Seismic signals of "wet" vs. "dry" ice shelves

Two seismograph stations were operated on the McMurdo Ice Shelf (McMIS) during the Austral summer of 2016/7 in an exploratory effort to compare and contrast ambient seismic signals on wet (surface ablating during melt season) and dry (covered by firn with significant air-content year round) ice shelves. The McMIS possesses both "wet" and "dry" zones because of local flow pattern and micro-climate that places a zone of extensive surface melting and hydrological features (streams and ponds, active debris cover) within about 25 km of where dry, cold Ross Ice Shelf ice enters the McMIS. At low-frequency, below 1 Hz, the ambient seismic signals on the wet and dry parts of the ice shelf are largely the same. Signals in the 1-Hz and below frequency range include the types and relative abundance of signals observed previously on many ice shelves including the RIS: 1) micro-tsunamis, or impulsive single wave fronts, probably generated by ice calving into the ocean surrounding the ice shelf; 2) teleseismic arrivals of the various classes of waves generated by earthquakes in the regions far from the ice shelf; and 3) sea swell that involves frequencies from below 0.05 Hz to 0.1 Hz over long, hours to days, periods of time. At high-frequencies, above 1 Hz, the ambient seismic signals between the wet and dry parts of the ice shelf were entirely different. The dry seismograph was very quiet, recording ice-generated quakes only rarely; whereas the wet seismograph recorded hundreds to thousands of ice-generated quakes every day. These abundant high-frequency ice-quakes were found to follow a diurnal cycle tied to the air temperature, suggesting a thermal mechanism for their source that is similar to what has been observed for the ambient hydroacoustic noise below sea ice. The wave forms of the high-frequency events reveal multiple propagation mechanisms, including reverse dispersed Rayleigh waves influenced by the water layer below the ice shelf, as well as possible full-thickness elastic modes of the ice shelf layer that have been previously reported on floating ice-islands of the Arctic Ocean. We present an overview of these results and an initial view of the data as a means of determining the possible utility of seismic observations on wet ice shelves in support of research on ice-shelf stability in response to atmospheric warming.