Abstract
AbstractStatic 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 isMw= 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 Banerjeeet 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 ofMw> 9.0.
Highlights
The great Sumatra-Andaman earthquake of 26 December, 2004 is the first earthquake to be observed by modern geodetic techniques across the globe
A third challenge is that, as we have shown for some stations, coseismic offsets might represent only a fraction of the total station motion caused by the earthquake
We have estimated and analyzed coseismic static offsets of far-field GPS stations from the 26 December, 2004, earthquake. In this process we found and analyzed significant postseismic displacements for nearby stations ( SAMP), which are consistent with a velocitystrengthening afterslip process
Summary
The great Sumatra-Andaman earthquake of 26 December, 2004 is the first earthquake to be observed by modern geodetic techniques across the globe. A much more detailed study was presented by Vigny et al (2005), who used a large amount of regional data (and some from far-field IGS stations) to put strong constraints on the extend and total slip along the rupture Their analysis was, hindered by not taking into account possible postseismic offsets for continuous sites in the 14 days after the event, and by using an elastic half-space model instead of a spherical model approach. For SAMP and NTUS we have used the times-series from 20 days before December, 2004 until March, 2005 (i.e., the day before the Mw = 8.7, Northern Sumatra event) to simultaneously solve for the horizontal coseismic offsets and a logarithmic function describing the afterslip process after the event (Fig. 2).
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