• Earthquakes and Structures
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• v.10 no.3
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• pp.669-680
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• 2016

In this paper, the underground box structure is discretized as a system with limited freedoms, and the explosion seismic wave is regarded as series of dynamic force acting on the lumped masses. Based on the local deformation theory, the elastic resistances of the soil are simplified as the effects of numbers of elastic chain-poles. Matrix force method is adopted to analyze the deformation of the structure in elastic half space. The structural dynamic equations are established and by solving these equations, the axial force, the moment and the displacement of the structure are all obtained. The influences of size ratio, the incident angle and the rock type on the dynamic response of the underground box structure are all investigated through a case study by using the proposed method.

• 한국방재학회:학술대회논문집
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• 2007.02a
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• pp.137-139
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• 2007

To enhance capability on discerning local and regional seismic phases, such as, Pn, Pg, Sn, Rg, etc, within the crust, 2-D numerical forward modeling will be applied to the data obtained from local seismic stations by simulating almost all waves including not only body wave but also surface wave generated without having to explicitly include them under consideration of Q factor. In this study, after getting rid of instrumental response by deconvolution, pseudo-spectral method instead of relying on typical numerical methods, such as, FEM(Finite Element Method) and FDM(Finite Difference Method), will be implemented for 2-D numerical forward modeling by considering velocities of P-wave and S-wave, density, and Q factors. Ultimately, the Power of reaching the enhanced capability on discerning local and regional seismic phases will make it easier for us to identify the seismic source, whether it is originated from man-made explosion or pure earthquake.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.10C
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• pp.844-850
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• 2008

This paper stated the proposed algorithm to certify the characteristics of artificial explosion sounds traveled through underground bedrock medium. Artificial explosion that travel through underground bedrock had an attenuation within high frequency bands in increase of a distance with multiple transmission paths phenomenon and inhomogeneity of geological status. In this paper, explosion experiment was made in underground tunnel to verify efficiency of proposed algorithm. The could certify the characteristics of artificial explosion sounds as extracted and numerically quantified the characterized parameter with collected sound sample that traveled through underground bedrock channel.

• Economic and Environmental Geology
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• v.18 no.2
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• pp.151-157
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• 1985

Events detected by the KIER microearthquake network operated in the Southern Part of Korea for 265 days in 1982~1984 were reviewed, and some of them were identified to be a dynamite explosion from several construction sites. The purpose of the present work is to determine the crustal structure of the Southern Korea using the time-destance data obtained from such explosion seismic records. The time·distance data can be well explained by a crustal model composed of four horizontal layers of which thickness, p and s-wave velocity ($V_p$ and $V_s$) are characterized as follows. 1st layer (surface) ; 0~2km, $V_p=5.5km/sec$, $V_s=3.3km/sec$ 2nd layer (upper crust) ; 2~15km, $V_p=6.0km/sec$, $V_s=3.5km/sec$ 3rd layer (lower crust) ; 15~29km, $V_p=6.6km/sec$, $V_s=3.7km/sec$ 4th layer (upper mantle) ; 29km~ , $V_p=7.7km/sec$, $V_s=4.3km/sec$ The relatively shallow crust·mantle boundary and low $P_n$ velocity compared with the mean values for stable intraplate region are noteworthy. Supposedely, it is responsible for the high heat flow in the South-eastern Korea or an anomalous subterranean mantle. The mean $V_p/V_s$ ratio calculated from the relation between p-wave arrival and s-p arrival times appears to be 1.735 which is nearly equivalent to the elastic medium of ${\lambda}={\mu}$. However, the ratio tends to be slightly larger with the depth. The ratio is rather high compared with that of the adjacent Japanese Island, and the fact suggests that the underlying crust and upper mantle in this region are more ductile and hence the earthquake occurrences are apt to be interrupted. As an alternative curstal model, a seismic velocity structure in which velocities are successively increased with the depth is also proposed by the inversion of the time·distance data. With the velocity profile, it is possible to calculate a travel time table which is appropriate to determine the earthquake parameters for the local events.

