• Geophysics and Geophysical Exploration
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• v.12 no.3
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• pp.225-232
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• 2009

As part of seismic experiments investigating crustal velocity structures of the Korean peninsula, permanent (fixed) seismographs of the Korea Meteorological Administration (KMA) network recorded seismic signals from four and eight large explosions in Korean Crustal Research Team (KCRT) profiles shot in 2004 and 2008, respectively. Among the seismograms recorded by 43 velocity sensors and 103 accelerometers at KMA stations distributed throughout the southern Korean Peninsula, 156 records with epicentral distances less than 120 km and high signal-to-noise ratios were analyzed to determine velocity anisotropy of the Pg phase. Relative elevation corrections of -101.6 to 105.3 ms were made using velocity information derived from the 2004 KCRT profile data and differences in elevation between the permanent KMA stations and the temporary stations in the KCRT profiles at the same source-receiver offsets. To remove site effects, receiver-station corrections of -89.6 to 192.2 ms were additionally made to the KMA station data by subtracting the average differences in traveltimes between KMA stations and portable stations at the same offsets for all available shots with different azimuths. With the exception of anomalously fast velocities along trends of the Chugaryeong fault zone and the Okchon fold belt and anomalously slow velocities in the regions of high terrestrial heat near Yeongduk and Ulsan, the analysis of crustal velocity anisotropy using the Pg phase indicates overall isotropy in the southern half of the Korean peninsula.

• Geophysics and Geophysical Exploration
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• v.25 no.4
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• pp.177-188
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• 2022

Recently, the application of distributed acoustic sensors (DAS), which can replace geophones and seismometers, has significantly increased along with interest in micro-seismic monitoring technique, which is one of the CO₂ storage monitoring techniques. A significant amount of temporally and spatially continuous data is recorded in a DAS monitoring system, thereby necessitating fast and accurate data processing techniques. Because event detection and seismic phase picking are the most basic data processing techniques, they should be performed on all data. In this study, a machine learning-based P, S wave phase picking algorithm was developed to compensate for the limitations of conventional phase picking algorithms, and it was modified using a transfer learning technique for the application of DAS data consisting of a single component with a low signal-to-noise ratio. Our model was constructed by modifying the convolution-based EQTransformer, which performs well in phase picking, to the ResUNet structure. Not only the global earthquake dataset, STEAD but also the augmented dataset was used as training datasets to enhance the prediction performance on the unseen characteristics of the target dataset. The performance of the developed algorithm was verified using K-net and KiK-net data with characteristics different from the training data. Additionally, after modifying the trained model to suit DAS data using the transfer learning technique, the performance was verified by applying it to the DAS field data measured in the Pohang Janggi basin.

• Economic and Environmental Geology
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• v.52 no.6
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• pp.555-560
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• 2019

One-dimensional shear wave velocity structure of North Korea is constrained using short (2-sec) to long period (30-sec) Rayleigh waves generated from four seismic events in China. 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 constrain 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 Yellow Sea. 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.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.197-202
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• 2005

The electromagnetic signals associated with the seismic activity in the south-east offshore of Kii peninsula, Japan, were clearly recorded at the MT sites in Jeju island, Korea. In this research, we have identified the co-seismic electromagnetic signals associated with the seismic activity and have analyzed the characteristics of significant electromagnetic variations. The analysis of phase velocity, power spectral density, MT impedance and polarization direction shows that the significant earthquake signals have the frequency band of about 0.05 to 0.5 Hz and that the sources of electromagnetic field are local effects of passing seismic waves. The simple approximation using electrokinetic effect successfully explains the co-seismic EM signals coincides with measured data but cannot explain the localities of electromagnetic variations.

• Journal of the Earthquake Engineering Society of Korea
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• v.14 no.2
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• pp.67-74
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• 2010

To a series of vibration records obtained from experimental modal testing using a fixed hammer and roving accelerometers for greenhouse arch structures, modal parameters such as natural frequencies, damping ratios and mode shapes are extracted by applying the two most advanced system identification methods in the frequency-domain up to now, so-called PolyMAX and FDD. The former involves both input and output data, while the latter utilizes only the output data. The possibility of determining the static buckling load, detecting damages, etc., for very slender steel-pipe arches by means of a non-destructive testing method based on vibration measurements is primarily investigated. The extracted modal parameters generally correlated well with those obtained using finite element analysis, demonstrating promising results for further on-going research.

