Do the 1998 Antarctic Plate earthquake and its aftershocks delineate a plate boundary?



Plate motion data along the eastern Southeast Indian Ridge (SEIR) are not fit by a rotation pole consistent with data along the western SEIR. DeMets et al. (1988) looked for, but did not find, evidence of a microplate north of the SEIR to explain the discrepancy. We test the hypothesis that the large (Mw=8.2), strike-slip, 1998 Antarctic earthquake lies on the boundary of a microplate (here termed “Balleny”) south of the SEIR that explains the plate motion discrepancy. A small circle about an Antarctic-Balleny Euler pole, determined by plate closure, fits the east-west band of aftershocks, consistent with a strike-slip boundary. However, the Antarctic-Balleny pole predicts right-lateral slip, while the seismicity exhibits left-lateral, strike-slip motion. The opposite sense of slip not only rules out the Balleny plate hypothesis, but requires deformation elsewhere to accommodate both the plate motion differences along the SEIR and the strain accumulated from the Antarctic event.