Attenuation Property of Coda Parts of Seismic Waves from Local Earthquakes

Abstract

Coda parts of the seismic waves from local earthquakes occurring in the Kinki district were analyzed to discuss the mechanisms of generation and attenuation of the coda waves. The attenuation and fluctuation of the logarithmic rms amplitudes of the band-pass filtered traces ranging from 1 to 20 HZ are characterized as follows; The decay curve of each band is well fitted to the functional form of the lapse time t measured from source origin time, t^-a e^-bt. The geometrical spreading factor a is 1～2 for the intervals 15～300 sec in each frequency band. The attenuation coefficient b depends on the frequency as well as the lapse time. When the lapse time is early (t<50 sec), the Q values derived from b under the condition where a=1 are 150 at 1 HZ and 780 at 16 HZ, while they increase to 210 at 1 HZ and 1000 at 16 HZ, respectively, for the large lapse time (t>50 sec). In each time interval the dependence of the Q values on frequency is remarkable. The turbidity coefficient derived from the fluctuation of amplitudes is rather constant for frequency, but decreases with time similarly to the attenuation property, showing the vertical variation of turbidity in the earth.