Shear wave velocity and attenuation structure for the shallow crust of the southern Korean peninsula from short period Rayleigh waves

Tectonophysics 429 (2007) 253 – 265 www.elsevier.com/locate/tecto

Shear wave velocity and attenuation structure for the shallow crust of the southern Korean peninsula from short period Rayleigh waves Heeok Jung a,⁎, Yong-seok Jang b , Jung Mo Lee c , Wooil M. Moon d , Chang-Eob Baag d , Ki Young Kim e , Bong Gon Jo b a

Department of Ocean System Engineering, Kunsan National University, Kunsan, 573-701, South Korea Department of Earth and Environmental Sciences, ChonBuk National University, JeonJu, South Korea c Department of Geology, Kyungpook National University, Daegu, 702-701, South Korea d School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-742, South Korea e Department of Geophysics, Kangwon National University, Chunchon. 200-701, South Korea

b

Received 4 February 2006; received in revised form 14 October 2006; accepted 16 October 2006 Available online 28 November 2006

Abstract We analyzed the short period Rayleigh waves from the first crustal-scale seismic refraction experiment in the Korean peninsula, KCRUST2002, to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the Korean peninsula and to examine if this can be related to the surface geology of the study area. The experiment was conducted with two large explosive sources along a 300-km long profile in 2002. The seismic traces, recorded on 170 vertical-component, 2-Hz portable seismometers, show distinct Rayleigh waves in the period range between 0.2 s and 1.2 s, which are easily recognizable up to 30–60 km from the sources. The seismic profiles, which traverse three tectonic regions (Gyeonggi massif, Okcheon fold belt and Yeongnam massif), were divided into five subsections based on tectonic boundaries as well as lithology. Group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method, respectively, were inverted for the shear wave models. We obtained shear wave velocity models up to a depth of 1.0 km. Overall, the shear wave velocity of the Okcheon fold belt is lower than that of the Gyeonggi and Yeongnam massifs by ∼ 0.4 km/s in the shallowmost 0.2 km and by 0.2 km/s at depths below 0.2 km. Attenuation coefficients, determined from the decay of the fundamental mode Rayleigh waves, were used to obtain the shear wave attenuation structures for three subsections (one for each of the three different tectonic regions). We obtained an average value of Q−β 1 in the upper 0.5 km for each subsection. Q−β 1 for the Okcheon fold belt (∼ 0.026) is approximately three times larger than Q−β 1 for the massif areas (∼ 0.008). The low shear wave velocity in the Okcheon fold belt is consistent with the high attenuation in this region. © 2006 Elsevier B.V. All rights reserved. Keywords: Shear wave velocity structure; Shear wave attenuation structure; Rayleigh waves; Upper crust

⁎ Corresponding author. Tel./fax: +82 63 469 1750. E-mail addresses: [email protected] (H. Jung), [email protected] (Y. Jang), [email protected] (J.M. Lee), [email protected] (W.M. Moon), [email protected] (C.-E. Baag), [email protected] (K.Y. Kim), [email protected] (B.G. Jo). 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.10.003

1. Introduction The seismic refraction method has been widely used to investigate the velocity structure of the crust and upper mantle. Whereas travel times and amplitudes of seismic refraction data are useful for modeling the

