distributions of gas hydrate and free gas accumulations associated with upward fluid flow in the sanriku oki forearc basin northeast japan abstract we applied automated velocity analysis to a 3d seismic data volume to obtain a high resolution seismic velocity model that we used to investigate the influence of fluid behavior on the subsurface distribution of gas in the sanriku oki forearc basin northeast japan we identified free gas accumulations as zones of low p wave velocity separated from overlying high velocity gas hydrates by the clear seismic boundary of the bottom simulating reflection bsr between them we then used conductive modeling to map upward heat flow in our study area from the depth of the bsr derived from the velocity model the estimated heat flow demonstrates that upward fluid flux has considerable influences on the distributions of both gas hydrate and free gas the areas of high heat flow representing high fluid flux correspond to underlying permeable geological features such as gas chimneys faults and the edges of porous slumps suggesting that these features provide fluid migration pathways from eocene to oligocene source rocks to free gas and hydrate accumulations in overlying sedimentary rocks the techniques we have employed can contribute to the high resolution mapping of gas hydrate and free gas accumulations in similar deep water reservoirs off the eastern continental margin of japan and at tectonically similar plate subduction margins in other regions

bottom simulating reflections and geothermal gradients across the western svalbard margin seismic reflection data reveal prominent bottom simulating reflections bsrs within the relatively young 0 78 ma sediments along the west svalbard continental margin the potential hydrate occurrence zone covers an area of c 1600 km2 the hydrate accumulation zone is bound by structural tectonic features knipovich ridge molloy transform fault vestnesa ridge and the presence of glacigenic debris lobes inhibiting hydrate formation upslope the thickness of the gas zone underneath the bsr varies laterally and reaches a maximum of c 150 ms using the bsr as an in situ temperature proxy geothermal gradients increase gradually from 70 to 115 c km 1 towards the molloy transform fault anomalies only occur in the immediate vicinity of normal faults where the bsr shoals indicating near vertical heat fluid flow within the fault zones amplitude analyses suggest that sub horizontal fluid migration also takes place along the stratigraphy as the faults are related to the northwards propagation of the knipovich ridge long term disturbance of hydrate stability appears related to incipient rifting processes

natural gas hydrates on the southeast u s margin constraints from full waveform and travel time inversions of wide angle seismic data strong bottom simulating reflectors bsr have been mapped over a region of approximately 50 000 km2 on the southeastern u s margin and have been associated with possible abundance of natural gas hydrates in june 1992 coincident single channel seismic and wide angle ocean bottom seismic data were acquired in the region focusing on the blake ridge and the carolina rise wide angle reflections from the bsrs were clearly observed at offsets up to 6 km joint travel time inversion was conducted with wide angle and vertical incidence data in order to explore possible regional variation and the resultant two dimensional average velocity models imply higher background velocities on the carolina rise full waveform inversion was then performed to determine the seismic origin of the bsrs the best fit model shows a similar low velocity 1 4 km s beneath the bsr at both sites indicating trapped free gas with low saturation 10 the inversion results also indicate that a thin high velocity wedge with a maximum velocity of 2 3 km s is present just above the blake ridge bsr sediment reflectivities were also calculated and higher reflectivities are observed on the carolina rise an increase in reflectivity below the bsr seems to correspond to the gas bearing zone at both sites concentration of hydrates were estimated based on these velocity models whereas average hydrate concentration of 3 of the total sediment volume is suggested for the lower half of the hydrate stability zone at the blake ridge only a very low average concentration of hydrate can be expected at the carolina rise the hydrates seem to be concentrated near the base of the hydrate stability zone and the maximum hydrate concentration is estimated as 20 at the blake ridge and 7 at the carolina rise both of which are too high to be explained by in situ biogenic activity only and require some secondary accumulation mechanism it is suggested that hydrate recycling caused by the stability field migration may have effectively condensed hydrates at both sites additional enhancement by upward fluid expulsion may also be viable for the blake ridge

estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

throughgoing crustal faults along the northern margin of the south china sea and their role in crustal extension a study of the rifled northern margin of the south china sea based on new single ship and two ship multichannel seismic data has revealed the presence of highly reflective steeply dipping normal faults which cut through the entire crust these faults with dips of 25 40 are associated with localized crustal thinning of 5 15 km and represent horizontal extension of the order of tens of kilometers a range of 5 20 km of horizontal extension has been measured along individual faults although the relative amount of horizontal extension taken up by these faults is modest compared to the total extension across the margin only about 15 or less the amount of localized crustal thinning which takes place along these fault surfaces is quite significant at one fault location the crustal thickness is reduced by 40 although the faults described have been recognized at three separate locations along the margin their trends appear to differ at each in two examples the faults are oblique to the inferred general north south extension direction of the margin this characteristic taken with the fact that some of the crustal faults also appear to serve as abrupt boundaries between broad regions of differing crustal thickness and reflectivity of the lower crust suggests that at least some of these faults were present prior to rifting such faults are postulated to have been former thrust faults perhaps associated with a triassic suture zone and or cretaceous subduction margin which have been reactivated as normal faults during subsequent rifting alternatively some of the throughgoing faults may represent crustal detachment surfaces that first formed as the result of a simple shear response to crustal extension

seismic evidence for shallow gas escape features associated with a retreating gas hydrate zone offshore west svalbard active gas venting occurs on the uppermost continental slope off west svalbard close to and upslope from the present day intersection of the base of methane hydrate stability bmhs with the seabed in about 400 m water depth in the inter fan region between the kongsfjorden and isfjorden cross shelf troughs from an integrated analysis of high resolution two dimensional pre stack migrated seismic reflection profiles and multibeam bathymetric data we map out a bottom simulating reflector bsr in the inter fan region and analyze the subsurface gas migration and accumulation gas seeps mostly occur in the zone from which the bmhs at the seabed has retreated over the recent past 1975 2008 as a consequence of a bottom water temperature rise of 1 c the overall margin parallel alignment of the gas seeps is not related to fault controlled gas migration as seismic evidence of faults is absent there is no evidence for a bsr close to the gas flare region in the upper slope but numerous gas pockets exist directly below the predicted bmhs while the contour following trend of the gas seeps could be a consequence of retreat of the landward limit of the bmhs and gas hydrate dissociation the scattered distribution of seeps within the probable hydrate dissociation corridor and the occurrence of a cluster of seeps outside the predicted bmhs limit and near the shelf break indicate the role of lithological heterogeneity in focusing gas migration

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope the tsunamigenesis of the 2011 tohoku earthquake off the east coast of northern japan abstract the gigantic 2011 march 11 mw 9 tohoku earthquake is examined from the viewpoint of the pre seismic forearc structure the seismic reflection properties of a megathrust around the usual up dip limit of the seismogenic zone the thermal state of a shallow subduction zone and the dehydration of underthrust sediments at the japan trench the pacific plate is subducting westward beneath northeast japan at a dip angle of 4 6 the middle and lower slopes of the landward side dip eastward at angles of 2 5 and 8 0 respectively the forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states respectively based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes the rapid uplift of the forearc that caused the 2011 tohoku tsunami may have been associated with this internal deformation of the prism the critical state of the prism indicates that the effective basal friction b of the plate boundary megathrust is 0 08 for the lower prism the megathrust especially under the middle slope is characterized by a prominent reflector with negative polarity one of multiple possible explanations is that the megathrust hosts highly pressurized fluids underthrust sediments in this part of the japan trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay the dehydration kinetics of opal a to quartz the clay transformation of smectite illite and the thermal structure of the japan trench suggest that maximum dehydration of the sediments would take place at 50 60 km horizontally from the deformation front where the temperature along the megathrust is 100 120 c the zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity we hypothesize a possible free slip along this portion of the megathrust during the 2011 tohoku earthquake caused by anomalously high fluid pressure resulting from fluid accumulation over centuries

propagation of a lithospheric tear fault step through the western boundary of the calabrian accretionary wedge offshore eastern sicily southern italy the rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault s along the lateral slab edge s and by strike slip movement in the upper plate defined as a step fault subduction tear edge propagator the neogene evolution of the central mediterranean is dominated by the interaction between the slow africa eurasia convergence and the se ward rollback of the ionian slab that leads to the back arc opening of the tyrrhenian sea here we present post stack time migrated and pre stack depth migrated archimede 1997 multichannel seismic lines that were acquired offshore eastern sicily at the foot of the malta escarpment first we identify the recent deformation along the lateral ramp of the calabrian accretionary wedge towards the east the calabrian wedge is formed by the accretion of the post evaporitic sediments above a decollement at the base of the messinian evaporites at the latitude of syracuse 50 km east of the malta escarpment a major n150 degrees e trending crustal scale and vertical fault slices through the entire accretionary wedge this fault cuts by several kilometers through the pre evaporitic messinian sediments and into the basement the vertical offset along this vertical fault decreases from north to south and the fault is no longer observed on the seismic lines 50 km se of the alfeo seamount a previously published moho depth isocontour map offshore sicily and the recent gps data combined with the presence of strike slip movements ne onshore sicily allow us to identify this 200 km long crustal scale fault as the surface expression of a step fault the presence of syntectonic pleistocene sediments on top this crustal scale fault suggests a recent lithospheric vertical movement of the step fault in response to the rollback of the ionian slab and to the se ward advance of the calabria peloritan block

