Abstract
In general, there are three mechanisms causing crustal deformation: elastic, viscous, and plastic deformation. The separation of observed crustal deformation to each component has been a challenging problem. In this study, we succeed in separating plastic deformation as well as viscous deformation in the northern Niigata–Kobe Tectonic Zone (NKTZ), central Japan, using GNSS data before and after the 2011 Tohoku-oki earthquake, under the assumptions that elastic deformation is principally caused by the plate coupling along the Japan trench and that plastic deformation ceased after the Tohoku-oki earthquake due to the stress drop caused by the earthquake. The cessation of plastic deformation can be understood with the concept of stress shadow used in the field of seismic activity. The separated strain rates are about 30 nanostrain/year both for the plastic deformation in the preseismic period and for the viscous deformation in both the pre- and post-seismic periods, which means that the inelastic strain rate in the northern NKTZ is about 60 and 30 nanostrain/year in the pre- and post-seismic periods, respectively. This result requires the revision of the strain-rate paradox in Japan. The strain rate was exceptionally faster before the Tohoku-oki earthquake due to the effect of plastic strain, and the discrepancy between the geodetic and geologic strain rates is much smaller in usual time, when the plastic strain is off. In order to estimate the onset timing of plastic deformation, the information on stress history is essentially important.
Highlights
Basic mechanisms of crustal deformation We generally consider the following three mechanisms for crustal deformation: elastic, viscous, and plastic deformation
Instead of it, the offset component (DC component) is subtracted from the strain rate profiles shown in Fig. 1e, f, because it is simpler and, more importantly, the offset component unambiguously corresponds to the longest wavelength component on the basis of the Fourier analysis
From the comparison of strain rate profiles in the northern Niigata–Kobe Tectonic Zone (NKTZ) before and after the Tohoku-oki earthquake (Fig. 2), we succeeded in separating plastic and viscous strain rates of short-wavelength components
Summary
Basic mechanisms of crustal deformation We generally consider the following three mechanisms for crustal deformation: elastic, viscous, and plastic deformation. Considering a one-dimensional case for simplicity, we can express the constitutive equation for elastic deformation as ε = σ/k, (1). Where k is an elastic modulus, ε and σ denote strain and stress, respectively, and represents the change from an initial condition. The constitutive equation for viscous deformation is ε = Aσ n, (2). Where n is the stress exponent, A is a coefficient that depends on various parameters, such as the temperature and grain size, and the dot denotes the differentiation with respect to time. This condition can be applicable to brittle deformation associated with earthquakes, and to aseismic plastic flow
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