Abstract
Recent global navigation satellite system (GNSS) data for the Japanese Islands have revealed a high-strain-rate region suggesting the existence of a region of broad-scale crustal deformation. The Niigata–Kobe Tectonic Zone (NKTZ), which is the high-strain-rate zone in central Japan, shows a short-term dextral strain rate of ~ 12 mm/year. The total slip rate of the Quaternary fault zones in the NKTZ has been estimated as ~ 6.7 mm/year, accounting for just over half the short-term strain rate of the zone. However, this slip rate underestimates the total slip rate on faults within the NKTZ owing to possible distributed deformation on minor faults. This study quantifies the slip rate attributable to these other faults in the southeastern-central NKTZ and reveals the unique deformation structure across the high-strain-rate zone, which comprises a Quaternary fault core, a Quaternary fault damage zone, an incipient brittle shear zone (active background), and an inactive background. The spatial characteristics of the incipient brittle shear zone can be explained in terms of fault density, which increases toward the central NKTZ. Minor faults located > 500 m from major Quaternary faults but within the NKTZ have sense of shear consistent with that of the major faults. In contrast, minor faults outside of the NKTZ show sense of shear that differ from the dextral displacement of the high-strain-rate zone and do not contribute to the slip rate of the zone. The total slip rate of minor faults in the southeastern-central NKTZ is estimated to be 0.46–2.88 mm/year (roughly equal to a major Quaternary fault in the zone), which implies 4–24% of crustal strain is stored in the active background.
Highlights
The Niigata–Kobe Tectonic Zone (NKTZ; Sagiya et al 2000) is one of the largest strain-concentration zones in Japan and extends from the northeastern (Niigata area) to southwestern (Kobe area) parts of Japan (Fig. 1a). Ohzono et al (2011) demonstrated a dextral motion of ~ 12 mm/ year across the NKTZ using dense global navigation satellite system (GNSS) observations
Crustal deformation triggered by infrequent, large-magnitude (M ≥ 9.0) earthquakes is proposed as a possible explanation for the strain-rate paradox (Ikeda et al 2012)
Because of the following reasons, we focus on the contribution of minor faults in the NKTZ which were not detected so far as the explanation of the crustal deformation. (I) In the NKTZ, paleomagnetic data suggest a broad deformation of Quaternary sediment adjacent to the Quaternary fault (Itoh et al 2004; Kimura et al 2004). (II) For the Ushikubi and the Atotsugawa Fault Zones in the NKTZ, Oohashi and Kobayashi (2008) and Niwa et al (2011) found minor faults both within and outside of the fault damage zones
Summary
The Niigata–Kobe Tectonic Zone (NKTZ; Sagiya et al 2000) is one of the largest strain-concentration zones in Japan and extends from the northeastern (Niigata area) to southwestern (Kobe area) parts of Japan (Fig. 1a). Ohzono et al (2011) demonstrated a dextral motion of ~ 12 mm/ year across the NKTZ using dense GNSS observations. Elastic strain in northeastern Japan crust was released during the 2011 Tohoku-Oki earthquake (Mw = 9) (Ikeda et al 2012). Angela-Menesses and Sagiya (2016) demonstrated continuity of E–W contraction in the NKTZ before and after the 2011 Tohoku-Oki earthquake by separating short- and long-wavelength components from GNSS data in the central Niigata region. Those authors inferred that this continuous strain state is related to aseismic slip of a deep crustal fault, which explained the existence of inelastic deformation in the NKTZ. The strain-rate paradox in the NKTZ is still open question
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