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Abstract

Previous observations have shown that rift propagation on the Amery Ice Shelf (AIS), East
Antarctica, is episodic, occurring in bursts of several hours with typical recurrence times of several
weeks. Propagation events were deduced from seismic swarms (detected with seismometers) concurrent
with rapid rift widening (detected with GPS receivers). In this study, we extend these results by
deploying seismometers and GPS receivers in a dense network around the tip of a propagating rift on the
AIS over three field seasons (2002/03, 2004/05 and 2005/06). The pattern of seismic event locations
shows that icequakes cluster along the rift axis, extending several kilometers back from where the rift tip
was visible in the field. Patterns of icequake event locations also appear aligned with the ice-shelf flow
direction, along transverse-to-rift crevasses. However, we found some key differences in the seismicity
between field seasons. Both the number of swarms and the number of events within each swarm
decreased during the final field season. The timing of the slowdown closely corresponds to the rift tip
entering a suture zone, formed where two ice streams merge upstream. Beneath the suture zone lies a
thick band of marine ice. We propose two hypotheses for the observed slowdown: (1) defects within the
ice in the suture zone cause a reduction in stress concentration ahead of the rift tip; (2) increased marine
ice thickness in the rift path slows propagation. We show that the size–frequency distribution of
icequakes approximately follows a power law, similar to the well-known Gutenberg–Richter law for
earthquakes. However, large icequakes are not preceded by foreshocks nor are they followed by
aftershocks. Thus rift-related seismicity differs from the classic foreshock and aftershock distribution
that is characteristic of large earth quakes.

Item Type:	Article
Authors/Creators:	Bassis, JN and Fricker, HA and Coleman, R and Bock, Y and Behrens, J and Darnell, D and Okal, M and Minster, JB
Journal or Publication Title:	Journal of Glaciology
Publisher:	International Glaciological Society
ISSN:	0022-1430
DOI / ID Number:	https://doi.org/10.3189/002214307784409207
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