Seafloor depressions, called pockmarks, are widespread morphologic features associated with fluid seepage; however, mega-pockmarks (> 1 km in diameter) are rare and their formation and long-term evolution remain poorly understood. In this study, we discover an uncharted mega-pockmark on the southwestern continental shelf of the East Sea (Sea of Japan) using multibeam echosounder (MBES), multi-channel seismic (MCS), and sub-bottom chirp seismic reflection data, and four piston cores. This mega-pockmark, which predates ~11 cal ky BP, covers an area of ~5 km2 on the seafloor, occurs in water depths of 125–142 m, and has steep walls reaching 3–4 m in height. With a diameter of 2.9 km, it ranks among the largest known fluid-escape structures documented on continental shelves. MBES and MCS reflection data show that the mega-pockmark occurs immediately above a NE-SW-tending growth anticline. The anticline deforms a ~ 80-m-thick sequence of shelf deposits that are disrupted by bending moment faults and vertical seismic chimneys. We interpret these extensional fractures and chimneys as primary conduits for upward migration of gas and pore fluids sourced from strata below the unconformity, and that this fluid migration led to formation of the mega-pockmark. Within the mega-pockmark, bathymetric mounds up to 13 m high occur above these chimneys; we interpret these as possible carbonate or carbonate-encrusted mounds based on their co-occurrence with gas plumes on the water column and seismic chimneys in the subsurface. Piston cores recovered inside the mega-pockmark contain clean, shelly and gravelly shelly sands, coral fragments, and bioclastic material with significant radiocarbon age reversals (ranging from ~3 to 46.5 cal ky BP). The mixed ages, together with spatial variability in siliciclastic grain size and the presence of erosional moats around the mounds, indicate that although the initial excavation was likely driven by pre-Holocene episodic focused fluid escape, possibly including catastrophic blowout events, the original pockmark morphology has been modified by bottom currents. Our study extends the global inventory of submarine mega-pockmarks and documents how actively growing anticlines can favor the location and longevity of mega-pockmarks on continental shelves.

Grain size analysis of the terrigenous fraction of sediments from IODP Site U1501 was used to characterize depositional patterns in the northern South China Sea since the Middle Eocene (similar to 35 Ma). Fine sand dominates the lower sequence (similar to 457-440 m CSF-A (core depth below sea floor - A)), reflecting turbidite influence on the Chinese continental shelf (similar to 35-34 Ma). This is followed by a fining-upward trend (4-8 phi) marking the transition to hemipelagic deposition mostly devoid of turbidites. A new silt-dominated sedimentation regime became established at similar to 30 Ma, which correlates with the initial expansion of the Pearl River. The regime is accompanied by stable source geochemical proxies (e.g. Zr/TiO2 values) and remains unaffected by an unconformity (T60) and hiatus at similar to 26.8-24.36 Ma. Grain size changes with stable geochemical proxies require variable intensities of terrigenous influx, which we propose reflect the dynamics of surface buoyant plumes and their controlling factors. At similar to 16 Ma, sediment fines abruptly (4.7-7.5 phi) and grain size remains constant to the present, reflecting drier conditions in the source area that began shortly afterward. The observed patterns suggest that grain size data from hemipelagic sediments devoid of turbidites can be combined with source tracers to help reconstruct the intensity of buoyant plumes over time.

This study reveals that submarine landslides could pose a significant tsunami threat to the northeast (NE) coast of South Korea, with potentials of causing large damages and casualties. Recent bathymetric surveys identified four large submarine landslides (namely SL1 to SL4) at water depth ranging from 400 to 600 m in the southwestern East Sea (Sea of Japan), with volumes ranging between 2.1 and 4.4 km3. We evaluated tsunami potentials of these landslides with the COMCOT model. Numerical results show that these landslides, particularly SL3, could cause significant tsunami impact along the NE coast, with tsunami heights up to 8.0 m and current speeds up to 8.2 m/s at the coastline. Major cities along this coast, such as Gangneung and Donghae, are at risk of very short tsunami travel times (6–12 min) in all the simulated scenarios. The simulations also predict 200 m inundation extent on land with flow depths of 2 m in low-lying areas of coastal cities (e.g., Gangneung), indicating high potential of damages to buildings and infrastructure. We also examine major parameters that dominate the interaction between the submarine landslide and the resulting tsunami heights. Our analyses indicate that the rate of displaced volume of material with time, is closely linked to the steepness of the slip surface, significantly affects the tsunami heights. Additionally, comparisons with historical tsunami records on the NE coast of Korea suggest that local submarine landslides could cause much greater coastal impacts than the past, distant earthquake-triggered tsunamis. Our findings provide new insights into the understanding of tsunami hazards in the NE coast of Korea, as well as valuable information for tsunami education, exposure analysis, and mitigation planning in this region.

