Broadscale postseismic gravity change following the 2011 Tohoku‐Oki earthquake and implication for deformation by viscoelastic relaxation and afterslip

Abstract

The analysis of GRACE gravity data revealed postseismic gravity increase by 6 μGal over a 500 km scale within a couple of years after the 2011 Tohoku-Oki earthquake, which is nearly 40–50% of the coseismic gravity change. It originates mostly from changes in the isotropic component corresponding to the Mrr moment tensor element. The exponential decay with rapid change in a year and gradual change afterward is a characteristic temporal pattern. Both viscoelastic relaxation and afterslip models produce reasonable agreement with the GRACE free-air gravity observation, while their Bouguer gravity patterns and seafloor vertical deformations are distinctly different. The postseismic gravity variation is best modeled by the biviscous relaxation with a transient and steady state viscosity of 1018 and 1019 Pa s, respectively, for the asthenosphere. Our calculated higher-resolution viscoelastic relaxation model, underlying the partially ruptured elastic lithosphere, yields the localized postseismic subsidence above the hypocenter reported from the GPS-acoustic seafloor surveying.

Monthly GRACE detection of coseismic gravity change associated with 2011 Tohoku-Oki earthquake using northern gradient approach

“…The model-predicted linear positive trend of 0.40 μGal year −1 for the postseismic gravity changes within two years after the earthquake probably underestimates the postseismic signals, compared with recent studies that demonstrate significant changes and biviscous rheology leading fast relaxation within a few months to a couple of years (e.g., Han et al 2014;Hu et al 2014;Sun et al 2014;Watanabe et al 2014). Han et al (2014) reveal by careful processing to GRACE observations that the postseismic gravity increases by 6 μGal over a 500-km scale within a couple of years after the earthquake, which supports a biviscous relaxation with both transient and steady-state viscosities for the asthenosphere. Thus, the postseismic gravity changes are probably much larger than 0.80 μGal in 2 years and should not be negligible in the GRACE observations.…”

Importance of rheological heterogeneity for interpreting viscoelastic relaxation caused by the 2011 Tohoku-Oki earthquake

“…For example, Ozawa et al (2012) argued that afterslip was the dominant mechanism; Diao et al (2014) and Han et al (2014) examined the combined effects of afterslip and viscoelastic relaxation; Hu et al (2014) explored the effect of poroelastic rebound; and several authors showed that viscoelastic relaxation plays an important role even in short-term deformation (Sun et al 2014;Sun and Wang 2015;Hu et al 2016a). The most important results from these studies are those obtained using seafloor geodetic observation (SGO) (Watanabe et al 2014;Tomita et al 2015).…”

Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands

“…The coseismic correction of the 2011 Tohoku‐Oki earthquake (−0.5 μGal) was applied in the GRACE time series following Han et al . []. The GRACE data are shown in black with its error estimate, the data fit in green, and the a priori model variation of nontidal atmosphere and ocean mass variability in magenta.…”

“…Compared to the postseismic gravity change after the 2004 Sumatra‐Andaman, 2011 Tohoku‐Oki, and 2012 Wharton Basin earthquakes [e.g., Han et al ., , , ], gravity change associated with the 2006–2007 Kuril events is distinctly different; namely, the magnitude of postseismic gravity change becomes by far larger than the coseismic change within a few years. We seek to explain such observations by testing alternate viscoelastic models of the asthenosphere beneath an elastic lithosphere.…”