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observed seismic signals, short period surface waves from refraction profiles also can be used to provide information about the uppermost layers. However, there are few examples of short period surface wave studies using refraction profile data partly because most refraction experiments are designed to record only the first part of the body wave arrivals. Also, artificial explosive sources generate mostly isotropic compressional waves unless there exists recognizable anisotropy in the vicinity of the explosive source. Short period surface wave energy generated by artificial explosive sources is usually band-limited and concentrated most in frequencies higher than 1 Hz. Nevertheless, some studies indicate that analysis of short period Rayleigh waves can be used to determine the shallow structure in upper crust; Mokhtar et al. (1988) used short period Rayleigh waves from large explosive sources to investigate the seismic velocity and attenuation for the shallow structure of the Arabian shield; Astrom (1998) investigated the shear wave velocity in the upper 0.5 km of crust in southern Sweden using short period fundamental mode Rayleigh wave dispersion, and P and S wave data; Sarrate et al. (1993) also studied the shallow structure of a part of northern Iberia above the depth of 1.5 km. There is no previous crustal scale seismic refraction survey of Korea, even though there have been several very shallow refraction and reflection seismic engineering surveys. Most previous studies on the velocity and attenuation of seismic waves in the Korean peninsula have been conducted using earthquake data. The following summary of the seismic structure of the Korean peninsula is based on the work of Kim et al. (2006) and Kim et al. (2000). The first velocity model of crustal structure in the Korean peninsula (Lee, 1979) was based on travel time data of the 1936 Ssanggue-sa earthquake. It comprised a 35-km thick crust with a P-wave velocity of 5.8 km/s overlying mantle (Vp = 7.7 km/s). A recent 2-D velocity model of the southern Korean peninsula (Park et al., 2003) indicates that the crust is generally thinner toward the east coast of the Korean peninsula. The thickness varies from greater than 33 km near Mt. Jiri in the central south to less than 31 km in the Gyeongsang sedimentary basin in the southwest of the peninsula. Three-dimensional velocity models using multiphase inversion in the lower crust show relatively small lateral variation in the central Korean peninsula (Kim, 1995). In the south, a distinct mid-crust interface exist in the depth range of 14.5–26 km and the Moho is shallower in the Gyeongsang basin than in the Yeongnam massif area (Park et al., 2003). According to a recent 3-D velocity model of the southern Korean peninsula (Kang and Park, 2003), the distinct mid-crust

interface with an average depth of 22.5 km is deeper in the central interior of the peninsula than near the eastern coast. At present, there are few studies on the shear wave attenuation in the Korean peninsula. A one-dimensional shear wave attenuation model (Kim et al., 2000) based on the Qp and Qc indicates a Qs value of ∼200 in the upper 3 km of the crust in the southeastern part of the Korean peninsula. In December 2002, Korean Crust Research Team (KCRUST team1) carried out the first crustal scale refraction experiment in the southern Korean peninsula, KCRUST2002. A 300-km deep seismic refraction profile of the experiment traversed the peninsula from WNW to ESE (Fig. 1). Using two-dimensional ray-tracing, the arrival times of phases Pg, PmP, Pn, Sg, and SmS were modeled to produce a two-dimensional seismic velocity model (Cho et al., 2006). The results indicate that the average thickness of the crust is 31 km with thinner crust at the margins and thicker crust in the middle of the survey line. The average compressional and shear wave velocities for the crust are 5.95 and 3.45 km/s, respectively. The depth of the Moho discontinuity beneath the peninsula increases from 30 km at the WNW end to 34 km in the middle, and it decreases to 28 km at the ESE end. The compressional and shear wave velocities in the upper mantle are approximately 8.1 and 4.8 km/s, respectively. The KCRUST2002 data also contained coherent, dispersed short period (0.2 s–1.2 s) Rayleigh waves which contain information on the seismic properties of the uppermost crust. The purpose of this paper is to analyze the short period Rayleigh waves of the refraction profiles to determine the shear wave velocity and attenuation structure of the upper 1 km in different tectonic zones of the Korean peninsula and to examine if the shear wave velocity and attenuation in the uppermost crust can be related to the surface geology of the study area. 2. Geology of the study area The Korean peninsula has a complex geologic history and composition. Since this study is focused on the area where the seismic refraction line traverses, only a brief description of the broader setting is given here, based on the studies of Kim (1988), Kim et al. (2006), and Chough et al. (2000). The Korean peninsula is a part of the Amura plate and represents a link between continental blocks of

1

A consortium of Korea Universities participating in the Crustal Seismic Experiment in the Korea.

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Fig. 1. Map of the Korean peninsula and the KCRUST2002 seismic refraction profile. Two shot points, SP1 and SP2, are denoted with stars (⋆). Four major tectonic units are denoted. Five subsections of the refraction profile investigated for the surface wave analysis in this study are marked with rectangle. The names of the five subsections are: GM1-Gyeinggi massif 1, GM2-Gyeonggi massif 2, OFB1-Okcheon fold belt 1, OFB2-Okcheon fold belt 2 and YM-Yeongnam massif.

