Constraints on Subglacial Conditions from Seismicity
Abstract:A family of physics-based models designed to explain emergent, bandlimited, “tremor-like” seismograms shed light onto subglacial and englacial conditions. We consider two such models. In the first, a water-filled fracture hosts resonant modes; the seismically observable quality factor and characteristic frequency of these modes constrain the fracture length and aperture. In the second model, seismicity is generated by repeating stick-slip events on a fault patch (portion of the glacier bed) with sliding described by rate- and state-dependent friction laws. Wave propagation phenomena may additionally generate bandlimited seismic signals.
These models make distinct predictions that may be used to address questions of glaciological concern. Laboratory friction experiments show that small, repeating earthquakes most likely occur at the ice-till interface and at conditions below the pressure melting point. These laboratory friction values, when combined with observed ice surface velocities, may also be used to constrain basal pore pressure. In contrast, seismic signals indicative of water-filled basal fractures suggest that, at least locally, temperatures are above the pressure melting point.
We present a simple diagnostic test between these two processes that concerns the relationship between the multiple seismic spectral peaks generated by each process. Whereas repeating earthquakes generate evenly spaced spectral peaks through the Dirac comb effect, hydraulic fracture resonance, as a result of dispersive propagation of waves along the crack, generates spectral peaks that are not evenly spaced.