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.