SOUTHBERG: An in-situ investigation of
the seismic symphony of Iceberg C-16,
Ross Sea, Antarctica
D.R. MacAyeal, E.A. Okal, and the SOUTHBERG Team
We present preliminary results of the operation, from
November 2003 to January 2004, of a
PASSCAL array of 4 short-period seismometers on Iceberg
C16, presently grounded North of Ross Island. A broadband
STS-2 instrument was also run at the central station.
Results from an uninterrupted 33-day window during
which all 4 stations were operational are:
(i) GPS tracking indicates that the 4-station polygon remained rigid
but drifted (about 20 m) and rotated (0.1 degree) abruptly
on Christmas Day; (ii) most of the background seismic noise is
concentrated in the 20-50 mHz range, which includes the natural
bobbing and rolling frequencies of the ice sheet; (iii)
a large variety of seismic signals originating from the ice
were detected, including: (Type I) short events featuring a white spectrum,
probably simple episodes of cracking
in the ice; (Type II) long episodes of relatively
broad-band noise, of variable amplitude, lasting hundreds
to thousands of seconds, occasionally superimposed over somewhat
preferential frequencies; and (Type III) resonant signals featuring
extremely narrow spectral lines (with overtones), similar
to those recorded in Polynesia in 2000 from drifting icebergs.
Type III signals are correlated
among the four stations of the network, but do not seem to
be interpretable as the mere passage of a seismic wavefront, thereby
suggesting the resonance of a normal mode of the whole structure.
A Type III signal on 15 January 2004 coincides
with an episode of collision with and friction against B-15A,
as documented from GPS data obtained on the latter.
We may present additional data to be retrieved from Station C16-A,
which wintered over, and will be revisited in October 2004.