Abstract
Obvious crustal deformation is observed during a postseismic period as well as a coseismic period associated with a large earthquake. Major mechanisms of transient postseismic deformation are known as afterslip and viscoelastic relaxation. Since the viscoelastic relaxation occurs as a response to a coseismic slip, postseismic deformation provides information on coseismic deformation through the viscoelastic response. However, most previous studies have not thoroughly utilized postseismic geodetic observational data for revealing coseismic slip behaviors. In this study, we developed a slip inversion method that simultaneously estimates coseismic slip and postseismic slip distributions from coseismic and postseismic geodetic observational data using viscoelastic Green’s function (viscoelastic inversion method). We investigated the performance of the viscoelastic inversion method via two synthetic tests: one assumed a strike–slip event along an inland fault, while the other assumed a dip–slip event along a plate interface in a subduction zone. Both synthetic tests demonstrated that when extensive postseismic observational data were given, the viscoelastic inversion method provided a superior spatial resolution of coseismic slip distributions compared to conventional elastic inversion distributions. We also applied the viscoelastic inversion method to co- and post-seismic deformations associated with the 2011 Tohoku-oki earthquake. The seafloor geodetic observational network of the off-Tohoku region has been widely extended after the occurrence of the mainshock. Using this extended seafloor geodetic observational data, we successfully improved the spatial resolution of the coseismic slip distribution through the viscoelastic inversion method. Furthermore, using the seafloor observational data during the postseismic period, our inversion method enables us to obtain high spatial resolution of the coseismic slip in the offshore area and a reasonable coseismic slip distribution even if seafloor observational data during the coseismic period are unavailable. These results clarify the importance of deploying a geodetic observational network even after large coseismic events to assess past coseismic slip behaviors by considering the viscoelasticity of the Earth.
Highlights
Associated with the occurrence of large earthquakes, characteristic crustal deformation is measured during both co- and post-seismic periods
The synthetic and calculated displacements are shown in Additional file 1: Figure S1, while those due to viscoelastic relaxation and postseismic slip in the postseismic period are shown in Additional file 1: Figure S2
The estimated postseismic slip distribution is distorted, the spatial resolution during the postseismic period is improved compared to the distribution during the coseismic period (Fig. 4b)
Summary
Associated with the occurrence of large earthquakes, characteristic crustal deformation is measured during both co- and post-seismic periods. Most previous studies that modeled coseismic slip distributions for large earthquakes based on observations have utilized only coseismic observational data assuming an elastic Earth. Some recent studies have simultaneously estimated contributions of afterslip and viscoelastic relaxation using a stress kernel, which is independent of a coseismic slip model (e.g., Tsang et al 2016; Moore et al 2017; Qiu et al 2018). This approach is quite useful and reasonable, but it cannot model coseismic behaviors by utilizing postseismic deformation data. Since the estimation of fault-slip distributions with better spatial resolution is crucial to investigate the process of strain accumulation and release and to evaluate the temporal change in the seismic potential along the faults during a seismic cycle, it is important to assess potential performance of the viscoelastic inversion method
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