Seismological Aspects of the December 2004 Great Sumatra-Andaman Earthquake

The 2004 Great Sumatra-Andaman earthquake had an average source duration of about 500 sec . and a rupture length of 1,200–1,300 km . The seismic moment, M 0 , determined with a finite source model, was 6.5× 1022 N - m , which corresponds to Mw =9.18. Allowing for the uncertainties in the current M 0 determinations, Mw is in the range of 9.1 to 9.3. The tsunami magnitude Mt is 9.1, suggesting that the overall size of the tsunami is consistent with what is expected of an earthquake with Mw =9.1 to 9.3. The short-period body-wave magnitude m^ b is 7.25, which is considerably smaller than that of large earthquakes with a comparable Mw . The m^ b versus Mw relationship indicates that, overall, the Great Sumatra-Andaman earthquake is not a tsunami earthquake. The tectonic environment of the rupture zone of the Great Sumatra-Andaman earthquake is very different from that of other great earthquakes, such as the 1960 Chile and the 1964 Alaska earthquakes. This difference may be responsible for the unique source characteristics of this earthquake. The extremely large size of the Great Sumatra-Andaman earthquake is reflected in the large amplitude of the long-period phase, the W phase, even in the early part of the seismograms before the arrival of the S wave. This information could be used for various early warning purposes.