The response of Black Rapids Glacier, Alaska, to the Denali earthquake rock avalanches

We describe the impact of three simultaneous earthquake-triggered rock avalanches on the dynamics of Black Rapids Glacier, Alaska, by using spaceborne radar imagery and numerical modeling. We determined the velocities of the glacier before and after landslide deposition in 2002 by using a combination of ERS-1/ERS-2 tandem, RADARSAT-1, and ALOS PALSAR synthetic aperture radar data. Ice velocity above the debris-covered area of the glacier increased up to 14% after the earthquake but then decreased 20% by 2005. Within the area of the debris sheets, mean glacier surface velocity increased 44% within 2 years of the landslides. At the downglacier end of the lowest landslide, where strong differential ablation produced a steep ice cliff, velocities increased by 109% over the same period. By 2007, ice velocity throughout the debris area had become more uniform, consistent with a constant ice flux resulting from drastically reduced ablation at the base of the debris. Without further analysis, we cannot prove that these changes resulted from the landslides, because Black Rapids Glacier displays large seasonal and interannual variations in velocity. However, a full Stokes numerical ice flow model of a simplified glacier geometry produced a reversal of the velocity gradient from compressional to extensional flow after 5 years, which supports our interpretation that the recent changes in the velocity field of the glacier are related to landslide-induced mass balance changes

Identification and characterization of alpine subglacial lakes using interferometric synthetic aperture radar (InSAR): Brady Glacier, Alaska, USA

The temporary storage and subsequent release of water at glacial margins can cause severe flooding in downstream areas and substantially impact glacier dynamics. Alpine subglacial lakes may not be identified until they become subaerially exposed or release a jökulhlaup. We use interferometric synthetic aperture radar (InSAR) to identify and characterize three dynamic alpine subglacial lakes of Brady Glacier, Alaska, USA. We quantify changes in vertical displacement of the glacier surface and lake volumes from September 1995 through March 1996 using European Remote-sensing Satellite-1/-2 (ERS-1/-2) tandem data. In the autumn, subsidence ranged from 4 to 26 cm d−1 and the volume of water discharged ranged from 22 000±2000 to 243 000±14 000 m3d−1. Subsidence and discharge rates declined significantly during the winter and continued at a lesser rate through March. Application of this technique may allow researchers to locate alpine subglacial lakes years or decades before they begin to release hazardous outburst floods and substantially impact glacier dynamics