gas hydrates at the storegga slide constraints from an analysis of multicomponent wide angle seismic data geophysical evidence for gas hydrates is widespread along the northern flank of the storegga slide on the mid norwegian margin bottom simulating reflectors bsr at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 outside but also inside the storegga slide scar area traveltime inversion and forward modeling of multicomponent wide angle seismic data result in detailed p and s wave velocities of hydrate and gas bearing sediment layers the relationship between the velocities constrains the background velocity model for a hydrate free gas free case the seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3 and 6 in a zone that is as much as 50 m thick overlying the bsr hydrates are most likely disseminated neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably average free gas concentrations beneath the hydrate stability zone are approximately 0 4 to 0 8 of the pore volume assuming a homogeneous gas distribution the free gas zone underneath the bsr is about 80 m thick amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone bghs this gives rise to enhanced reflections that terminate at the bghs the stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations gas bearing layers can be as thin as 2 m

compressional velocity structure and poisson s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data south shetland islands antarctica abstract a bottom simulating reflector bsr on the south shetland margin antarctic peninsula was surveyed within the progetto nazionale di ricerche in antartide pnra high resolution multichannel mcs reflection profiles were acquired and a three component ocean bottom seismograph obs was deployed where the bsr was particularly strong the compressional velocity field associated to the bsr was determined by the inversion of travel times of reflected mcs data and refracted obs data events information on shear wave velocities was extracted from the horizontal components of the obs records the analysis indicates the presence of velocity anomalies both above and below the bsr that are not related to the lithologic characteristics of the sedimentary column but can be associated to gas hydrate and free gas presence in pore space amplitude versus offset avo analysis was performed on common depth point gathers which correspond to the obs position the study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance a theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas the resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion a quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves normally consolidated marine terrigenous sediments the comparison of the results obtained by these independent methods seismic inversion avo analysis and theoretical model gives consistent values of poisson s ratio and gas hydrate and free gas estimation

velocity structure of a bottom simulating reflector offshore peru results from full waveform inversion abstract much of our knowledge of the worldwide distribution of submarine gas hydrates comes from seismic observations of bottom simulating reflectors bsrs full waveform inversion has proven to be a reliable technique for studying the fine structure of bsrs using the compressional wave velocity we applied a non linear full waveform inversion technique to a bsr at a location offshore peru we first determined the large scale features of seismic velocity variations using a statistical inversion technique to maximise coherent energy along travel time curves these velocities were used for a starting velocity model for the full waveform inversion which yielded a detailed velocity depth model in the vicinity of the bsr we found that the data are best fit by a model in which the bsr consists of a thin low velocity layer the compressional wave velocity drops from 2 15 km s down to an average of 1 70 km s in an 18 m thick interval with a minimum velocity of 1 62 km s in a 6 m interval the resulting compressional wave velocity was used to estimate gas content in the sediments our results suggest that the low velocity layer is a 6 18 m thick zone containing a few percent of free gas in the pore space the presence of the bsr coincides with a region of vertical uplift therefore we suggest that gas at this bsr is formed by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure

relationship between outer forearc subsidence and plate boundary kinematics along the northeast japan convergent margin 1 tectonic erosion along convergent plate boundaries whereby removal of upper plate material along the subduction zone interface drives kilometer scale outer forearc subsidence has been purported to explain the evolution of nearly half the world s subduction margins including part of the history of northeast japan here we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern japan a synthesis of newly updated analyses of outer forearc subsidence the timing and kinematics of upper plate deformation and the history of plate convergence along the japan trench demonstrate that the onset of rapid fore arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the sea of japan and with an acceleration in convergence rate at the trench in plio quaternary time relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate the coincidence of these changes across the forearc arc backarc system appears to require an explanation at the scale of the entire plate boundary similar observations along other western pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule rather than the exception we suggest that a significant component of forearc subsidence at the northeast japan margin is not the consequence of basal tectonic erosion but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics if correct this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins

