Fundamental research on attenuation and coherency characteristics of body waves in a sedimentary layer-basement system

点击翻译按钮获取中文摘要

A fundamental issue in strong-motion seismology and earthquake engineering is to determine the frequency band in which body waves propagate coherently in a sedimentary layer-basement system. To respond this issue, we investigated down-hole array recordings including deep-borehole seismograms obtained in the Kanto region, Japan, and obtained the following results:(1) Coherence characteristics of body waves from local events and traveling in a sedimentary llayer-basement system are empirically modeled by γ^2(f)=γ_0^2/1+(f/f_c)^n, where n is approximately 2.(2) The corner frequency, f_c in the ω^<-2> - model intends to increase with a decrease in travel-time. However, the values of f_c/2 becomes nearly constant, independent of travel time for travel timeless than 2.5 s. In the case of SH-wave, the values of f_c/2 are about 2.5 -3Hz in the travel time range from 0.5 to 2.5 s and the values of γ_0^2 are larger than 0.9. This means that the SH waves in the pre-Tertiary basement are getting on toward incoherent in frequencies higher than 2.5 to 3 Hz and the use of stochastic methods for the study of strong-motion prediction becomes effective in frequencies higher than 2.5 to 3 Hz.(3) In frequencies where body waves propagate coherently in a sedimentary layer-basement system, Q_p(f) and Q_s(f) are approximately proportional to frequency. The average value of Q_s(f)/Q_p(f) is about 1.5 in the frequency band between 0.5 and 2Hz.