Abstract

More than six years after the great (Mw 9.2) Sumatra–Andaman earthquake, postevent processes responsible for relaxation of the coseismic stress change remain controversial. Modeling of Andaman Islands Global Positioning System (GPS) displacements indicated early near‐field motions were dominated by slip down‐dip of the rupture, but various researchers ascribe elements of relaxation to dominantly poroelastic, dominantly viscoelastic, and dominantly fault slip processes, depending primarily on their measurement sampling and modeling tools used. After subtracting a pre‐2004 interseismic velocity, significant transient motion during the 2008.5–2010.5 epoch confirms that postseismic relaxation processes continue in Andaman. Modeling three‐component velocities as viscoelastic flow yields a weighted root‐mean‐square (wrms) misfit that always exceeds the wrms of the measured signal (26.3  mm/yr). The best‐fitting models are those that yield negligible deformation, indicating the model parameters have no real physical meaning. GPS velocities are well fit (wrms 4.0  mm/yr) by combining a viscoelastic flow model that best fits the horizontal velocities with ∼50  cm/yr thrust slip down‐dip of the coseismic rupture. Both deep slip and flow respond to stress changes, and each can significantly change stress in the realm of the other; it therefore is reasonable to expect that both transient deep slip and viscoelastic flow will influence surface deformation long after a great earthquake.

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