Strain-rate Field of Japan Estimated from GNSS Data and Its Interpretation Based on Inter- and Intra-arc Deformation

― ― Abstract How to estimate a strain­rate field from spatially discrete geodetic data has been a long­ standing issue. In this paper, a method based on basis function expansion with Akaike’s Bayes­ ian information criterion （ ABIC ） is introduced, by which strain­rate fields can be obtained objec­ tively and stably. By applying the method to GNSS data in Japan, strain­rate fields are obtained for three periods: 1997 ­ 1999, 2006 ­ 2009, and 2017 ­ 2020. Except for deformation related to volcanic activity and large earthquakes, the obtained strain­rate fields are roughly stationary in time, while showing large variations in space. In order to interpret such spatially heterogeneous deformation, a framework of inter­arc and intra­arc deformation is used, considering Japanese Islands to be composed of five island arcs （ Kuril, northeast Japan, west Japan, Izu ­ Bonin, and Ryukyu ） and that these island arcs are defined with little ambiguity, though the northeast­ and west­Japan arcs are collectively treated as the Honshu arc in this study. Inter­arc deformation between the Kuril and Honshu arcs is characterized by EW contraction, the Izu ­ Bonin and Honshu arcs by NS to NW ­ SE contraction, and the Ryukyu and Honshu arcs by NS extension with EW contraction. Regarding intra­arc deformation, the Kuril arc shows high strain rates from the Pacific coast to the back of the volcanic arc, the northernmost part of the Izu ­ Bonin arc shows significant EW to NE ­ SW extension, and the Ryukyu arc shows NS extension with EW contraction similar to the inter­arc deformation with the Honshu arc, although the EW contrac­ tion is weaker to the south. The Honshu arc shows zones of high strain rates along the eastern margin of the Japan Sea via the Niigata ­ Kobe tectonic zone （ NKTZ ） to the Median Tectonic Line and along the Ou­backbone Range, while it also shows low strain rates in the Chugoku district and in the zone from northern Ibaraki prefecture via the northern Kanto district to northern Aichi prefecture, which is named the Hitachi ­ Mikawa forearc low strain­rate zone （ HMLSZ ） .