Shallow gas escaping from the seafloor is normally observed in poor sedimentary layers and geological structures accompanied by faults, cracks, and fractures. Gas venting, the migration of the fluid between pores, causes seafloor deformation such as pockmarks and can trigger large-scale geohazards such as submarine sliding and tsunamis, so multi-year monitoring is required.After first discovering a gas flare in the southeastern continental shelf area of the East Sea, Korea in 2019, we conducted multi-scaled seismic and acoustic surveys using R/V TamhaeⅡ from 2021 to 2023, except for 2020. In 2019, EK60, sub-bottom profiler (SBP), and high-resolution seismic (HRS) data were acquired, and EK60 and SBP data were acquired in 2021. In 2022, EK60, multi-beam echo sounder (MBES), SBP, and conventional seismic data were acquired, and in 2023, EK60, MBES, and SBP data were acquired. In 2019 and 2021, MBES data was only acquired to detect seafloor deformation such as the pockmark, while water column data using MBES began to be recorded to detect flares from 2022. The flare size from the seafloor to the sea surface was measured in the EK60 data, while the quantification study on the gas flow rate using the ESP3 software and the VBALab plugin has been tried since 2022. Through EK60 data acquired over 4 years, it can be estimated that gas venting periodically rather than continuously. MBES data presents evidence of a lot of gas-related seafloor deformation in the study area. Since the first exploration of a new R/V TamhaeⅢ installed EK80, parametric SBP, and acoustic Doppler current profiler (ADCP) will begin this May, we expect that a high-quality seismic and acoustic dataset will be obtained for the site of gas flare. For further research, it will be necessary for sea-water and geological sampling to analyze gas components, and detailed monitoring using ROV and seafloor observation systems installed with a camera should be accompanied to quantify the gas flow rate.

Tephra layers embedded in marine sediments can be significant in controlling submarine landslide dynamics and seafloor morphology, but their preconditioning effects on slope failure remain uncertain. Here, we study the morphology and preconditioning factors of submarine landslides from a volcanically active region, the Eastern South Korea Plateau (ESKP) using a recently acquired multibeam echosounder (MBES), high-resolution sub-bottom chirp profiler, and piston core data, together with previous geophysical data. At least 50 translational landslides were identified on the southern margin of the ESKP in an area of-470 km2, with a relatively small volume of remobilized sediment (-10 km3). The landslide headscarps are arcuate, up to 400 m in height, and lie at water depths of 980 to 2300 m. Landslides on the upper ESKP margin are more disintegrative (debris flow type) than those on the lower margin which consists largely of blocky type failures with small runouts. Landslide deposits identified in the cores consist of debris flows and slides/slumps while background deposits contain hemipelagic mud interbedded with ca. 10-cm-thick coarse-grained, poorly-sorted pumiceous tephra. Seismic and core integration indicate that the glide planes of the observed slides correspond to the pumice-rich tephra layers. These tephras are predominantly composed of fresh volcanic glass devoid of clay, indicating that the composition of the tephra did not control the formation of weak layers or slide planes. We infer therefore that the weakness of the glide planes resulted from high hydraulic (pore) pressure at the interface between some of the porous tephra layers and their overlying, relatively impermeable, hemipelagic mud. In addition to burial compaction and build-up of pore pressure in the tephra layers, fluid overpressure could have been facilitated by earthquakes causing granular convection within, and compaction of, the poorly sorted tephra. The landslide predominantly formed > ca. 84 ka ago, which suggests time-restricted triggering associated with a temporary increase in seismicity and/or sediments reaching their overpressure threshold during burial compaction. Our study suggests that regional occurrences of discrete layers of porous tephra deposits within finer grained hemipelagic sediments can potentially control the generation of landslides in subaqueous environments affected by explosive volcanism.