high heat flow anomalies on an old oceanic plate observed seaward of the japan trench thirty three new measurements on the seaward slope and outer rise of the japan trench along a parallel of 38 45 n revealed the existence of high heat flow anomalies on the subducting pacific plate where the seafloor age is about 135 m y the most prominent anomaly with the highest value of 114 mw m2 is associated with a small mound on the outer rise which was reported to be a kind of mud volcano on the seaward slope of the trench heat flow is variable high 70 90 mw m2 at some locations and normal for the seafloor age about 50 mw m2 at others the spatial variation of heat flow may be related to development of normal faults and horst graben structures due to bending of the pacific plate before subduction with fluid flow along the fault zones enhancing the vertical heat transfer possible heat sources of the high heat flow anomalies are intra plate volcanism in the last several million years like that discovered recently on the pacific plate east of the japan trench

fault parameters of the 1896 sanriku tsunami earthquake estimated from tsunami numerical modeling the june 15 1896 sanriku earthquake generated devastating tsunamis with the maximum run up of 25 m and caused the worst tsunami disaster in the history of japan despite its moderate surface wave magnitude ms 7 2 and weak seismic intensity this is a typical tsunami earthquake which generates anomalously larger tsunamis than expected from its seismic waves previously proposed mechanisms of tsunami earthquakes include submarine slumping and slow rupture in the accretionary wedge or in the subducted sediments in this paper we estimate the fault parameters of the 1896 tsunami earthquake by numerically computing the tsunami and comparing the waveforms with those recorded at three tide gauge stations in japan the result indicates that the tsunami source is very close to the japan trench and the fault strike is parallel to the trench axis the fault width is about 50 km suggesting that the tsunami earthquake is a slow rupture in the subducted sediments beneath the accretionary wedge

seismic indicators of gas hydrate and associated gas in the ulleung basin east sea japan sea and implications of heat flows derived from depths of the bottom simulating reflector abstract analysis of multi channel seismic reflection data from the ulleung basin east sea japan sea reveals various seismic indicators of gas hydrate and associated gas including the bottom simulating reflector bsr enhanced reflections below the bsr and seismic chimneys the recent recovery of massive gas hydrate by drilling together with these seismic indicators strongly suggests favorable conditions for the formation of gas hydrate in the area the bsr is most common and of a wide range of amplitude and continuity seismic chimneys characterized by columnar zones of amplitude reduction terminate mostly within the gas hydrate stability zone ghsz but a few extend to pockmarks or mounds on the seafloor seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the ghsz the increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate filled coated fractures in the ghsz bsr derived heat flows range from about 65 to over 115 mw m2 comparable to those from direct measurements heat flows in the northern part of the study area are very high for the age of the basin suggesting hotter than normal mantle temperature the area of high 105 mw m2 heat flows also coincides largely with the distribution of the presumed incipient oceanic crust formed during the earliest stage of seafloor spreading

distribution of methane hydrate bsrs and its implication for the prism growth in the nankai trough abstract detailed mapping of a bottom simulating reflector bsr which marks the phase transition from the methane hydrate layer above the reflector to free gas below was conducted in the nankai accretionary prism off shikoku and tokai bsrs are widely distributed in the prism slope from the toe region to the forearc basin bsr positions provide us with the regional heat flow variations using pressure temperature conditions for methane hydrate stability estimated heat flows generally show constant values about 50 mw m2 shallower than the middle slope of the prism and gradually increase seaward in the lower prism slope occurrences of bsrs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field bghs and or concentration of methane hydrate above the bghs these conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon moreover sedimentation uplifting and sediment stacking by thrust faulting cause upward migration of the bghs and migration of methane from the dissociated hydrate to new bghs such recycling of methane gas may have actively occurred in accretionary prisms in contrast there are five regions of no bsrs the nankai trough floor prism toe slope basin steep slope and deep sea canyon the trough floor the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub horizontal strata unsuitable for migration of gases and fluids erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the bghs bsr distribution and thermal structure estimated from bsr positions offer information about active processes occurring in accretionary prisms

heat flow variations from bottom simulating reflectors on the cascadia margin abstract a bottom simulating reflector bsr representing the base of the gas hydrate stability field was observed over a closely spaced grid of seismic lines on the continental slope of the northern cascadia margin a simple conductive model was used to calculate heat flow from the depth of the bsr a regional trend was observed in which heat flow decreased landward across the margin from an average of 80 mw m2 at a distance of 15 km from the deformation front to 65 mw m2 at a distance of 40 km this trend reflects the processes of tectonic thickening of accretionary wedge sediments and subduction of the juan de fuca plate and contrasts with the near constant heat flow across the oregon margin where the dip of the subducting plate and thus the rate of sediment thickening are slightly smaller consistent local variations were also observed notably low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs at some localities over a horizontal distance of 1 2 km heat flow increased by as much as 50 from typical values of 65 to 100 mw m2 much of this variation may be due to focusing and defocusing effects of the topography alone and indicates the importance of carrying out topographic corrections to heat flow in regions of significant relief such as the continental slope however a component of the local heat flow patterns may be due to dynamic effects including the displacement of isotherms by thrust faulting and the upward migration of warm fluids along permeability channels such as faults

