Geosciences Journal Vol. 14, No. 1, p. 57 - 66, March 2010 DOI 10.1007/s12303-010-0007-3 ⓒ The Association of Korean Geoscience Societies and Springer 2010 1D crustal velocity structure beneath broadband seismic stations in the Okcheon Fold Belt of Korea by receiver function analysis ABSTRACT: The crustal velocity structure is one of the funda- mental and important subjects for seismology. The receiver func- tion analysis technique is applied to the data from six broadband stations (HKU, JSB, MGB, NPR, SND, and TJN) in the Okcheon Fold Belt of Korea. The genetic algorithm that is independent of the initial model is adopted as the inversion method. The surface- wave dispersion data are used as constraints in the receiver func- tion inversion. Among these stations, the receiver function analy- ses on the velocity structures beneath stations JSB, MGB, and SND have not been carried out before. Since recent large teleseis- mic earthquake data of magnitudes larger than 6.5 are included in the analyses, the velocity structures found in the present work are expected to be more reliable than those by the previous indepen- dent works. The characteristics of crustal velocity structures in the Okcheon Fold Belt are revealed. The crustal thickness in the Okcheon Fold Belt is estimated to be 32-34 km. The Moho depths are 32, 34, 32, 32-34, 32, and 32 km beneath stations HKU, JSB, MGB, NPR, SND, and TJN, respectively. These results are similar to those found in the previous independent works. The low veloc- ity layers around the depth of 10 km are also found. Geological interpretation or correlations are, however, skipped since they are not always distinctive. Midcrustal velocity discontinuities at the 18-km depth are found beneath stations NPR and TJN although the former is only distinctive. Those at the 22-km depth are also found beneath stations HKU and SND. The crustal velocity structures estimated in this study will be able to offer fundamental informa- tion to other geologists, and the geological and tectonic interpre- tation of the velocity structures is expected. Key words: crustal velocity structure, Okcheon Fold Belt, receiver function, surface-wave dispersion, genetic algorithm 1. INTRODUCTION The crustal velocity structure is used as basic information in various geological and geophysical researches such as geological structures, earthquake locations, seismic hazard analysis, and so on. It is one of the fundamental and impor- tant subjects for seismology. However, there are some dif- ficulties on the study of crustal velocity structures in Korea: 1) the seismicity of the country is relatively low, 2) since most of earthquakes occurred in Korea are of magnitudes less than 5.0, signal-to-noise ratio (SNR) is poor, and 3) well-developed seismic phases in records cannot be easily observed due to sparsely distributed seismic stations. There lacks a priori information about the crustal velocity struc- ture, too. Since mid-1990s, broadband seismographs have been installed and spread all over the country. Almost thirty broadband seismic stations are presently in operation. It is possible to use teleseismic earthquake data in these days. Using teleseismic data to estimate crustal velocity structures might overcome the low seismicity in Korea. Among var- ious techniques to estimate the velocity structures using teleseismic data, the receiver function analysis technique (e.g., Langston, 1979; Owens et al., 1984) is chosen. The receiver function analysis has a velocity-depth trade-off. It does not include absolute traveltimes but relative travel- times, and the result is not unique. For example, a thin and slow layer produces the same relative traveltime as a thick and fast layer. Recently, researchers have used the surface- wave dispersion data in the receiver function analysis. Receiver functions are sensitive to the shear-wave velocity contrasts between the layers while surface-wave velocities are sensitive to the average shear wave velocities of the lay- ers, and they complement each other. We adopt the genetic algorithm (GA) as the inversion method. As GA proceeds, the “crossover” procedure gen- erates new models from randomly selected pairs of the pre- vious generation. In addition, the “mutation” procedure makes model parameters arbitrarily jump to other points in the model space. GA is more independent of the initial model than other inversion methods since it can generate initial models diversely and cover a wider model space. The purpose of the present work is to estimate the crustal velocity structures beneath the broadband seismic stations in the Okcheon Fold Belt. The Okcheon Fold Belt is a Phanerozoic orogenic belt which is developed along the northeast -southwest trend in the southern Korean Peninsula. From the viewpoint of lithologic formation, the Okcheon Fold Belt can be divided into the Okcheon Basin to the southwest and the Taebaeksan Basin to the northeast. The Okcheon Basin is mainly composed of metasedimentary Sung Il Han Jung Mo Lee* Tae-Seob Kang } Department of Geology, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu 702-701, Republic of Korea Department of Earth Environmental Sciences, Pukyong National University, 599-1 Daeyeon-dong, Nam-gu, Busan 608-737, Republic of Korea *Corresponding author: jung@knu.ac.kr