炭山地震観測室における地震観測 (2) -局所地震のcoda parts の減衰とスペクトルの性質-

Abstract

Coda parts of the seismograms obtained at the Sumiyama Seismic Station were analyzed tostudy the characteristics of attenuation and spectra of the coda waves from local earthquakesoccurred in the Kinki district.The attenuation of the logarithmic rms amplitude of the band pass filtered traces ranging from1 to 20 HZ are characterixed as follows; The decay curve of each band is well fitted to the func-tional form of the lapse time t measured from source origin time, c t-ae-bt. The geometricalspreading factor a is 1～2 for the intervals 15～300 sec in each frequency band. The attenuationcoefficient b depends on the frequency as well as the lapse time. When the lapse time is early(t<60 sec), Q derived from b under a condition of a=1 are 150 at 1 HZ and 780 at 16 HZ, whilethey increase to 210 at 1 HZ and 1000 at 16 HZ, respectively, for the large lapse time (t>50sec). The dependency of Q on frequency may be shown with a form of Q=q√f, where q=160 for t<60 sec and 230 for t>50 sec, respectively.The spectral shape of the coda source factor c varies with earthquake magnitude as wellas source location. The rate of increase in the frequency range of 1～4 HZ are larger than thoseof 8～16 HZ for the observed magnitude range of 2.3～4.9. The amplitudes of the lower fre-quency range of the events, which occurred at the seismic active region, are larger than those inthe region where seismic activity is low. The deeper events (H=60～70 Km) are characterizedwith relatively large high frequency contents than shallowew ones (H<30 Km).The observational system at the Sumiyama Seismic Station, which consists of a symmetricalthree-component seismograph and a digital data logger utilizing cartridge tape, is described inconnection to the way of data sampling according to the predominant frequency of the codaparts of seismic waves.