identification of the static backstop and its influence on the evolution of the accretionary prism in the nankai trough abstract to reveal the origin of a backstop and its influence on the evolution of the accretionary prism we analyzed reflection seismic data acquired in the nankai trough off the kii peninsula the deformation features of the forearc basin sequence show that the landward accretionary prism close to the coast was not deformed after the development of the forearc basin about 2 4 ma the surface of the landward prism can be identified as strong amplitude reflector indicating that the landward prism has higher seismic velocity therefore the landward accretionary prism inferred to be of higher strength constitutes a static backstop based on seismic and geologic observations we interpret that the backstop was generated due to the large age differences of accreted material resulting from an inferred hiatus in subduction between 13 and 6 ma the time dependent processes such as the igneous activity in middle miocene further contribute to the development of the backstop a ridge structure beneath the forearc basin located trenchward of this backstop and running roughly parallel to it appears to reflect activity on an ancient splay fault the strike of the ancient splay fault runs parallel to the backstop identified in this study and oblique to the current trench this geometry suggests that location and mechanical behavior of this splay fault system is influenced by the backstop and its distribution could be related to the coseismic rupture area

migration of methane gas through the hydrate stability zone in a low flux hydrate province new high resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable low methane flux blake ridge crest this movement of gaseous methane through a thermo dynamic regime where it should be trapped as hydrate suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean in the study area none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor instead they terminate at the base of a highly reflective unfaulted capping layer seismic inversions of anomalous regions show 1 increased velocities beneath the hydrate stability zone suggesting less gas and 2 increased velocities within the hydrate stability zone associated with observed low amplitude chimneys and bright spots indicating increased hydrate concentrations these observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate the observations strongly imply that given appropriate permeable pathways free gas can escape into the ocean even in a low flux environment the hydrate stability zone is not an impermeable barrier to free gas migration

erosion of the seafloor at the top of the gas hydrate stability zone on the hikurangi margin new zealand 1 the dissociation of gas hydrates in sediment pores is thought to decrease seafloor strength potentially facilitating submarine slides because of the generation of overpressured gas and the melting of load bearing or cementing solid hydrate here we present findings that suggest gas hydrates may lead to a previously unknown mechanism of seafloor erosion gas hydrate bearing sub sea ridges on the hikurangi margin are eroded close to the top of the hydrate stability field in the ocean we hypothesize that gas hydrate instability may lead to ridge erosion by a combination of two processes hydrate destabilization caused by depressurization during ridge uplift and repeated pore volume expansion and contraction from hydrate dissociation and formation triggered by fluctuating water temperatures

possible strain partitioning structure between the kumano fore arc basin and the slope of the nankai trough accretionary prism 1 a 12 km wide 56 km long three dimensional 3 d seismic volume acquired over the nankai trough offshore the kii peninsula japan images the accretionary prism fore arc basin and subducting philippine sea plate we have analyzed an unusual trench parallel depression a notch along the seaward edge of the fore arc kumano basin just landward of the megasplay fault system this bathymetric feature varies along strike from a single steep walled 3 5 km wide notch in the northeast to a broader 5 km wide zone with several shallower linear depressions in the southwest below the notch we found both vertical faults and faults which dip toward the central axis of the depression dipping faults appear to have normal offset consistent with the extension required to form a bathymetric low some of these dipping faults may join the central vertical fault s at depth creating apparent flower structures offset on the vertical faults is difficult to determine but the along strike geometry of these faults makes predominantly normal or thrust motion unlikely we conclude therefore that the notch feature is the bathymetric expression of a transtensional fault system by considering only the along strike variability of the megasplay fault we could not explain a transform feature at the scale of the notch strike slip faulting at the seaward edge of fore arc basins is also observed in sumatra and is there attributed to strain partitioning due to oblique convergence the wedge and decollement strength variations which control the location of the fore arc basins may therefore play a role in the position where an along strike component of strain is localized while the obliquity of convergence in the nankai trough is comparatively small 15 we believe it generated the kumano basin edge fault zone which has implications for interpreting local measured stress orientations and suggests potential locations for strain partitioning related deformation in other subduction zones

