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

强震地震学和地震工程学中的一个基本问题是确定体波在沉积层-基底系统中相干传播的频带。为了回答这个问题，我们研究了包括在日本关东地区获得的深井地震记录在内的井下阵列记录，得到了以下结果：(1)局部事件和在沉积地层-基底系统中传播的体波的相干特征被经验地模拟为γ^2(F)=γ_0^2/1+(f/f_c)^n，其中n约为2。(2)ω^&lt；-2&gt；模型中的拐角频率f_c随走时的减小而增加。当走时小于2.5 S时，f_c/2几乎不变，与走时无关，对于SH波，f_c/2在走时0.5~2.5 S范围内约为2.5~3赫兹，γ_0^2大于0.9。这意味着前第三纪基底中的SH波在高于2.5~3 Hz的频率上趋于非相干，用随机方法研究强震预报在高于2.5~3 Hz的频率下变得有效。(3)在沉积层-基底系统中体波相干传播的频率中，Q_p(F)和Q_S(F)近似与频率成正比。在0.5~2赫兹频段，Q_S(F)/Q_p(F)的平均值约为1.5。

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.