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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.37 No.5 pp.286-294
DOI : https://doi.org/10.5467/JKESS.2016.37.5.286

# Rayleigh Wave Group Velocities with an Enhanced Resolution in the Northern Korean Peninsula

Heeok Jung*, Yong-Seok Jang
Department of Ocean Engineering, Kunsan National University, 1170 Daehangno Kunsan, Korea 54150
Corresponding author: hjjung2616@gmail.com Tel: +82 63 469 1750 Fax: +82 63 469 1750
July 31, 2016 September 21, 2016 September 26, 2016

## Abstract

Using a method suggested by Yanovskaya, we obtained Rayleigh wave group velocities with a resolution of 1.0o × 1.0o in a period range between 10 and 80 s in and around the Korean peninsula. Both regional and distant earthquake data sets were used together in analysis of group velocities. The resolution of the group velocity maps has been remarkably enhanced by the method, especially in the sparse/non-station region in the northern Korean peninsula. Some qualitative geophysical information was inferred from the group velocity maps. In the East Sea, the slow group velocities at periods longer than 40 s suggest the existence of an oceanic lithosphere at depths of 50-70 km, assuming 4 km/s of S wave velocity at a period of 40 s. On the other hand, a thick lithosphere can be inferred in the continental area from the fast group velocities at periods longer than 50 s. For most periods, the group velocities change rapidly over a short distance of about 200 km across the eastern coast of Korean peninsula, which may suggest a rapid change in the thickness of lithosphere in this area.

## Introduction

The North East Asia including Korean peninsula have a complex tectonic history and experienced severe crustal deformation. Seismic velocities of the crust and upper-mantle can be used as one tool to understand the tectonic evolution and to clarify and predict the mechanism of intra-plate seismicity and volcanism. Many P and S tomography studies have been performed in the NE Asia including the Korean peninsula, China, and Japan. Sun and Toksöz (2006), Chang and Van der Hilst (2010), and Huang and Zhao (2006) were able to resolve the high-velocity stagnant Pacific plate and low-velocity area in the big mantle wedge using body wave tomography. Shear-wave tomography maps were constructed by Priestly et al. (2006), Yanovskaya and Kozhevnikov (2003), Huang et al. (2009), and Yoshizawa et al. (2010). Tomographic methods using ambient seismic noise also have been used to produce 3D shear-wave velocity maps in this area (Zheng et al., 2008; Sun et al., 2010; Zheng et al., 2011). However, the resolutions of these studies are low (>2° ). The resolution for Korean peninsula is even worse than 2° because it is a fringe area in these studies. The low resolution in and around the Korean peninsula also comes from low seismicity and sparse seismic stations. Installation of modern seismometers began in the mid-1990s, and the number of seismic stations has gradually increased since then. Presently in South Korea, Korea Meteorological Administration (KMA) and Korean Institute of Geology and Mineral Resources (KIGAM), operate ~30 broadband seismometers, ~50 short-period seismometers, and ~150 accelerometers.

On the other hand, very limited information on both seismic stations and seismic data recorded in North Korea is available to scientists outside of North Korea. The lack of seismic information on North Korea renders the area very sparse in ray path coverage for seismic tomography in and around the Korean peninsula. As a result, the seismic structure beneath the northern Korean peninsula has not been revealed in detail yet.

Meanwhile, various efforts have been made to obtain the crustal seismic-velocity and attenuation of the peninsula with high resolution using a number of different methods. They include cross-correlation technique (Kang and Shin, 2006; Cho et al., 2007; Choi et al., 2009), joint inversion of surface-wave dispersions and receiver functions (Chang et al., 2004; Chang and Baag, 2005; Yoo et al., 2007; Park et al., 2009). These studies have improved our understanding of the tectonic development of the Korean peninsula, opening of the East Sea, and earthquake mechanisms in and around the Korean peninsula (Hong and Kang, 2009; Hong, 2010).

The purpose of this study is to suggest a method to fill the surface wave group velocity gap in the northern Korean peninsula due to the absence of earthquake data and to test the plausibility of this method. Rayleigh wave group velocities are generally used to obtain S wave velocity structures. In this study, however, we will limit ourselves to the acquisition of group velocity maps with a resolution of 1.0° ×1.0° in the study area including the northern Korean peninsula where almost no seismic information has been available. Some qualitative interpretations of the resulting Rayleigh wave group velocity maps will be given briefly.

### Geology and Tectonic Setting

While the Korean peninsula is characterized by low seismicity at shallow depths, there is a high possibility of volcanism and seismicity in the surrounding regions (Fig. 1). Brief geologic and tectonic characteristics related to volcanic activity, opening of East Sea, and the velocity structures of the Korean peninsula are presented here.

