1867 Bulletin of the Seismological Society of America, 91, 6, pp. 1867–1874, December 2001 Attenuation of High-Frequency P and S Waves in the Crust of Southeastern South Korea by Tae-Woong Chung and Haruo Sato Abstract The seismicity of the Yangsan fault in southeastern S. Korea has re- ceived increasing attention recently because the fault lies in an industrial area. For this fault region, we first measured Q P - 1 and Q S - 1 simultaneously by applying the extended coda-normalization method to seismograms at nine stations of a network deployed by the Korea Institute of Geology, Mining, and Materials. We analyzed 707 seismograms of local earthquakes that occurred between December 1994 and February 2000. From velocity seismograms, bandpass-filtered traces were made by applying a phaseless four-pole butterworth filter with five octave-width frequency bands, 1–2, 2–4, 4–8, 8–16, and 16–32 Hz. Estimated Q P - 1 and Q S - 1 values decrease from (7  2)  10 3 and (5  4)  10 3 at 1.5 Hz to (5  4)  10 4 and (5  2)  10 4 at 24 Hz, respectively. By fitting power-law frequency dependence to the estimated values over the whole stations, we obtained 0.009 f 1.05 for Q P - 1 and 0.004 f 0.70 for Q S - 1 , where f is frequency in Hz. These results indicate that Q P - 1 and Q S - 1 in the crust of southeastern S. Korea are the lowermost of the reported values in the world, although the exponent values agree well with those in the other areas. Introduction The Q 1 value, amplitude attenuation with travel dis- tance of seismic waves, is a very important parameter used to infer the material and physical conditions of the earth’s interior. In particular, the knowledge of the Q 1 value is indispensable for the quantitative prediction of strong ground motion from the viewpoint of earthquake-resistant design of construction. Hence numerous studies of Q 1 have been undertaken worldwide and concentrated mainly on seismically active zones and/or densely populated industrial areas (e.g. Frankel et al., 1990; Kinoshita, 1994). These stud- ies show relatively higher Q 1 in seismically active areas than in stable areas (Sato and Fehler, 1998). There are sev- eral mechanisms of seismic-wave attenuation. One is scat- tering due to small-scale heterogeneities widely distributed in the lithosphere, and another is intrinsic absorption mostly controlled by thermal condition (Aki, 1980; Blakeslee et al., 1989; Lees and Linley, 1994). In S. Korea, the Yangsan fault (Fig. 1) has been receiv- ing increasing attention because the fault lies in the industrial region where nuclear power plants are located. In Korean history, this fault is believed to have been responsible for a large earthquake that caused more than 100 deaths in A.D. 779 (Lee, 1998). On 26 June 1997, an earthquake with mag- nitude (M) 4.3 took place near the fault (star in Fig. 1), al- though S. Korea is far from a seismically active plate bound- ary, and earthquakes over M 4 have not frequently occurred in this century. The Yangsan fault trending NNE–SSW is about 200 km long and runs parallel with other subsidiary faults such as Miryang, Moryang, Ulsan, and Tongre faults (solid and dashed lines in Fig. 1). These faults are developed mainly in the area of Cretaceous sediments that were intruded by igneous rocks in the Cretaceous to the Tertiary (Chang, 1987). Right-lateral displacements of these faults took place in the early Quaternary (Sillitoe, 1977; Otsuki and Ehiro, 1978), and a possibility for their recurrence is suggested (Lee et al., 1984; Lee and Jin, 1991; Choi et al., 1998). The focal mechanism analysis of several events in Korea suggests that the direction of the maximum compressional stress is ENE– SWS (e.g., Baag et al., 1998). In the Yangsan fault area, a regional seismic network has been in operation by the Korea Institute of Geology, Mining and Materials (KIGAM) since December 1994 (solid squares in Fig. 1). Based on the network data, Kim et al. (1999) first measured very low Q 1 for P waves (hereafter, Q P - 1 ); however, the value was derived from only 10 earth- quake data points. There have been no reports on Q 1 for S waves (hereafter, Q S - 1 ) in S. Korea. In this article, using more than 120 earthquakes recorded by the network from 1994 to 2000, we first make the simultaneous measurement of Q P - 1 and Q S - 1 by means of the coda-normalization method.