North and South China and the island arc of Japan. The KCRUST2002 refraction line traverses four major tectonic provinces in the Korean peninsula (Fig. 1): Gyeonggi massif, Okcheon fold belt, Yeongnam massif, and Gyeongsang sedimentary basin, from WNW to ESE. The Gyeonggi massif and the Yeongnam massif are separated by the Okcheon fold belt. The basement rocks, exposed in the Kyeonggi and Yeongnam massifs, mostly consist of 2.7 to 1.1 Ga high grade gneiss and schist. Okcheon fold belt comprises meta-sedimentary rocks and meta-volcanic rocks younger than 650 Ma. The Okcheon fold belt is a NE-trending fold and thrust belt. The Okcheon fold belt can be divided into two zones, the Okcheon basin to the southwest and the Taebacksan basin to the northeast, mainly based on lithology and metamorphic grade. The Okcheon basin consists of nonfossiliferous, low to medium grade meta-sedimentary and meta-volcanic rocks, and the Taebacksan basin comprises fossiliferous, non-or-weakly metamorphosed sedimentary rocks of the Paleozoic to early Mesozoic. The Gyeongsang basin is composed of Cretaceous terrestrial sediments, felsic to intermediate volcanic rocks and granites of late Cretaceous to early Tertiary age.

3. Data Three shots had been planned along the survey line, but only two shots, SP1 at the western end (36°37′ 34.03ʺN, 126°23′4.21ʺE) and SP2 (36°13′26.85ʺN, 127°47′2.32ʺE) in the middle of the profile, were detonated (Fig. 1). The shot planned at the eastern end of the profile could not be detonated because of serious concerns of the local residents near the shot point. The shot holes were drilled to the depth of 100 m and the size of explosives at SP1 and SP2 was 1000 and 500 kg, respectively. 170 vertical-component, 2-Hz portable seismometers (PRS-1) were placed at an interval of approximately 1.5 km along the survey line and the data were sampled at a rate of 120 points per second. The instrument response was removed from the seismic signals by correcting for PRS-1 transfer function (Asudeh, 1996) using SAC2000 programs (Goldstein, 1998). The seismograms low-pass filtered at 5 Hz are shown in Fig. 2 with the geologic cross-sections along the profiles. Coherent dispersive Rayleigh waves can be recognized up to 60 km from SP1 and 30 km from SP2.

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4. Analysis of data The seismic record sections which contain recognizable Rayleigh waves run through three different tectonic regions (Fig. 1). They were divided into five subsections based on tectonic boundaries as well as lithological boundaries; two in the Gyeonggi massif (Fig. 2a), two in the Okcheon fold belt and one in the Yeongnam massif (Fig. 2b). We divided the record section east of the shot point SP1 into two subsections, GM1 and GM2, where lithology changes from the Jurassic granite to the

Precambrian gneiss. We also divided the record section east of the shot point SP2 at the tectonic boundary between the Okcheon fold belt and the Yeongnam massif. The geologic cross-sections in the vicinity of the shot points SP1 and SP2 are shown in Fig. 2a and b, respectively. GM1 traverses an area composed of mainly Jurassic granite while the GM2 runs though an area where Precambrian gneiss and schist are dominant. Pebble-bearing phyllite (age unknown, but younger than 650 Ma) and Jurassic granite are the major rock types in the OFB1 area while OFB2 area is covered by entirely

Fig. 2. Seismic record sections from the shot points SP1 and SP2, low-pass filtered at 5 Hz. Location of the subsections and the corresponding geologic cross-sections along the profiles are shown at the bottom. The plotted seismic traces are normalized with respect to the maximum amplitude of each trace. Two shot points, SP1 and SP2, are denoted with a triangle (▾). (a) For SP1. The lithologic boundary dividing the GM1 and GM2 is denoted with F. (b) For SP2. The tectonic boundary between the Okcheon fold belt and the Yeongnam massif is denoted with a thick solid line. The age of pebble-bearing phyillte is unknown, but younger than 650 Ma.

