pISSN 1225-7281 자원환경지질, 제52권, 제6호, 555-560, 2019 eISSN 2288-7962 Econ. Environ. Geol., 52(6), 555-560, 2019 http://dx.doi.org/10.9719/EEG.2019.52.6.555

1-D Shear Wave Velocity Structure of Northwestern Part of Korean Peninsula

Tae Sung Kim* Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Republic of Korea

한반도 북서부의 1차원 전단파 속도구조

김태성* 한국지질자원연구원 지진연구센터, 대전광역시 유성구 과학로 124 (Received: 18 December 2019 / Accepted: 23 December 2019)

중국 지역 4개 지진에서 발생한 2~30초 범위의 레일리파를 이용하여 북한 지역의 1차원 전단파 속도구조를 구하였다. 레일리파는 남북한의 국경선 인근에 위치한 5개의 광대역 관측소(BRD, SNU, CHNB, YKB, KSA)에 양호하게 기록되 었다. 다중필터분석를 이용하여 레일리파 기본모드 군속도가 계산되었고 이는 다시 위상부합필터를 적용하여 개선되었 다. 2.9~3.2 km/s의 범위에 이르는 평균 분산곡선을 역산하여 전단파 속도를 구하였다. 지진원으로부터 BRD 관측소의 경로에서 추출된 4~6초 주기의 상대적으로 느린 군속도는 서해 서한만분지의 퇴적층군과 연관되었을 가능성이 있다. 14~20km의 상대적으로 깊은 지역 전단파 저속도층은 평남분지와 관련된 것으로 생각되며 낭림육괴와 평남분지에 분포 하는 변성암 및 화강암체는 표면부터 14km 깊이의 빠른 전단파 속도와 일치한다. 주요어 : 전단파 속도구조, 레일리파 군속도 분산곡선, 서한만분지, 평남분지, 낭림육괴

One-dimensional shear wave velocity structure of is constrained using short (2-sec) to long period (30- sec) Rayleigh waves generated from four seismic events in . Rayleigh waves are well recorded at the five broadband seismic stations (BRD, SNU, CHNB, YKB, KSA) which are located near to the border between North and South Korea. Group velocities of fundamental-mode Rayleigh waves are estimated with the Multiple Filter Analysis and refined by using the Phase Matched Filter. Average group velocity dispersion curve ranging from 2.9 to 3.2 km/s, is inverted to con- strain the shear wave velocity structures. Relatively low group velocity dispersion curves along the path between the events to BRD at period from 4 to 6 seconds may correspond to the sedimentary sequence of the West Korea Bay Basin (WKBB) in the . The low velocity zone in deep layers (14-20 km) may be related to the deep sedimentary structure in Pyongnam basin. The fast shear wave velocity structure from the surface to the depth of 14 km is consistent with the existence of metamorphic rocks and igneous bodies in Nangrim massif and Pyongnam basin. Key words : shear wave velocity structure, Rayleigh wave group velocity dispersion curve, West Korea Bay Basin, Pyongnam basin, Nangrim massif

1. Introduction regional seismic velocity model to understand the crustal structure of North Korea and to decode the It is necessary to have a well-constrained source parameters of the natural and man-made

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*Corresponding author: [email protected]

