Compound rupture of the great 1998 Antarctic plate earthquake

The March 25, 1998, Antarctic plate earthquake ruptured a portion of the Antarctic plate more than 200 km west of its boundary with the Australian plate. The Harvard centroid moment tensor solution indicates that the earthquake was primarily a strike-slip event, but the large non-double-couple component of the moment tensor suggests considerable complexity in the rupture process. We use a finite fault inversion method to determine details of the rupture process from teleseismic body waves recorded by the Global Seismic Network. The P waves are poorly fit by one or more subevents having only a strike-slip mechanism. We find that the presence of a large oblique-normal faulting subevent located to the east of the hypocenter is necessary to improve the fit. This subevent combines with a larger strike-slip subevent to the west to comprise the main moment release in the earthquake and is the cause of the large non-double-couple component in the long-period focal mechanism. The earthquake exhibited very high slip and high stress drop compared with most interplate strike-slip events and the rupture was largely confined to the upper 15-20 km of the lithosphere. Both constituent focal mechanisms indicate that this part of the Antarctic plate is under NW-SE oriented tension, although the origin of these stresses is unknown.