• Economic and Environmental Geology
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• v.44 no.6
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• pp.493-502
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• 2011

We study the deep structure of Korean Peninsula by estimating Moho depth and crustal thickness from using land and oceanic topography and free-air gravity anomaly data. Based on Airy-Heiskanen isostatic hypothesis, the correlated components between the terrain gravity effects and free-air gravity anomalies by wavenumber correlation analysis(WCA) are extracted to estimate the gravity effects that will be resulted from isostatic compensation for the area. With the resulting compensated gravity estimates, Moho depth that is a subsurface between the crust and mantle is estimated by the inversion in an iterative method with the constraints of 20 seismic depth estimates by the receiver function analysis, to minimize the uncertainty of non-uniqueness. Consequently, the average of the resulting crustal thickness estimate of Korean Peninsula is 32.15 km and the standard deviation is 3.12 km. Moho depth of South Korea estimated from this study is compared with the ones from the previous studies, showing they are approximately consistent. And the aspects of Moho undulation from the respective study are in common deep along Taebaek Mountains and Sobaek Mountains and low depth in Gyeongsang Basin relatively. Also, it is discussed that the terrain decorrelated free-air gravity anomalies inferring from the intracrustal characteristics of the crust are compared to the heat flow distributions of South Korea. The low-frequency components of terrain decorrelated Free-air gravity anomalies are highly correlated with the heat flow data, especially in the area of Gyeongsang basin where high heat flow causes to decrease the density of the rocks in the lower crust resulting in lowering the Moho depth by compensation. This result confirms that the high heat sources in this area coming from the upper mantle by Kim et al. (2008).

• Economic and Environmental Geology
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• v.53 no.5
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• pp.597-608
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• 2020

Even after the Gyeongju earthquake and the Pohang earthquake, hundreds of aftershocks and micro-earthquakes are still occurring in the southeastern part of the Korean Peninsula. These phenomena mean that the stress is constantly working, implying that another huge earthquake may occur in the future. Therefore, the gravity field interpretation method was used to analyze the deep geological structure of the Pohang-Ulsan region in the southeastern Korean Peninsula. First, a gravity survey was performed to collect the insufficient data and to calculate the detailed Bouguer gravity anomaly in the study area. Based on the gravity anomaly data, the location, direction, and maximum depth of deep fault lines were analyzed using the inversion methods "Curvature analysis" and "Euler deconvolution method". As a result, it is interpreted that at least six fault lines(C1~C6) exist in deep depth. The deep fault line C1 is well correlated to the Yeonil Tectonic Line(YTL), suggesting that YTL is extended up to about 4000m deep. The deep fault line C2 consists of several segment faults and well correlated to the fault lines on the surface. Inferred fault lines C3, C4, and C5 have an NW-SE direction, which is parallel to the Ulsan fault. The deep fault line C6 has the direction of NE-SW, and it is interpreted that the eastern boundary fault of Eoil Basin is extended to the deep. Comparing the inferred fault lines with the distribution of micro-earthquakes, the location of the deep fault line C1 is well correlated to the hypocenter of micro-earthquakes. This implies that faults in deep depth are related to the recent earthquakes in the southeastern 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.