Jung Mo Lee

Helicopter-borne and ground-towed radar surveys of the Fourcade Glacier on King George Island, Antarctica

To determine subglacial topography and internal features of the Fourcade Glacier on King George Island in Antarctica, helicopter-borne and ground-towed ground-penetrating radar (GPR) data were recorded along four profiles in November 2006. Signature deconvolution, f-k migration velocity analysis, and finite-difference depth migration applied to the mixed-phase, single-channel, ground-towed data, were effective in increasing vertical resolution, obtaining the velocity function, and yielding clear depth images, respectively. For the helicopter-borne GPR, migration velocities were obtained as root-mean-squared velocities in a two-layer model of air and ice. The radar sections show rugged subglacial topography, englacial sliding surfaces, and localised scattering noise. The maximum depth to the basement is over 79 m in the subglacial valley adjacent to the south-eastern slope of the divide ridge between Fourcade and Moczydlowski Glaciers. In the ground-towed profile, we interpret a complicated conduit above possible basal water and other isolated cavities, which are a few metres wide. Near the terminus, the GPR profiles image sliding surfaces, fractures, and faults that will contribute to the tidewater calving mechanism forming icebergs in Potter Cove.

Determination of hypocentral parameters of local earthquakes using weighting factor based on take-off angle

This paper introduces an improved method of determining hypocentral parameters of local earthquakes by applying a weighting factor based on take-off angle at a source. The hypocentral parameters determined by the existing methods contain errors if the true velocity structure is not used, and they are not identical depending on the velocity models used. Variation in the traveltime caused by a focal depth may indicate the degree of focal depth information contained in the data. The degree of information will decrease rapidly as the epicentral distance increases. The take-off angle at the source is related to the epicentral distance. Therefore, a weighting factor representing the degree of focal depth information could be described in terms of the take-off angle at the source. The accuracy of hypocentral parameters, especially the focal depth and the origin time, is improved by applying the take-off angle as the weighting factor and two-point ray tracing to the existing methods. Synthetic traveltime data both with noise and without noise are generated using the two-point ray tracing technique with a model in which velocity structure and hypocentral parameters are known. The effect of the weighting factor is examined by comparing the estimated hypocentral parameters with the true, known values. The computational results show that the focal depth and the origin time estimated by the improved method (MHYPO) in this work are more accurate than those estimated by other existing methods.

Seismic reflection image of the crust structure along the KCRT-2002 profile in the southern Korean peninsula

In order to reveal the perspective of velocity structure in the southern part of the Korean peninsula, a seismic experiment was carried out along a WNW-ESE profile of 294-km length in December, 2002. In 100-m deep drill-holes, seismic explosives of 1000 and 500 kg were detonated on the west coast and near the center of the profile, respectively. The seismic signals were detected by 198 portable seismometers spaced approximately at an 1.5 km interval and digitally recorded for 115 s at an 120-Hz sampling rate. Normal moveout corrections were applied to yield reflection images for P, S, and PS converted waves followed by time-to-depth conversion through forward modeling of travel times using the fourth-order equation for wide angle reflections. Three distinct reflection events are identified at average two-way travel times of 4.90, 8.02, and 10.99 s on the NMO corrected P-wave sections. Their converted depths are 13.71, 23.37, and 33.67 km, respectively, and the P-wave interval velocities between the depth boundaries including the free surface are 5.60, 6.19, and 6.96 km/s from top to bottom. The reflection Moho is imaged as a sharp boundary in the middle of the profile while multi-layers are featured near both ends of the profile.

Probabilistic seismic hazard analysis using a synthetic earthquake catalog: comparison of the Gyeongju City Hall site with the Seoul City Hall site in Korea

Probabilistic seismic hazard analysis (PSHA) generally requires the identification of line sources (active faults) and area sources where seismicity parameters must be determined. It is not easy to identify seismic sources and to estimate seismicity parameters for sources in the intraplate region. We perform an alternative PSHA with a synthetic earthquake catalog, which does not require the delineation of seismic sources. A synthetic catalog of the Korean Peninsula for 500,000 years is constructed by simulating earthquakes using the Monte Carlo method in such a way that an earthquake is randomly selected from the observed earthquake catalog, its epicenter is perturbed by a normally distributed random number, and a magnitude is assigned to the simulated event with a random value generated based on the Gutenberg-Richter magnitude-frequency relation with specified minimum and maximum magnitudes. The seismicity of the Korean Peninsula is thoroughly investigated for estimation of parameters required for synthesizing the catalog. The SMSIM computer program is used to simulate ground motions at sites instead of estimating the ground motion prediction equation. Two sites are chosen for PSHA; the one is the Seoul City Hall site (Seoul site), a densely populated metropolitan site, and the other is the Gyeongju City Hall site (Gyeongju site) close to the epicenter of the ML 5.8 earthquake on 12 September 2016. Seismic hazard curve, uniform hazard spectra (UHSs), and deaggregation of hazard for two sites are presented. The mean return periods for 0.1, 0.2, and 0.3 g at the Seoul site appears to be 1640, 6170, and 17650 years, respectively while those at the Gyeongju site appears to be 327, 1197, and 3548 years, respectively. The mean return periods for the Seoul site are around five times longer than those for the Gyeongju site, and this fact implies that the seismic hazard level of the Gyeongju site is considerably higher than that of the Seoul site. UHSs for 10% and 2% probabilities of exceedance over 50 years are constructed at both sites. The spectra show peak amplifications around the 0.05-s period.