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Cretaceous conglomerate. YM in the Youngnam massif traverses Precambrian Gneiss and Mesozoic granite. 4.1. Group and phase velocity determination The measurement methods of group velocity have been improved by many researchers (e.g., Dziewonski et al., 1969; Cara, 1973; Nyman and Landisman, 1977; Yamada and Yomogida, 1997; Chen et al., 2002). For the windowed Fourier method, recorded data are windowed, Fourier transformed, filtered, and reconstructed in the time domain (Dziewonski et al., 1969). The instantaneous amplitude and phase of the wave packets in the reconstructed time signal is used to measure the group velocity. A problem inherent in all Fourier transform-based analyses is that the product of frequency uncertainty and arrival time uncertainty is constant. In addition, there is a problem arising from the assumption of the signal stationarity; the Fourier transform requires continuous input data, but Rayleigh waves, as used in this study, are transient. To overcome these limitations, we used a time series analysis called “continuous wavelet transform” to obtain the group velocity. Whereas the Fourier transform decomposes a signal into infinitelength sines and cosines, effectively losing all timelocalization information, the continuous wavelet transform is a convolution of a data sequence with a scaled and translated version of the mother wavelet, Ψ function (Combs et al., 1989). Therefore, the continuous wavelet transform defines the coverage of time and frequency uniquely and measurement errors can be set in both time and frequency without any ambiguity; as contrasted with the windowed Fourier analysis, where the resolution in frequency is arbitrary depending on the choice of the width of band-pass filter (Pyrak-Nolte and Nolte, 1995).

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The definition of the continuous wavelet transform of a seismic signal (xn′) is: rffiffiffiffi   yt ⁎ ðnV−nÞyt Wn ðsÞ ¼ xn V W S 0 s n V¼0 N −1 X

ð1Þ

In Eq. (1), n is the time index of the location of the window, s is the wavelet scale, and Ψ is the mother wavelet. We have used a Gaussian derivative function as the mother wavelet. The real component of Ψ function is defined: ð−1Þmþ1 d m −g2 =2 W0 ðgÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Þ   m ðe C m þ 12 dg

ð2Þ

In Eq. (2), Γ is the gamma function and m is the derivative order. The complex wavelet is generated by the addition of a Heaviside function in the frequency domain. The Gaussian derivative wavelet produced the better-determined maximum on the group velocity dispersion plot than other frequently used wavelets, such as Morlet and Paul wavelets. The Gaussian derivative wavelet is adjustable with the derivative order; a derivative order of 6 resulted in the best-determined maximum on the group velocity dispersion plot of the Rayleigh waves for all five subsections. We were able to obtain the fundamental and the first higher mode group velocity using the continuous wavelet transform method. Modes higher than the first were difficult to isolate. The group velocity dispersion contours for GM2 subsection obtained by the wavelet analysis is shown in Fig. 3a. The dispersion curve in the period less than 0.4 s is enlarged (Fig. 3b) to show its details at the higher

Fig. 3. (a) Group velocity dispersion contour plot for GM2 subsection generated by the continuous wavelet transform method. (b) Enlarged section for higher frequencies. (c) A Gaussian derivative mother wavelet with order 6. Solid and dotted lines represent the real and imaginary parts, respectively.

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frequencies. The Gaussian mother wavelet used for the wavelet transform is also illustrated (Fig. 3c). The group velocities for GM2 and YM subsections were determined by the two-station method using the arrival time difference between two stations in each subsection. Average group velocity was used for the GM1, OFB2 and OFB1 subsections. Phase velocities were determined by the method given by McMechan and Yedlin (1981). The method consists of performing a slant stack on the data to obtain a wave field in the p − τ (phase slowness-intercept time) domain. A one-dimensional Fourier transform over τ is followed to obtain the wave field in the p − ω plane. The surface wave field in p − ω plane for subsection GM2 is shown in Fig. 4. The maximum value for a given frequency on the energy contour in p − ω plane defines the phase velocity at that frequency. From the p − ω analysis, we were able to determine the fundamental mode phase velocity for the period range between 0.3 and 1.0 s. For periods shorter than 0.3 s, the signal begins to lose coherence and the wave field function no longer has a well-defined maximum due to the increase of noise level and the effect of spatial aliasing at high frequencies. Fig. 5a and b show the shear wave group velocity dispersion and phase velocity dispersion for the five subsections, respectively. The 1st higher mode group velocity was obtained for all five subsections except for OFB2 because we could not determine the distinct maxima in the period range shorter than 0.4 s for this subsection (Fig. 5a). It is noticeable in Fig. 5 that OFB2 in the Okcheon fold belt area has both the lowest shear wave fundamental group velocity (Fig. 5a) and phase velocity (Fig. 5b) among the five subsections. Of the remaining 4 subsections, OFB1 in the Okcheon fold belt has the lowest value in both shear wave group and phase velocity. Overall, the shear wave velocities for