Glide planes, the basal surface or failure surface upon which submarine landslides initiate, commonly develop along weak, distinctive stratigraphic horizons but their lithological/mechanical characteristics and genetic mechanisms remain largely unknown. We use 2-D multi-channel seismic reflection data, integrated with multibeam bathymetry and deep drilling data from the Ulleung Basin margins, East (Japan) Sea, to: (1) identify and characterize the nature of glide planes associated with submarine landslides; (2) understand the influence of climate-modulated factors in preconditioning slope failures; and (3) document the post-failure evolution of the landslides. 24 glide planes were identified among 38 submarine slides (SL1 – SL38), which correspond to regionally continuous, positive-polarity high-amplitude seismic reflections. Well-seismic integration support ca. 340 ka – 1,200 ka ages of formation of the major glide planes in the southwestern and western margins of the basin. These glide planes developed at the interface between clay-rich sediment deposited during glacial periods and biogenic diatom-rich sediments deposited during interglacial periods. Physical, mineralogical and geochemical properties determined by density, porosity, gamma-ray, shear strength, X-ray diffraction, and X-ray fluorescence data reveal significant lithological and mechanical changes at the interface between these two lithologies. We therefore infer that these interfaces dictate the position of failure surfaces, with the diatom-rich layers acting as a weak layer. Excess pore pressure in these layers is likely due to initial high-water contents (up to 75%) and high compressibility; this is considered an important pre-condition for failure. In contrast, the glide planes along the northwestern margin of the Ulleung Basin (SL34 – 37) are older (ca. 1,200 ka – 2,140 ka). Seismic data further reveal three distinct contrasting styles of landslide post-failure behavior throughout the margins: (1) evacuated slide scars with areas of smooth seafloor; (2) slide scars with residual debris consisting of blocky sediments; and (3) slide scars with buried intact sediment blocks in front of the headwalls. Lateral variability of fluid flow, sediment composition, and mechanical properties of basal 'weak' layer(s), or the magnitude of earthquakes may have contributed to forming different types of mass-transport deposits (MTDs). Overall, these results show that landslide formation in the East (Japan) Sea result from a complex climatic, volcanic and tectonic interplay that controlled the formation of weak layers. Some of these layers extend regionally and can be identified and mapped by remote geophysical methods and targeted drilling.

Sedimentary beds of alternating red and green colour are commonly interpreted to reflect orbitally‐forced cyclic climatic, syn‐depositional conditions, although colour changes caused by post‐depositional fluids are also documented. Results from IODP Hole U1502A marine sediments in the South China Sea exemplify post‐depositional reducing fluid–rock interactions that locally changed the sediment colour from red to green. Petrographic, rock magnetic and paleomagnetic data on cores show that the red colouration originates from an early, basin‐wide, pervasive diagenetic oxidation event (forming haematite), whereas the green colouration results from subsequent fluid‐driven reduction (forming pyrrhotite‐magnetite). The dense sulfidic stockwork in the basaltic basement underlying these sediments was the likely source of reducing fluids. Drilling deep holes into marine basin basements can thus provide useful information on fluid transfer from the basement to the overlying sedimentary layers.

Since most of the shallow gas is composed of methane, studies on its availability as a resource and global warming are being conducted. A gas flare is called a phenomenon in which shallow gas escapes from the sub-bottom into the seawater. Gas flares cause seafloor deformation and can trigger large-scale geohazards such as landslides and tsunamis. We discovered the gas flares in 2021 and 2022 by conducting seismic and acoustic surveys using R/V TamaheⅡ seismic vessel at the southeastern continental shelf of the East Sea in Korea. The gas flares were detected on the water column data obtained by an EK60 of 38 kHz frequency and a multi-beam echo sounder of 20 – 40 kHz frequency bands (Kongsberg EM2040). We observed the deformation of the seafloor and sub-bottom using a Chirp sub-bottom profiler (SBP) of 2 – 7 kHz frequency bands (FalMouth HMS-622 CHIRPceiver). The water depth of the survey area ranges from 130 to 140 m. Four gas flares are distributed within approximately 3.5 km in a northwest-southeast direction. The height of the gas flare is about a maximum of 100 m from the seafloor. The seafloor where the gas escaped was observed to deform into dome shapes and pockmark. Additionally, we performed the seismic survey using a 60 in3 mini GI gun and a 48-channel streamer cable with a 12.5 m group interval to detect the source layer of shallow gas and the migration pathways. We tried using VBALab software to quantify the gas flow rate on the acoustic data of EK60.