• The Journal of Engineering Geology
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• v.6 no.3
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• pp.137-153
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• 1996

The purpose of this study is to investigate the possibilities of identifying and detecting underground cavities using seismic waves recorded by the fixed and mobile stations. During 18 months of field work we recorded chemical explosions near the Bongdarn station. Seismic Stations were installed on the free surface and underground inside the Samba mine. The seismograms at the fixed(lorg-term) seismic station show abrupt change of polarization characteristics which can he associated with the appearance of P-to-S converted phase(PS) at 150 ~ 200 msec after the first P arrival. This result indicates that converted phases are generated very near to the Bongdarn station at a depth of 190m. Shear-wave splitting phenomena have also been observeci The time delay between fast shear(fS) and slow shear(sS) waves ranges between 30 and 60 msec(average is 42 msec). However, exact time delay between the fast and the slow shear waves can not be accurately measured because of the very short time delay and limitation of sampling rate. Chemical explosion experiments were recorded at stations along various paths to contrast the seismic response of areas with and without cavities. The seismograms recorded at the stations installed at cavity areas show an abrupt change of polarization characteristics but not on the other stations. Seismic waves propagating through the cavity are characterized by the attenuation of high frequency waves and predominantly low frequency seismic waves after the S wave arrivals.

• Journal of the Korean Geophysical Society
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• v.2 no.1
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• pp.1-8
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• 1999

Using the microearthquake data acquired from 1995 to 1996 through the seismic network operated by the Korea Institute of Geology, Mining & Materials (KIGAM), the three P-wave velocity models proposed by Lee (1979), Kim·Kim (1983) and Kim·Jung (1985) concerning the structure of the southeastern part of the Korean peninsula were examined in terms of the least square errors of the P-wave arrival times. The three models do not differ significantly in arrival time residuals except that the Lee's model gives slightly deeper focuses than the others. The layering of the crust of the peninsula is not clear as yet and to be studied by more earthquake and explosion data in the future.

• Geophysics and Geophysical Exploration
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• v.13 no.3
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• pp.286-294
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• 2010

Korea Institute of Geoscience and Mineral Resources (KIGAM) has installed and operated seven seismoacoustic (infrasound) arrays as well as seismic stations in Korea. The seismo-acoustic array, which consists of co-located seismometers and micro-barometers, can observe both seismic and infrasonic signals from distant explosive phenomena. The infrasound is defined as low frequency (<20 Hz) acoustic waves in atmosphere. In particular, it can be detectable at long distance due to its low energy attenuation during propagation in atmosphere. KIGAM adopted the infrasound technology to discriminate surface explosions from earthquakes only because the surface explosion generally generates infrasound following seismic signal. In addition to surface explosions, these arrays have detected diverse geophysically natural and artificial phenomena, such as infrasound signal from the North Korean nuclear test. This review introduced the state-of-the-art studies and examples of infrasonic signals in and around the Korean Peninsula. In conclusion, infrasound technology would be clearly accepted itself as a new Earth monitoring technology by expanding its detectable regime to lithosphere-Earth surface-atmosphere. In future, an advanced technology, which allows to analyze seismic and infrasonic wave fields together, will enlarge the understanding of geophysical phenomena and be used as a robust analysis method for remote explosive phenomena in the broad infrasound regime.

• Geophysics and Geophysical Exploration
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• v.13 no.3
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• pp.243-248
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• 2010

Two suspicious events, which were claimed as underground nuclear tests by North Korea, were detected in the northern Korean Peninsula on October 9, 2006 and May 25, 2009. The KIGAM and Korea-China Joint seismic stations are distributed uniformly along the boundaries between North Korea and adjacent countries. In this study, the data from broadband stations with the distance of 200 to 550 km from the test site are used to analyze and compare two nuclear tests of North Korea. By comparing the time differences of the Pn-wave arrival times of 1st and 2nd tests at multiple stations, the relative locations of two test sites could be calculated precisely. From the geometrical calculation with the velocity of Pn wave $V_{Pn}$ = 8 km/s, the 2nd test site is estimated to move in the WNW direction from 1st one with the distance of 2 km. Body wave magnitude, mb of the 2nd test, which was announced officially as the network average of 4.5, varies widely with the directional location of stations from 4.1 to 5.2. The magnitude obtained from Lg wave, $m_b$(Lg), shows less variation between 4.3 to 4.7 with the average of 4.6. The moving-window spectra of time traces of 1st and 2nd tests show very similar pattern with different scale level. In addition, the corner frequencies of P wave of 1st and 2nd tests at each station show no or negligible difference. This indicates the burial depths of two tests might be very similar. The relative yield amount of the 2nd test is estimated 8 times larger than that of the 1st from the weighted average of ground-velocity amplitude ratios.