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The Formosa Ridge cold seep is among the first documented active seeps on the northern South China Sea passive margin slope. Although this system has been the focus of scientific studies for decades, the geological factors controlling gas release are not well understood due to a lack of constraints of the subsurface structure and seepage history. Here, we use high‐resolution 3D seismic data to image stratigraphic and structural relationships associated with fluid expulsion, which provide spatio‐temporal constraints on the gas hydrate system at depth and methane seepage at modern and paleo seafloors. Gas has accumulated beneath the base of gas hydrate stability to a critical thickness, causing hydraulic fracturing, propagation of a vertical gas conduit, and morphological features (mounds) at paleo‐seafloor horizons. These mounds record multiple distinct gas migration episodes between 300,000 and 127,000 years ago, separated by periods of dormancy. Episodic seepage still seems to occur at the present day, as evidenced by two separate fronts of ascending gas imaged within the conduit. We propose that episodic seepage is associated with enhanced seafloor sedimentation. The increasing overburden leads to an increase in effective horizontal stress that exceeds the gas pressure at the top of the gas reservoir. As a result, the conduit closes off until the gas reservoir is replenished to a new (greater) critical thickness to reopen hydraulic fractures. Our results provide intricate detail of long‐term methane flux through sub‐seabed seep systems, which is important for assessing its impact on seafloor and ocean biogeochemistry. Plain Language Summary: Gas hydrates are ice‐like compounds that form in marine sediments. They can reduce the permeability of the sediments by clogging up the pore spaces, and influence how methane gas flows through sediments and then seeps out of the seafloor. Seepage of methane into the water column plays an important role in seafloor biology and ocean chemistry. In this study, we use 3D seismic imaging to investigate the subseafloor sediments of a ridge in the South China Sea where gas is currently seeping into the ocean. Our data show, in high detail, how gas migrates upward through the sediments due to the buoyancy of gas. Our data also reveal mound structures at certain depths beneath the seafloor. We interpret that these mounds represent distinct phases in the geological past where gas was seeping out of the seafloor. This indicates that gas seepage at this ridge has switched on and off (episodically) throughout geological time. We speculate that the episodic seepage is associated with rapid seafloor sedimentation, which changes pressure conditions beneath the seafloor. Our work improves the understanding of how gas seepage processes can change on geological timescales. Key Points: Gas has accumulated beneath the base of gas hydrate stability, causing vertical gas conduit formation and seabed moundsMounds imaged within the conduit record episodic seepage between 300 and 127 kyrs agoQuiescence may be associated with enhanced seafloor sedimentation that increases effective stress at the top of the gas reservoir [ABSTRACT FROM AUTHOR]

Characterizing properties of marine subsurface sediment helps with siting for offshore infrastructure. Shear‐wave velocity (Vs) provides information on the geotechnical properties of the seabed. We present our initial efforts to obtain a detailed two‐dimensional model of Vs for a large‐offset multi‐channel seismic (MCS) transect collected in shallow waters across the Taiwan Strait using surface waves excited by a large volume airgun. We derived the dispersion curves of the Scholte waves along the 37.5‐km‐long transect using the phase‐shift method and then conducted multimodal inversion to obtain a Vs model down to a depth of 150 m. To estimate the dynamic Poisson's ratio across the transect, we combined the Vs model with a compressional wave velocity model derived from the traditional MCS semblance velocity analysis. Lastly, we approximated the seismic attenuation of the profile. Our results show a large lateral variation in shear‐wave velocity. In the north, a low‐velocity zone with shear‐wave velocities of about 150 m/s was identified, while in the south, the shear‐wave velocity was found to be 300 m/s. With synthetic data, several sensitivity tests were performed to derive optimal parameters for offshore large‐offset streamer data. We particularly focused on the depth of the streamer and source and the water depth in combination with different seabed properties. Our results show that we can robustly derive the shear‐wave velocity, along with the Poisson's ratio, using large‐offset streamer data elsewhere based on the criteria we have tested using field and synthetic data sets. Plain Language Summary: With an underwater air gun and underwater receivers, we can stimulate waves that travel through the water to the seafloor and back. At the seafloor, the wave splits into different types of waves (P‐wave, S‐wave, and Scholte wave). Previous works are mostly focused on P‐waves as S‐waves cannot travel through the water column. The Scholte wave travels along the seafloor and is sensitive to the shear properties of the top 150 m shallow sediments. By using a technique called the phase‐shift method, we can infer the S‐wave velocity from the Scholte wave. Combined with the P‐wave velocity, it can provide critical geotechnical information. We found an area with a relatively low S‐wave velocity of 150 m/s in the north and another area in the south with velocities of 300 m/s. This information can be used to weigh how and where marine constructions have to be built so that they are safe even under different environmental hazards (typhoons, earthquakes, and tsunamis). We are showing that existing data sets in shallow water can be used to estimate geotechnical properties; we further give suggestions in designing surveys to efficiently measure these wave types. Key Points: Scholte wave inferred shear‐wave velocity of shallow seabedLarge‐offset seismic streamer data as a tool to derive S‐wave and P‐wave velocity simultaneouslySensitivity tests for seismic streamer data on Scholte waves using synthetic waveforms [ABSTRACT FROM AUTHOR]