An uncharted field of sand waves was discovered in a low-relief submarine canyon incised in the outer shelf on the southeastern continental margin of the Korean Peninsula in water depths of 180-190 m. We characterize the nature and origin of the waves and the sand forming them using sub-bottom chirp profiles, eXpendable bathythermograph (XBT) profile, multibeam echosounder (MBES) data, and sediment samples from four piston cores. Two types of sand waves characterized by distinct height versus wavelength relationships were found in the study area. The sand waves in the upper, narrower part of the shelf-incised canyon are sinuous-crested, with amplitudes of 0.3-2.1 m (mean: similar to 1 m) and wavelengths of 10-45 m (mean: similar to 24 m). Their asymmetry indicates migration upslope in a southwesterly direction, opposite to the surface currents. In contrast, the lower part of the canyon that is wider and closer to the margin of the continental shelf hosts nine long (ca. 1 km) curvilinearcrested sand waves with symmetrical crests; these waves likely reflect transient bedforms forming under fluctuating current conditions. The sediment of the sand waves consists of a variable mixture of siliciclastic and carbonate materials. The carbonate fraction (similar to 22-55%; mean: similar to 34%) is derived mainly from the remains of bryozoans, bivalves, echinoderms, foraminifers, gastropods, and serpulids. Six bioclasts were dated by the radiocarbon method between ca. 41.3 and 11.8 ka BP. These relatively old ages and palaeontological data supports reworking from a shallowmarine environment during the last glacial transgression and limited sedimentation/sediment supply in the study area. The siliciclastic fraction (similar to 44-79%; mean: similar to 37%) is composed of rounded to subrounded quartz and feldspar of moderate to good sorting and a mean grain size of similar to 1.3 phi (medium sand). The uppermost similar to 30 cm of all the sand wave cores reveals a decrease in the grain size of the siliciclastic fraction coupled with an increase in the carbonate/siliciclastic ratio, suggesting episodic sediment reworking and migration of the sand waves in response to fluctuating bottom currents. The coarser sediment that forms the core of the sand waves records bedload transport during periods of stronger currents. Finer carbonate-rich pelagic sediment (i.e., plankton) accumulated at the top of the sand waves during periods of weaker bottom currents. Significantly, our results show that the grain size and mineralogy of the sediment composing the sand waves are controlled by changes in hydrodynamic conditions. Our study provides novel geomorphological evidence for the influence of SW-flowing cold-water incursions (Korean Strait Bottom Cold Water) on the seafloor sediments.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Recommended Citation Jian, Z., Larsen, H., Zarikian, C., Jian, Z., Christian Larsen, H., Alvarez Zarikian, C., Bowden, S., Cukur, D., Dadd, K., Ding, W., Ferré, E., Ferreira, F., Gewecke, A., Huang, E., Jiang, S., Jin, H., Kurzawski, R., Li, Y., Li, B., Lin, J., Liu, C., Liu, C., Mohn, G., Ningthoujam, L., Osono, N., Peate, D., Persaud, P., Qiu, N., Satolli, S., Schindlbeck, J., Straub, S., Tian, L., & Van Der Zwan, F. (2018). International ocean discovery program expedition 368 preliminary report: South China Sea Rifted Margin Testing hypotheses for lithosphere thinning during continental breakup: Drilling at the South China Sea rifted margin. Integrated Ocean Drilling Program: Preliminary Reports (368), 1-54. https://doi.org/10.14379/iodp.pr.368.2018

