A great earthquake of Mw9.0 (Harvard) occurred off of northwestern Sumatra on December 26, 2004 (UTC), causing an unprecedented tsunami disaster. An earthquake of Mw8.6 (Harvard) then occurred on March 28, 2005 (UTC), about 160 km to the southeast of the December event's epicenter. The Matsushiro Seismological Observatory of Japan Meteorological Agency determined magnitudes of M8.8 and M8.7 respectively for these events using the Global Seismic Network's (GSN) Live Internet Seismic Server (LISS) data. The West Coast/Alaska Tsunami Warning Center (WC/ATWC), and the Pacific Tsunami Warning Center (PTWC) both use Mwp to rapidly evaluate moment magnitude. The WC/ATWC calculated a mean Mwp of 8.0 for the multiple event on Decmber 26, 2004. Using data from the IRIS station MAJO, we determined an Mwp of 8.5 by using a distancedependent apparent P-wave velocity (α = 0.16 + 7.9 km/sec.) instead of a constant apparent P wave velocity (α = 7.9 km/sec.), for α in the original equation for Mwp. The corrected Mwp value of 8.5 is much closer to the total moment magnitude of the multiple ruptures of the complex December 26 main-shock, and is useful as a first magnitude estimation to evaluate possible tsunami generation.

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