• Journal of the Earthquake Engineering Society of Korea
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• v.14 no.2
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• pp.57-65
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• 2010

To determine the static buckling loads and evaluate the structural performance of slender steel pipe-arches such as for greenhouse structures, a series of modal tests using a fixed hammer and roving sensors was carried out, by providing no load, then a range of vertical loads, on an arch rib in several steps. More attention was given to an internal arch where vertical and horizontal auxiliary members are not placed, unlike an end arch. Modal parameters such as natural frequencies, mode shapes and damping ratios were extracted using more advanced system identification methods such as PolyMAX (Polyreference Least-Squares Complex Frequency Domain), and compared with those predicted by commercial FEA (Finite Element Analysis) software ANSYS for various conditions. A good correlation between them was achieved in an overall sense, however the reduction of natural frequencies due to the existence of preaxial loads was not apparent when the vertical load level was about up to 38% of its resistance. Some difficulties related to the field testing and parameter extraction for a very slender arch, as might arise from the influences of neighboring members, are carefully discussed.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.43-46
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• 2005

The local magnitude scales yield in the Southern Korean Peninsula following the Richter's original definition are reviewed. In order to compare the previous $M_L$ scales, the new distance correction term of $M_L$ scale is derived as $-logA_{0}$ = 1.017log(r/17)+0.00028(r-17)2+2.0 using broadband velocity seismograms from 126 local events occured from 2000 to 2004. The attenuation rate of this formula fall between those of western and eastern North America. This result is in nearly accord with the $M_L$ scales proposed by Kim and Park(2002). The differences between various $M_L$ scales is owing to insufficient seismic data not to distribute whole area of Southern Korea

• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.78-84
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• 2008

Recent earthquakes near nuclear power plants in Korea have triggered public concerns about possible seismicity of the Ulsan Fault Zone in the south-eastern part of the Korean peninsula. To reveal subsurface structures of this fault zone, we conducted high-resolution seismic refraction and reflection surveys, and closely spaced gravity measurements in the Dongchon River valley north of Ulsan, Korea. Here alluvium covers the north-south trending fault zone in a 1-km wide valley. Both source points and receivers were spaced at 5-m intervals for the 24-channel seismic refraction and reflection methods, along two profiles of 835 m and 415 m length. Gravity data were also measured along these profiles at 131 stations using a 10-m interval. Synergetic interpretation of seismic refraction, high-resolution seismic reflection, and gravity surveys across the valley indicates that the Ulsan Fault Zone was formed by apparent north-south strike-slip motions during the Cretaceous, and that some faults may have been reactivated by east-west compressional or transpressional stresses during the Tertiary or Quaternary.

• Computational Structural Engineering
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• v.8 no.3
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• pp.113-122
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• 1995

The purpose of this study is to evaluate performance and safety of structures designed according to current seismic code or provisions (e.g., Uniform Building Code(UBC), NEHRP provisions, etc.) during lifetime of structures. The performance is represented in terns of reliability in this paper. To perform reliability analyses, a large number of time history response analyses for a given structure are usually required. In this study, to perform reliability analyses ground motions are generated based on nonstationary random process and structures are designed based on UBC. In this paper, responses of structures under a given earthquake is evaluated using dynamic nonlinear time history analyses and also an equivalent nonlinear system (ENS) with response scaling factors. The ENS system is described in the companion paper. Therefore, this paper evaluates the seismic performance of structures and also verify the accuracy of ENS.

• Journal of the Korean earth science society
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• v.24 no.8
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• pp.684-690
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• 2003

Recently Choongchung provinces in the central part of South Korea have received increasing attention because of the newly Planned administrative capital construction. In this area, a seismic network of Korea National University of Education has been installed since September 1996, and analyzed Q$_P^{-1}$ and Q$_S^{-1}$ by the extended Coda normalization method based on 60 events recorded by 2 stations of the network. To compensate for insufficient data, we combined the data from 33 events observed at 1 of the stations of the network of Korea Institute of Geology, Mining & Materials. Estimated Q$_P^{-1}$ and Q$_S^{-1}$ showed frequency dependence that decrease from (1.9${\pm}$3.0)${\times}$10$^{-3}$ and (2.4${\pm}$1.4)${\times}$10$^{-3}$ at 3.0 Hz to (5.4${\pm}$1.5)${\times}$10$^{-4}$ and (6.3${\pm}$1.1)${\times}$10$^{-4}$ at 24 Hz, respectively. Using a power law dependent on frequency, the best fit of Q$_P^{-1}$ and Q$_S^{-1}$ are 0.003f$^{-0.62}$ and 0.006f$^{-0.71}$ respectively. These values correspond to those of seismically stable regions, and are slightly less dependent on frequency than those of the southeastern part of Korea due to high Q$^{-1}$ values in high frequencies. Further observations are required in the central part of S. Korea to evaluate the difference of Q$^{-1}$ between central and southeastern parts of S. Korea.