the two subsections in the Okcheon fold belt are lower than those for the three subsections in the massif areas. The fundamental mode group velocities range between 1.9 km/s at 0.3 s and 2.6 km/s at 1.0 s (Fig. 5a). For the first higher mode, the group velocities range between 2.5 km/s at 0.2 s and 3.1 km/s at 0.4 s. The fundamental mode phase velocity ranges between 2.2 km/s at 0.25 s to 3.0 km/s at 1 s (Fig. 5b). An example of the fundamental and the first higher mode group velocities and the phase velocities for GM2 subsections is shown in Fig. 6 with their error estimates. 4.2. Anelastic attenuation determination The continuous wavelet transform method was also used to determine the attenuation of the Rayleigh waves. Due to factors such as local site effects, lateral inhomogeneity, and scattering, the decay of surface wave amplitude with distance in the uppermost crust is difficult to measure. To obtain a meaningful estimate of amplitude decay, it is usually necessary to have accurate amplitude data at closely spaced stations over a considerable distance range. Of the five subsections used for the shear wave velocity analysis, we examined only three subsections (GM2 in the Gyeonggi massif, OFB1 in the Okcheon fold belt, and YM in the Yeongnam massif) for the attenuation structure because the distance range of the GM1 and OFB2 subsections were not long enough for a meaningful measurement of amplitude decay. All signals were analyzed for each center filter frequency in the frequency range between 1 and 5 Hz using the Gaussian derivative mother wavelet. Spectral amplitudes were then determined at each center filter frequency and multiplied by the square root of the distance to correct for geometrical spreading. The decay of the corrected amplitude spectrum with distance was used to estimate the anelastic attenuation coefficient of

Fig. 4. Phase velocity dispersion plot on τ − ω space for GM2 subsection. The maximum value for each frequency is denoted with a circle.

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period for the three subsections with error estimates. In general, γR is larger at higher frequencies and decreases monotonically in all of the three subsections. However, there are considerable differences between the attenuation values of the Okcheon fold belt and those of the two massif areas; for the period of 0.5 s, the γR for OFB1 in the Okcheon fold belt is almost twice as large as those for GM2 and YM in the massif areas. Average γR decreases from 0.04/km at 0.2-s period to 0.02/km at 0.5-s period for the two subsections in the massif areas while the value of γR for the OFB1 decreases from 0.08/km at 0.3-s period to 0.03/km at 1-s period. 4.3. Inversion for shear wave velocity and attenuation A general linear inversion (Wiggins, 1972) was used to invert the dispersion data of Rayleigh waves for the shear wave velocity structure. Because of the nonlinear nature of the problem, an iterative linear inversion was performed in which the model was updated after each iteration. The general linear inverse problem is governed by the equation: y ¼ Ax þ e ∼

ð4Þ

In Eq. (4), y is a data vector containing phase velocities, group velocities, and attenuation coefficients. x represents the model to be determined. In the inversion for shear wave velocity, the shear wave velocity is considered as an independent variable because dispersion

Fig. 5. (a) Plot of fundamental group velocity for the five subsections. The 1st higher mode group velocities for four subsections (OFB2 is missing) appear in the upper left corner in the figure. (b) Plot of phase velocities for the five subsections.

Rayleigh waves, γR, in the frequency range between 1 and 5 Hz, using a linear regression analysis: ln Aðd; xÞ ¼ ln A0 −gR ðxÞd

ð3Þ

In Eq. (3), d is the distance of a signal from the shot point and A0 is the amplitude of the signal near the shot point. Fig. 7a shows examples of the amplitude ratio as a function of distance for the three subsections. Strong amplitude variations due to site effects are noticeable. Fig. 7b shows γR of Rayleigh waves as a function of

Fig. 6. The fundamental (open circles) and first higher mode (diamonds) group velocity and the phase (small solid circles) velocity plot for GM2 with error estimates.

