Han-Joon Kim

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data : evidence for rifting affected by the hotter than normal mantle

Abstract Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.

A seismic experiment in the Ulleung Basin (Tsushima Basin), southwestern Japan Sea (East Sea of Korea)

Understanding the crustal structure of the Ulleung basin (Tsushima basin) in the southwestern Japan sea (East sea of Korea) is important for reconstructing the opening tectonics of the Japan sea. A Korea and Russia collaborative seismic experiment was carried out in 1991 to investigate the crustal structure of this basin using ocean bottom seismometers (OBSs) and large capacity air guns. The results of the experiment reveal that the Ulleung basin is floored by distinctive upper and lower crustal layers with velocities typical of layers 2 and 3. The lower crust, however, is unusually thick (7–8 km) and the Moho lies flat at 15–16 km depths. Though similar to the Yamato basin in crustal thickness, the Ulleung basin has a much thicker sedimentary sequence and a slightly thinner lower crust in the center. Geophysical evidence, thus, shows that the crust under the Ulleung basin has velocity range and crustal layering falling closer to those of normal oceanic crust than the Yamato basin. The formation mechanism of the thicker than normal lower crust should be clarified to better understand the complex opening process in the southern Japan sea.

Seabed classification from acoustic profiling data using the similarity index

We introduce the similarity index (SI) for the classification of the sea floor from acoustic profiling data. The essential part of our approach is the singular value decomposition of the data to extract a signal coherent trace-to-trace using the Karhunen-Loeve transform. SI is defined as the percentage of the energy of the coherent part contained in the bottom return signals. Important aspects of SI are that it is easily computed and that it represents the textural roughness of the sea floor as a function of grain size, hardness, and a degree of sediment sorting. In a real data example, we classified a section of the sea floor off Cheju Island south of the Korean Peninsula and compared the result with the sedimentology defined from direct sediment sampling and side scan sonar records. The comparison shows that SI can efficiently discriminate the bottom properties by delineating sediment-type boundaries and transition zones in more detail. Therefore, we propose that SI is an effective parameter for geoacoustic modeling.

Contrasting Development of the Latest Quaternary Slope Failures and Mass-Transport Deposits in the Ulleung Basin, East Sea (Japan Sea)

In order to understand how the variations in shelf morphology and sediment supply to the shelf within a basin can change the occurrence styles of slope failures and mass-transport deposits (MTDs), this chapter details dimensions and morphology of the latest Quaternary slope failures and MTDs on the western and southern margins of the Ulleung Basin. On the western margin, the slides and slumps have relatively small dimensions with a few small, scoop-shaped scars and gullies deeper than 700 m water depth. The downslope mass-flow deposits occur as small, solitary lobes restricted at the base-of-slope. On the western margin, the small sediment input to the shelf and the prominent Hupo Bank and Hupo Trough blocking sediment delivery to the slope probably caused relatively low accumulation of muddy sediments in the slope, most likely resulting in the small dimensions of slope failures, and the restricted occurrence of small MTDs at the base-of-slope. In contrast, the southern margin is characterized by large dimensions of gullied scars with huge slides and slumps deeper than 250 m water depth. These catastrophic failures evolved into extensive mass flows, which travelled downslope for several tens of kilometers. On the southern margin, the flat, broad shelf and the high sediment supply to the shelf during the last glacial period probably caused relatively high accumulation of mixed muddy and sandy sediments in the upper slope. These conditions could have promoted large-scale slope failures along the entire upper slope, forming the extensive occurrence of MTDs in the middle to lower slopes. This study provides an example that the variations in shelf morphology and sediment supply to the shelf within a basin can affect the styles of slope failures and MTDs by controlling sediment input and sediment types to the slope.

Analytic minimum information deconvolution and its application to ocean bottom seismometer data

Seismic refraction/wide angle reflection data for the study of the earths crust generally lack resolution due to the low frequency wavelets from seismic sources. Standard deconvolution techniques to improve resolution of the data are limited by phase-shifted waveforms such as supercritical reflections from the Moho as well as various refraction phases from the crust. We extended the minimum information deconvolution technique to the analytic formulation to deconvolve seismic data that contain phase-shifted wavelets. When applied to a seismogram, analytic deconvolution yields the amplitude of the earths impulse response function in the form of the signal envelope. An application example with ocean bottom seismometer data acquired along the Canadian continental margin of the Atlantic shows a remarkable improvement in resolution by compressing the wavelets effectively.

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.

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.