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  • 1
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    Observed correlation between the depth to base and top of gas hydrate occurrence from review of global drilling data (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 18 (7). pp. 2543-2561.
    Details
    Publication Date: 2020-02-06
    Description: A global inventory of data from gas hydrate drilling expeditions is used to develop relationships between the base of structure I gas hydrate stability, top of gas hydrate occurrence, sulfate-methane transition depth, pressure (water depth), and geothermal gradients. The motivation of this study is to provide first-order estimates of the top of gas hydrate occurrence and associated thickness of the gas hydrate occurrence zone for climate-change scenarios, global carbon budget analyses, or gas hydrate resource assessments. Results from publically available drilling campaigns (21 expeditions and 52 drill sites) off Cascadia, Blake Ridge, India, Korea, South China Sea, Japan, Chile, Peru, Costa Rica, Gulf of Mexico, and Borneo reveal a first-order linear relationship between the depth to the top and base of gas hydrate occurrence. The reason for these nearly linear relationships is believed to be the strong pressure and temperature dependence of methane solubility in the absence of large difference in thermal gradients between the various sites assessed. In addition, a statistically robust relationship was defined between the thickness of the gas hydrate occurrence zone and the base of gas hydrate stability (in meters below seafloor). The relationship developed is able to predict the depth of the top of gas hydrate occurrence zone using observed depths of the base of gas hydrate stability within less than 50 m at most locations examined in this study. No clear correlation of the depth to the top and base of gas hydrate occurrences with geothermal gradient and sulfate-methane transition depth was identified.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2017GC006805
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Defining megathrust tsunami source scenarios for northernmost Cascadia (2018)
    Springer
    In:  Natural Hazards, 94 (1). pp. 445-469.
    Details
    Publication Date: 2021-02-08
    Description: For assessing tsunami hazard in northernmost Cascadia, there is an urgent need to define tsunami sources due to megathrust rupture. Even though the knowledge of Cascadia fault structure and rupture behaviour is limited at present, geologically and mechanically plausible scenarios can still be designed. In this work, we use three-dimensional dislocation modelling to construct three types of rupture scenarios and illustrate their effects on tsunami generation and propagation. The first type, buried rupture, is a classical model based on the assumption of coseismic strengthening of the shallowest part of the fault. In the second type, splay-faulting rupture, fault slip is diverted to a main splay fault, enhancing seafloor uplift. Although the presence or absence of such a main splay fault is not yet confirmed by structural observations, this scenario cannot be excluded from hazard assessment. In the third type, trench-breaching rupture, slip extends to the deformation front and breaks the seafloor by activating a frontal thrust. The model frontal thrust, based on information extracted from multichannel seismic data, is hypothetically continuous along strike. Our low-resolution tsunami simulation indicates that, compared to the buried rupture, coastal wave surface elevation generated by the splay-faulting rupture is generally 50–100% higher, but that by trench-breaching rupture is slightly lower, especially if slip of the frontal thrust is large (e.g. 100% of peak slip). Wave elevation in the trench-breaching scenario depends on a trade-off between enhanced short-wavelength seafloor uplift over the frontal thrust and reduced uplift over a broader area farther landward.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1007/s11069-018-3397-6
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Gas hydrates in the Ulleung Basin, East Sea of Korea (2017)
    Meteorological Society of the Republic of China
    In:  Terrestrial, Atmospheric and Oceanic Sciences, 28 (6). pp. 943-963.
    Details
    Publication Date: 2020-02-06
    Description: To develop gas hydrates as a potential energy source, geophysical and geological studies were carried out in the Ulleung Basin, East Sea. Bottom simulating reflectors (BSRs) were initially used indicator for the potential presence of gas hydrates across the basin. Based on these early results, 12367 line-km of multichannel seismic data, 38 piston cores, and multibeam data were collected. The cores showed high amounts of total organic carbon and high residual hydrocarbon gas levels. Gas composition and isotope ratios define it as of primarily biogenic origin. In addition to the BSRs, numerous chimney-structures were found in seismic data. These features indicate a high potential of the Ulleung Basin to host significant amounts of gas hydrate. Dedicated geophysical surveys, geological and experimental studies were carried out culminating in two deep drilling expeditions, completed in 2007 and 2010. Sediment coring (including pressure coring), and a comprehensive well log program complements the regional studies and were used for a resource assessment. Two targets for a future test-production are currently proposed: pore-filling gas hydrate in sand-dominated sediments and massive occurrences of gas hydrate within chimney-like structures. An environmental impact study has been launched evaluating any potential risks to production.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.3319/TAO.2017.10.21.01
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Elongate fluid flow structures: Stress control on gas migration at Opouawe Bank, New Zealand (2018)
    Elsevier
    In:  Marine and Petroleum Geology, 92 . pp. 913-931.
