• Journal of the Korean Geophysical Society
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• v.1 no.1
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• pp.23-30
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• 1998

Fault-plane solutions of the December 13, 1996 Yeongweol earthquake with magnitude 4.5 is obtained using the grid test technique. Thirty polarities of P waves recorded at KMA, KIGAM, KSRS and JAPAN stations are used for the event. The obtained fault plane solution shows strike-slip motion with significant amount of thrust component. The orientation of the fault is 180±20° in strike, 50±5° in dip and 150±5° in rake, or 292±3° in strike, 65±5° in dip and 30±10° in rake. These solutions are similar to those of earthquakes occurred at Sagju (Jan. 7, 1980), Pohang (Apr. 15, 1981) and offshore Gunsan (Oct. 6, 1976). The compressional axis of the stress field is trending from ENE to WSW, which is consistent with the previously defined typical regional tectonic stress orientation in and around Korean Peninsula.

• Economic and Environmental Geology
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• v.44 no.3
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• pp.229-238
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• 2011

S-wave, which provides lithology and pore fluid information, plays a key role in estimating gas-hydrate saturation. In general, P- and S-wave velocities increase in the presence of gas-hydrate and the P-wave velocity decreases in the presence of free gas under the gas-hydrate layer. Whereas there are very small changes, even slightly increases, in the S-wave velocity in the free gas layer because S-wave is not affected by the pore fluid when propagating in the free gas layer. To verify those velocity properties of the BSR (bottom-simulating reflector) depth in the gas-hydrate prospect area in the Ulleung Basin, P- and S-wave velocity profiles were derived from multi-component ocean-bottom seismic data which were acquired by Korea Institute of Geoscience and Mineral Resources (KIGAM) in May 2009. OBS (ocean-bottom seismometer) hydrophone component data were modeled and inverted first through the traveltime inversion method to derive P-wave velocity and depth model of survey area. 2-D multichannel stacked data were incorporated as an initial model. Two horizontal geophone component data, then, were polarization filtered and rotated to make radial component section. Traveltimes of main S-wave events were picked and used for forward modeling incorporating Poisson's ratio. This modeling provides S-wave profiles and Poisson's ratio profiles at every OBS site. The results shows that P-wave velocities in most OBS sites decrease beneath the BSR, whereas S-wave velocities slightly increase. Consequently, Poisson's ratio decreased strongly beneath the BSR indicating the presence of a free gas layer under the BSR.

• The Journal of Engineering Geology
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• v.19 no.4
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• pp.509-515
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• 2009

Three earthquakes with local magnitude ($M_L$) greater than 3.0 occurred on April 24, June 2 and September 12 in 1999 nearby the Gyeongju area. Redetermined epicenters were located within the radius of 1 km. We carried out waveform inversion analysis to estimate focal mechanism of June 2 event, and P and S wave polarity and their amplitude ratio analysis to estimate focal mechanisms of April 24 and September 12 events. June 2 and September 12 events had similar fault plane solutions each other. The fault plane solution of April 24 event included those of other 2 events, but its distribution range was relatively broad. Focal mechanisms of those events had a strike slip faulting with a small normal component. P-axes of those events were ENE-WSW which were similar to previous studies on the P-axis of the Korean Peninsula. Considering distances between epicenters, similarities of seismic waves and sameness of polarities of seismic data recorded at common seismic stations, these events might occurred at the same fault. The seismic moment of June 2 event was estimated to be $3.9\;{\times}\;10^{14}\;N{\cdot}m$ and this value corresponded to the moment magnitude ($M_W$) 3.7. The moment magnitude estimated by spectral analysis was 3.8, which was similar to that estimated by waveform inversion analysis. The average stress drop was estimated to be 7.5 MPa. Moment magnitudes of April 24 and September 12 events were estimated to be 3.2 and 3.4 by comparing the spectrum of those events recorded at common single seismic station.

• Journal of the Korean earth science society
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• v.27 no.3
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• pp.341-347
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• 2006

Focal mechanisms were analyzed for the seven earthquakes which occurred in the southwestern part of the Korea Peninsula from 2001 to 2005. Grid searches are performed to fit distributions of P-wave first-motion polarities and SH/P amplitude ratios for each event. The focal mechanism solutions imply that most of the events have strike-slip sense including partially thrust component. The compressional axes of the solutions are predominantly ENE-WSW or NE-SW indirections. This result is similar to the directions of the principal compressional axes for major earthquakes occurred around the Korea Peninsula.

