Gwang Hoon Lee

Distribution and Acoustic Characteristics of Shallow Gas in the Korea Strait Shelf Mud off SE Korea

Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, are accompanied by cone-shaped acoustic masking, which is probably the reflection from a narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly in the upper, recent mud of the KSSM. Neither permeable beds nor faults, which can act as vertical migration pathways for deep thermogenic gas, are evident in the recent mud. We interpret that the bacterial degradation of organic matter in situ is the main source for the gas in the KSSM. The upwelling off SE Korea may be an important source for the increased organic matter in the area.

Timing of trap formation in the southwestern margin of the Ulleung Basin, East Sea (Japan Sea) and implications for hydrocarbon accumulations

Contractional tectonism, associated with the backarc closure of the East Sea (Japan Sea), resulted in a series of thrust and anticline along the southern margin of the Ulleung Basin, providing numerous traps that include the Dolgorae and Gorae structures. Restoration of depth-converted seismic profiles suggests two phases of contractional deformation in the area: (1) the latest Early to late Late Miocene phase that created the Dolgorae structures and (2) the late Late Miocene to Quaternary (?) phase that created the Gorae structures. The second phase of contractional deformation was accompanied by transpressional strikeslip movements. The formation of the Dolgorae II and Gorae structures postdated the main phase of oil generation but overlapped and/or predated the main phase of gas generation. The Dolgorae I and III structures began to form during the later stage of oil generation but did not have significant relief until the beginning of the main phase of gas generation. Thus, potential for large oil accumulations in the area is probably limited. Older and deeper structures, predating the contractional deformation, may have accumulated oil if later tectonic movements had not disrupted their trap integrity.

Seismic reflection imaging of Quaternary faulting offshore the southeastern Korean Peninsula

The Yangsan Fault System (YFS) is a dominating tectonic structure in the southeastern part of the Korean Peninsula. The YFS consists of NNE-striking dextral strike-slip faults that are traced to the southeastern coast of the Korean Peninsula. We acquired high-resolution seismic profiles offshore the southeastern Korean Peninsula to investigate how the YFS extends offshore and constrain the age of fault activity using stratigraphic interpretation. The seismic profiles image near-vertical faults trending NE to NNE that constitute a fault zone similar to a duplex structure at a releasing bend of a right-lateral strike-slip fault. The faults are interpreted as an offshore extension of the Ilgwang fault that is a member of the YFS. Stratigraphic interpretation of seismic profiles indicates that the offshore faults were activated repeatedly in the Pliocene and Quaternary. The right-lateral activity of the Ilgwang fault is consistent with the current stress regime in and around the southeastern Korean Peninsula that dictates the P-axis direction in the E-W or ENE-WSW since the Pliocene.

Neotectonics of the Eastern Korean Margin Inferred from Back-arc Rifting Structure

Earthquakes occur frequently in the continental shelf and slope area of the Korean Peninsula in the East Sea (Japan Sea) although they are mostly not large in magnitude. This area constitutes the eastern Korean margin, marking a transitional structure from rifted continental crust to oceanic crust that resulted from lithospheric extension into breakup in a back-arc. We reviewed how the crustal structure of the eastern Korean margin was emplaced to understand its correlation with the present seismicity. Back-arc extension that caused rifting and breakup at the Korean margin took place sequentially from the northern to southern parts in the Late Oligocene through the Early Miocene. The stress regime of the Korean margin switched from extension to compression in the Middle Miocene, resulting from the collision of the Philippine Sea Plate with the Japan Arc. The structural lineations at the Korean margin inherited from backarc rifting and breakup are interpreted to be prone to earthquakes by showing a close spatial correlation with ongoing seismicity. The changing geometry of the estimated locus of breakup at the Korean margin that follows a curvilinear path appears to induce diverse focal mechanisms of the earthquakes under the present compressive stress field.