    Details
    Publication Date: 2021-02-08
    Description: Highlights • Elongated fault structures are conduits for focused fluid flow. • Gas migration occurs only along a sub-set of faults across Opouawe bank. • Stress state deduced from 3D fault structures appears partially stratigraphically controlled. Abstract High-resolution 2D and 3D seismic data from Opouawe Bank, an accretionary ridge on the Hikurangi subduction margin off New Zealand, show evidence for exceptional gas migration pathways linked to the stress regime of the ridge. Although the ridge has formed by thrusting and folding in response to a sub-horizontal principal compressive stress (σ1), it is clear that local stress conditions related to uplift and extension around the apex of folding (i.e. sub-vertical σ1) are controlling shallow fluid flow. The most conspicuous structural features are parallel and horizontally-elongated extensional fractures that are perpendicular to the ridge axis. At shallower depth near the seafloor, extensional fractures evolve into more concentric structures which ultimately reach the seafloor where they terminate at gas seeps. In addition to the ridge-perpendicular extensional fractures, we also observe both ridge-perpendicular and ridge-parallel normal faults. This indicates that both longitudinal- and ridge-perpendicular extension have occurred in the past. The deepest stratigraphic unit that we image has undergone significant folding and is affected by both sets of normal faults. Shallower stratigraphic units are less deformed and only host the ridge-parallel normal faults, indicating that longitudinal extension was limited to an older phase of ridge evolution. Present-day gas migration has exploited the fabric from longitudinal extension at depth. As the gas ascends to shallower units it ‘self-generates’ its flow pathways through the more concentric structures near the seafloor. This shows that gas migration can evolve from being dependent on inherited tectonic structures at depth, to becoming self-propagating closer to the seafloor.
    Type: Article , PeerReviewed
    Format: text
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    DOI: 10.1016/j.marpetgeo.2018.03.029
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    A case study on swell correction of Chirp sub-bottom profiler (SBP) data using multi-beam echo sounder (MBES) data (2017)
    Elsevier
    In:  Journal of Applied Geophysics, 145 . pp. 100-110.
    Details
    Publication Date: 2020-02-06
    Description: High-resolution marine seismic data acquisition and subsequent analyses are highly influenced by sea conditions, directly affecting data quality and interpretation. Traditional swell effect correction methods are effective in improving reflector continuity; however, they are less useful for enhancing travel time consistency at intersection points of crossing lines. To develop a robust swell-removal technique for a set of crossing lines multi-beam echo sounder (MBES) data and Chirp sub-bottom profiler (SBP) data were acquired. After generation of a time structure map of the sea-bottom converted from the final processed multi-beam data, a moving average was used to improve the event continuity of the sea-bottom reflection of the Chirp SBP data. Using the position of the Chirp SBP data, the difference between the travel time of the sea-bottom from the smoothed map and the original travel time of the sea-bottom is calculated as a static correction. The static correction method based on the MBES data was compared and verified using three different cases: (i) simple 2D swell effect correction on a line-by-line basis, (ii) comparing the swell corrections at the crossing positions of 2D lines acquired from different dates, and (iii) comparison of ties of intersection points between 2D lines after new swell correction applied. Although a simple 2D swell correction showed great enhancement of reflector continuity, only the full static correction using the newly proposed method using MBES data produced completely corrected reflection events especially at the crossing points of 2D lines.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1016/j.jappgeo.2017.08.001
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming (2018)
    Nature Research
    In:  Nature Communications, 9 (Article number: 83 ).
    Details
    Publication Date: 2021-03-19
    Description: Methane seepage from the upper continental slopes of Western Svalbard has previously been attributed to gas hydrate dissociation induced by anthropogenic warming of ambient bottom waters. Here we show that sediment cores drilled off Prins Karls Foreland contain freshwater from dissociating hydrates. However, our modeling indicates that the observed pore water freshening began around 8 ka BP when the rate of isostatic uplift outpaced eustatic sea-level rise. The resultant local shallowing and lowering of hydrostatic pressure forced gas hydrate dissociation and dissolved chloride depletions consistent with our geochemical analysis. Hence, we propose that hydrate dissociation was triggered by postglacial isostatic rebound rather than anthropogenic warming. Furthermore, we show that methane fluxes from dissociating hydrates were considerably smaller than present methane seepage rates implying that gas hydrates were not a major source of methane to the oceans, but rather acted as a dynamic seal, regulating methane release from deep geological reservoirs.
    Type: Article , PeerReviewed
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    DOI: 10.1038/s41467-017-02550-9
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Distributed natural gas venting offshore along the Cascadia margin (2018)
    Nature Research
    In:  Nature Communications, 9 (3264).
    Details
    Publication Date: 2021-03-19
    Description: Widespread gas venting along the Cascadia margin is investigated from acoustic water column data and reveals a nonuniform regional distribution of over 1100 mapped acoustic flares. The highest number of flares occurs on the shelf, and the highest flare density is seen around the nutrition-rich outflow of the Juan de Fuca Strait. We determine similar to 430 flow-rates at similar to 340 individual flare locations along the margin with instantaneous in situ values ranging from similar to 6 mL min(-1) to similar to 18 L min(-1). Applying a tidal-modulation model, a depth-dependent methane density, and extrapolating these results across the margin using two normalization techniques yields a combined average in situ flow-rate of similar to 88 x 10(6) kg y(-1). The average methane flux-rate for the Cascadia margin is thus estimated to similar to 0.9 g y(-1) m(-2). Combined uncertainties result in a range of these values between 4.5 and 1800% of the estimated mean values.