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Fig. 7. (a) Amplitude ratio as a function of distance for GM2, OFB1 and YM subsections. (b). Attenuation coefficients of Rayleigh waves, γR, for the three subsections with error estimates for OFB1. Observed (symbols) and predicted (lines) attenuation values calculated using the attenuation models are shown for the three subsections.

is more sensitive to shear wave velocity than to compressional wave velocity (Russell, 1987). The ratio of compressional wave velocity to shear wave velocity was kept as 1.73 throughout the inversion. The inversion problem was solved by finding the minimum of the error vector e in Eq. (4). An iterative procedure and the firstorder difference technique were used to solve the Eq. (4). We followed the procedure of Rodi et al. (1975), which utilizes the relationship between the phase velocity and group velocity, to compute the partial derivatives ∼ A in Eq. (4). Singular value decomposition of ∼ A was performed as

described by Lawson and Hanson (1974). An initial model comprising 100 m thick layers with the same model parameters at all depths was used for unbiased inversion. We started with the S-wave velocity of 3.5 km/s. To estimate shear wave attenuation Qβ models, we followed Mitchell's (1975) formulation where the spatial attenuation coefficient is written as: p gðxÞ ¼ T

n n X AcR ai −1 X AcR bi −1 Q þ Q ai 2 Aai cR Abi c2R bi i¼1 i¼1

! ð5Þ

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In Eq. (5), n is the total number of layers; cR is the phase velocity of Rayleigh waves; α and β are the compressional and shear wave velocities of the layer i, respectively; and T is the period. Following Anderson et al. (1965), the compressional wave quality factor, Qα, can be related to the shear wave quality factor Qβ by   4 a 2 Qb ¼ Qa ð6Þ 3 b Using Eqs. (5) and (6), the attenuation coefficients can be related to Qβ as: "   !# n pX 4 bi 2 ai AcR gðxÞ ¼ ð7Þ Q−1 bi þ e T i¼1 3 ai c2R Aai This equation can be written in the form of a linear (4) forthe purpose of inversion. Eq. 2 n X −1 ai AcR 4 bi gðxÞ; Tp correspond to y, and Qβi 3 ai c2 Aai i¼1

R

A and x in Eq. (4), respectively. The partial derivatives of the phase velocity are then determined using the inverted shear velocity model for the corresponding subsection. 5. Results and discussion The inverted shear velocity models are shown in Fig. 8a for the five subsections together with an example of the resolving kernels (Fig. 8b) for the GM2 subsection. Fig. 8c and d show good agreement between the data and theoretical values obtained from the models for the group and phase velocity, respectively. The error bars in the velocity models are relatively small because we selected the final shear wave velocity models with 100-m thick layers and broad resolution kernels; small errors in the models were traded off with broad resolution kernels. The shape of the resolution kernels becomes broader with depth, which means poorer resolution at increased depth. Use of the higher mode group velocity in the inversion improved the resolution and produced a stable model for depths greater than 400 m. The effect of the first higher mode group velocity on resolution is shown in Fig. 9. We interpreted the shear wave velocity structure only down to 1-km depth because the resolution below this depth was very poor; the peak of the resolution kernel for a given layer appears at the corresponding depth for that layer in the upper 1 km, but it changes little with depth below 1.0 km. In Fig. 8a, the region bound by the shear wave velocities of the two models (OFB1 and OFB2) for Okcheon fold belt area is dotted, and the