555 556 Tae Sung Kim events occurring in North Korea. Particularly the shear wave velocity structure in the northwestern attention on the analysis using regional seismic part of Korean Peninsula. signals has been intensively increased since North Korea’s first nuclear test on October 9, 2006. 2. Data and Method However, the essential part, the velocity structure of North Korea is not well constructed until now. The Haicheng area in China, which belongs to The estimates on the crustal structure of southern southern Province, is a seismically active part of Korean Peninsula have been carried out by area because of natural earthquakes from active utilization of travel times (Kim and Kim, 1983; faults (Deng et al. 2003) and man-made events Kim and Jung, 1985; Kim, 1995; Song and Lee from large mining activities. Historically there was 2001; Cho et al., 2006), joint inversion using shear a M 7.3 event in 1975, which was caused by the wave phase velocity dispersion and receiver left-lateral slip of a northwest-trending fault (Wang function (Chang et al., 2004; Chang and Baag, et al., 2006). 2005), joint inversion using surface wave dispersion Four earthquake greater than ML 4.0 have curves from ambient noise and receiver function occurred in the Haicheng area of China since for constraining a 3-D shear wave velocity structure November 13, 2008 (Table 1). Seismic Signals of southern part of Korean Peninsula (Yoo et al., from these earthquakes were recorded at the 2007) and shear wave group velocity variation seismic stations in South Korea. Between these derived from noise correlation function (Kang and stations, five broadband seismic stations (BRD, Shin, 2006). The crustal studies on the northern SNU, CHNB, YKB, KSA) in South Korea were part of Korean Peninsula, however, were limited in chosen for the analysis (Fig. 1). The stations are number of studies. For hydrocarbon and petroleum located near to the border between North and resources exploration, several studies reported basin South Korea. The stations are equipped with STS- structures of Northern Yellow Sea (Massoud et al., 2 seismometers except for YKB in which a 1991; Wu et al., 2008). These studies didn’t include CMG3TB is installed. The data stream of 20 Hz the mainland of North Korea and shear wave sampling rate is selected for the analysis. velocity model. A simplified two-layer P wave Clear surface waves are observed as well as body velocity model of North Korea was reported (Paek wave phases Pn, Pg, Lg in the raw data (Fig. 2). et al., 1996). The report, however, didn’t describe Two data streams recorded at BRD from the two shear wave velocity structure of North Korea. 2012 events are not included for the analysis due In this paper, one-dimensional shear wave velocity to the poor data quality (Fig. 2b). The period of structure of northwestern part of Korean Peninsula surface wave ranges from short (~2 seconds) to is presented based on surface wave dispersion long period (~30 seconds). In this study, we focus analysis. The seismic signals generated from four on short to long period Rayleigh waves to constrain earthquakes in China mainly propagated through the shear wave velocity structure in study area. the crust and upper mantle of North Korea and the Fundamental-mode group velocity dispersion curves Yellow Sea. The surface waves from the events of Rayleigh wave are extracted by using the gave an opportunity to constrain one-dimensional Multiple Filter Analysis (MFA; Dziewonski et al.,

Table 1. Event Information

Event Time (UTC) Latitude (°N) Longitude (°E) Magnitude (ML) Region Reporting Organization Nov 13, 2008 A 40.6194 122.9709 4.1 Haicheng area, China KIGAM 22:53:26 Feb 01, 2012 B 40.4347 122.5278 4.2 Haicheng area, China KIGAM 21:16:56 Feb 01, 2012 C 40.4422 122.5018 4.2 Haicheng area, China KIGAM 21:43:31 Nov 22, 2015 D 40.8255 122.3697 4.2 Haicheng area, China KIGAM 19:57:52 1-D Shear Wave Velocity Structure of NW Korean Peninsula 557

