Subsurface S-Wave Velocity Structure at Tenjin Area in Chuo Ward, Fukuoka City

Abstract

We investigate the shallow subsurface S-wave velocity structure across the Kego fault at the area of Tenjin in Chuo Ward, Fukuoka, Japan. We observed microtremors for several minutes by small arrays consisting of 7 sensors at 8 sites using wireless LAN data loggers. These sites were situated at the same locations of observation sites by Yamanaka et al. (2005) for aftershocks of the 2005 north-west off Fukuoka prefecture MJ 7.0 earthquake and with K-NET station FKO006. The micro-tremor array data were processed to obtain Rayleigh wave phase velocities in the period range of 0.04-0.4 s by SPAC analysis. Then the 1D S-wave velocity structures having three layers with a Vs of about 130-170 m/s, 220-260 m/s and 800 m/s are estimated from hybrid inversions at each site. The depth to the top of the layer of Vs 800 m/s is only 5 m at the site west of the Kego fault, while the depth is 25-40 m at the sites east of the fault. The layer with a Vs of 800 m/s has a steep slope with a thickness difference of about 20 m at the fault. These 1D velocity structure models show amplifica-tions characterized by dominant peaks in period range of longer than 0.2 s. However, the distribution of the AVS30s for the profiles shows no correlation to those of the ratios of PGA and PGV and the difference of seismic intensity. This indicates the limit of understanding only by 1D structure model and necessity of the consideration of 2D or 3D subsurface structure. The profile for FKO006 at site 08 is different from previous study, which has a deeper depth of the top of the engineering bedrock in our model. These results must be included to explain the observed strong ground motion records of the 2005 earthquake around the central Fukuoka area in short period range.