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 longstanding issue. In this paper, a method based on basis function expansion with Akaike’s Bayesian information criterion （ABIC） is introduced, by which strainrate fields can be obtained objectively and stably. By applying the method to GNSS data in Japan, strainrate fields are obtained for three periods: 19971999, 20062009, and 20172020. 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, IzuBonin, and Ryukyu） and that these island arcs are defined with little ambiguity, though the northeastand 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 IzuBonin and Honshu arcs by NS to NWSE 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 IzuBonin arc shows significant EW to NESW extension, and the Ryukyu arc shows NS extension with EW contraction similar to the interarc deformation with the Honshu arc, although the EW contraction is weaker to the south. The Honshu arc shows zones of high strain rates along the eastern margin of the Japan Sea via the NiigataKobe 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 HitachiMikawa forearc low strainrate zone （HMLSZ）