Abstract
AbstractThis study uses borehole array recordings to measure the propagation characteristics of bedrock waves traveling from pre-Tertiary basement (seismic bedrock) to engineering bedrock with anS-wave velocity of approximately 0.5–0.8 km/s. To avoid the destructive interference of surface-reflected down-going waves and incidence waves in seismic and engineering bedrock, borehole data recorded at sufficient depth levels are used in this study. This is the most important aspect of the fundamental basis of this study. The propagation characteristics, i.e., the transfer function, of bedrock motions for S-waves are well represented by a Butterworth-type low-pass filter model with a high corner frequency in excess of 15 Hz and a low decay rate of —3 power of frequency. The use of such a filter model is based on the concise representation of the transfer function from an engineering viewpoint. Simple one-dimensional ray theory with a plane wave approximation explains the characteristics of the model filter at low frequencies of less than approximately 5 Hz; however, one-dimensional ray theory with a plane wave approximation at high frequencies in excess of 5 Hz requires the unusual frequency characteristics ofQS−1(f), which increases with increasing frequency, to explain the frequency characteristics of the model filter. These facts imply that the filter gain can be determined using the impedance ratio of seismic bedrock to engineering bedrock and the attenuation characteristics of the intervening media at low frequencies less than 5 Hz. However, the cutoff frequency and decay rate of the filter must be determined from observational data.
Highlights
The use of reference sites plays an important role in seismology and earthquake engineering, in the evaluation of site effects (Steidl et al, 1996; Kawase, 2003)
Simple one-dimensional ray theory with a plane wave approximation explains the characteristics of the model filter at low frequencies of less than approximately 5 Hz; one-dimensional ray theory with a plane wave approximation at high frequencies in excess of 5 Hz requires the unusual frequency characteristics of QS−1( f ), which increases with increasing frequency, to explain the frequency characteristics of the model filter
Seismic waves recorded upon or within seismic bedrock are used for studies of source characteristics (e.g., Kinoshita and Ohike, 2002a), whereas seismic waves recorded upon or within engineering bedrock, i.e., engineering bedrock waves, are used in making design code employed in the construction of important buildings and civil engineering structures
Summary
The use of reference sites plays an important role in seismology and earthquake engineering, in the evaluation of site effects (Steidl et al, 1996; Kawase, 2003). There are two commonly used types of references sites: those installed upon or within pre-Tertiary basement, termed seismic bedrock, and those installed upon or within hard Pleistocene or Pliocene rocks, termed engineering bedrock; the choice of site depends on the purpose of the investigation. Seismic hazard analysis makes use of seismic bedrock waves or engineering bedrock waves as reference waves. Body waves generally propagate from pre-Tertiary basement to the Pleistocene or Pliocene rocks; the propagation characteristics of the bedrock waves, i.e., the relation between seismic bedrock and engineering bedrock
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