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

three dimensional splay fault geometry and implications for tsunami generation megasplay faults very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism are thought to play a role in tsunami genesis we imaged a megasplay thrust system along the nankai trough in three dimensions which allowed us to map the splay fault geometry and its lateral continuity the megasplay is continuous from the main plate interface fault upwards to the sea floor where it cuts older thrust slices of the frontal accretionary prism the thrust geometry and evidence of large scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time contrary to the usual seaward progression of accretionary thrusts the megasplay fault has progressively steepened substantially increasing the potential for vertical uplift of the sea floor with slip we conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis such as the 1944 moment magnitude 8 1 tonankai event and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis

heat flow along the nantroseize transect results from iodp expeditions 315 and 316 offshore the kii peninsula japan mal conductivity varies between about 1 0 w m 1 k 1 near the seafloor to 1 6 w m 1 k 1 at a depth of 1 km thermal conductivity generally increases with depth and correlates with variations in porosity and lithology thermal gradients along the transect are characterized by 48 sediment temperature measurements from 6 sites thermal corrections for the effects of bathymetric relief and sedimentation improve the confidence with which the heat flow values can be interpreted heat flow generally decreases with landward distance from the deformation front and varies from 70 mw m 2 just landward of the deformation front to 54 mw m 2 at sites characterizing the outer fore arc high and to 57 mw m 2 at the kumano basin site iodp heat flow measurements are significantly lower than nearby seafloor heat flow measurements this difference is most likely due to variations in bottom water temperature that have a large effect on values of seafloor heat flow thus the heat flow of the nankai accretionary prism is lower than previously thought we present thermal models of subduction along this transect and explore the impact of the initial geotherm conductive plate cooling based on the age of subducting seafloor 20 myr under predicts the observed heat flow we find a good fit to the data using a geotherm appropriate for 10 myr seafloor the extra heat is interpreted in terms of back arc thermal environments

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

gas hydrate distribution and hydrocarbon maturation north of the knipovich ridge western svalbard margin a bottom simulating reflector bsr occurs west of svalbard in water depths exceeding 600 m indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern north atlantic margin regional bsr mapping shows the presence of hydrate and free gas in several areas with the largest area located north of the knipovich ridge a slow spreading ridge segment of the mid atlantic ridge system here heat flow is high up to 330 mw m 2 increasing towards the ridge axis the coinciding maxima in across margin bsr width and heat flow suggest that the knipovich ridge influenced methane generation in this area this is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination to evaluate the source rock potential on the western svalbard margin we applied 1d petroleum system modeling at three sites the modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons the bulk petroleum mass produced since the eocene is at least 5 kt and could be as high as 0 2 mt most likely source rocks are miocene organic rich sediments and a potential eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the knipovich ridge the presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the svalbard margin varies spatially

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

thermal properties and thermal structure in the deep water coalbed basin off the shimokita peninsula japan abstract heat transport properties are significant parameters that affect geothermal processes in deep sedimentary basins we measured the thermal properties of deep sediment core samples to a depth of approximately 2500 m below sea floor mbsf whereas most previous studies have only presented data 1 k 1 and calcite cemented sandstone siltstone exhibits the highest conductivity of approximately 3 wm 1 k 1 the other thermal properties are also influenced by the porosity and lithological contrast the correlation between thermal conductivity and porosity is explained by the mixed law model of the geometric mean the one dimensional temperature depth profile at site c0020 estimated from measured thermal conductivity reveals a regression of the thermal gradient with depth and the surface heat flow of 30 mwm 2 which is consistent with the heat flow data near this site was evaluated the obtained results suggest that the linear geothermal gradient estimated from the average thermal conductivity value at shallow sediment materials could overestimate the temperature at greater depths and underestimate the heat flow