About 1000 km of high-resolution seismic reflection profiles were used to study the morphology, distribution, nature, and sequence stratigraphy of incised valleys and valley fills within the Quaternary (<ca. 2.0 Ma) sedimentary section of the southeastern Korean continental shelf, East Sea. Seven sequences, S1 to S7 from bottom to top, were analyzed on the modern continental shelf. These sequences comprise a -300 m thick sedimentary section containing, at minimum, one cycle of sea-level lowstand, transgression, and high stand deposits. The lowstand sections are composed mainly of laterally and vertically accreting fluvial channel deposits (i.e., point bars, channel lags) formed on an alluvial plain during a lowstand in sea level. The transgressive sections consist of confined, low-to moderate-amplitude continuous reflections interpreted as estuarine facies. The high stand sections, in contrast, are comprised of laterally extensive low amplitude reflections interpreted as open marine facies. Each of these sequences rests unconformably on its? predecessor, particularly at the mid-southeastern edge of the shelf where angular, toplap, and onlapping relationships can be discerned. The sequence boundaries (SB1 to SB7) bounding these sequences are produced by fluvial incision (i.e., valleys and channels) during sea-level fall. SB4, SB5, and SB7 form the most prominent unconformities on the shelf and are incised by extensive paleo-incised valleys and channels. These valleys and distributary channels display simple U- or V-shaped cross sections or complex morphologies; they are -1?8 km wide and -10?60 m deep. Incised-valley fills reach a maximum thickness of over 80 m. The largest incised valley fills (-8 km wide, -60 m thick) are found near the shelf break in the SE, indicating relative sea-level fall on this location (ca. 150 m lower than the present sea level). In the SE, the incised valleys are comprised dominantly of stacked lowstand successions (point bars and channel lags) and a lesser degree of transgressive successions (estuarine mud and levees) that are capped by subsequent high stand deposits (marine mud). The youngest channel above the valley was inferred to have been formed during the Last Glacial Maximum (LGM) on the shelf edge trajectory about 140 m lower than the present. These large incised valleys are thought to have developed through a complex history of repeated avulsion and reoccupation during the lowstand sea level. The most important factors regulating the valley morphology are the duration of sea-level fall, lithologic variations, climate (i.e., fluvial discharge), and the river gradient relative to the shelf gradient. The ancient drainage patterns are more-or-less northeast-southwest orientated, consistent with the trend of the present-day submerged fluvial channel. Superimposed upon this general pattern are the effects of tectonic activity (uplift, folds, and faults), which appear to have modified the courses of the rivers. Our results, therefore, provide a good example of how external factors (i.e., climate, tectonics, sediment supply, and paleogeomorphology) can determine the location and geometry of the valleys and the architecture of the valley-filling deposits of continental shelves.

High resolution multi-channel seismic reflection data (-1000 km) and multibeam echosounder bathymetry from the southeastern Korean continental shelf of the East Sea (Japan Sea) reveal numerous shallow gas indicators and seepage-related features, such as bright spots, enhanced reflections, seismic chimneys, acoustic blanking, pockmarks, and bathymetric mounds. Bright spots, indicating gas-charged layers, appear as local negativepolarity reflection anomalies (up to 5 km wide) and occur at various stratigraphic levels within a subsurface depth of -320 m. Bright spots covering an area of -60 km2 are clustered at the tip of NE-SW-trending reverse faults in the northeastern and southeastern part of the investigated region, suggesting gas entrapment. Enhanced reflections (ca. 20-km-long) are developed along erosional unconformities and tilted sedimentary layers below them. This suggests that unconformities formed during sea-level low stands in the study area are potential reservoirs and may have acted as potential conduits for lateral migration of gas-rich fluids due to their permeable nature. Some enhanced reflections are formed along interfluves of channels where channel walls cut them, and thus they may potentially act as fluid reservoirs. Seismic chimneys, expressed as vertical disturbances in seismic data, are interpreted as the upward movement of fluids (i.e., either in liquid or gaseous form). Lack of faulting in some seismic chimneys suggests higher permeability in the sedimentary interval, which would allow the migration of deeper-sourced fluids. Pockmarks (up to 500 m in diameter) are typically associated with seismic chimneys in the sub-seabed, suggesting that they were formed by the explosive emission of gas or gas fluids. Some exhibit mound-like features near their crests that are interpreted as carbonate mounds. The locations of mounds above uplifted fault blocks in the central part suggest a structural control on the formation of these seabed features.

