Current Research in EEPS: Dr. Grace Barcheck, Cornell University
The Whillans Ice Stream is a large stick-slipping glacier in West Antarctica that provides a unique opportunity to observe many repeats of a field-scale stick-slip cycle in just a few weeks. This ~100 km wide glacier has 1-2x daily “ice stream earthquakes”, in which the ice surges forward ~0.5 m in only ~20 minutes. With broadband seismometers and GNSS instrumentation installed across this glacier fault, many hard-to-observe parts of the stick-slip cycle —including nucleation— can be observed repeatedly in just a matter of weeks. This glacier fault is therefore a uniquely valuable natural (vs. laboratory) analog to earthquake cycles.
On this glacier fault, GNSS and seismic data from dozens of events reveal that there is nearly always a minutes to hours-long period of slower slip that precedes the mainshock ice stream earthquake. These precursors appear to be part of the nucleation process, and they tend to occur far from the eventual mainshock epicenter, which is evidence for dominantly migratory nucleation in this low-stress system. Interestingly, the duration of these slow slip precursors varies inversely with event recurrence time. A variety of mainshock rupture characteristics also vary positively with event recurrence time: peak rupture speed, peak velocity, and total slip. Because total slip scales neatly with recurrence time (time available for loading), the ice stream earthquake cycle appears to be slip predictable, and recurrence time can be considered a proxy for fault stress. This implies that higher fault stress results in a shorter duration nucleation process on the glacier, as well as a mainshock that slips faster and farther. Understanding these relationships for this simple glacial fault will inform understanding of slip in fault zones with more complex stressing histories and material heterogeneity.