1969). MFA is a moving-window technique applied in the frequency domain. Signal’s amplitudes in the frequency (period) and group velocity domain are estimated through the MFA. After applying MFA to Rayleigh waves, Phase Matched Filter (PMF; Herrin and Goforth, 1977) is adopted to remove multi-pathing or higher mode surface waves that may bias group velocity estimation. Fig. 3 illustrates an example of fundamental- mode Rayleigh dispersion curve and waveforms after applying the MFA and PMF to the vertical component seismograms recorded at CHNB. All the group velocity dispersion curves in study area are used to calculate average dispersion curve with standard deviation at each period (Fig. 4a). The average group velocity dispersion curves range from 2.9 to 3.2 km/s. Fig. 1. Map of study area is illustrated. The red circles Rayleigh dispersion curve is primarily affected represent four earthquakes occurred in Haicheng area while by the P-wave velocity, shear-wave velocity, layer five blue triangles represent broad band seismic stations in thickness, density and quality factor Q. The shear- South Korea. A yellow triangle is MUS station at which the wave velocity, however, has the strong influence on et al velocity profile of the initial model 2 was estimated (Yoo ., the Rayleigh wave dispersion curve (Mooney and 2007). Tectonic boundaries (white) within Korean Peninsula et al (Lee, 1988) and part of West Korea Bay Basin boundaries Bolt, 1966; Busher and Smith, 1971; Bache ., (WKBB, red) in the Yellow Sea (Wu et al., 2008) are 1978) and will be the main parameter determined shown. The numbered regions are the names of the main from the inversion procedure with fixed layer geological blocks; 1. Tumangang Basin, 2. Paektusan Volcanic thickness. The inversion method is an iterative, Plateau, 3. Kwanmobong Uplift, 4. Kilju-Myoungchoen Basin, 5. Hyesan-Iwon Basin, 6. Nangrim Massif, 7. Pyongnam Basin, stochastic least squares method for estimating 8. Anju Basin, 9. Imjingang Foldbelt, 10. Kyonggi Massif, shear wave velocity at each layer. The inversion 11. East Sag of WKBB, 12. West Sag, 13. West Swell, 14. method for this study used the inversion scheme North Sag, 15. North Swell, 16. South Sag, 17. South Swell.

Fig. 2. Seismic signals from the four events recorded at five stations are displayed with total length of 300 seconds. The event information is summarized in Table 1. (a) Top trace is the vertical component seismic signal recorded at CHNB from the event A. Second trace is the signal from the event B. Third trace is the signal from the event C. Bottom trace is the signal from the event D. (b) Seismograms at BRD. The same relationship as demonstrated in Fig. 2a. The second and third traces show poor data quality. These data are not included in the analysis. (c) Seismograms at SNU. The same relationship as demonstrated in Fig. 2a. (d) Seismograms at KSA. The same relationship as demonstrated in Fig. 2a. (e) Seismograms at YKB. The same relationship as demonstrated in Fig. 2a. 558 Tae Sung Kim

Fig. 3. Examples of multiple filter analysis (MFA) and phase matched filter (PMF) at CHNB are illustrated. (a) A dispersion curve of the fundamental-mode Rayleigh wave at CHNB from the event A is extracted through MFA. (b) Fundamental-mode Rayleigh waves after applying PMF for the four events recorded at CHNB are shown. The signals arranged in the order of ascending from event A to D are shown.

Fig. 4. Summary of Rayleigh wave dispersion curves, initial models and inverted model. (a) Dispersion curves along the path of inland (gray), yellow sea (green) and the average dispersion curve (red) are compared to each other. The blue bars represent standard deviations. (b) Two initial shear wave velocity models (broken lines) are compared with the inverted shear wave velocity model (red). The initial model 1 was from Paek et al., 1996 while the initial model 2 was from Yoo et al., 2007. written by Herrmann (2006). surface down to 15 km while the second layer is Before starting inversion process, a careful the basaltic crust from 15 km to the crust-mantle consideration for choosing initial model was taken boundary at 32 km. The P wave velocities for the into account because a proper initial model will first and second layer are 5.96 km/s and 6.7 km/s, lead to the optimum velocity model which is near respectively. The shear wave velocity is assumed to the real velocity structure in study area. First, a using Poisson’s ratio of 0.25 (Fig. 4b). Second, a two-layer shear wave velocity model is considered five-layer shear wave velocity model from a joint as a candidate for the initial model. This model inversion study (Yoo et al., 2007) is considered as was attained based on the P wave velocity model the other candidate for the initial model. According of northeastern part of Korean Peninsula (Paek et to Yoo et al. (2007), profiles of the 3D shear wave al., 1996). The first layer is the granitic crust from velocity structure in the southern part of Korean 1-D Shear Wave Velocity Structure of NW Korean Peninsula 559

