lake Drainage at Lemon Creek Glacier, Alaska

Abstract:

Glacier hydrology can significantly influence the overall dynamics of glaciers, with hydrologic activity affecting behaviors throughout the glacier system. In recent years, studies have demonstrated the utility of seismometers in observing both glacier hydrology (through glaciohydraulic tremor) and sliding/fracturing (through icequakes), making seismology a useful tool to observe the interactions between these parts of the glacier system. In this study, we monitor cryoseismic activity at Lemon Creek Glacier, Alaska using a dense array of seismometers deployed for a two week period over the majority of the 1.5 km by 5.5 km glacier’s surface in 2017. During the deployment, a supraglacial lake at the glacier’s head margin drained over the course of 36 hours directly into the glacier’s subglacial hydrologic system. Throughout the deployment, the seismometers detect continuous background noise (composed of glaciohydraulic tremor and other ambient seismic signals) and discrete events (icequakes, earthquakes, rockfalls, etc). We use the distribution of amplitudes at each station in the array to statistically determine the amplitude of continuous background noise and assign threshold amplitudes for events. We record a large number of small local icequakes among the events, and the rate of event occurrence varies both spatially and temporally throughout the array. The event rate detected in the upper portion of the glacier is dominated by a quasi-diurnal pattern, while the event rate detected in the lower portion of the glacier reflects the water flux through the glacier due to the lake drainage. These two regions are separated by an increase in the glacier’s slope (from ~3° to ~6°), a boundary that also divides different subglacial hydrologic behavior as detected through glaciohydraulic tremor. This hydrologic boundary and the correlation between icequakes and subglacial water flux lead us to believe that differences in cryoseismic activity may be at least in part related to the glacier’s hydrology, even for small icequakes.