Abstract
The Philippine Sea (PHS) slab is severely deformed beneath central Japan, but it is subject to little deformation before the subduction along the Nankai Trough. Thus, it is considered that the currently observed deformation of the PHS slab has been formed only after the subduction along the Nankai Trough. This study estimates the contraction rate of the PHS slab along segments roughly parallel to the Nankai Trough, assuming that the configuration of the PHS slab is stationary in time in the coordinate system fixed to the overriding plate. Results show that the contraction rate is small beneath most parts of Shikoku (≤ 0.3 mm/year) and the offshore fore-arc (≤ 0.6 mm/year), while it increases from the Pacific coastal region (3–6 mm/year) to the inland region (approximately 20 mm/year) beneath central Japan. Considering various error sources, the strain rate is estimated to be about 4–7 × 10−8/year beneath the inland region of central Japan, although it could be reduced by about 20% if the PHS slab has a tear beneath the Kii Strait. Due to the geographical concordance of the deformation between the PHS slab and the upper crust above the slab, the contraction rate of the PHS slab could be used as a proxy to provide an independent estimate of the crustal deformation rate of central Japan over the long term, which is slower than the geodetic estimate while faster than the geologic one.ᅟ
Highlights
Geodetic data (Hashimoto 1990; Sagiya et al 2000; Heki and Miyazaki 2001), seismological data (Townend and Zoback 2006; Terakawa and Matsu’ura 2010), and geological and geomorphological data (Huzita 1980; Research Group for Active Faults of Japan 1991) have consistently shown that most parts of the Japanese islands are currently situated under an east-west (E-W) compression stress field
In this study, the contraction rate of the Philippine Sea (PHS) slab is estimated from the plate configuration models, assuming that the configuration of the PHS slab is stationary in time in the coordinate system fixed to the overriding plate (Honshu arc)
The advantage of this method consists in the simplicity of the initial configuration of the slab, that is, it is considered that the currently observed deformation of the PHS slab has been formed only after the subduction along the Nankai Trough
Summary
Geodetic data (Hashimoto 1990; Sagiya et al 2000; Heki and Miyazaki 2001), seismological data (Townend and Zoback 2006; Terakawa and Matsu’ura 2010), and geological and geomorphological data (Huzita 1980; Research Group for Active Faults of Japan 1991) have consistently shown that most parts of the Japanese islands are currently situated under an east-west (E-W) compression stress field. Traditional triangulation and trilateration data cover more than a century, though they are not as accurate as GNSS data. From seismic moment release rates due to historical large inland earthquakes, Wesnousky et al (1982) estimated the E-W contraction rate of Honshu and Shikoku islands for the last 400 years. 400 years may still be too short to accurately estimate the contraction rates considering that the recurrence intervals of large inland earthquakes are much longer than several hundred years, which might be even longer than thousands of years. It is possible to estimate the contraction rate from slip rates on active faults (Wesnousky et al 1982; Kaizuka and Imaizumi 1984). Large inland earthquakes have often happened on faults that were previously not identified as active faults, as in the cases of the 2000 Western Tottori earthquake (Asano and Hasegawa 2004)
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