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

pore size controls on the base of the methane hydrate stability zone in the kumano basin offshore japan the base of the methane hydrate stability zone mhsz in the kumano basin offshore japan is marked by a bottom simulating reflection bsr on seismic data at integrated ocean drilling program site c0002 which penetrates this bsr the in situ temperature profile combined with bulk seawater methane equilibrium conditions suggest that the base of the mhsz is 428 m below seafloor bsf which is 28 m deeper than the observed bsr 400 m bsf we found that submicron pore sizes determined by mercury injection capillary pressure are sufficiently small to cause 64 of the observed uplift of the base of the mhsz by the gibbs thomson effect this is the most thorough characterization of pore sizes within the mhsz performed to date and illustrates the extent to which pore size can influence mhsz thickness our results demonstrate the importance of considering lithology and pore structure when assessing methane hydrate stability conditions in marine sediments

deformation of the nankai trough inner accretionary prism the role of inherited structures funding for this research was provided by the national science foundation grant oce 0451790 and a u s science support program post expedition award

heat flow distribution and thermal structure of the nankai subduction zone off the kii peninsula detailed heat flow surveys were carried out in the central part of the nankai trough southeast of the kii peninsula off kumano for investigation of the thermal structure of the subducting plate interface at stations in the kumano trough forearc basin and its vicinity long term monitoring of temperature profiles in surface sediments was conducted because bottom water temperature variations btv significantly disturb subbottom sediment temperatures heat flow values were successfully determined at seven stations by removing the influence of btv from temperature records for 300 to 400 days the surface heat flow data were combined with estimates from depths of methane hydrate bottom simulating reflectors to construct a heat flow profile across the subduction zone heat flow decreases from 90 110 mw m2 on the floor of the nankai trough to 50 60 mw m2 at around 30 km from the deformation front while it is rather uniform 40 60 mw m2 in the kumano trough the values measured on the nankai trough floor are concordant with the value estimated from the age of the subducting philippine sea plate about 20 m y taking into account the effect of sedimentation the obtained heat flow profile was used to constrain thermal models of the subduction zone the subsurface thermal structure was calculated using a two dimensional steady state model in which the frictional heating along the plate interface and the radioactive heat production are treated as unknown parameters comparison of the calculated surface heat flow in the kumano trough with the observed data indicates that the effective coefficient of friction is small about 0 1 or less and thus the shear stress on the plate interface is very low in this subduction zone

a low velocity zone with weak reflectivity along the nankai subduction zone three dimensional seismic reflection data reveal the presence of a low seismic velocity zone lvz with weak reflectivity character along the nankai accretionary prism this lvz is intercalated between an upper offscraped layer and a lower underthrusting layer in the outer accretionary wedge wide angle ocean bottom seismograph data also support the presence of the lvz which is estimated to be a maximum of 2 km thick 15 km wide and 120 km long the lvz could be an underthrust package underplated in response to the lateral growth of the nankai accretionary prism underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward the lvz could have elevated fluid pressure leading to rigidity reduction of the entire outer accretionary wedge the rigidity lowered outer wedge containing the lvz may be more easily uplifted and thus eventually foster tsunami generation during a nankai megathrust earthquake if the fluid rich lvz supplies a significant amount of the fluid to the megasplay fault zone at depth it may affect stick slip behavior of the fault

gas hydrate stability and the assessment of heat flow through continental margins summary a prominent feature across some continental margins is a bottom simulating reflector bsr this seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field because the occurrence of gas hydrates is controlled by temperature and pressure conditions it has been suggested that bsrs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins crucial parameters are the temperature at bsr depth and at the seafloor and the thermal conductivity structure between the bsr and the seabed however very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities here we use downhole temperature thermal conductivity porosity and logging data from 10 ocean drilling program odp sites drilled into and through the gas hydrate field to investigate the quality of estimates our analyses and application of constraints to the makran margin off pakistan indicate the following i the temperature at bsr depth could be approximated by a seawater methane system although capillary forces chemical impurities or non equilibrium conditions can lower or increase the temperature if calibration by heat probe measurements is possible errors of geothermal gradients are less than 10 per cent otherwise uncertainties of 20 per cent or even higher may arise in addition seasonal variations of bottom water temperature have to be considered because they may affect thermal gradients by up to y10 per cent ii the impact of typical quantities of low thermal conductivity gas hydrate on the bulk thermal conductivity is insignificant iii the thermal conductivity profile between the bsr and the seabed can generally be approximated by a mean value thus iv seabed measurements should be used instead of empirical relationships which may produce errors of 5 30 per cent consequently in addition to high quality seismic data a prerequisite should be a large data set of thermal conductivities and oceanographic data heat probe measurements are recommended to constrain geothermal gradients in this case the uncertainty of heat flow is 5 10 per cent of the estimated heat flow if these data are not available errors uncertainties can reach 50 60 per cent of the calculated value

