The March 25, 1998 Antarctic plate earthquake

The March 25, 1998, Antarctic plate earthquake (M_w=8.1) occurred ∼250 km from the nearest plate boundary, in oceanic lithosphere with an age of 35-55 my. Analysis of aftershock patterns shows that the earthquake ruptured a fault, or series of strike-slip fault segments, nearly 300 km long. The strike of the fault(s) is nearly perpendicular to the north-south trending fossil fracture zones which are the most marked bathymetric features of this region. Moment release during the mainshock was concentrated in two large subevents, clearly visible in the teleseismic body wave waveforms. Modeling of these body waves using a finite fault source places the first of the two subevents near the point of rupture initiation, on the eastern end of the fault, with the second large subevent occurring 220-280 km to the west. The two pulses of moment release are found to be separated in time by ∼65 s. Comparison of the relative S wave amplitudes of the first and second pulses suggests that a rotation of the focal mechanism by ∼10° occurred between the two subevents.