A comprehensive study of the Kappa value by borehole and surface strong-motion acceleration records

The spectral decay parameter of the amplitudes of high-frequency ground motion, Kappa (k), is widely used in many seismological and earthquake engineering studies. This parameter is dependent on the local characteristics of the site, the propagation path, and the earthquake source. In this paper, the values of k and its variation with subsurface anisotropy and the direction of energy propagation of seismic waves at the source are studied. For this purpose, k values for 508 accelerograms of 141 earthquakes with a moment magnitudes between Mw 3.3 to 7 and epicentral distances less than 100 km are calculated. Various portions of records (P- and S-wave) are used. Records were collected at 20 surface and borehole accelerometery stations of KiK-net strong-motion seismograph networks of Japan’s National Research Institute for Earth Science and Disaster (NIED). Our results show that for all the wave segments considered in this study, k values for the horizontal component are larger than the vertical component at surface stations. In addition, the estimated value of this parameter for surface stations is greater than that of the borehole stations. Differences in k values for the surface and the borehole records also show that k values for S-wave are greater than of the P-wave. The effect of the radiation pattern of seismic waves and source directivity on the k values is another parameter that assessed for our database. The results of this study clearly show that the k changes with the angle at which the wave reaches the station.