• Journal of the Korean Geotechnical Society
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• v.35 no.11
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• pp.121-129
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• 2019

An Earthquake Early Warning (EEW) system is a technology that alerts people to an incoming earthquake by using P waves that are detected before the arrival of more severe seismic waves. P-wave analysis is therefore an important factor in the production of rapid seismic information as it can be used to quickly estimate the earthquake magnitude and epicenter through the amplitude and predominant period of the observed P-wave. However, when a large-magnitude teleseismic earthquake is observed in a local seismic network, the significantly attenuated P wave phases may be mischaracterized as belonging to a small-magnitude local earthquake in the initial analysis stage. Such a misanalysis may be sent to the public as a false alert, reducing the credibility of the EEW system and potentially causing economic losses for infrastructure and industrial facilities. Therefore, it is necessary to develop methods that reduce misanalysis. In this study, the possibility of seismic misclassifying teleseimic earthquakes as local events was reviewed using the Filter Bank method, which uses the attenuation characteristics of P waves to classify local and outside Korean peninsula (regional and teleseismic) events with filtered waveform depending on frequency and epicenter distance. The data used in our analysis were analyzed for maximum P_v values using 463 events with local magnitudes (2 < M_L ≦ 3), 44 (3 < M_L ≦ 4), 4 (4 < M_L ≦ 5), 3 (M_L > 5), and 89 outside Korean peninsula earthquakes recorded by the KMA seismic network. The results show that local and telesesimic earthquakes can be classified more accurately when combination of filtering bands of No. 3 (6-12 Hz) and No. 6 (0.75-1.5 Hz) is applied.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2003.03a
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• pp.45-49
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• 2003

The velocity structure beneath the CHNB broadband station is determined by receiver function analysis using by from teleseismic P waveforms. The detailed broadband receiver functions are obtained by stacking method for source-equalized vertical, radial and tangential components of teleseismic P waveforms. A time domain inversion uses the stacked radial receiver function to determine vertical P wave velocity structure beneath the station. The crustal velocity structures beneath the stations are estimated using the receiver function inversion method in the case at the crustal model parameterized by many thin, flat-tying, homogeneous layers. The result of crust at model inversion shows the crustal velocity structure beneath the CHNB station varies smoothly with increasing depth, and there are six discontinuity around 2.5km, 6.25km, 12.5km, 22.5km and 27.5km depth, with Moho discontinuity at about 32.5km depth.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2003.09a
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• pp.3-7
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• 2003

The velocity structure beneath the CHNB broadband station is determined by receiver function analysis using by from teleseismic P waveforms. The detailed broadband receiver functions are obtained by stacking method for source-equalized vertical, radial and tangential components of teleseismic P waveforms. A time domain inversion uses the stacked radial receiver function to determine vertical P wave velocity structure beneath the station. The crustal velocity structures beneath the stations are estimated using the receiver function inversion method in the case at the crustal model parameterized by many thin, flat-lying, homogeneous layers. Events divide into 4 groups. four azimuths corresponding to events in group a(southwest), b(south), c(southeast), d(northeast). The result of crust at model inversion shows the crustal velocity structure beneath the CHNB station varies smoothly with increasing depth. The conard discontinuity lies around 18 km and moho discontinuity lies range from 30 to 34 km.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 1997.04a
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• pp.21-26
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• 1997

For 262 locations throughout the southern part of Korean peninsula, intensities of 13 December 1996 Yeongweol earthquake are estimated to make an isoseismal map and investigate attenuation properties in the southern part of Korea. Due to the inherent uncertainties in the estimation of intensities, obtained intensity map show quite scattered pattern of intensity distribution. Estimated intensities range from III to possibly Ⅷ. In case of intensity larger than Ⅵ, considerable damages such as fracturing of walls are reported one of the most significant feature of the intensity map is, considering its magnitude 4.7 reported by KMA, the felt area is appeared to be unusually large covering most of the Korean peninsula except Cheju island. This result indicates ether the magnitude is underestimated or the focus of this earthquake is much deeper. Assuming shallow(less than 10km) intraplate earthquake, we obtained average magnitude 5.6 by using the area encircled by isoseismal contour lines from intensity IV to intensity Ⅶ. This ambiguity can be clarified if more reliable focal depth is estimated by using teleseismic earthquake records in the future.

• 한국지구물리탐사학회:학술대회논문집
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• 2009.10a
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• pp.54-59
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• 2009

Seismic station of high noise level has difficulties detecting relatively weak ground motions due to small earthquakes or teleseismic events because earthquake detectability of seismic station depends on seismic noise level. To figure out the capability of earthquake detection of a seismic network, therefore, seismic noise level of each station also needs to be considered, including the distribution of seismic stations. Recently, it has been known that most of broadband seismic stations in South Korea have affected by cultural noise in the frequencies higher than 1 Hz and show diurnal variations of noise level. In order to analyze the effect of diurnal variation of seismic noise level on earthquake detectability, we used the result of background seismic noise level analysis of seismograms of 30 broadband stations of KIGAM and KMA from 2005 to 2007. This study shows that earthquakes greater than magnitude 2.4 occurring within the Korean Peninsula can be detected at night while those greater than magnitude 2.6 can be detected in the daytime.

