Seismic Recordings Reveal the Timing and Extent of Subglacial Water Pressurization

Date: 4/26/2019

Time: 04:45 PM

Room: Pike

The evolving state of the subglacial hydrologic system is of fundamental importance for the regulation of fast glacier flow, subglacial bedrock erosion, and, in the case of marine-terminating glaciers, submarine melt of glacier termini by warm ocean water. However, the state of this subglacial hydrologic system is notoriously difficult to assess. Here we report on the use of broadband seismic recordings to reveal subglacial water pressurization and track subglacial discharge over three months of the melt season at Lemon Creek Glacier, in Southeast Alaska, USA. Simultaneous Global Positioning System (GPS), meteorological, proglacial runoff, and active-source seismic measurements, as well as glaciohydraulic modeling, support and inform our interpretations.

We find that, over the significant majority of the summer (approximately 90%), water flow beneath 190 m-thick Lemon Creek Glacier is likely unpressurized, since the relationship between 1.5 – 10 Hz glaciohydraulic tremor and proglacial runoff follows a 5/4^th power relationship predicted theoretically. However, during storms in which at least 4 cm of rain fall in day-long periods, the subglacial hydrologic system appears to pressurize and drive increased basal motion. During these apparently pressurized time periods, we observe broadband seismic power increases, an increase in power spectral slopes, and nonlinearity between tremor power and proglacial runoff consistent with theoretically-predicted power relationships near 14/3rds. This nonlinearity, and the observation of high frequency seismic signals produced from flowing, turbulent water, indicates that hysteresis between tremor and discharge is necessarily the result of bedload sediment transport. These results expand our understanding of melt-season glacier hydrology (often assumed to include continuously-pressurized, full subglacial conduits), provide empirical support for theoretical predictions in seismology, and represent a new tool for the validation of subglacial hydrologic models.

Presenting Author: Timothy C. Bartholomaus

Authors