    Type: Article , PeerReviewed
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    Format: other
    Format: other
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    DOI: 10.1038/s41467-018-05736-x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Bayesian inversion of marine controlled source electromagnetic data offshore Vancouver Island, Canada (2016)
    Oxford Univ. Press
    In:  Geophysical Journal International, 204 (1). pp. 21-38.
    Details
    Publication Date: 2019-02-01
    Description: This paper applies nonlinear Bayesian inversion to marine controlled source electromagnetic (CSEM) data collected near two sites of the Integrated Ocean Drilling Program (IODP) Expedition 311 on the northern Cascadia Margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents. The Cascadia margin, off the west coast of Vancouver Island, Canada, has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics. Gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling. Here, we invert time-domain CSEM data collected at Sites U1328 and U1329 of IODP Expedition 311 using Bayesian methods to derive subsurface resistivity model parameters and uncertainties. The Bayesian information criterion is applied to determine the amount of structure (number of layers in a depth-dependent model) that can be resolved by the data. The parameter space is sampled with the Metropolis–Hastings algorithm in principal-component space, utilizing parallel tempering to ensure wider and efficient sampling and convergence. Nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude. Marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties. One-dimensional inversion of time-domain CSEM data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure. The data sets can be generally explained by models with 1 to 3 layers. Inversion results at U1329, at the landward edge of the gas hydrate stability zone, indicate a sediment unconformity as well as potential cold vents which were previously unknown. The resistivities generally increase upslope due to sediment erosion along the slope. Inversion results at U1328 on the middle slope suggest several vent systems close to Bullseye vent in agreement with ongoing interdisciplinary observations.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1093/gji/ggv437
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Submarine landslides offshore Vancouver Island along the northern Cascadia margin, British Columbia: why preconditioning is likely required to trigger slope failure (2016)
    Springer
    In:  Geo-Marine Letters, 36 (5). pp. 323-337.
    Details
    Publication Date: 2019-02-01
    Description: Bathymetric data reveal abundant submarine landslides along the deformation front of the northern Cascadia margin that might have significant tsunami potential. Radiocarbon age dating showed that slope failures are early to mid-Holocene. The aim of this study is the analysis of slope stability to investigate possible trigger mechanisms using the factor of safety analysis technique on two prominent frontal ridges. First-order values for the earthquake shaking required to generate instability are derived. These are compared to estimated ground accelerations for large (M=5 to 8) crustal earthquakes to giant (M=8 to 9) megathrust events. The results suggest that estimated earthquake accelerations are insufficient to destabilize the slopes, unless the normal sediment frictional resistance is significantly reduced by, for example, excess pore pressure. Elevated pore pressure (overpressure ratio of 0.4) should significantly lower the threshold for earthquake shaking, so that a medium-sized M=5 earthquake at 10 km distance may trigger submarine landslides. Preconditioning of the slopes must be limited primarily to the mid- to early Holocene as slope failures are constrained to this period. The most likely causes for excess pore pressures include rapid sedimentation at the time of glacial retreat, sediment tectonic deformation, and gas hydrate dissociation as result of ocean warming and sea level rise. No slope failures comparable in size and volume have occurred since that time. Megathrust earthquakes have occurred frequently since the most recent failures in the mid-Holocene, which emphasizes the importance of preconditioning for submarine slope stability.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1007/s00367-016-0452-8
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Paleoseismicity derived from piston coring methods, Explorer and Juan de Fuca Plate systems, Bristish Columbia (2015)
    Natural Resources Canada
    In:  Geological Survey of Canada Current Research, 2015-10 . Natural Resources Canada, 11 pp.
    Details
    Publication Date: 2020-01-08
    Description: Coring of marine sediments has revealed deposits related to slope instability induced by seismicity on the western margin of Canada. Debris flows and turbidite sequences related to megathrust earthquakes have been recovered in six piston cores on the Juan de Fuca and Explorer tectonic plates, allowing comparison of the response of each plate to shaking during great earthquakes. Analyses of the recovered cores show that turbidite sequences associated with a megathrust quake occur on the Juan de Fuca Plate and do not occur in cores collected 90 km away at a similar site on the Explorer Plate. The record of subduction-related earthquake turbidite sequences is not complete at the Juan de Fuca study area and no reconstruction of megathrust quake periodicity is thus possible using this site alone. These results indicate that strong ground shaking is probably not experienced during large subduction earthquakes on the Explorer Plate.
    Type: Report , NonPeerReviewed
    Format: text
    DOI: 10.4095/297317
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    OceanRep: Report - Cruise Report
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