Seismicity associated with icebergs calving from glaciers near Mawson, East Antarctica

Icebergs produced from the termini of non-floating or partly floating outlet glaciers at the seaward margin of the Antarctic ice sheet near Mawson (67°36’S, 62°53’E) were photographed and measured. Seismic records at Mawson are interpreted in terms of catastrophic calving of these icebergs from ice cliffs into the sea to produce low frequency seismic signals of varying amplitude and duration. It is shown how parameters of foreshocks to these ice falls could have been used to predict the approximate time and location of one of the falls. These parameters included the directions of first motions on the Mawson seismographs, the S-P times, as well as the rate of change of smoothed event frequency data. The frequency and amplitude distributions of the foreshocks can be represented as n(t) - 180(24-t)^-t.65and N(A)= 225^-t.85 where n(r) is the number of foreshocks per day, t days after the foreshocks began and N(A) is the number of events whose amplitudes were in the range A to (A+ 2) mm on the Mawson seismograph. These data were compared with those from other icequake and earthquake studies. Exponent 1.65 is within the range 1.53 and 2.60 obtained in a study on reservoirassociated foreshocks. Exponent 1.85 in the second relation compares with corresponding values obtained for natural earthquakes and icequakes studied elsewhere, but is low when compared with values for Antarctic snowquakes (2.21-2.61). Rewards for further studies of this type are expected to include greater understanding of ice dynamics, assistance with estimation of the mass of ice leaving Antarctica and ice fall prediction which has immediate ramifications for safety of base personnel.