dynamics of gas hydrate case of the congo continental slope a numerical model is developed to study the temperature effect on stability and phase transformation of gas hydrate the model uses a mathematical formulation based on the enthalpy form of the conservation law of energy the use of the enthalpy form instead of the temperature form as often done in the literature has made the problem numerically simpler the model is then applied to describe the effect of sea bottom temperature variations on the stability of gas hydrate occurrences and on the seafloor reflectivity in sediments of the congo continental slope indeed a migrating seafloor reflectivity front is observed on 3d seimic images from two surveys performed 6 months apart and interpreted as a migrating gas hydrate stability zone seawater temperature variations were recorded over a 230 day period at the seafloor the amplitude of these variations could explain a 75 m shift in water depth of the upper limit of the gas hydrate stability zone however the response of the sediment to a perturbation applied at the seafloor is not immediate because of the effects of thermal diffusion and of latent heat of hydrate dissociation thermal modelling shows that the depth of penetration of these perturbations is around 2 m for the saturated sediment and around 6 m for hydrate bearing sediments hydrate fraction of 0 1 the switch between gas and gas hydrate generated by these temperature fluctuations concern only the first meter of sediment these appear too small to explain the migration of the seafloor reflectivity considering that the wavelength of the seismic signal is significantly larger around 10 m however for temperature fluctuations with larger wavelength the switch between gas and gas hydrate will be more important and consequently could explain the reflectivity contrast as a possible migrating gas or gas hydrate front

thermal effects of hydrothermal circulation and seamount subduction temperatures in the nankai trough seismogenic zone experiment transect japan 1 we examine the thermal effects of seamount subduction seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position prior to subduction seamounts provide high permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust as the seamount is subducted the high permeability pathway is closed shutting down the advective transfer of heat if significant fluid flow occurs it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system additionally subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge we apply these features of seamount subduction to a thermal model for the nankai trough seismogenic zone experiment transect on the margin of japan models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench they yield temperatures of 100 c 295 c for the rupture area of the 1944 tonankai earthquake temperatures in the region of episodic tremor and slip are estimated at 290 c 325 c 70 c cooler than a model with no fluid circulation

spatial distribution of gas hydrate deposits in krishna godavari offshore basin bay of bengal abstract to delineate the occurrences of gas hydrates and free gas in the krishna godavari kg basin bay of bengal we used 3d seismic data to construct a velocity model of high spatial density we used an automated version of migration velocity analysis to estimate p wave velocities using kirchoff prestack time migration we manually checked all common image gathers cigs to assess the reliability of the auto picking algorithm the velocity model was validated against available downhole information from drilling sites nghp 01 02 nghp 01 03 and nghp 01 10 the model shows a anomalously high velocities at site nghp 01 10 where significant gas hydrates were reported and b background velocities at sites nghp 01 02 and nghp 01 03 where a negligible amount of hydrate was recovered a bottom simulating reflector bsr is identified in the 3d seismic data and an average velocity map is prepared above the bsr background velocities of marine sediments in the kg basin vary from 1 43 km s to 1 6 km s velocities higher than 1 65 km s indicate gas hydrate bearing sediments while velocities lower than 1 43 km s indicate free gas bearing sediments high velocities observed below seafloor mounds and ridges suggest the presence of gas hydrate in a structurally deformed area slope basins are generally devoid of gas hydrates free gas is observed at depths of up to several hundred meters below the bsr suggesting the migration of deeper fluid gas into the gas hydrate stability zone ghsz zones of gas hydrates and free gas are preferentially oriented along major faults indicating the role of faults in the transportation of free gas and formation of gas hydrates deposits we identify a free gas bearing horizon at depths 300 m below the bsr which might contribute to the lateral migration of free gas gas hydrate deposits in kg basin are classified into two categories i leaky deposits which occur in the areas of active seafloor faults and are characterized by heterogeneous distribution of gas hydrates and ii non leaky deposits which are formed in areas of subsurface faults and show a relatively homogeneous distribution of gas hydrates site nghp 01 10 is a good example of leaky gas hydrate deposits above a deep seated gas reservoir

