Seismic vertical array analysis of the velocity structure and phase decomposition during aftershocks of the 2007 Chuetsu-oki earthquake

Abstract

Abstract A velocity profile derived from PS-logging was examined using the mainshock and aftershock records from KK5 and KSH seismic vertical arrays in the Kashiwazaki-Kariwa nuclear power plant, which is located on the focal region. A seismic phase decomposition analysis method was applied to aftershock vertical array records. Peak frequencies of spectral ratios of the aftershock records from the KSH array between various depths were slightly different from those of theoretical transfer functions. A comparison of observed and theoretical S-wave travel times at various depths indicated that travel times for the mainshock measured by picking S-wave arrival times were identical to the theoretical travel times calculated from the PS-logging profile, although travel times for the aftershocks were different from the theoretical travel times. The decomposition analysis of seismic vertical array records from KK5 array for a small aftershock shows that a clear wave packet of surface waves follows just after the direct S-wave arrival. In contrast, a decomposition analysis of the KSH array records show that a wave packet of surface waves are not clearly discernable.