Postseismic Deformation from the 1995 Kobe Earthquake Detected with InSAR

Abstract:

About 20 years have passed since the 1995 earthquake of Mw 6.9 hit the Kobe and Awaji areas in Japan. This earthquake was a right lateral faulting on the nearly vertical Rokko fault trending in the NE-SW direction. Coseismic deformations were studied using campaign GPS, leveling and InSAR, but postseismic deformations have not been well investigated. Nakano and Hirahara (1997) and Kato et al. (1997) reported postseismic deformation detected by GPS up to 1996, while Ozawa et al. (2004) presented the results of the analysis of JERS-1 SAR images acquired during 1995 - 1998.

We reexamined postseismic deformation from the 1995 Kobe, Japan, earthquake using ERS/Envisat and JERS-1 SAR data. 14 ERS-1/2, 22 Envisat and 7 JERS-1 images acquired during 1992 to 2007 that cover the Kobe and northern Osaka areas were collected and analyzed. We applied 2-pass interferometry. We divided whole period into 5 intervals, preseismic, coseismic and 1995 - 2004 for ERS-1/2, 2003 - 2004 and 2004 - 2007 for Envisat, and stacked interferograms during each period.

We found that rapid subsidence in a wedge shaped region near the northeastern tip of the source fault. This wedge shaped area is bounded by two active faults; the Arima-Takatsuki fault on the north side, and the Rokko fault on the other side. This deformation is already pointed out by Ozawa et al. (2004). It is noteworthy that this subsidence is not clear in the preseismic interferogram, which implies the Kobe earthquake may have triggered this subsidence. Considering the shape of subsided region, the configuration of basement structure is closely related to the deformation, while it is difficult to explain any model of simple afterslip or viscoelastic relaxation with the assumption of homogeneous structure. This subsidence decayed slowly and became less clear after 2007 according our previous study using ALOS/PALSAR.

Furthermore there are two spots of further rapid subsidence of > 10 mm/yr in this region. These two spots are located in the old river terraces, while the Osaka plain is covered with marine sediments. Our previous study with ALOS/PALSAR showed that the subsidence at these spots still continues up to 2010. This infers the difference in surface geology affects the postseismic deformation siginificantly.