Peninsula is calculated at 81 stations by combining tectonic unit developed in the Nangrim massif, and the benefits of surface wave dispersion curves and mainly consists of Late Mesozoic and Early Cenozoic receiver function. This model is robust compared sequences with Quaternary unfolded sedimentary to the first candidate of initial model in minimizing top layers. The WKBB between Haiyang Island tradeoff between shear wave velocity and crustal Rise and Liugong Island Rise shows the series of thickness with joint inversion method. Therefore, graben, host, fault structure, and extends to the we choose second candidate’s shear wave velocity Anju onshore of the North Korea. Based on the profile at MUS station as the initial model. MUS interpreted seismic sequences, the thickness of station is near to our study area (Fig. 1). strata is reported as 5.3 to 6.6 km (Wu et al., 2008; Massoud et al., 1991). Deep sags exist along active 3. Results and Discussion faults and sediments deposit during rifting since Late Cretaceous. Inversion process is performed iteratively until The group velocity dispersion curve for the path the fit between observed and calculated dispersion to BRD from 4 to 6 seconds may be closely related curve reaches more than 99.5 %. Fig. 4a shows to the sedimentary basin structure of WKBB even that observed average group velocity dispersion though an inverted velocity model is not calculated curve and calculated fit for the path of northwestern (Fig. 1, Fig. 4). The LVZ calculated in the inverted part of Korean Peninsula. The shear wave velocity 1-D model, estimated for the path from the four in the inverted one-dimensional model increases events to five stations, is located from 14 to 20 km. generally from 3.23 km/s to 3.93 km/s with depth This LVZ may be correlated with the Pyongnam increase from the surface to the crust-mantle Paleozoic basin composing of marine sedimentary boundary at 32 km except a low velocity zone sequences at lower crust. The relationship between (LVZ) for the depth from 14 to 20 km (Fig. 4b). LVZ and tectonic structure needs further investigation The geological setting in study area can be in the future. The relatively fast shear wave divided into two environments; geology in North velocity from surface to the depth of 14 km, which Korea’s mainland and the Yellow Sea. The Nangrim varies from 3.23 to 3.37 km/s, can be related to the massif and Pyongnam basin composes most part existence of igneous and metamorphic rocks such of North Korea’s mainland (Fig. 1). Nangrim as mafic intrusions and quartzite in study area. massif consists of Precambrian crystalline schist These rock types have relatively higher shear and gneiss with the intrusion of Triassic to Jurassic modulus (Kim and Bae, 2006), which explains fast granitoid while the Pyongnam Paleozoic basin is shear wave velocity along the path in study area. overlying the massif with the lower marine and the upper terrestrial sedimentary sequences (Lee, 4. Conculsion 1988). On the other hand, the geological feature of the Yellow Sea is quite different from that of the Short (2-sec) to long (30-sec) period Rayleigh mainland. Wu et al., 2008, reviewed the research waves generated from the four earthquakes from results on the Yellow Sea from 1950s when the Haicheng area were well recorded at the five study on the Yellow Sea was motivated due to the broadband stations in South Korea. The fundamental- need for exploring hydrocarbon. They grouped mode group velocity dispersion curves from sedimentary basins into three groups; the North Rayleigh waves were estimated using MFA and Yellow Sea Basin (NYSB) or the West Korea Bay PMF. The average group velocity dispersion curves Basin (WKBB), the northern basin of the South were inverted to constrain one-dimensional shear Yellow Sea (SYSNB) and the southern basin of wave velocity structure in northwestern part of the South Yellow Sea (SYSSB). The WKBB is Korean Peninsula by using an iterative, stochastic located in the northern area of the Yellow Sea, linear inversion method. which is western offshore of North Korea. As it is Relatively low group velocity dispersion curve illustrated in Fig. 1, the propagation path from the along the path to BRD at period from 4 to 6 events to BRD crosses the eastern depression of seconds may correspond to the Late Mesozoic and WKBB. The WKBB is the one of significant Early Cenozoic sedimentary sequences in the 560 Tae Sung Kim

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