Conditions at the base of an ice stream control the ability of basal material to resist ice sliding, which affects ice stream mass balance. Yet, basal properties are notoriously hard to constrain. Tiny magnitude ~-2 to -1 stick-slip basal icequakes occurring near or on the basal sliding surface hold valuable information about this basal environment. In this dissertation, I investigate spatiotemporal patterns in these basal icequakes occurring beneath the Whillans Ice Plain (WIP), in West Antarctica, and interpret these patterns to gain insight into basal material heterogeneity and temporally evolving basal conditions.
In Chapter 2, I determine where basal icequakes happen beneath the entire WIP. I find spatially variable seismicity rates, with basal seismicity most common in a ~40 km wide area surrounding a dynamically important region where ice-plain-wide unstable slip nucleates. This result implicates icequake-generating bed conditions in large-scale ice stream stick-slip. Additionally, I propose that basal icequakes occur where basal erosion exposes over-consolidated till to the ice base.
In Chapter 3, I use back-projection to detect basal icequakes beneath a small seismic network near the nucleation region. Here, basal icequakes occur in streaks elongated along ice flow and in conjunction with low-amplitude (~2m) undulating basal topography. These patterns suggest the presence of mega-scale glacial lineations (MSGL), elongate bedforms common on paleo-ice stream beds. One icequake streak may occur in a shallow trough beside an MSGL, suggesting that these icequakes occur in erosion-impacted lows between MSGL where over-consolidated till or stiff sediment outcrops contact the ice base.
In Chapter 4, I analyze an improved basal icequake catalog generated by cross correlating icequakes detected in Chapter 3. I estimate icequake moment magnitudes of Mw = -2.1 to -1.2 and fault rupture areas of 1-100 m^2 for several large basal icequakes. Families of nearly-identical repeating basal icequakes continue for ice sliding distances of typically <0.5 m, and most <0.2 m. If this distance also represents the approximate size of an icequake-generating fault, then basal icequake faults have dimensions of cm to m. I explore evidence for ice-bed interface healing between unstable slip events. Lastly, I discuss four possible mechanisms for basal seismicity.
