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Monitoring of basal seismicity rates of ice stream C, West Antarctica: Preliminary results of the Antarctic Microearthquake Project, 1994-1995 and 1995-1996

Sridhar Anandakrishnan, Earth System Science Center and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802-2711

Because of their large ice flux, the ice streams of the Siple Coast have been and continue to be the focus of much research. One of them, ice stream C, largely stagnated about two centuries ago (Rose 1979; Retzlaff and Bentley 1993). Understanding the reasons for this shutdown is crucial to understanding the dynamics of the west antarctic ice sheet and its response to climate change. The speed of ice-streaming appears to be inversely related to the rates of basal seismicity (Anandakrishnan and Bentley 1993), and to quantify this relationship, an intensive program of seismic monitoring was initiated on ice stream C. Alley et al. (1994) hypothesized that ongoing drawdown of the ice sheet caused ice stream C to migrate into a region that caused a diversion of basal water from the lower parts of ice stream C. The Antarctic Microearthquake Project (AMP) was designed to test that hypothesis because one consequence of the lack of basal water is high seismicity (under the right conditions; see Anandakrishnan and Alley [1994]).

Over the past two field seasons a series of seismic arrays has been deployed along the length of the ice stream from the grounding line to the head of the ice stream at a nominal spacing of 75 kilometers. The arrays consisted of four or five short-period seismometers deployed in a diamond pattern 6 kilometers wide by 8 kilometers long. During the first season (1994-1995), CWA, SH, and CC sites were occupied. During the second season (1995-1996), STC, WB, EF, and BC were occupied, and SH and CC re-occupied (see table). In addition to the seismic work, we surveyed the position of a steel pole at the center of each array at the beginning of the austral summer and then resurveyed the sites at the end of the summer when we retrieved the instruments. We have determined iceflow velocities for each site. Because of the short time between occupations, our measurements of the velocities are not precise, but we hope to return to the sites and resurvey them at a later time.

The work was mainly carried out from a base camp at CWA during 1994-1995 and a base camp at STC during 1995-1996. The instruments were successfully installed at all the sites, and most (40 out of 44) ran for the whole period. The seismometers were configured to begin recording when an event was detected. Most triggers were due to earthquakes; very few wind-noise, crevassing, or firnquake events were recorded. The arrival times of the events were picked, and their hypocenters were located.

Preliminary analysis shows that most are basal events and the rates of those events (in number per day) are listed in the table. The pattern along ice stream C is one of low seismicity at the sites that have relatively high velocity and vice-versa. The ice stream is divided into two regions and the boundary between the high- and low-seismicity and between the low- and high-velocity regions is centered on site CC, about 350 kilometers from the grounding line. This boundary corresponds closely with a basal-water diversion zone (Alley et al. 1994; Anandakrishnan and Alley 1994) that was hypothesized to have caused the stagnation of ice stream C. We confirm that the data are consistent with that hypothesis.

Ice stream C has stagnated in its lower part because of the loss of basal lubricating water, and this stagnation has had two consequences: the ice has coupled to localized hard spots at the bed, causing microearthquakes, and has slowed to less than 10 meters per year because of the friction from those "sticky spots." The region between the sticky spots is relatively soft and deformable and supports little of the basal shear stress resulting in a concentration of the shear stress on the sticky spots. Ice stream C stagnated because of an accident of topography which has withdrawn water from the lower part of ice stream C and has probably increased the water supply to ice stream B.

I thank the members of my field team, Peter Burkholder, Paul Friberg, Anton Wopereis, and John Witzel, for their dedication and hard work. I thank Bjorn Johns, Antarctic Support Associates, and VXE-6 for their professional and efficient support. I thank the Incorporated Research Institutions for Seismology and University Navstar Consortium (UNAVCO) for their equipment support.

References

Alley, R.B., S. Anandakrishnan, C.R. Bentley, and N. Lord. 1994. A water-piracy hypothesis for the stagnation of ice stream C, Antarctica. Annals of Glaciology, 20, 187-194.

Anandakrishnan, S., and R.B. Alley. 1994. Ice stream C, Antarctica, sticky-spots detected by microearthquake monitoring. Annals of Glaciology, 20, 183-186.

Anandakrishnan, S., and C.R. Bentley. 1993. Microearthquakes beneath ice streams B and C, West Antarctica: Observations and implication. Journal of Glaciology, 39(133), 455-462.

Retzlaff, R., and C.R. Bentley. 1993. Timing of stagnation of ice stream C, West Antarctica, from short-pulse-radar studies of buried surface crevasses. Journal of Glaciology, 39(133), 553-561.

Rose, K. 1979. Characteristics of ice flow in Marie Byrd Land, Antarctica. Journal of Glaciology, 24(90), 63-74.