ABSTRACTWe analyzed data from seven piston cores, multi-channel seismic-reflection (MCS) and chirp profiles, and multibeam echosounder (MBES) data to study the distribution, emplacement time, sedimentary facies, and depositional processes of sediment-gravity-flow deposits in the Onnuri Basin, a confined basin in the East Sea. These data reveal that debris flows have traveled ca. 30 km downslope, forming a seismic facies consisting of stacked, wedge-shaped, transparent units separated by high-amplitude continuous reflectors. Analysis of piston cores shows three distinct sedimentary units, throughout the basin. The lowest unit, I, is a debrite containing numerous mud clasts of varying size and color distributed in a mud-rich matrix; it is absent over elevated basinal highs or ridges, such as the Onnuri Ridge, suggesting that local topography controls its distribution. The debrite forms a recognizable acoustically transparent layer on subbottom chirp profiles (av. 7 m thick), covers approximately 500 km2, and has an estimated volume of ∼ 3.5 km3.The overlying unit, II, contains normally graded beds composed of massive sand, laminated and cross-laminated sand and silt, and a thick cap of structureless mud. This unit is interpreted to be a megaturbidite deposited from turbidity currents that originated from the flow transformation of debris flows on the upper continental slope. The megaturbidite covers the entire basin (at least 650 km2), and has an average thickness of 2.8 m (maximum thickness of 4.35 m), and comprises a volume of 1.8 km3. Variations in grain size and sedimentary structures suggest that the megaturbidite was deposited by progressively waning flows that reflected off basin flanks and ridges. The thick (up to 3.65 m) structureless mud cap further indicates deposition in a confined basin. The sharp basal contact, together with the lack of hemipelagic sediments between debrite and overlying megaturbidite, suggest that both were deposited during the same flow event, likely to have originated from a single catastrophic slope failure. Collapsing slide material evolved into a debris flow, from which a turbidite formed by dilution of the debris flow. Radiocarbon dates suggest that the slope failure occurred about 13–11 ka, a time when sea level was ca. 50 m lower than at the present day. Hemipelagic sediments in the topmost unit, III-2, above the megaturbidite indicate that the basin has been stable since ca. 11 ka.We provide robust evidence that submarine slope failures evolve downslope into slides, debris flows, and finally, thick megaturbidites. This contribution highlights the importance of seafloor morphology on the distribution and stratigraphy of submarine flows in confined basins.

This study investigates the evidence of shallow gas from a newly collected dataset comprising 2D multi-channel seismic (MCS), single-channel seismic (SCS), Chirp sub-bottom profiler (SBP), and multi-beam echo sounder (MBES) data from the southwestern continental shelf of the Ulleung Basin, East Sea, Korea. Various indicators of shallow gas were identified in this part the shelf, including seismic chimneys, acoustic flares in the water column, pockmarks, enhanced reflections, and acoustic blanking. Seismic chimneys, which are related to fluid leakage within the subsurface, are characterized weak to high amplitude, upturned or concave-upward internal reflection with polarity reversal in MCS section, whereas they are faint or transparent on SCS section with 700 Hz and Chirp SBP sections with 3.5 kHz frequency. Acoustic flares, which are diagnostics of active gas venting, were detected only on the Chirp SBP sections. There is also evidence of small-scale depressions, immediately below the acoustic flares; these are interpreted as formed by the sudden explosion of gas. Enhanced reflections (similar to 3 km-long) concordant with stratification are observed at different levels. Acoustic blanking anomalies, caused by absorption of acoustic energy due to the presence of gas, are seen in the range 50-200 ms TWT below seabed. Amplitude versus offset (AVO) analysis indicates the presence of free gas ca. 350 ms TWT below sea surface using the MCS dataset recorded by a 600 m-long streamer cable. The AVO cross-section shows that the lower repetitive signals in the MCS data are due to the gas plume, not lithological contrast. Our study highlights that AVO analysis is an extremely useful tool for identifying free gas, helping to discriminate water-gas contacts and bright events among the chaotic signals on the MCS data.

In marine seismic surveys, various acquisition systems are used depending on the survey purpose, target depth, survey environment, and conditions. A 3D survey of oil and/or gas exploration, for instance, require large-capacity air-gun arrays and six or more streamers with a minimum length of 6 km. In contrast, a high-resolution seismic survey for the shallow-water geological research and engineering needs a small capacity source such as air-gun, sparker, and boomer, deployed with a single-channel or multi-channel (24-channel) streamers. The main purpose of our seismic survey was to investigate the Quaternary geology and stratigraphy of offshore, Korea. Because the Quaternary is the most recent geological period, our target depth was very shallow at about 50 m below the sea-bottom. We used a high-frequency seismic source including a sparker of 2,000 J capacity or a 60 in3 mini GI-gun and an eight-channel streamer with a 3.125 m group interval or a single-channel streamer that included 96 elements. To compare the resolution of seismic data according to the seismic source, a boomer or sparker systems were used with the single-channel streamer on a small survey ship. The seismic data processing was performed at the Korea Institute of Geoscience and Mineral Resources (KIGAM) with ProMAX, and the data processing and resolution of each survey were compared based on their acquisition systems.