Mt. Baekdu (Changbai in Chinese), located on the border of China and North Korea, is considered the most dangerous and the most likely to erupt in the NE Asia (Fang et al., 2010; Liu et al., 2011) and has a 5 km-wide summit caldera, Cheonji (Tianji in Chinese). Magmatism beneath Mt. Baekdu has been ascribed to the hot upwelling in the big mantle wedge (BMW) above the stagnant Pacific plate in the mantle transition zone and deep slab dehydration as well (Lei and Zhao, 2005; Zhao et al., 2009). Intense volcanic eruption took place around A.D. 1000 (Nakamura et al., 1989; Yoon and Chough, 1995) depositing tephra as far away as northern Japan and the summit caldera was formed. Four historical eruptions have been recorded since the 15th century (1413, 1597, 1668, and 1702). The basalt fields in this area are scattered along the failed rift system in the NE-SW direction (Tasumi and Kimura, 1991). Recently, it has been reported that Mt. Baekdu is experiencing an unrest episode (Liu et al., 2011; Hetland et al., 2004; Zhao et al., 2003).

The opening of the East Sea started in the Oligocene and was completed in the mid-Miocene. It developed a crustal structure between the continental and the oceanic crust. Also, three basins (Japan, Yamato, and Ulleung basins in Fig. 1) were formed during this opening period. The Ulleung basin shows crustal velocity structure typical of oceanic with a thickness of ~10 km. Rapid transition from the continental to the oceanic across the continental margin take place over a distance of ~100 km (Kim et al., 2003). The crustal structure of the Japan basin is that of a typical oceanic basin. Hirata et al., (1992) reported a crustal thickness of 8.5km. Sediment thickness varies considerably from basins (5-11 km) to plateaus (0-300 m).

The lithospheric structure of the Korean peninsula is composed of three Precambrian massif blocks (i.e., Nangnim, Gyeonggi, and Yeongnam). The southern Korean peninsula shows a typical continental crust with thickness of ~32 km (Chang and Baag, 2006, 2007; Cho et al., 2006; Hong et al., 2008; Cho et al., 2012). Crustal structures of the northern Korean peninsula have not been investigated in detail yet as mentioned earlier in the introduction section of this study.

## Data and Method

### Data

In order to improve the resolution of the group velocity map, the number of earthquake paths within the study area must be increased and the study area should be covered evenly with ray paths. We adopted a tomographic inversion method by Yanovskaya (2009) to increase the number of earthquake paths in the northern Korean peninsula. The method can be summarized as follows. The traditional standard method of surface wave tomography uses earthquakes of which paths wholly located inside the region being investigated and oriented in different directions. Earthquake sources also should be situated within the limits of this region. If the region is small, then this requirement restricts the range of periods of surface waves and the number of great-circle paths suitable for the tomography. The use of data from distant earthquakes enlarges the range of periods and the number of great-circle paths. However, it does not enable one to use the usual tomography method because of an inappropriate configuration of the system of paths. Outside of the region in which the stations are located, the paths do not intersect. However, the paths from a particular earthquake source toward the different stations of networks are very close over a large distance. Therefore, the mean correction to the velocity in the path sections (lk) outside of the network of stations can be accepted as identical (Fig. 2). This assumption forms the basis of the surface wave tomography proposed by Yanovskaya (2009), in which group velocities measured at stations of the local network from distant earthquakes are used. The lateral variations in the velocity are determined within the limits of the network, and the mean corrections to the velocity are determined on the paths from different earthquake sources in the sections outside of the network. In the method, the relative correction to the slowness for the kth earthquake is designated as μk=δck-1 /c0-1 , where c0 is the average velocity for the entire territory. On the area occupied by the group of stations, the relative variations of slowness as a function of coordinates (x, y) is searched: m(x, y)= δc-1 (x, y)/c0-1 . Then, the time discrepancy, the correction on the ith path from the kth earthquake source, relative to the model with the constant velocity c0 is determined as follows:

$δ t k i = L k c 0 μ k + ∫ l k i m ( x , y ) d s c 0 ,$

where lki is the length of the part of the ith path obtained by subtraction of the length Lk, common for all paths, from the overall path.

Figure 3 shows the effect of Yanovskaya’s method applied in this study very clearly. Figure 3(a), 3(b) represent the ray paths and the checkerboard test obtained by the traditional group velocity analysis. Ray path coverage is very poor, especially in the northern Korean peninsula and Manchuria. Consequently, the checkerboard test in this area is also very poor in Fig. 3(b). We used regional earthquakes from January 2001 to February 2012 on and around the Korean peninsula in Fig. 3(a), 3(b). Recording stations consist of KMA (Korea Meteorological Administration), Japanese F-net, and global seismographic network of IRIS. The total number of stations was 163 (61 KMA, 81 F-net, and 21 IRIS stations). The number of ray paths was 4810 and they are shown in grey lines.

