Investigation on afterslip and steady state and transient rheology based on postseismic deformation and geoid change caused by the Sumatra 2004 earthquake

Abstract

[1] The commonly used rheological model for the Earth's mantle when considering geological time scales (mantle convection) is the viscoelastic Maxwell model, which assumes a steady state creep process. However, application of this model to phenomena on shorter time scales, such as postglacial rebound or postseismic relaxation, leads to difficulties in finding a consistent interpretation of obtained viscosities. Using standard Maxwell viscosity of 1e19 Pa s to analyze postseismic near-field GPS time series from the 2004 Sumatra-Andaman earthquake requires large time-dependent afterslip with a relaxation time of about 1 year. We show that using linear biviscous Burgers rheology for the asthenosphere, together with a refined coseismic slip model, we can drastically reduce the amount of apparent afterslip. Comparison of predicted geoid change to observations by the GRACE satellite mission shows that a univiscous Maxwell model with afterslip is not compatible with observations, since even large afterslip has a more localized effect than transient relaxation due to the main earthquake, which in turn is in agreement with observations. Thus, a combination of ground-and space-based geodetic observations is very useful in differentiating between rheological models. An additional independent discrimination between afterslip and biviscous relaxation could be obtained by installing ocean bottom pressure gauges close to the trench.

New analogue materials for nonlinear lithosphere rheology, with an application to slab break-off

“…Asthenosphere viscosity † Asthenosphere steady-state viscosities estimates for subduction zones are commonly on the order of 10 19 Pa s, based on observations of postseismic deformation (generally estimates are in the range [10 18 -10 20 ] see table 1 of Wang (2007)). This viscosity estimate is in line with results from recent studies that interpret postseismic deformation after recent megathrust earthquakes, such as the 2004 Sumatra-Andaman earthquake (Hoechner et al, 2011;Broerse et al, 2015), the 2010 Maule earthquake (Klein et al, 2016;Bedford et al, 2016) or 2011 Tohoku earthquake (Hu et al, 2016). However, the asthenosphere viscosities reported for subduction zones may be more applicable to the mantle wedge than the suboceanic asthenosphere Hirth and Kohlstedt (2003), for which we adopt the higher value of ≈ 10 20 Pa s (Hu et al, 2004;Broerse et al, 2015).…”

Combined logarithmic and exponential function model for fitting postseismic GNSS time series after 2011 Tohoku-Oki earthquake

“…Moreover, postseismic displacements calculated by models with biviscous Burgers rheologies are often described by logarithmic functions (Hetland and Hager 2006). Hoechner et al (2011) also proposed that the response of a Burgers body consists of two superposed exponential decays.…”

GRACE, time-varying gravity, Earth system dynamics and climate change

“…A few papers [183,192,193] have modelled the observed postseismic signal after the Sumatra-Andaman event as the result of viscoelastic relaxation. All studies agree that relaxation is characterized by a transient phase with fast flow followed by a slower steady-state phase.…”

“…Panet et al [192], however, invoked the presence of a small amount of afterslip, on the basis of GRACE data and a few GPS sites at about 500-1000 km from the fault. Following a different approach, Hoechner et al [193] started from GPS data to refine the coseismic model and to reduce the number of candidate postseismic models, and to estimate the optimal crustal thickness. Then, they used GRACE data to discriminate between two alternatives, the combination of a Maxwell model and afterslip vs. a Burgers model, and found that the Burgers model provides a much better fit to gravity observations ( Figure 18).…”