Tracking high-frequency seismic source evolution: 2004 M_w 8.1 Macquarie event

The 2004 M_w 8.1 event on 23 December 2004 near the Macquarie Ridge is a very large intraplate event that has been overshadowed by the M _w 9.3 Sumatra-Andaman event only 3 days later. We are able to track the progress of source evolution by estimating the progression of the points of energy emission, exploiting the good azimuthal distribution of available stations. The results indicate that this event ruptured on two nearby fault systems reactivating fossil fracture zones, with the second subevent to the west triggered by the first. The total duration of high-frequency radiation is quite short, about 60 s, for such a large event. Much of the high-frequency radiation occurs on a fault subparallel to that inferred from long-period studies. This composite fault behavior with displaced triggered failure appears to be a characteristic of large intraplate events beneath the oceans. Key Points M _w 8.1 2004 Macquarie event triggers secondary event Separation of high-frequency and low-frequency radiation Energy projection without use of large arrays