Not all icequakes are created equal: basal icequakes suggest diverse bed
deformation mechanisms at Rutford Ice Stream, West Antarctica
Abstract
Microseismicity, induced by the sliding of a glacier over its bed, can
be used to characterize frictional properties of the ice-bed interface,
which are a key parameter controlling ice stream flow. We use naturally
occurring seismicity to monitor spatiotemporally varying bed properties
at Rutford Ice Stream, West Antarctica. We locate 230000
micro-earthquakes with local magnitudes from –2.0 to –0.3 using 90
days of recordings from a 35-station seismic network located
~40 km upstream of the grounding line. Events
exclusively occur near the ice-bed interface and indicate predominantly
flow-parallel stick-slip. They mostly lie within a region of interpreted
stiff till and along the likely stiffer part of mega-scale glacial
landforms. Within these regions, micro-earthquakes occur in spatially
(<100 m radius) and temporally (mostly 1-5 days activity)
restricted event-clusters (up to 4000 events), which exhibit an
increase, followed by a decrease, in event magnitude with time. This may
indicate event triggering once activity is initiated. Although ocean
tides modulate the surface ice flow velocity, we observe little periodic
variation in overall event frequency over time and conclude that water
content, bed topography and stiffness are the major factors controlling
microseismicity. Based on variable rupture mechanisms and spatiotemporal
characteristics, we suggest the event-clusters relate to three
end-member types of bed deformation: (1) continuous creation and
seismogenic destruction of small-scale bed-roughness, (2) ploughed
clasts and (3) flow-oblique deformation during landform-formation or
along bedrock outcrops. This indicates that multiple processes,
simultaneously active during glacial sliding, can accommodate stick-slip
behaviour and that the bed continuously reorganizes.