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

fluid flow and origin of a carbonate mound offshore vancouver island seismic and heat flow constraints a 1 5 km long 1 km wide and 70 80 m high carbonate mound was identified on the mid slope region of the subduction accretionary sedimentary prism offshore vancouver island 3 5 km west of ocean drilling program odp site 889 and integrated ocean drilling program iodp site u1327 seabed video images show the presence of seafloor carbonate as well as chemosynthetic communities a high resolution single channel seismic survey with close line spacing recording coherent reflectivity down to about 400 m beneath the seafloor provided acoustic images of this mound and of the gas hydrate bottom simulating reflector bsr beneath it the mound is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope the fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near surface sediments to examine the thermal effect of possible upward fluid flow beneath the mound heat flow at the mound and in the neighbouring region was calculated from the depth of the bsr below the seafloor these data were combined with heat flow calculated over a broader region from previous multi channel seismic data heat flow within the flattest portion of the surrounding 4 km by 8 km region averages 74 mw m2 taking this value to represent the regional or background heat flow a simple 2d analytical method was used to calculate theoretical heat flow variations due to topography across the mound most of the variability is explained by topographic effects including a local 6 mw m2 negative anomaly over the central mound and a large 20 mw m2 positive anomaly over the mound steep side slope however just south of the mound there is a 6 7 mw m2 positive anomaly in a 2 km long band that has predominantly flat seafloor most of this anomaly is probably unrelated to topographic effects but rather likely due to warm upward fluid flow along faults or fracture zones

authigenesis of magnetic minerals in gas hydrate bearing sediments in the nankai trough offshore japan gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation a high resolution rock magnetic study was conducted in iodp expedition 316 hole c0008c located in the megasplay fault zone of the nankai trough offshore japan in this particular zone no bottom simulating reflectors bsr indicating the base of the gas hydrate stability field have been identified two hundred and eighteen pleistocene samples were collected from 70 to 110 m csf in order to document the changes in the concentration grain size and rock magnetic parameters of magnetic minerals through the gas hydrate bearing horizons two different populations of magnetic grains are recognized in the pseudosingle domain range three types of magnetic mineral assemblages are identified iron oxides magnetite ferrimagnetic iron sulfides greigite and pyrrhotite and their mixture greigite and pyrrhotite are authigenic and constitute six layers called is1 is6 is1 is3 is4 and is6 are associated with pore water anomalies suggesting the occurrence of gas hydrates and anoxic conditions is2 and is5 are probable gas hydrates horizons although there is no independent data to confirm it the remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval the results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in hole c0008c and counterbalances the low resolution of pore water analyses and the absence of a bsr

origin and evolution of a splay fault in the nankai accretionary wedge subduction zones are often characterized by wedge shaped sedimentary complexes called accretionary prisms that form when sediments are scraped off the subducting plate and added to the overriding plate large landward dipping thrust faults can cut through such a prism these faults known as megasplay faults 1 2 originate near the top of the subducting plate and terminate at the shallow landward edge of the prism1 3 4 5 6 megasplay faults have been the subject of numerous geological and geophysical studies4 5 6 7 8 9 10 11 12 13 14 15 but their initiation and evolution through time remains poorly constrained here we combine seismic reflection data from the nankai accretionary wedge with geological data collected by the integrated ocean drilling program iodp and find that the splay fault cutting this wedge initiated approx1 95 million years myr ago in the lower part of the prism as an out of sequence thrust oost after an initial phase of high activity the movement along the fault slowed down but uplift and reactivation of the fault resumed about 1 55 myr ago the alternating periods of high and low activity along the splay fault that we document hint at episodic changes in the mechanical stability of accretionary prisms

heat flow estimated from bsr and iodp borehole data implication of recent uplift and erosion of the imbricate thrust zone in the nankai trough off kumano heat flow values were estimated in the nankai trough fore arc slope region off kii peninsula from the bottom simulating reflector bsr and other data obtained during iodp expeditions 315 and 316 the heat flow has an uncertainty of 20 mainly due to ambiguities in the temperature estimate at the bsr and uncertainties on the thermal conductivity bsrs occur intermittently in the imbricate thrust zone itz they are significantly shallower below anticlines designated anticlinal high zone ahz than in the adjacent synclines basal low zone blz the heat flow in the blz 55 65 mw m2 is consistent with the regional heat flow trend the shallow bsr in the ahz produces an apparent high heat flow anomaly of 70 90 mw m2 and discontinuities are observed across thrust faults the most likely cause is the transient effect of thrust faulting followed by erosion on the hanging wall side a one dimensional time dependent numerical model with latent heat for hydrate and gas transition considered indicates that the relaxation time for the bsr to reequilibrate after such a disturbance is 10 kyr although the possibility that the shallow bsr is a remnant base of hydrate stability zone cannot be ruled out if the shortening has occurred in the recent 10 kyr the convergent rate at the itz would be 30 m kyr which is much larger than the recent horizontal shortening across the megasplay fault and could even exceed the portion of the plate convergence than that accommodated at the frontal thrust