hyporheic exchange with heterogeneous streambeds laboratory experiments and modeling 1 hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone in reality streambed sediments have a heterogeneous structure and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns to assess the basic effects of sediment structure on hyporheic exchange we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative exponential correlated random hydraulic conductivity fields dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high and low permeability regions of the streambed as no existing model could represent these effects we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form induced boundary head distribution and the heterogeneous sediment structure a numerical particle tracking approach was then used to assess the resulting hyporheic exchange the combined flow transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t 30 hours the heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed the layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth as a result solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed the combination of faster near surface transport and shallower solute penetration produced a shorter mean hyporheic residence time on the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate

excess pore pressure resulting from methane hydrate dissociation in marine sediments a theoretical approach 1 this study quantifies the excess pore pressure resulting from gas hydrate dissociation in marine sediments the excess pore pressure in confined pore spaces can be up to several tens of megapascals due to the tendency for volume expansion associated with gas hydrate dissociation on the other hand the magnitude of excess pore pressure in well connected sediment pores is generally smaller depending primarily on the hydrate dissociation rate and the sediment permeability volume expansion due to gas hydrate dissociation in well connected pore spaces is related via darcy s law to an increase in pore pressure and its gradient in sediment which drives an additional upward fluid flow through the sediment layer overlying the gas hydrate dissociation area the magnitude of this excess pore pressure is found to be proportional to the rate of gas hydrate dissociation and the depth below seafloor and inversely proportional to sediment permeability and the depth below sea level the excess pore pressure is the greatest at low initial pressures and decreases rapidly with increasing initial pressure excess pore pressure may be the result of gas hydrate dissociation due to continuous sedimentation tectonic uplift sea level fall heating or inhibitor injection the excess pore pressure is found to be potentially able 1 to facilitate or trigger submarine landslides in shallow water environments 2 to result in the formation of vertical columns of focused fluid flow and gas migration and 3 to cause the failure of a sediment layer confined by low permeability barriers in relatively deep water environments

active gas venting through hydrate bearing sediments on the vestnesa ridge offshore w svalbard abstract gas hydrate systems offshore western svalbard are extensive and include the whole vestnesa ridge an elongated sediment drift north of the molloy transform and just east of the molloy ridge one of the shortest segments of the slow spreading north atlantic ridge system the crest of the vestnesa ridge at water depth between 1200 1300 m is pierced with fluid flow features seafloor pockmarks vary in size up to 700 m in diameter pockmarks are generally larger at the eastern start of the vestnesa ridge than at its western termination a recent cruise with r v jan mayen discovered methane flares in the water column above the pockmark field at the onset of the vestnesa ridge over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m it is still unclear what has triggered the increase in gas expulsion from seafloor sediments high resolution 3d seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems at the western end of the vestnesa ridge sub seafloor fluid flow features resemble well described chimney structures however the seismic expression of the active fluid flow features is much broader much more chaotic dome shaped and is not very similar to a typical chimney structure the vestnesa ridge gas hydrate and free gas system occurs within few kilometers of a mid oceanic ridge and transform fault which makes this gas hydrate system unique on earth the close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas hydrate and free gas system the high heat flow together with the high tectonic activity of this region a thick sedimentary cover a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free gas system leading to high variability in gas supply gas migration and gas hydrate build up and dissociation

conductive heat flow variations from bottom simulating reflectors on the hikurangi margin new zealand 1 bottom simulating reflectors bsrs represent the base of a gas hydrate zone underlain by widespread free gas the extent of bsrs 40 000 km2 has been mapped on the continental slope of the hikurangi margin using industry and research seismic data a conductive model was used to calculate heat flow from the depth of the bsr to the surface a regional trend is observed in which heat flow uncorrected for sedimentation effects decreases from an average of 50 mw m 2 at the toe of the accretionary prism to 35 mw m 2 at a distance 100 km landward from the deformation front this trend reflects the processes of thickening accretionary wedge sediments and subduction of the pacific plate and is in general agreement with heat flow values obtained using bottom hole temperature data local variations of heat flow were also observed notably low and high values over the crest and flanks of anticlines numerical modeling indicates that much of this heat flow variation is due to a combination of variable seabed bathymetry and sedimentation effects the dynamic effects of thrust faulting are found to be small and fluid flow along near surface faults is not predicted to contribute significantly to the observed heat flow anomalies along the modeled section