Recent advancement in the understanding of snow-microwave interactions has helped to isolate the considerable potential for radar-based retrieval of snow water equivalent (SWE). There are however, few datasets available to address spatial uncertainties, such as the influence of snow microstructure, at scales relevant to space-borne application. In this study we introduce measurements from SnowSAR, an airborne, dual-frequency (9.6 and 17.2 GHz) synthetic aperture radar (SAR), to evaluate high resolution (10 m) backscatter within a snow-covered tundra basin. Coincident in situ surveys at two sites characterize a generally thin snowpack (50 cm) interspersed with deeper drift features. Structure of the snowpack is found to be predominantly wind slab (65%) with smaller proportions of depth hoar underlain (35%). Objective estimates of snow microstructure (exponential correlation length; lex), show the slab layers to be 2.8 times smaller than the basal depth hoar. In situ measurements are used to parametrize the Microwave Emission Model of Layered Snowpacks (MEMLS3&a) and compare against collocated SnowSAR backscatter. The evaluation shows a scaling factor (ϕ) between 1.37 and 1.08, when applied to input of lex, minimizes MEMLS root mean squared error to <1.1 dB. Model sensitivity experiments demonstrate contrasting contributions from wind slab and depth hoar components, where wind rounded microstructures are identified as a strong control on observed backscatter. Weak sensitivity of SnowSAR to spatial variations in SWE is explained by the smaller contributing microstructures of the wind slab.
Iceberg calving is known to release substantial seismic energy, but little is known about the specific mechanisms that produce calving icequakes. At Yahtse Glacier, a tidewater glacier on the Gulf of Alaska, we draw upon a local network of seismometers and focus on 80 hours of concurrent, direct observation of the terminus to show that calving is the dominant source of seismicity. To elucidate seismogenic mechanisms, we synchronized video and seismograms to reveal that the majority of seismic energy is produced during iceberg interactions with the sea surface. Icequake peak amplitudes coincide with the emergence of high velocity jets of water and ice from the fjord after the complete submergence of falling icebergs below sea level. These icequakes have dominant frequencies between 1 and 3 Hz. Detachment of an iceberg from the terminus produces comparatively weak seismic waves at frequencies between 5 and 20 Hz. Our observations allow us to suggest that the most powerful sources of calving icequakes at Yahtse Glacier include iceberg‐sea surface impact, deceleration under the influence of drag and buoyancy, and cavitation. Numerical simulations of seismogenesis during iceberg‐sea surface interactions support our observational evidence. Our new understanding of iceberg‐sea surface interactions allows us to reattribute the sources of calving seismicity identified in earlier studies and offer guidance for the future use of seismology in monitoring iceberg calving.
The 2002 Denali fault earthquake was the largest on-land strike-slip earthquake in the United States since 1857. It ruptured three faults in sequence, with over 300 km total rupture length. Geophysical investigations have focused on determining the geometry of the fault(s) at depth, deriving a slip model for the earthquake from geodetic displacements and seismic records, determining the preand post-earthquake stress fields, and studying the postseismic deformation that was caused by the earthquake. Aftershocks outline, in general, the extent of the rupture, but often result from failure of nearby faults. The aftershocks following this event show considerable geometric complexity. Slip models show that most of the seismic energy was released in two or three zones along the fault, with most slip occurring in the eastern part of the rupture. Geophysical slip models are in good agreement with the surface offset observations made by geologists. Earthquake focal mechanisms before and after the earthquake show that the regional stress field is compressional, with the maximum compressional axis located nearly normal to the Denali fault. The stress field after the earthquake shows almost the same orientation. This suggests that the Denali fault is probably mechanically weak (or has very high fluid pressures within it) to allow it to slip under such a stress field. Finally, following the earthquake we have observed rapid deformation persisting for months, caused by a combination of post-seismic processes including continued slip on the fault, poroelastic relaxation, and viscous relaxation of the lower crust and/or upper mantle.
