Duk Kee Lee

Deep geoelectrical structure in and around the southern Korean Peninsula by GDS study

This paper presents lateral conductivity variations in the Earths crust in and around the southern Korean Peninsula, which were mapped using magnetic field variations recorded at ten sites in Korea. GDS data can provide an efficient way to identify crustal units having different conductivities. And a three-dimensional (3-D) magnetotelluric (MT) modeling based on the induction arrows revealed a major conductivity anomaly in and around the Korean Peninsula. The comparison of observed and modeled induction arrows strongly suggests evidence of a relationship between main tectonic belts and the deep conductivity anomalies in this area. The overall pattern of induction arrows in this area appears to indicate a northwest-southeast direction, which is similar to that of the so-called ‘sea effect’, affected by the surrounding sea. However, the results of observations in the middle of the peninsula imply an anomalous pattern in the tectonic area near the Imjin River Belt. Induction arrows in the mid-southern area appear to be related to a thick sedimentary pile, the Ogcheon Belt, which forms another tectonic boundary in the Korean Peninsula. The overall pattern of arrows observed in the coastal area and islands was affected by the sea effect. The pattern, which shows southward arrows similar to those in the previous study of the southern coastline of the peninsula and Jeju Island near Kyushu, Japan, might indicate highly conductive anomalies (HCL) (Shimoizumi et al., 1997). However, the sea effect seems to be weak in the western coastal region, probably because the Yellow Sea is much shallower than the East Sea (Sea of Japan), and this phenomenon leads to the possibility of discerning an anomalous pattern beneath the Yellow Sea.

Gravity models for the South Shetland Trench and the Shackleton Fracture Zone, Antarctica

Four deep crustal models of the South Shetland Trench (SST) and the Shackleton Fracture Zone (SFZ) off the northern Antarctic Peninsula are presented based on gravity data. The gravity models of the SST suggest that the dip of the subducting crust increases from southwest to northeast ranging from 25° in Line KSL93-5 to 30° in Line KSL93-6 as the age of crust increases along the trench axis. The gravity low observed near the island are is directly associated with the deep forearc basin bounded seaward by a large fault. In the SFZ, the thin crust is concentrated primarily beneath the ridge, where Moho shallows by 2.5 km. A low-density material (about 2.45 g/cm3) assigned to the SFZ ridge is presumably due to serpentinite intrusion. The gravity low associated with the trough is due to the relatively low-density sediments filled in the trough. The crustal thickening to the western slope of the ridge near the triple junction was probably caused by the collision of the SFZ ridge with the Shetland Platform.

Did the 12 September 2016 Gyeongju, South Korea earthquake cause surface deformation?

An earthquake with a local magnitude (ML) of 5.8 occurred on 12 September 2016 near Gyeongju, South Korea. This earthquake was the largest event on record in Korea since 1978. A relatively large (ML 5.1) foreshock preceded the main shock by about 50 min, and numerous aftershocks followed. In this study, we performed seismological and geodetic analyses to determine the possibility of the occurrence of surface deformation. Estimated surface deformation using seismological analysis was less than 1 cm, and that observed by geodetic (GNSS and InSAR) data was within the range of error. These results indicate that no surface deformation occurred due to this earthquake. This may have been due to relatively small size of the fault plane (4 km × 4 km), moderate moment magnitude (Mw 5.5) or deep focal depth (15.4 km) of the earthquake.

Numerical Simulations of the 2011 Tohoku, Japan Tsunami Forerunner Observed in Korea using the Bathymetry Effect

The 2011 Tohoku, Japan Tsunami, which occurred on March 11, 2011, reached the Korean Peninsula and was recorded at numerous tide stations. In the records of the north-eastern tide stations, tsunami forerunners were found in only about a few minutes after the earthquake, which was much earlier than the expected arrival time based on a numerical simulation. Murotani et al. (2015) found out that the bathymetry effect is related to the tsunami forerunners observed in Japan and Russia. In this study, the tsunami forerunners observed in Korea were well reproduced by a numerical simulation considering the bathymetry effect. This indicates that it is important to consider the bathymetry effect for a tsunami caused by an earthquake on shallowly dipping fault plane(e.g. 2011 Tohoku, Japan Earthquake). However, since the bathymetry effect requires additional computation time, it is necessary to examine the problems that results from applying the bathymetry effect to the tsunami warning system.

Real-time linked common-view time transfer for monitoring GNSS system time differences

This paper presents results obtained by conducting real-time linked common-view (CV) Global Navigation Satellite System (GNSS) time transfer between stations that are supposed to maintain various GNSS system times including those of GPS and Galileo. The distance between those stations is typically more than 5000?km and the conventional CV GPS time transfer may not be feasible for such long-distance time transfer. We used the real-time linked CV time transfer technique to monitor the GNSS system time differences, and the time links examined in this study are the USNO?PTB link for the GPS?Galileo system time difference and the USNO?NICT link for the GPS?QZSS system time difference. The performance of the developed time transfer method was assessed by comparing against other time transfer solutions such as TWSTFT. Finally, global closure was formed to examine the inherent noise level of the method in a self-sufficient way. Results showed that the real-time linked CV time transfer solution yields sub-nanosecond-level agreement with the reference solutions including TWSTFT, and its frequency stability is generally at the level of (3?4)???10?15 in terms of the modified Allan deviation at the averaging time of one day, which seems to be sufficient to support civil interoperability between different GNSSs for sub-metre-level accuracy.