The disastrous Sumatra−Andaman earthquake of 26 December 2004 was one of the largest ever recorded. The damage potential of such earthquakes depends on the extent and magnitude of fault slip. The first reliable moment magnitude estimate¹ of 9.0 was obtained several hours after the Sumatra−Andaman earthquake, but more recent, longer-period, normal-mode analyses have indicated that it had a moment magnitude of 9.3, about 2.5 times larger². Here we introduce a method for directly imaging earthquake rupture that uses the first-arriving compressional wave and is potentially able to produce detailed images within 30 min of rupture initiation. We used the Hi-Net seismic array in Japan as an antenna to map the progression of slip by monitoring the direction of high-frequency radiation. We find that the rupture spread over the entire 1,300-km-long aftershock zone by propagating northward at roughly 2.8 km s^-1 for approximately 8 minutes. Comparisons with the aftershock areas of other great earthquakes indicate that the Sumatra−Andaman earthquake did indeed have a moment magnitude of 9.3. Its rupture, in both duration and extent, is the longest ever recorded.

1. Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, IGPP 0225, University of California San Diego, La Jolla, California 92093, USA
2. Department of Earth and Space Sciences, IGPP, University of California Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA
3. *These authors contributed equally to this work

Correspondence to: Miaki Ishii^1,3 Correspondence and requests for materials should be addressed to M.I. (Email: mishii@ucsd.edu).

Received 13 February 2005; Accepted 18 April 2005

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