• Geophysics and Geophysical Exploration
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• v.16 no.3
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• pp.190-195
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• 2013

We estimated near-surface shear-wave velocity (${\nu}_s$) at the Hwacheon seismic station using a geologic log of a well, microtremors recorded during a period of 56 days, and records of three teleseismic events ($M_w{\geq}6.0$). The vs of the 10-m thick soil layer (${\nu}^s_s$= 296 m/s) was determined from horizontal-to-vertical spectral ratios of microtremors recorded at the surface. The average ${\nu}_s$ ($\bar{\nu}_s$= 1,309 m/s) from the surface to the 96-m depth of a borehole sensor, was computed using spectral coherence analyses of data recorded by surface- and borehole-sensors for the three teleseismic events. Using these calculated values of ${\nu}^s_s$ and $\bar{\nu}_s$, the computed bedrock ${\nu}_s$ is 2,150 m/s and the time-averaged ${\nu}_s$ to a 30-m depth is 696 m/s. Accordingly the Hwacheon seismic station is regarded as a relatively good site. The deduced near-surface ${\nu}_s$ can be used for further quantitative evaluation of site amplification and earthquake hazard.

• The Journal of Engineering Geology
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• v.2 no.2
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• pp.155-165
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• 1992

The purpose of this study is to evaluate compatability of seismic source characteristics of the Kern County earthquake to those of Korean Peninsula seismotectonics. The compatability could be used to make Korean type response spectrum from the strong ground motions observed from the assingned earthquake. The July 21, 1952, Kern County, California, earthquake is the largest earthquake to occur in the western U.S. since 1906, and the repeat of this event poses a significant seismic hazard. The Kern County event was a complex thrusting event, with a surface rupture pattern that varied from pure leftlateral strike-slip to pure dip-slip. A time dependent moment tensor inversion was applied to ten observed teleseismic long-period body waves to investigate the source complexity. Since a conventional moment tensor inversion(constant geometry through time) returns a non-double-couple source when the seismic source changes(fault orientation and direction of slip) with time, we are required to use the time dependent moment tensor which allows a first-order mapping of the geometric and temporal complexity. From the moment tensor inversion, a two-point seismic source model with significant overlap for the White Wolf fault, which propagates upward(20 km to 5 km) from SW to NE, fits most of the observed seismic waveforms in the least squares sense. Comparison of P, T and B axes of focal mechanisms and focal depths suggests that seismic source characteristics of the Kern County earthquake is consistant with those of Korean Peninsula Seismotectonics.

• Geophysics and Geophysical Exploration
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• v.14 no.3
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• pp.254-260
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• 2011

We studied magnitude determination method using high-frequency energy radiation duration to calculate rapidly magnitude of large earthquakes that occurred around Japan. Fourteen earthquakes were analyzed using Korea Meteorological Administration (KMA) data. We calculated duration of high-frequency energy radiation with 2~4 Hz band pass filter at each data and estimated magnitude. As a result, duration becomes longer as magnitude becomes larger and the magnitude estimated using regional earthquake data are similar to that using teleseismic data. Therefore when an earthquake occurs around Japan we will be able to estimate the magnitude in a relatively short time using KMA data and it may be possible to determine if the earthquake is large enough to produce tsunami.

• Journal of the Earthquake Engineering Society of Korea
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• v.25 no.2
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• pp.71-81
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• 2021

This paper presents a real-time, false-pick filter based on deep learning to reduce false alarms of an onsite Earthquake Early Warning (EEW) system. Most onsite EEW systems use P-wave to predict S-wave. Therefore, it is essential to properly distinguish P-waves from noises or other seismic phases to avoid false alarms. To reduce false-picks causing false alarms, this study made the EEWNet Part 1 'False-Pick Filter' model based on Convolutional Neural Network (CNN). Specifically, it modified the Pick_FP (Lomax et al.) to generate input data such as the amplitude, velocity, and displacement of three components from 2 seconds ahead and 2 seconds after the P-wave arrival following one-second time steps. This model extracts log-mel power spectrum features from this input data, then classifies P-waves and others using these features. The dataset consisted of 3,189,583 samples: 81,394 samples from event data (727 events in the Korean Peninsula, 103 teleseismic events, and 1,734 events in Taiwan) and 3,108,189 samples from continuous data (recorded by seismic stations in South Korea for 27 months from 2018 to 2020). This model was trained with 1,826,357 samples through balancing, then tested on continuous data samples of the year 2019, filtering more than 99% of strong false-picks that could trigger false alarms. This model was developed as a module for USGS Earthworm and is written in C language to operate with minimal computing resources.

• Geophysics and Geophysical Exploration
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• v.18 no.1
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• pp.31-34
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• 2015

A gravity meter has been used for exploring subsurface mineral resources and monitoring long-period events such as Earth tides. Recently, researchers found several other intriguing features that we could even detect large teleseismic earthquakes and monitor seismic ambient noise using gravimeters. The zero-length spring suspension technology gives the gPhone (Micro-g LaCoste) excellent low frequency sensitivity, which may have implications for investigating much longer-period natural events (e.g., Earth's hum, tsunami waves, etc.). In this study, we present preliminary results through temporary operation of the gPhone at Geumsan in South Korea for 9 months (Nov. 2008-Jul. 2009). The gPhone successfully recorded large teleseismic events and showed a clear seasonal variation of the Double frequency microseisms during its operation period.