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region bound by the minimum and maximum values for each layer of the three shear wave velocity models (GM1, GM2 and YM) for the massif areas is shaded in grey. The average shear wave velocity in the upper 1 km of the five subsections increases from ∼ 2.4 km/s at the surface to ∼ 3.0 km/s at 1.0 km-depth. The velocity gradient of the three models for the two massif areas is almost constant in the upper 1 km while that of the two models for the Okcheon fold belt is steeper in the upper 0.5 km. This may reflect rapid closing of cracks and pores in the upper 0.5 km of the crust in this fold belt area. In the shallowmost 0.2 km, the shear wave velocity models for the five subsections present a large variance of ∼ 10%. Overall, shear wave velocities for the two subsections in the Okcheon fold belt are lower than those for the three subsections in the Gyeonggi and Yeongnam massifs by ∼0.4 km/s in the shallowmost 0.2 km. Below 0.2-km depth, the difference in the average shear wave velocity between the two massifs and the fold belt becomes small, approximately 0.2 km/s. Shear wave velocity differences also exist between the areas within the same tectonic region when there are distinct lithological differences; within the Okcheon fold belt, the shear wave velocity of OFB2 area is lower than that of OFB1 area by ∼0.15 km/s throughout the upper 1.0 km of the crust. While OFB2 area is composed of entirely Cretaceous conglomerate, OFB1 area comprises pebblebearing phyllite and Jurassic granite. Shear wave velocity structures for the three subsections in the massif areas are complicated. The shear wave velocities of YM and GM1 areas are lower than those of GM2 area in the upper 0.4 km and higher in the lower 0.6 km of the depth range investigated. Although there exist differences in the rock type among these three subsection areas, it is apparent that the shear wave velocity is not easily related to the rock type within these massif areas. Qβ− 1 models for the three subsections (GM2, OFB1 and YM), one for each of the three different tectonic regions, are shown in Fig. 10a with the resolution kernels for GM2 subsection. We could not obtain attenuation coefficients in GM1 and OFB2 areas because of short distance range of the subsections (Fig. 2). The resolution of the attenuation inversion in the lower 0.5 km of the model was so poor that we were able to obtain only an average value of Qβ− 1 in the upper 0.5 km for each subsection, instead of Qβ− 1 structures in the upper 1.0 km. The resolution kernels in Fig. 10b indicate that the values of Qβ− 1 models in the upper 0.5 km (5 top layers) are averaged values over these layers; the location of the peaks of resolution kernels in the upper 5 layers changes little with depth. Fig. 7b shows that the predicted attenuation values from the models match the

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Fig. 8. (a) Inverted shear wave velocity models for the five subsections with error estimates. Dotted area is for the Okcheon fold belt and shaded area is for the Gyeonggi and Yeongnam massif areas. (b) Plot of resolution kernels for the GM2. Plots of shear wave group velocity and phase velocity for the five subsections are shown in (c) and (d), respectively. Symbols and lines represent the observed and predicted values, respectively.

observed attenuation coefficients reasonably well. The value of Qβ− 1 for the Okcheon fold belt (∼0.026) is almost three times larger than that of the Gyeonggi and Yeongnam massifs (∼ 0.008). The results of attenuation

analysis – low attenuation in the massif areas and high attenuation in the fold belt – are consistent with high shear wave velocity in the massif areas and low shear wave velocity in the fold belt.

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Fig. 9. Effects of the higher mode group velocity on the shear wave velocity inversion. (a) Fundamental and the first higher mode group velocity for OFB1 subsection. Open circles and solid lines are for the data and the predicted values based on the shear wave velocity model, respectively. (b) Comparison of the two models; one with fundamental mode only and the other with both fundamental and the first higher mode group velocity. (c) Resolution kernels obtained using fundamental mode group velocity only. (d) Resolution kernels obtained using both fundamental and the first higher mode group velocity. The resolution below 0.4-km depth is much improved when both fundamental and the first higher mode group velocity are used in the inversion.

As mentioned in the ‘Analysis of data’ section, the major rock types in the Okcheon fold belt are metasedimentary rocks and meta-volcanic rocks younger than 650 Ma, while 2.7 to 1.1 Ga high grade Precambrian gneiss and schist are prevalent in the two massif areas. In detail (Fig. 2), the major rock types of the OFB1 and OFB2 are pebble-bearing phyllite and Cretaceous conglomerate, respectively; the major rock

types of the GM1, GM2, and YM are Jurassic granite, Precambrian gneiss and schist, and Precambrian gneiss and Jurassic granite, respectively. Shear wave velocities in the upper 1 km of the crust can be affected by many factors such as rock type, degree of weathering, porosity, the shape and orientation of cracks, etc. However, when we consider, the differences in shear wave velocity and attenuation between the massif areas and the fold

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Fig. 10. (a) Q−β 1 models for the three subsections with error estimates. (b) Resolution kernels for GM2 subsection.