• Journal of the Korean earth science society
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• v.34 no.1
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• pp.59-68
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• 2013

Focal mechanism solutions in the southwestern region of the Korean Peninsula ($34^{\circ}N-36^{\circ}N$, $126^{\circ}E-128^{\circ}E$) were obtained from the analysis of the recent 22 earthquakes ($M{\geq}2.0$) occurred from January, 2005 to March, 2011. The spatial differences between the epicenters recalculated by this study and those by KMA (Korea Meteorological Administration) and KIGAM (Korea Institute of Geoscience and Mineral Resources) are less than $0.05^{\circ}$, indicating a small deviation. However, they become a little bit larger in the coastal area due to a biased arrangement of seismic stations. Redetermined depths of hypocenters show a difference less than 12.7 km by comparison with the depth data announced by KIGAM. Most epicenters in inland area are located closely to the lineaments. Fault plane solutions were obtained from the analysis of P and SH wave polarities, and SH/P amplitude ratios. They show strike-slip faulting or strike-slip faulting with reverse components dominantly. The P-axes trends are mainly ENE-WSW or E-W directions. The direction of fault plane and auxiliary plane with 'NNE-SSW and WNW-ESE' or 'NE-SW and NW-SE' are dominant and almost parallel to the general trends of lineaments in the study area.

• The Journal of Engineering Geology
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• v.23 no.4
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• pp.457-465
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• 2013

We integrated and correlated datasets from surface and subsurface geophysics, drilling cores, and engineering geology to identify geological interfaces and characterize the joints and fracture zones within the rock mass. The regional geometry of a geologically weak zone was investigated via a fence projection of electrical resistivity data and a borehole image-processing system. Subsurface discontinuities and intensive fracture zones within the rock mass are delineated by cross-hole seismic tomography and analyses of dip directions in rose diagrams. The dynamic elastic modulus is studied in terms of the P-wave velocity and Poisson's ratio. Subsurface discontinuities, which are conventionally identified using the N value and from core samples, can now be identified from anomalous reflection coefficients (i.e., acoustic impedance contrast) calculated using a pair of well logs, comprising seismic velocity from suspension-PS logging and density from logging. Intensive fracture zones identified in the synthetic seismogram are matched to core loss zones in the drilling core data and to a high concentration of joints in the borehole imaging system. The upper boundaries of fracture zones are correlated to strongly negative amplitude in the synthetic trace, which is constructed by convolution of the optimal Ricker wavelet with a reflection coefficient. The standard deviations of dynamic elastic moduli are higher for fracture zones than for acompact rock mass, due to the wide range of velocities resulting from the large numbers of joints and fractures within the zone.

• Journal of the Korean earth science society
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• v.32 no.2
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• pp.161-169
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• 2011

Focal mechanism solutions in the central and western areas of the Korean Peninsula (36-37.8$^{\circ}N$, 126-128$^{\circ}E$) were obtained from the analysis of the recent seventeen earthquakes (M${\geq}$2.2) which occurred from January, 2005 to May, 2010. The spatial differences between the epicenters recalculated by this study and those announced by the Korea Meteorological Administration are less than $0.03^{\circ}$, indicating a small deviation. Focal mechanism solutions were obtained from the analysis of P wave polarities, SH wave polarities and SH/P amplitude ratios. The focal mechanism solutions show dominant strike-slip faulting or oblique slip faulting with strike-slip components. The P-axes trends are mainly ENE-WSW or E-W directions. The direction of fault plane and auxillary fault plane with NNE-SSW and WNW-ESE are almost parallel to the general trends of lineaments in the study area. The results also show that focal mechanism solutions and the main axis of stress field in the Kyonggi massif and Okchon belt are almost same.

• Journal of the Korean earth science society
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• v.28 no.2
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• pp.202-213
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• 2007

The Odaesan earthquake $(M_L=4.8)$ occurred near Mt. Odae, Jinbu-Myon, Pyongchang-Gun, Kangwon Province on January 20, 2007. It has a shallow focal depth about 10 km. Its felt area covers most of the southern peninsula except some southern and western inland area. The maximum MM intensity was VI in the areas including Jinbu, Doam, Kangreung, Jumunjin, and Pyongchang. In these areas, there was a very strong shaking that caused several cracks on the walls of buildings and houses, slates falling off the roof, tiles being off the wall, things falling off the desk, and rock falling from the mountains. In order to get fault plane solutions, grid searches were performed by fitting distributions of P-wave first-motion polarities and SH/P amplitude ratios for each event. The results showed that the main shock represented right-lateral strike-slip sense and two aftershocks, reverse sense. It seems that the seismogenic fault may be the NNE-SSW trending Weoljeongsa fault near the epicenter based on the distribution of epicenters (foreshock, main shock, and aftershocks), damage area, and fault plane solution. The distribution of the epicenters indicates that the length of the subsurface rupture is estimated to be about 2 km.