Earth Planets Space, Vol. 58 (No. 2), pp. 141-148, 2006
Corné Kreemer, Geoffrey Blewitt, William C. Hammond, and Hans-Peter Plag
Nevada Bureau of Mines and Geology, and Seismological Laboratory, University of Nevada, Reno, MS 178, NV 89557-0088, U.S.A.
(Received June 30, 2005; Revised October 17, 2005; Accepted November 9, 2005; Online published February 17, 2006)
Static coseismic offsets >1 mm are observed up to 7800 km away from the great Sumatra-Andaman earthquake of 26 Dec. 2004 using global GPS network data. We investigate the rupture process based on far-field continuous GPS data. To reduce error in the coseismic offset estimates due to post-seismic deformation in the days following the main shock, we simultaneously fit a model of co- and postseismic offsets for nearby stations SAMP (500 km) and NTUS (900 km). The 3-month cumulative postseismic displacement for station SAMP amounts to 20% of the coseismic displacement, and can be well modeled by velocity-strengthening afterslip. We find that coseismic slip on the northern rupture segment is ~3 m, which is consistent with seismic estimates. Our best estimate of the moment magnitude is Mw = 9.13 if we take into account the expected increase of the shear modulus with depth (for uniform μ = 30 GPa, the moment-magnitude would only be 8.97). Our geodetic results, and thus our inferred rupture model, are different from a similar study using far-field data of Banerjee et al. (2005). These differences highlight the challenge in earthquake studies on a global scale in terms of the sensitivity of far-field offset estimates to the analysis strategy and reference frame treatment. Our predicted coseismic offsets from this event are at least 1 mm across almost the entire globe. This warrants a reconsideration of how to maintain the global terrestrial reference frame affected by earthquakes of Mw > 9.0.
Key words: GPS, Great Sumatra-Andaman Earthquake, earthquake rupture, coseismic displacements, postseis-mic deformation.