Geology topics

J. T. Harper

Publications and source records attributed to J. T. Harper.

2 recordsLinked to original sources

USGS2012

Monitoring glacier surface seismicity in time and space using Rayleigh waves

Sliding glaciers and brittle ice failure generate seismic body and surface wave energy characteristic to the source mechanism. Here we analyze continuous seismic recordings from an array of nine short-period passive seismometers located on Bench Glacier, Alaska (USA) (61.033°N, 145.687°W). We focus on the arrival-time and amplitude information of the dominant Rayleigh wave phase. Over a 46-hour period we detect thousands of events using a cross-correlation based event identification method. Travel-time inversion of a subset of events (7% of the total) defines an active crevasse, propagating more than 200 meters in three hours. From the Rayleigh wave amplitudes, we estimate the amount of volumetric opening along the crevasse as well as an average bulk attenuation ( = 42) for the ice in this part of the glacier. With the remaining icequake signals we establish a diurnal periodicity in seismicity, indicating that surface run-off and subglacial water pressure changes likely control the triggering of these surface events. Furthermore, we find that these events are too weak (i.e., too noisy) to locate individually. However, stacking individual events increases the signal-to-noise ratio of the waveforms, implying that these periodic sources are effectively stationary during the recording period.

Journal of Geophysical Research F: Earth Surface↗

USGS2008

Mass balance of a cirque glacier in the U.S. Rocky Mountains

Glacier National Park, Montana, USA, contains 27 cirque glaciers, most less than 1 km 2 and together comprising about 17 km 2 . These glaciers lie at relatively low elevation (2000 – 3000 m a.s.l.) and latitude (48 o N) and have undergone dramatic retreat since the mid-nineteenth century, when an estimated 150 glaciers existed. Continuing volume losses and the disappearance of glaciers in recent decades are used as key indicators of regional warming. Here we present initial results from a long-term study initiated in 2005 on Sperry Glacier (48.6 o N, 113.75 o W), a 0.8 km 2 cirque glacier that has undergone an 80% reduction in size since the mid 1800s. We calculated seasonal and annual balances using the direct glaciological method augmented with data from an automated weather station adjacent to the glacier and data from a nearby automated snow pillow. The net annual balance averaged -1.0 m w. eq. for the 2005 and 2006 balance years. Specific balances showed significant transverse spatial variability due to site-specific processes that augment accumulation or mute ablation. These processes have a significant effect on the mass balance of the glacier. Proxy measures showed that the 2005 and 2006 balance years likely had very low accumulation or very high ablation, respectively, relative to other years in recent decades.

Montana↗