Detailed crustal deformation and tectonic loading of the Atotsugawa fault system, central Japan

The Atotsugawa fault system (AFS) is located in the northern part of central Japan. It consists of three ENE- WSW trending right lateral strike slip faults over 80 km long. AFS is considered as a part of the Niigata-Kobe Tectonic Zone (NKTZ) (Sagiya et al., 2000), a 50-200 km wide, NE-SW trending intraplate large strain rate (lager than 0.1 ppm/yr) belt that was identified by the nationwide continuous GPS (GEONET) observation. In order to investigate the stress accumulation process of AFS and the deformation mechanism of NKTZ, a dense continuous GPS array observation around AFS has been continued since 1997. As of June 2006, the 30 dual-frequency and 5 single-frequency GPS sites covers the whole area of AFS with an interval of 2-10km. We analyzed daily carrier phase data of these sites from 1997 to June 2006 to calculate daily precise coordinates. Based on daily coordinates data, we estimate displacement rates with a precision better than 1 mm/yr, and a detailed crustal deformation pattern is revealed. We interpret the velocity data by using the block-fault model (Hashimoto and Jackson, 1993) in which the crustal deformation is expressed as a superposition of rigid block motion and fault interaction. Main conclusions from our analysis are as follows. 1) The deformation of NKTZ takes place within a 50km-wide zone, in which AFS accounts for the northern half only. 2) ~10mm/yr right-lateral motion between the northern and the southern blocks is accommodated by NKTZ. 3) Significant strain accumulation occurs in the southern half of NKTZ around Takayama city. 4) Estimated relative block motion along the Atotsugawa fault is 2.8 mm/yr and a little larger than the geological estimate (2-3mmyr). 5) There is no evidence of aseismic fault creeping at the eastern part of the Atotsugawa fault. On the other hand, the western part may have an aseismic slip of ~1mm/yr. This result is in discord with the previous studies. 6) At the both ends of AFS, lateral heterogeneity in the deformation pattern is evident. Inelastic deformation may play an important role since there are Quaternary volcanoes (Tateyama and Hakusan).