Low frequency icequakes as the signature of transient subglacial water flow underneath the Greenland Ice Sheet

The Greenland Ice Sheet (GrIS) is a major contributor to global sea-level rise. However, predicting its future contribution remains complicated due to large uncertainties in modeling seasonal velocity variations. One of the key challenges is to better constrain the role of isolated subglacial water cavities. Over the melting season, water pressure within these cavities fluctuates, causing the glacier base to be coupled or decoupled from the bedrock. This process modulates basal friction, thereby influencing the velocity of the glacier.
Yet, the mechanisms by which these cavities depressurize by connecting to efficient drainage systems remain poorly understood, particularly for rapid drainage events where viscous creep cannot play a role (Mejia et al., 2021).
To investigate this phenomenon and identify key hydrological parameters, a dense seismic array (Gimbert et al., 2021; Nanni et al., 2021) and a GNSS station was deployed over purported subglacial lakes at Isunguata Sermia, West Greenland. In early September, the GNSS station highlighted a sharp decrease in ice surface elevation, accompanied by intense seismicity at the ice-bed interface. We used unsupervised machine learning to explore recorded seismic signals and identified Low-Frequency Icequakes (LFI), which do not follow classical rupture scaling laws. Additionally, many of these events were followed by a tremor with very low frequencies (~2 Hz). These tremors migrated spatially over time, following a diffusion pattern correlated with the ice surface subsidence, suggesting water migration at the ice-bed interface and thus, a lake drainage. However, the estimated diffusion coefficient is two orders of magnitude higher than predicted by Darcy's flow law. This suggests a hydrofracturing mechanism, facilitating rapid connections between multiple isolated cavities rather than a simple, localized lake drainage process.
Unlike previous interpretations of LFIs observed on glaciers (Thelen et al., 2013; Helmstetter, 2022), our findings suggest that these events do not represent stick-slip mechanisms at the glacier base but are instead, generated by fluid pressure diffusion due to the pressure difference between two isolated cavities. To further support this idea, we propose a theoretical forward model for seismic noise generation driven by pressure diffusion. The model is based on the geometric and hydrological characteristics of the cavities, including cavity width, inter-cavity spacing, the diffusion coefficient and the water volume. Accurate identification of these parameters, based on observations allows us to reproduce the key features of the observed power spectrum.