Submarine landslides are common geomorphological features of continental margins. Some of the largest submarine landslides occurred on low-angle (< 4 degrees), sediment-starved margins, yet their preconditioning and trigger mechanisms are still largely unconstrained. The southwestern continental margin of the East Sea (between 37.5 degrees N and 38.0 degrees N), Korea, occupies a narrow shelf (< 10 km), is characterized by low sedimentation rates (similar to 3-7 cm /ka) with an average gradient of less than 2 degrees. Here, we investigate submarine landslides using newly collected datasets including multibeam echosounder (MBES), chirp sub-bottom profiler, multichannel seismic (MCS) data and ten piston cores. MBES data from the margin reveal at least four major submarine landslides initiated at depths of 400 m to over 600 m. These landslides left clear headwall scarps on the seafloor with reliefs reaching over similar to 130 m and extend for over 40 km. MCS data show that some of the failures have resulted in the complete disintegration of the failed mass, while others have resulted in the deposition of well-defined hummocky debris flows. Sediments recovered downslope of the headwall scarps contain slides and debris flow deposits and turbidites that are overlain by bioturbated hemipelagic layers. Radiocarbon dating from hemipelagic units overlying MTDs within the headwall scarps reveal that major failures occurred at ca. 11 to 19 ka, coinciding with the time of the Last Glacial Maximum (LGM) to early deglaciation. Since then, hemipelagic sedimentation has prevailed throughout the sediment starved slope. Slope stability analyses based on geotechnical properties of sediments indicate that all areas are stable under static, and even stable under loads derived from earthquakes in instrumental records, but there were probably earthquakes in pre-historical records (i.e., with a longer recurrence interval) of potentially significant larger magnitude. We suggest that the preferential occurrence of major failures adjacent to the major faults on the lower slope may ultimately be tectonic-controlled although other factors may have contributed as well. Our work shows that coarse-grained clastic sediments are abundant in the shallow subsurface and that these higher-permeability units, often identified as weak layers, would focus fluid flow and could act as slip planes for slope failure. Our data also indicate that tectonic steepening and gas charging are other key parameters for controlling instability in sediment-starved margins.

In the western margin of the Ulleung Basin, a detailed analysis of cores with geophysical data from a fan-shaped body, just downslope of a submarine gully associated upslope with failure scars, reveals various modes of mass-transport processes. The arcuate failure scars occurs in water depths exceeding 600 m. The fan-shaped body, less than ca. 10 km long in radius, displays strong backscatter intensity in sonar images, and corresponds to the uppermost transparent mass in Chirp sub-bottom profiles. Sediment cores penetrating to the uppermost transparent mass consist mostly of various facies of mass-transport deposits (MTDs), causing the strong back-intensity in the sonar images. The interval of MTD facies comprises the upper and lower units without hemi-pelagic muds between them, implying that the fan-shaped body was probably deposited during a single event separated in at least two stages without a significant time break. The lower unit shows brittle to plastic deformation of soft muds (slides/slumps), whereas the upper units exhibits fully fragmented soft mud clasts (low viscous debris flows). Both the upper and lower units involve same original lithology (i.e., soft hemi-pelagic mud) prior to failures, suggesting that the lithology could not significantly affect depositional processes. The fully fragmented soft mud clasts of the upper unit are probably indicative of more shearing than the brittle to plastic deformation of soft muds in the lower unit. Considering the small dimension of the failure scars/gully and the same original lithology, the more shearing of the upper unit was most likely caused by longer transport distance than that of the lower unit. The rare turbidites with absence of channellevee systems in the fan-shaped body and the failure scars confined in the upper to middle slopes suggest that the submarine gully probably formed by slope failures, not by erosion of turbidity currents.

In this study, the distribution and provenance of sandy surface sediments were investigated along the northeastern continental shelf of Korea. A total of 154 surface sediment samples from river, beach, continental shelf and shelf edge substrates were collected and analyzed for their grain size and mineral composition. Coarse-grained sandy sediments (mean grain size, -1.3 to 1.0 phi) showed that the inner shelf and shelf break were clearly separated by outer shelf silt sediment (mean grain size, similar to 5 phi). The mineral composition of continental shelf sandy sediments differed between Gangneung and Donghae sites. Sandy sediments from Gangneung site consist mainly of quartz, feldspar, and mica, whereas those from Donghae site are composed chiefly of opaque minerals, including quartz and feldspar. Differences in mineral composition between the Gangneung and Donghae sites reflect the influence of geology on land. Sandy sediments were supplied directly to shelf edges during the low stand by small rivers, and not mixed by coastal currents.