A simple method of correction for profile-length water-column height variations in high-resolution, shallow-water seismic data

In high-resolution, shallow-water seismic surveys, correction for water-column height variations caused by tides, weather, and currents is an important part of data processing. In this study, we present a very simple method of correction for profile-length (i.e., long-wavelength) water-column height variations for high-resolution seismic data using a reference bathymetric grid. First, the difference between the depth of the seafloor picked from seismic data and the bathymetry from the bathymetric grid is computed at the locations where the shot points of seismic profiles and the bathymetric grid points are collocated or closest. Then, the results are gridded and smoothed to obtain the profile-length water-column height variations for the survey area. Next, the water-column height variations for each seismic profile are extracted from the smoothed grid and converted to two-way traveltimes. The corrections for the remaining mis-ties at the intersections, computed within a circular region around each tie shot point, are added to the corrections for the water-column height variations. The final, mistie corrected water-column height corrections are loaded to the SEGY trace header of seismic data as a total static. We applied this method to the sparker data acquired from the shallow-water area off the western-central part of Korea where the tidal range is over 7 m. The corrections for water-column height variations range from -10 to 4 m with a median value of about -2 m. Large corrections occur locally between and near the islands probably due to the amplification and shortening in tidal wavelength caused by rapid shoaling toward the islands.

Crustal Structure and Tectonic Evolution of the East Sea

The East Sea (Japan Sea) is a back-arc basin lying on the southeastern Amur plate, separated from the Philippine Sea, Eurasia, and Pacific-Okhotsk plates by a complex border. The opening of the East Sea was initiated in the Early Oligocene by rifting and extension in the Japan Basin, followed by seafloor spreading in the Late Oligocene. The opening of the Japan Basin was achieved by a counterclockwise rotation of the NE Japan Arc. The extension in the Japan Basin propagated southwestward toward the northern part of the eastern Korean margin, inducing NW-SE rifting in the South Korea Plateau. In contrast, the southern part of the Korean margin underwent E-W extension before the Early Miocene, probably due to the N-S trending subduction off the SW Japan Arc. Back-arc rifting at the Korean margin gave way to breakup at the base of the continental slope, after which the SW Japan Arc was separated southeastward with a clockwise rotation, opening the Ulleung Basin. Part of the Ulleung Basin is underlain by thicker-than-normal oceanic crust generated by seafloor spreading with hotter-than-normal mantle temperature. The plate reorganization in East Asia in the early Middle Miocene led to the East Sea closure. The anticlinal structures in the southwestern Ulleung Basin margin show NW-SE or N-S compression in the Middle Miocene-Early Pliocene and NE-SW or E-W compression since the Late Pliocene. The incipient subduction along the eastern margin of the East Sea also suggests E-W compression since the Late Pliocene. The E-W compression is probably due to the eastward movement of the Amur plate that began in the Pliocene.

The Presence of the Breakup Unconformity in the Korea Plateau in the East Sea and its Tectonic Implications

한국대지(Korea Plateau)는 울릉분지의 북쪽에 위치한 고지형대로서 동해가 후열도해로서 형성되기 시작할 때 한반도의대륙주변부에서 리프팅이 발생한 대륙지각의 조각이다(Fig. 1)(Kim et al., 2007). 한국대지는 리프트분지들과 이들을 둘러싸고 있는 고지형대로 이루어져 있다. 한국대지에서 가장 두드러진 리프트분지는 반달분지(Bandal Basin)인데 융기된 리프트측면부가 북동-남서 방향으로 발달한 것은 이 분지가 북서-남동 방향으로 확장되었음을 지시한다(Fig. 1). Kim et al. (2007)은 다중채널 탄성파 자료로부터 한국대지의 리프트구조가 동해의 열림과 관련되어 형성되었다고 해석하였다. 그들은 특히반달분지의 북서-남동방향의 확장이 울릉분지의 열림 직전에진행된 대륙지각 리프팅의 최종단계를 나타내며 한반도의 대륙주변부는 후열도해에 속하지만 그 지구적 진화과정은 비활성 대륙주변부에서 발생하는 것과 매우 유사하다고 제시하였다. 대륙지각이 분리되고 새로운 해양지각이 형성된 비활성 대륙주변부의 경우 리프트가 일어난 시기(syn-rift)에 쌓인 퇴적층의 상부 경계면이 융기에 의해 대기에 노출된 후 침식을 받아서 부정합을 이루고 있는 것이 보고되고 있다(e.g., Braunand Beaumont, 1989). 이러한 부정합은 지각분리 부정합(breakup unconformity)으로 불리며 비활성 대륙주변부의 특징중 하나로서 인식된다(e.g., Falvey, 1974). 이 연구에서는 한국대지 내 반달분지에 쌓인 퇴적층내에 존재하는 지각분리 부정합을 설명하고 이와 관련한 지구조적 운동을 해석하였다.