Presentation by Seth Saltiel
TITLE:
What can subglacial seismicity tell us about bed conditions and mechanics?
Abstract:
Observations of glacier bed slip show abundant seismicity ranging from repeater microearthquakes and tremor to large slow‐slip events. A complete understanding of the mechanisms capable of producing seismic signals in these environments represents a strong constraint on bed conditions at the epicentral location and time, since the widely assumed conditions have been experimentally shown not to produce seismic instability. To measure the frictional strength and stability of ice sliding, we performed a series of double‐direct‐shear experiments at controlled temperatures slightly above and below the ice melting point. Our results confirm velocity‐strengthening ice–till slip at melting temperatures, while friction increases significantly and becomes velocity-weakening below freezing. Suggesting local basal freeze-on could be a mechanism for seismic slip on till beds. Ice-on-rock also slips unstably at similar conditions, but with much different friction magnitude and evolution. By de-stiffening the machine and freezing in piezoelectric sensors, we record acoustic emission waveforms and mechanical stress drops from stick-slip cycles on frozen rock and till beds. Although waveforms from each bed type are remarkably similar, machine learning classification algorithms are able to correctly predict the bed type (rock or till) of up to 80% of event waveforms or spectra. The Random Forest Classifier is interpretable, showing the importance of initial oscillation peaks and higher frequency energy. Till beds have generally higher friction and resulting stress-drops, with more impulsive first arrivals and more high frequency content compared to rock emissions, but rock beds can produce till-like events. Better understanding the mechanisms of unstable ice slip and their seismic signatures could greatly enhance interpretation of bed conditions and mechanics from subglacial seismicity.
Zoom link:
https://columbiauniversity.zoom.us/j/92721173078?pwd=fad18QQXN6kNTgfDkguCIlbZaR1rLq.1