Estimates of the sub‐seabed fluid flow rates are important for understanding hydrological budgets, biogeochemical cycles, and physical properties of the sediments. Fluid flow rates and directions, however, are difficult to measure, particularly beneath the seafloor. We developed a rapid method to estimate regional fluid migration rates using an extensive database of seismic reflection profiles taken offshore SW Taiwan. We observe bottom‐simulating reflector (BSR) that deflects toward the seafloor near thrust faults that indicate localized heat flow variations. At these sites, advecting warm pore fluids transport heat to shallower depths and force the BSR shallower. Our 2D steady‐state numerical method quantifies the fluid flow rates required to cause such thermal anomalies. We found that fluid flow rates near the trench of the accretionary wedge range between 0.1 and 16 m3 yr−1 m−1, with slower and faster rates generally associated with slope basin discontinuities and faults, respectively. To evaluate the fluid pattern evolution from subduction to collision, we studied three transects: one along the Manila subduction zone in the south and two in Taiwan's initial collision zone in the north. We quantified the fluid budget and partitioning of fluid flow between focused discharge through faults and diffusive flow through the wedge. Faults in Taiwan's accretionary wedge capture on average 25% of the total dewatering flux in the younger subduction zone and 38.5% in the tectonically mature collision zone. Our method provides estimates of fluid migration rates along convergent plate boundaries, and contributes to our understanding of focused fluid flow processes in many other regions. Plain Language Summary: Fluids play a key role in many subduction zone processes. However, quantitative constraints on flow expulsion rates and directions are limited. Efficient upward fluid migration through subbottom conduits can be generated tectonically, such as faults. Faults are ubiquitous along convergent margins; yet, a quantitative understanding of their impact on regional fluid budgets, flow rates, and distribution at vent sites remains unclear. We developed a rapid numerical method to constrain 2D focused fluid flow rates using seismically derived thermal structure and applied it to the subduction‐collision zone system off SW Taiwan. To study the influence of long‐term tectonic processes on the fluid budget, we remotely mapped the distribution and amount of focused fluid flow across the convergent margin, using a widespread shallow subbottom temperature field derived from a spatially dense seismic data set covering an area of more than 25,000 km2. We combined the results with other previously published geophysical data sets to calculate the margin fluid budget. We found stronger fluid advection from depth along the collision zone, where thicker sediments are deformed more intensively. Our approach to quantify fluid fluxes is applicable to a range of tectonic regimes and can provide critical insight into local, regional, or even global fluid budget estimates. Key Points: Developed a method to quantify fluid flow rates from BSR‐based temperature dataIdentified fluid partitioning patterns off SW Taiwan from subduction to collisionFocused fault‐related flow controls the local depth of the hydrate stability zone [ABSTRACT FROM AUTHOR]

Large amounts of methane, a potent greenhouse gas, are stored in hydrates beneath the seafloor. Sea level changes can trigger massive methane release into the ocean. It is not clear, however, whether surficial seafloor processes can cause comparable discharge. Previously, fluid migration was difficult to study due to a lack of spatially dense seismic and thermal observations. Here we examine a gas hydrate site at Four‐Way‐Closure Ridge off SW Taiwan using a high‐resolution 3‐D seismic cube, together with bottom‐simulating reflections (BSRs) mapped in the cube, a thermal probe data set, and 3‐D thermal modeling results. We document, on a scale of tens of meters, the interaction between surficial sedimentary processes, fluid flow, and a dynamic gas hydrate system. Fluid migrates upward through dipping permeable strata in the limb, the slope basin, and along thrust faults and ridge‐top normal faults. The seismic data also reveal several double BSRs that underlie seabed sedimentary sliding and depositional features. Abrupt changes in subsurface pressure and temperature due to the rapid seabed sedimentary processes can cause a rapid shift of the base of the gas hydrate stability zone. This shift may be either downward or upward and would result in the accumulation or dissociation of hydrate in sediments sandwiched by the double BSRs, respectively. We propose that dynamic surficial processes on the seafloor together with shallow focused fluid flow affect hydrate distribution and saturation at depth and may even result in methane expulsion into the ocean if such localized features are common along convergent plate boundaries. Plain Language Summary: Gas hydrates are ice‐like compounds in marine sediments. Shallow surface dynamic processes may affect the hydrate saturation beneath the seabed. We combine 3‐D seismic and thermal probe data, with numerical geothermal modeling to investigate the geological processes controlling the distribution and formation of gas hydrates beneath thrust ridge anticlines. We also study fluid flow patterns under the seabed and found that localized fluid flow and rapid surficial erosional processes have significantly altered the temperature and pressure conditions of hydrate bearing sediment strata at depth, ultimately influencing gas hydrate formation and dissociation. We propose to conduct hydrate exploration close to thrust anticlines, where such active processes might enrich the saturation of gas hydrates or even influence fluid emission into the ocean if similar processes are widespread along continental margins. Key Points: Mass wasting and rapid sedimentation can generate double BSRs and enhance hydrate saturationRapid seabed processes might trigger hydrate dissociation and active venting of methane and other fluids in shallow sedimentary sectionFocused fluid flow updip along the slope basin strata can locally enhance the hydrate saturation [ABSTRACT FROM AUTHOR]

