Broadband strong-motion observation in Kikai Island for aftershocks of the 1995 Amamioshima-Kinkai earthquake

Two mainshocks of the Amamioshima-Kinkai earthquakes occurred at the southeast from Kikai Island on October 18 (M6.7) and 19 (M6.6), 1995. Those were observed by the two seismometers (STS-2 and VSE11C/12C) which had been installed at temporal station CVC (Chojabaru visitor center) near Kuju volcano, about 560 km north from the epicenters. In order to observe near-field seismic waves of aftershocks we installed the broadband strong-motion seismometer (VSE11C/12C) at temporal station TKG (Takigawa elementary school) in Kikai Island, the nearest land to the aftershock region. We observed 40 events including the largest aftershock of M5.7 on November 1 during observation period from October 21, 1995 to February 18, 1996. The purpose of this paper is to describe the observational conditions, and to report the result of preliminary analysis of the observed seismograms. We transformed the seismograms from the two horizontal components of N30°W and N60°E to the radial and transverse components. We analyzed the Fourier amplitude spectra of SH waves by using FFT. We found that the SH wave of the largest aftershock has the two predominant frequencies of 0.5-0.7 Hz and 1.8-2.5 Hz. Further, we integrated the velocity seismograms to get the displacement waveforms. Using the amplitude ratio of the vertical and radial components for the first pulses, we also evaluated the P-wave incident angles to the observation point. They show that the rays of P waves came up nearly perpendicularly to the surface of the observation point.