belt, together with the major rock types of the corresponding areas, the differences are most likely attributed to the differences in the major rock types of the areas — low shear wave velocity and high attenuation in the fold belt consisted of sedimentary and volcanic rocks and high shear wave velocity and low attenuation in the massifs comprising Precambrian gneiss and schist. Unfortunately, at present, no laboratory measurements of shear wave velocity or Qβ− 1 for the study area are available. 6. Conclusions 1) Shear wave velocity models in the upper 1 km and an average value of Qβ− 1 in the uppermost 0.5 km of the crust were obtained in three different tectonic zones across the southern Korean peninsula using the short period 0.2–1.2 s Rayleigh waves in KCRUST2002 data. The continuous wavelet transform method enabled us to acquire the first higher mode group velocity of the Rayleigh waves. The use of the higher mode group velocity data in the velocity inversion resulted in better resolution and a stable model for depths greater than 0.4 km. 2) The average shear wave velocity of the five subsections in three different tectonic regions (Okcheon fold belt and Gyeonggi and Yeongnam massifs)

increases from ∼ 2.4 km/s at the surface to ∼ 3.0 km/s at 1.0-km depth. The average shear wave velocity of the Okcheon fold belt is lower than that of the Gyeonggi and Yeongnam massifs by ∼0.4 km/s in the shallowmost 0.2 km. Below this depth, the difference in the average shear wave velocity between the massif and fold belt areas becomes small, approximately 0.2 km/s. Shear wave velocity differences also exist between the areas within the same tectonic region when there are distinct lithological differences; Shear wave velocity models for OFB2 area in the Okcheon fold belt, which composed of entirely Cretaceous conglomerate, shows lower shear wave velocities than shear wave velocity models for OFB1 in the same tectonic region. 3) Due to the poor resolution below 0.5 km, only the average values of Qβ− 1 in the upper 0.5 km of the crust for three different tectonic regions were obtained using the decay of the amplitude of Rayleigh waves. The value of Qβ− 1 (∼0.026) for the Okcheon fold belt is almost three times larger than that (∼ 0.008) of the Gyeonggi and Yeongnam massifs. The high attenuation of the Okcheon fold belt is consistent with the low velocity in this region. 4) Shear wave velocities in the upper 1 km of the crust can be affected by many factors such as rock type,

H. Jung et al. / Tectonophysics 429 (2007) 253–265

degree of weathering, porosity, the shape and orientation of cracks, etc. However, the differences in shear wave velocity and attenuation between the massif areas and the fold belt are most likely caused by differences in the major rock type of the areas. 5) In this study, we have shown that short period Rayleigh waves from seismic refraction profiles can be used to examine the shear wave velocity and attenuation structure in the uppermost crust and that the results can be related to the surface geology of the study area. The KCRUST2002 is the first refraction profile of a long-term project of which main purpose is to reveal the 3-D crustal structures of the Korean peninsula and the surrounding areas. With more refraction and reflection profiles in various directions, the KCRUST Team should be able to analyze the relationships between the shear wave velocity and attenuation structure in the uppermost crust and the surface geology in different tectonic regions in the Korean peninsula. Acknowledgements This work was funded by the Korea Meteorological Administration Research and Development Program under Grant CATER 2006-5204. We thank more than 200 students from 8 National Universities in Korea, who volunteered to join the KCRUST2002 experiment. The manuscript was much improved by the constructive comments from two anonymous reviewers, Brian T.L. Lewis (University of Washington, USA) and Eric Kunze (Victoria University, Canada) to whom we would like to express our great thanks. References Anderson, D.L., Ben-Menahem, A., Archambeau, C.B., 1965. Attenuation of seismic waves in the upper mantle. Journal of Geophysical Research 70, 1441–1448. Astrom, K., 1998. Seismic signature of the Lake Mien impact structure. Geophysical Journal International 135, 215–231. Asudeh, I., 1996. The PRS-1 Response, in Lithoseis, Version 4.10. Geological Survey of Canada. Cara, M., 1973. Filtering dispersed wave trains. Geophysical Journal of the Royal Astronomical Society 33, 65–80. Chen, C., Li, C., Teng, T., 2002. Surface wave dispersion measurement using Hilbert–Huang transform. TAO 13, 171–184. Cho, H., Park, C., Lee, J., Moon, W., Jung, H., Kim, K.Y., 2006. Crustal velocity structure across the southern Korean Peninsula from seismic refraction survey. Geophysical Research Letters 33, L06307, doi:10.1029/2005GL025145. Chough, S.K., Kwon, S.-T., Ree, J.-H., Choi, D.K., 2000. Tectonic and sedimentary evolution of the Korean peninsula: a review and new view. Earth-Science Reviews 52, 175–235.

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