Shallow magnetic survey of the Younghae Basin area, South Korea: Evaluation of structural setting

To evaluate the subsurface structural setting and the correlation between the surface and subsurface geological structure of the Younghae basin area, magnetic survey was carried out. The measurements were taken in and between rice fields to avoid the artificial objects that can lower the overall signal to noise (S/N) ratio. The total magnetic intensity map was reduced to the pole (RTP) map, which was then processed to smooth out field variations caused by various noise sources, and to facilitate the qualitative and quantitative interpretation. The structural elements deduced from the RTP, band-pass filtered and downward continuation maps show the same trends as the local geological structure. The igneous basement rocks are the main causative bodies for the magnetic anomalies. Fifteen profiles showing the significant magnetic anomalies were analyzed for target depth estimations. Two methods: the slope and power spectrum methods were employed and the basement depths deduced range from 63 meters to 354 meters.

The equation and properties of a ray path in a linearly varying velocity field

The equation of ray path in a linearly varying velocity field in one-dimensional (1-D) case corresponds to an equation of circle whose center is defined by the initial take-off angle and the constant parameters of the velocity field. The physical ray path can exist only on the half circle, where the velocity is positive. The initial take-off angle of a ray passing through the two specified points is derived, the expression of which assures an analytical two-point ray tracing. The linearly varying velocity field in two-dimensional (2-D) or three-dimensional (3-D) can be transformed to that in 1-D. This transformation reduces the equation of ray path in a linearly varying velocity field in 2-D or 3-D to one in 1-D in the transformed coordinates. The results can be implemented for two-point ray-tracing calculation as well as analytical ray path and travel time calculation, both in forward and inverse problems.

Imaging the crustal structure along the KCRT-2008 profile in the Southern Korean Peninsula using regularisation

Information regarding crustal structure is important for geological and geophysical research. Several attempts have been made to delineate the crustal structure of the Korean Peninsula, but the resolution in these studies has generally been too low. To acquire detailed images of the regional subsurface structure, we applied seismic data processing tools that are commonly used in oil exploration to crustal refraction data recorded along the Korea Crust Research Team 2008 (KCRT-2008) profile at irregular intervals. To use conventional seismic data processing tools, the data should be sampled regularly in both time and space domains. In this study, we applied the matching pursuit interpolation technique to regularise the KCRT-2008 data in the space domain, followed by dip filtering to attenuate coherent noise. Then, Kirchhoff prestack depth migration was applied using both refracted and reflected waves to image the subsurface structure at higher signal-to-noise ratios and increase spatial resolution using far offset data. The migrated section shows the interfaces of three mid-crustal layers at depths of 14.27 and 24.37–25.34 km and the downwarped Moho at a depth range of 29.3–36.1 km. In this study, high-resolution subsurface structures of the southern Korean Peninsula were delineated. To acquire images, we applied seismic data processing tools that are commonly used in oil exploration to irregularly sampled data after regularisation by matching pursuit interpolation.

External Gravity Field in the Korean Peninsula Area

The free-air anomalies are computed using a data set from various types of gravity measurements in the Korean Peninsula area. The gravity values extracted from the Earth Gravitational Model 2008 are used in the surrounding region. The upward continuation technique suggested by Dragomir is used in the computation of the external free-air anomalies at various altitudes. The integration radius 10 times the altitude is used in order to keep the accuracy of results and computational resources. The direct geodesic formula developed by Bowring is employed in integration. At the 1-km altitude, the free-air anomalies vary from -41.315 to 189.327 mgal with the standard deviation of 22.612 mgal. At the 3-km altitude, they vary from -36.478 to 156.209 mgal with the standard deviation of 20.641 mgal. At the 1,000-km altitude, they vary from 3.170 to 5.864 mgal with the standard deviation of 0.670 mgal. The predicted free-air anomalies at 3-km altitude are compared to the published free-air anomalies reduced from the airborne gravity measurements at the same altitude. The rms difference is 3.88 mgal. Considering the reported 2.21-mgal airborne gravity cross-over accuracy, this rms difference is not serious. Possible causes in the difference appear to be external free-air anomaly simulation errors in this work and/or the gravity reduction errors of the other. The external gravity field is predicted by adding the external free-air anomaly to the normal gravity computed using the closed form formula for the gravity above and below the surface of the ellipsoid. The predicted external gravity field in this work is expected to reasonably present the real external gravity field. This work seems to be the first structured research on the external free-air anomaly in the Korean Peninsula area, and the external gravity field can be used to improve the accuracy of the inertial navigation system.