Using the method by Yanovskaya (2009), 827 ray paths were added to 4810 ray paths in Figure 3(a). They are shown in black lines in Fig. 3(c). The white area, still not covered with ray paths, has been remarkably reduced. As a result, the checkerboard test in Fig. 3(d) shows a noticeable improvement in resolution, except for a northwest corner of the study area. We used distant earthquakes with epicentral distances greater than 20° as well as regional earthquakes in Fig. 3(c), 3(d). Even though a small number of ray paths was added to the traditional analysis, we obtained a remarkable improvement in resolution. Especially, the resolution improvement in the northern Korean peninsula, where almost no seismic information was available, is noticeable.

Information on the earthquakes and the great-circle paths used in the study is summarized in Tables 1 and 2.

### Determination of Group Velocity

The Rayleigh-wave group velocities of the selected paths were estimated by an analysis of the continuous wavelet transform method (Pyrak-Nolte and Nolte, 1995; Yamada and Yomogida, 1997). The advantages of the continuous wavelet analysis have been documented in many studies of surface-waves (e.g., Yamada and Yomogida, 1997; Jung et al., 2007; Jung et al., 2011). We applied the method to our Rayleigh waves and obtained smooth and concentrated Rayleigh-wave group velocity energy distributions. The definition of the continuous wavelet transform of a seismic signal ($x n ′$) is

$W n ( s ) = ∑ n ′ N − 1 x n ′ δ t s Ψ 0 ∗ [ ( n ′ − n ) δ t s ] ,$

where n is the time index of the location of the window, s the wavelet scale, and Ψ the mother wavelet. Gaussian derivatives were used as the mother wavelet. The real component of Ψ function is

$Ψ 0 ( η ) = ( − 1 ) m + 1 Γ ( m + 1 2 ) d η m exp ( − η 2 / 2 ) ,$

where Γ is the gamma function and m is the derivative order. The complex wavelet is generated by addition of a Heaviside function in the period domain. By using several trials, the optimum derivative order m was determined to be 6, and this value was applied to estimate Rayleigh-wave dispersion curves.

Quality inspection of group velocity was performed based on the degree of energy concentration in the period-energy plot shown in Fig. 4(a). When energy concentration patterns of some data were too weak and broad to determine a dispersion curve, they were eliminated from our data set. After inspection of the degree of energy concentration, the ray paths were divided into 3 groups: Group I which traveled mostly in the oceanic area; Group II in the continental area; Group III both in the oceanic and the continental areas. After we obtained the average dispersion curve for each group, dispersion curves showing a deviation larger than ~0.3 km/s from the average velocities of its corresponding group were eliminated from the data set. Some examples of dispersion curves for the three groups are shown in Fig. 4b.

## Results and Discussion

We have obtained the group velocities with a resolution of 1.0° ×1.0° for a period range from 10 s to 80 s using the method by Yanovskaya (2009) in the study area. Group velocity maps at periods of 12, 20, 25, 30, 40, 50, 60, 80 s are presented in Fig. 5.

We will discuss the horizontal group velocity distribution in two areas - the East Sea area and the continental area including the Korean peninsula and Manchuria area. At shorter periods from 20 s to 40 s (Fig. 5(b), 5(c), 5(d)), group velocities are faster in the East Sea area than in the continental area. At 40 s (Fig. 5(d)), slow velocities begin to appear in the Japan basin and occupy the entire East Sea at 80 s. If this slow velocities imply the existence of the oceanic asthenosphere in this area, we can suggest approximate depths corresponding to the top of the oceanic asthenosphere by assuming that most energy of Rayleigh waves propagates in the depths corresponding to 1/3~1/2 of their wave lengths. Using 4 km/s of S wave velocity at a period of 40 s, we can infer that the oceanic asthenosphere appear at depths of 50~70 km.

In the continental area, the group velocities are slower than those of the oceanic area for a period range from 20 to 40 s. However, the area with slow velocities becomes smaller for longer periods and restricted in the high mountain area around Mt. Baekdu and Changbai Mt. range at a period of 40 s. Slow velocities also appear in patches along the mountain range in the eastern coast of the Korean peninsula. For periods longer than 50 s group velocities are faster in the continental area than those in the East Sea area. This suggests a thicker lithosphere in the continental area than in the East Sea area. At a period of 80 s the area with fast velocities mirrors the area with slow velocities at a period of 40. Although this may suggest an equilibrium state of lithosphere in this area, more information on gravity and other seismic data including phase velocities are required to confirm the state of lithospheric equilibrium.