Multi-scale reflection seismic data, from deep-penetration to high-resolution, have been analyzed and integrated with near-surface geophysical and geochemical data to investigate the structures and gas hydrate system of the Formosa Ridge offshore of southwestern Taiwan. In 2007, dense and large chemosynthetic communities were discovered on top of the Formosa Ridge at water depth of 1125 m by the ROV Hyper-Dolphin. A continuous and strong BSR has been observed on seismic profiles from 300 to 500 ms two-way-travel-time below the seafloor of this ridge. Sedimentary strata of the Formosa Ridge are generally flat lying which suggests that this ridge was formed by submarine erosion processes of down-slope canyon development. In addition, some sediment waves and mass wasting features are present on the ridge. Beneath the cold seep site, a vertical blanking zone, or seismic chimney, is clearly observed on seismic profiles, and it is interpreted to be a fluid conduit. A thick low velocity zone beneath BSR suggests the presence of a gas reservoir there. This “gas reservoir” is shallower than the surrounding canyon floors along the ridge; therefore as warm methane-rich fluids inside the ridge migrate upward, sulfate carried by cold sea water can flow into the fluid system from both flanks of the ridge. This process may drive a fluid circulation system and the active cold seep site which emits both hydrogen sulfide and methane to feed the chemosynthetic communities. [ABSTRACT FROM AUTHOR]

Abstract: A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements. [Copyright &y& Elsevier]

[Display omitted] • Acoustic remote sensing can be used to determine different types of sedimentary seabeds using quantitative classification. • Three attributes extracted from acoustic waveforms reveal four types of sedimentary seabeds. • The proposed method differentiates sedimentary seabed types with similar grain size distribution. • Core data from the offshore area of southwestern Taiwan validates the method. • This study provides a sensitive and robust quantitative method and reduces the uncertainty of manual identification between different seabeds. The sedimentary seabed properties are direct indicators of its stability, whose measure has several important implications in understanding and forecasting geological events. This study investigates the classification of sedimentary seabed types in different depositional environments using the three attributes: reflection coefficient (RC), similarity index (SI), and amplitude ratio (AR) values extracted from sub-bottom profiling data. Four sedimentary seabed types were evaluated automatically according to the waveforms in the horizontal and vertical directions. To infer the seabed properties, the attribute SI is used to identify the occurrence of homogeneity of sediment transportation in an area, while RC and AR are used to determine the seabed characteristics in terms of the variation of acoustic energy reflected from the sediment. To validate the results of this quantitative analysis method, sixteen seafloor sediment samples collected at different sites in the offshore area of southwestern Taiwan were analyzed. The automatic classification results suggest that the different sub-bottom profiling images are associated with the grain size distributions and compositions, such as the concentrated distribution, sand, silt, and clay percentages, and the mean bulk density. In addition, an inhomogeneous environment with similar grain size distribution can be identified. The proposed seabed classification is an effective method to study seabed characteristics, especially for identifying chaotic sediments. [ABSTRACT FROM AUTHOR]

