Broadband strong-motion observation for aftershocks of the 1997 northwestern Kagoshima earthquakes

An earthquake with M_JMA 6.5 occurred at the northwest region of Kagoshima prefecture on March 26, 1997. We started observing the aftershocks at KKN (31.9794°N 130.4442°E) on March 28, 1997 using the broadband strong-motion seismometer (VSE 11 C/12 C). The observation was finished on September 30, 1997. Also another VSE 11 C/12 C was installed at the station SIBI (31.9677°N 130.3524°E). After all, we observed 100 events at KKN, including the second large earthquake (M_JMA 6.2) on May 13. The purpose of this paper is to describe the detail of observational conditions, and to report the result of the preliminary analysis of the observed seismograms. We transformed the horizontal components of seismograms observed at SIBI into the radial and transverse components, and analyzed the Fourier amplitude spectra for S-wave parts of the aftershocks observed at KKN have a sharp peek at the frequency of 4.0-6.0 Hz, which is not seen for the seismograms observed at SIBI. Also, we integrated the velocity seismograms to get the displacement waveforms. The observation in short epicentral distances made it possible to recognize the near-field term in all of the displacement waveforms.