Q-values for P and S waves in the southern Korean Peninsula based on the coda-normalization method

The attenuation of seismic waves can be measured by the quality factor Q, which is an important parameter of the physical properties of the Earth's interior. Using the extended coda-normalization method, we estimated Q-values for P waves (Q(p)) and S waves (Q(s)) in the southern Korean Peninsula as well as for the three sub-tectonic provinces (the Gyeonggi Massif, the Ogcheon Belt and the Yeongnam Massif, and the Gyeongsang Basin) in it. We chose 675 seismograms from 189 earthquakes that occurred in Korea from January 2001 to October 2003. Most earthquakes had small magnitudes (M-L <= 3.0) and shallow focal depths (<= 15 km). Assuming that Q is independent of frequency, the Q-value and geometric spreading factor (gamma) were calculated simultaneously using least squares inversion. The resulting values for the southern Korean Peninsula were Q(p)=1384 and Q(S)=2036. The Gyeongsang Basin had the lowest value, indicating that its crustal structure is relatively heterogeneous and might be highly fractured. The gamma values varied from 0.73 to 0.87. Assuming that Q is dependent on frequency and gamma has a constant value of 1.0, the values of Q at the central frequencies of 1, 2, 3, 5, 10, 20, and 30 Hz yielded Q(P)=188.6 f(0.8110) and Q(s)=201.4 f(0.7509) for the southern Korean Peninsula. We analyzed the variation of Q(s) for two hypocentral distance ranges: 0-90 km and 90-180 km, respectively. The estimated Q-values for both P and S waves in the Gyeongsang Basin are the lowest, except for the case of Q(s)(f), in the low frequency range. The frequency-dependent Q(s) for the hypocentral distance range of 90 to 180 km appeared to be the larger than that for the shorter distance range. The values of Q(s)(f) estimated in this study were comparatively higher than for a seismically active area.