Minimum and maximum group velocities at 8 different periods are shown in Table 3 with the differences between the two velocities. At a period of 12 s the difference between the maximum and minimum group velocities is ~300 m/s. It becomes greater and reaches at ~600 m/s at 20 and 25 s. Then, it decreases to 400 and 180 m/s at 40 and 50 s, respectively. The differences between the two group velocities decrease to ~80 m/s at periods greater than 50 s.

At 60 and 80 s, group velocities change from 3.88 to 3.80 km/s (~2% pertubation) over a very short distance of ~200 km across the eastern coast of Korean peninsula. The rapid group velocity change over a short distance seems to reflect a rapid change in lithospheric thickness across the eastern coast of the Korean peninsula. This result is in accordance with the investigation of Yoshizima et al. (2010).

## Conclusions

Using the method suggested by Yanovskaya we obtained the Rayleigh wave group velocities with a resolution of 1.0° ×1.0° in a period range between 10 and 80 s in the Korean peninsula and surrounding regions. Distant as well as regional earthquake data sets were used together in the analysis of group velocities. The resolutions of the group velocity maps have been remarkably enhanced by the method in the northern Korean peninsula. The characteristics of the group velocity distribution in the study area can be summarized as follows.

• 1. The differences in the minimum and the maximum group velocities decrease from ~600 m/s at periods of 20-25 s to ~100 m/s at periods longer than 50 s.

• 2. In the period range shorter than 40 s, group velocities are faster in the East Sea area than in the continental area and slower for the periods longer than 40 s. The existence of oceanic lithosphere at depths of 50-70 km can be inferred assuming 4 km/s of S wave velocity at a period of 40 s.

• 3. In the continental area, the group velocities are slower than those of the oceanic area for the period range from 20 to 40 s. However, the area with slow velocities becomes smaller for longer periods and restricted in the high mountain area around Mt. Baekdu and Changbai Mt. range at a period of 40 s. For the periods longer than 50 s group velocities are faster in the continental area than those in East Sea area. This suggests a thicker lithosphere in the continental area than in the East Sea area.

• 4. Rayleigh wave group velocity transition is observed at periods of 12 and 40 s in and around the Korea plateaus. At periods of 60 to 80 s, group velocities show a rapid change over a short distance of ~200 km across the eastern coast of the Korean peninsula, which may reflect the rapid change in the lithospheric thickness in this area.

## Figure

Location map of the Korean peninsula and surrounding regions. The boundary of the study areais denoted by black-solid line. Seismicity is presented by locations of epicenters with the magnitudes andfocal depths. Earthquake information comes from the catalogs of IRIS and KIGAM (Korea Institute ofGeoscience and Mineral Resources). Convergent plate boundaries are blackdashed lines and the globalsubduction model of USGS (Slab 1.0) is illustrated by contour lines. Geological features are denoted byacronyms; JB for Japan basin, KPlt. for Korea plateau, HB for Hupo basin, UB for Ulleung basin, OBfor Oki bank, YB for Yamato basin, YR for Yamato rise, KS for Korea strait, KP for Korean peninsula, BMtn. for Mt. Baekdu, GM for Gyeonggi massif, YM for Youngnam Massif, and NM for Nangnim massif.

Diagram of wave propagation from the epicenter toward stations. Definition of Lk and lki are givenin the text.

Checkerboard test maps showing the improvement of spatial resolution by adding great-circlepaths from teleseismic data. Locations of epicenters and seismic stations are denoted by red-circles andblue-triangles, respectively. The great-circle paths and the resolution test using the regional seismic dataare presented in (a) and (b) respectively, and those obtained by the addition of the teleseismic data in (c) and (d), respectively. Black lines are the great-circle paths from the teleseismic data.

(a) An example of estimation of the Rayleigh-wave group velocity energy contour and dispersiondata. (b) Three different types of Rayleigh-wave group velocity grouped by the propagation path.Solid lines are for the continental type, dashed lines for the oceanic type, and dotted lines for mixed typeof the continental and the oceanic.

(a)-(h) Group velocities at 12, 20, 25, 30, 40, 50, 60, and 80 s. Periods and average group velocities are shown at the left-top and left-bottom of each figure.

## Table

Distribution of the Earthquake Magnitude

Distribution of the Epicentral Distances of Seismic Observations

Minimum and Maximum group velocities (km/s) and differences between the two velocities (m/s) at 8 periods from 12 s to 80 s

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