I study Earth processes, primarily concerning the ice sheets, using geophysical data and mechanical models.
Most Antarctic ice loss occurs as ice slides off the continent into the sea. Understanding this sliding is therefore essential to understanding the contribution of ice sheets to past, present, and future sea level rise. Despite this importance, most ice sheet models still rely on ad-hoc sliding laws that omit important physics and exhibit pathological behaviors. My recent work has employed more physically realistic frictional sliding laws. Such sliding laws describe the resistance to sliding provided by a finite strength ice--bed interface. The difference between frictional sliding laws and traditional, unbounded sliding laws has important consequences in the context of global change: if the ice--bed interface has a finite strength, then its capacity to resist the forces driving ice loss is fundamentally limited.
Stick--slip motion of the Whillans Ice Plain, West Antarctica. The Whillans Ice Plain (WIP) section of the Whillans Ice Stream is unique amongst the Antarctic ice streams in that it experiences twice daily tidally modulated stick-slip cycles reminiscent of the tectonic earthquake cycle. Slip events last about 30 min, have sliding velocities as high as ~0.5 mm/s (15 km/yr), and have total slip ~0.5 m. Slip events tend to occur during falling ocean tide: just after high tide and just before low tide. To reproduce these characteristics, we use rate-and-state friction, which is commonly used to simulate tectonic faulting, as an ice stream sliding law. This framework describes the evolving strength of the ice-bed interface throughout stick-slip cycles. The principal finding of this work is that if pore pressures are raised above a critical value, our simulations predict that the WIP would exhibit quasi-steady tidally modulated sliding as observed on other ice streams. This study furthermore validates rate-and-state friction as a sliding law to describe ice stream sliding styles.
Envronmental seismology is the study of seismic signals that originate from Earth surface processes. My work in environmental seismology includes the study of repeating earthquakes in within glaciers and landslides, the seismic wave field associated with water flowing through the Greenland Ice Sheet, and of hydraulic fracturing in geothermal systems, glaciers, and volcanoes.
Fluids in Earth's subsurface are of great societal interest. Petroleum, fracturing, and geothermal fluids are basic components of the energy system; magmatic fluids in volcanoes are associated with natural hazards; the fossilized remains of ancient volcanic intrusions provide insights into past tectonic environments; and liquid water in ice plays a critical role in the response of the cryosphere to a changing climate. Such fluids are commonly contained in fractures. Fractures are pervasive in geologic media, and fluid-filled fractures are the dominant fluid pathway in media with low intrinsic permeability. In the cryosphere, fluid-filled fractures occur as glacial crevasses as well as thin sheets of water at the bed of glaciers. In volcanoes, such fractures occur as magma-filled dikes and sills, while in geothermal and hydrocarbon reservoirs they provide either preexisting or stimulation-induced fracture space.