The 2 July 2013 Mw 6.1 Aceh, Indonesia, earthquake
GPS data after the 2 July 2013 Mw 6.1 Aceh earthquake provide valuable information for understanding the postseismic behavior of a strike-slip fault in northern Sumatra, Indonesia. The analysis of postseismic de- formation following the 2004 Sumatra-Andaman earthquake and the 2012 Indian Ocean earthquake suggest that the ongoing deformation affected northern Sumatra between July and December 2013. The postseismic deformation of the 2013 Aceh earthquake is herein investigated, with the contribution of poroelastic rebound and viscoelastic relaxation evaluated. The results suggest that the model displacement of those two mechanisms failed to estimate the data. The afterslip analysis of the 2013 Aceh earthquake, on the contrary, fits the data very well with a seismic slip occurring at multiple fault segments of the Sumatran fault in northern Sumatra, Indonesia.
The 2012 Mw 8.6 Indian Ocean earthquake
Postseismic motion in the middle-field (100–500 km from the epicenter) geodetic data resulting from the 2012 Indian Ocean earthquake exhibited rapid change during the two months following the rupture. This pattern probably indicates multiple postseismic deformation mechanisms and might have been controlled by transient rheology. Therefore, the relative contribution of transient rheology in the oceanic asthenosphere and afterslip in the oceanic lithosphere should be incorporated to explain short- and long-term transitional features of post- seismic signals. In this study, using two years of post-earthquake geodetic data from northern Sumatra, a three- dimensional spherical-earth finite-element model was constructed based on a heterogeneous structure and in- corporating transient rheology. A rheology model combined with stress-driven afterslip was estimated. Our best- fit model suggests an oceanic lithosphere thickness of 75 km with oceanic asthenosphere viscosity values of 1 × 1017 Pa s and 2 × 1018 Pa s for the Kelvin and Maxwell viscosity models, respectively. The model results indicate that horizontal landward motion and vertical uplift in northern Sumatra require viscoelastic relaxation of the oceanic asthenosphere coupled with afterslip in the lithosphere. The present study demonstrates that transient rheology is essential for reproducing the rapidly changing motion of postseismic deformation in the middle-field area.
The 2006 Mw 7.8 Java tsunami earthquake
We investigate three available coseismic fault models of the 2006 M7.8 Java tsunami earthquake, as reported by Fujii and Satake (2006), Bilek and Engdahl (2007), and Yagi and Fukahata (2011), in order to find the best coseismic model based on mechanisms of postseismic deformation associated with vis- coelastic relaxation and afterslip. We construct a preliminary rheological model using vertical data, obtaining a final rheological model after we include horizontal and vertical components of afterslip in the further process. Our analysis indicates that the coseismic fault model of Fujii and Satake (2006) pro- vides a better and more realistic result for a rheological model than the others. The best-fit rheological model calculated using the coseismic fault model of Fujii and Satake (2006) comprises a 60 ± 5 km elastic layer thickness with a viscosity of 2.0 ± 1.0 1017 Pa s in the asthenosphere. Also, we find that afterslip is dominant over the horizontal displacements, while viscoelastic relaxation is dominant over the vertical displacement. Additionally, in comparison to the coseismic displacement found through GPS data taken at BAKO station, our calculation indicates that Fujii and Satake (2006) modeled coseismic displacements with less GPS data misfit than the other examined models.
The 2004 Mw 9.2 Sumatra-Andaman earthquake
We investigate the postseismic deformation of the 2004 Sumatra–Andaman earthquake using 5 years of Global Positioning System (GPS) data located in northern Sumatra. Continuous GPS data from northern Sumatra suggest that the relaxation time in the vertical displacement is longer than horizontal displacements. This implies that there are multiple physical mechanisms that control the postseismic deformation, which refer to afterslip and viscoelastic relaxation. In this study, we introduce an analysis strategy of postseismic deformation to simultaneously calculate multiple mechanisms of afterslip and viscoelastic relaxation. The afterslip inversion results indicate that the distribution of the afterslip and the coseismic slip are compensatory of each other. Also, afterslip has a limited contribution to vertical deformation in northern Sumatra. In our rheology model, we use a gravitational Maxwell viscoelastic response and the result indicates that the elastic layer thickness is 65 ± 5 km and the Maxwell viscosity is 8.0 ± 1.0 1018 Pa s. We find that afterslip plus Maxwell viscoelastic relaxation are appropriate to explain the deformation in northern Sumatra. Finally, we showed that our rheology model is applicable to the GPS datasets of postseismic deformation of the 2004 SAE located in northern Sumatra, Thailand, and Andaman–Nicobar, respectively.