In this study, P- and S-wave velocity models were built based on two pre-stack depth migration (PSDM) profiles of long-offset reflection data and 25 four-component ocean-bottom seismometers (OBS) data at Jiulong Methane Reef off SW Taiwan in the passive margin of the northern South China Sea (SCS). According to the velocity models, the average P-wave velocity and Vp/Vs ratio of the free gas beneath the bottom-simulating reflector (BSR) are 1.52–1.58 km/s and 4.5–5.10, respectively. The depth of the BSR is found at 80–300 m below the sea floor and the sedimentary thickness of the hydrate and the free gas are about 50–100 m and 70–100 m, respectively. P-wave velocity of about 1.75 km/s in the hydrate-bearing sediment is southeastward increased to about 1.85 km/s above the BSR. Similarly, Vp/Vs ratio of about 3.06 in the hydrate-bearing sediment is increased southeastward to about 3.48 above the BSR. Based on the pseudo-3D map of gas-hydrate saturation estimated from the PSDM and OBS models, the average saturations of hydrate and free gas at Jiulong Methane Reef are about 7% and 0.9–2.4%, respectively. The highest hydrate saturation (11%) is located at 5–20 m above the BSR in the SE portion of the Jiulong Methane Reef. On the other hand, the highest gas saturation of about 2% is observed at 10–70 m below the BSR in the NW portion of the Jiulong Methane Reef. We suggested that several normal faults dipping southeastward beneath the continental slope provided conduits for gas migrating northwestward at Jiulong Methane Reef. Therefore, the highest gas saturation is observed below the anticline in the NW portion of the Jiulong Methane Reef and the highest hydrate saturation (high P-wave velocity and high Vp/Vs ratio) is identified above the BSR in the SE portion of the Jiulong Methane Reef. [ABSTRACT FROM AUTHOR]

Fluid migration patterns are important for understanding gas hydrate and hydrocarbon systems. However, conducting experiments on or below the seafloor is difficult because crustal fluid flow rates are usually very slow, so long term observations are needed. Temperature can be used as a good tracer for studying fluid flows. Temperatures derived from bottom-simulating reflectors (BSRs) might help to understand fluid migration patterns in shallow marine sediments. In this study, we studied 2D fluid flow patterns in two potential gas hydrate provinces offshore southwestern Taiwan: the Yung-An Ridge in the active margin and Formosa Ridge in the passive margin. We used 2D bathymetry, average seafloor temperatures and regional geothermal gradients measured by thermal probes, as constraints to construct 2D theoretical conductive temperature fields using finite element methods. We then compared the BSR-based temperature with the theoretical conductive temperature field. The results show a temperature discrepancy attributed to advective heat transfer due to fluid migration. For the Yung-An Ridge, the BSR-based temperatures are about 2°C higher than the model: Especially in (1) near a fault zone, (2) under the eastern flank where there are strong seismic reflectors in a pseudo-3D seismic dataset, and (3) near a fissure zone. For the Formosa Ridge, our results showed a distinct decrease in temperatures around the southern peak of the ridge, where an active gas plume was found. BSR-based temperatures predict on average 2°C lower than the model. At these two sites, the shallow temperature fields are strongly affected by 2D bathymetry. However, new insights regarding fluid flow patterns can be obtained using this model approach. [ABSTRACT FROM AUTHOR]

Abstract: A large number of sediment cores collected during 2005–2010 from the Taiwan Strait were analyzed for radionuclides (210Pb, 137Cs and 7Be) to elucidate sedimentation dynamics in this all-important gateway linking two largest marginal seas in the western Pacific (namely, the South China Sea and the East China Sea). Apparent sediment accumulation rates derived from 210Pb and 137Cs profiles vary from <0.1 to >2cm/yr, averaging ∼0.4cm/yr and showing a spatial pattern closely related to hydrodynamics and sediment source-to-sink pathways. Spatial-temporal variation of 7Be activity in surface sediments off Taiwan’s west coast indicates episodic deposition of flood layers and their mobility from river estuaries toward the north. In conjunction with particle size distribution in surface sediments and the structure of sediment strata revealed by sub-bottom echo images; the radionuclide data can be used to outline three different sediment source-to-sink dispersal systems. Based on sediment loads of surrounding rivers and the distribution of sediment accumulation rates, lateral transport is required to account for the budget and size distribution of sediments in the strait. [Copyright &y& Elsevier]

We re-examined the cessation of the South China Sea seafloor spreading by using radioactive dating and fossil assemblage of the ETO, and the Te-derived age. The youngest available age for the fragment of the South China Sea is ∼15 Ma and is more consistent with those suggested by previous studies. Moreover, recent IODP Expedition preliminary results suggested that the hemipelagic claystone layer stratigraphically bounded by igneous sequences of massive lava flow rock is Early Miocene in age. Therefore, the previous interpretations are more inclusive than those provided by Barckhasen et al. (2014) and indicate that a slow spreading ridge was more plausible to have occurred during the seafloor spreading of the South China Sea. [ABSTRACT FROM AUTHOR]