erosion and sedimentation as processes controlling the bsr derived heat flow on the eastern nankai margin in 2000 a 3d mcs survey was carried out on the eastern nankai trough a strong and widespread bottom simulating reflector bsr was observed and mapped after a preserved amplitude 3d prestack migration of the seismic data we use this bsr to calculate a heat flow map over the 3d survey area this map presents some high and low heat flow areas that cannot be correlated to known active wedge faults and previously observed fluid venting sites since the heat flow anomalies are not correlated to known major faults or fluid venting sites large fluid circulation is not likely to explain the varying bsr derived heat flow distribution in order to explain this heat flow distribution quantitatively we calculate the rates of erosion and sedimentation that are necessary to create these anomalies these rates are in good agreement with observations on seismic profiles and are locally consistent with a sedimentation rate calculated from slope basin depth and reflect the distribution of erosion landsliding and deposition on the margin slope we therefore propose that the observed bsr derived heat flow anomalies are mainly due to active erosion and sedimentation and that heat flow distribution on the margin section of the 3d survey is controlled by surface processes fluid migration could occur where a bsr is absent but is probably episodic or limited where a bsr is present and does not affect the bsr derived heat flow in this part of the margin these surface processes are the consequence of the tectonic and seismic activity of the wedge and are influenced by the subduction of a wide oceanic ridge below the eastern nankai margin

hydrocarbon seepage and its sources at mud volcanoes of the kumano forearc basin nankai trough subduction zone twelve submarine mud volcanoes mv in the kumano forearc basin within the nankai trough subduction zone were investigated for hydrocarbon origins and fluid dynamics gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from mvs 2 4 5 and 10 molecular ratios c1 c2 40 v pdb indicate that hydrate bound hydrocarbons hcs at mvs 2 4 and 10 are derived from thermal cracking of organic matter considering thermal gradients at the nearby iodp sites c0009 and c0002 the likely formation depth of such hcs ranges between 2300 and 4300 m below seafloor mbsf with respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of hcs fueling the mvs is derived from sediments of the cretaceous to tertiary shimanto belt below pliocene pleistocene to recent basin sediments considering their sizes and appearances hydrates are suggested to be relicts of higher mv activity in the past although the sporadic presence of vesicomyid clams at mv 2 showed that fluid migration is sufficient to nourish chemosynthesis based organisms in places distributions of dissolved methane at mvs 3 4 5 and 8 pointed at fluid supply through one or few mv conduits and effective methane oxidation in the immediate subsurface the aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth

estimation of seismic velocities and gas hydrate concentrations a case study from the shenhu area northern south china sea abstract in the shenhu area of the northern south china sea scs canyon systems and focused fluid flow systems increase the complexity of the gas hydrate distribution in the region it also induces difficulties in predicting the hydrate reservoir characteristics and quantitatively evaluating reservoir parameters in this study several inversion methods have been executed to estimate the velocities of strata and gas hydrate concentrations along a profile in the shenhu area the seismic data were inverted to obtain the reflection coefficient of each stratum via a spectral inversion method stratigraphic horizons were then delineated by tracking the inverted reflectivities based on the results of spectral inversion a low frequency velocity field of the strata was constructed for acoustic impedance inversion using a new iterative algorithm for acoustic impedance inversion reflection coefficients were converted into velocities and the velocity variations of the strata along a 2d seismic line were then obtained subsequently gas hydrate saturations at well sh2 were estimated via the shale corrected resistivity method the chloride ion concentration method and three different rock physics models the results were then compared to determine the optimal rock physics model and the modified wood equation mwe was found to be appropriate for this area finally the inverted velocities and mwe were used to predict the distribution and concentrations of gas hydrates along the seismic line the estimated spatial distribution of gas hydrates is consistent with that from sonic logging and resistivity data at well sh2 and with the drilling results therefore this method is applicable in areas with no well data or with few wells and provides an effective tool for predicting and evaluating gas hydrates using seismic data

injection transport and deposition of tephra during event 5 at redoubt volcano 23 march 2009 abstract among the events of the 2009 eruption at redoubt volcano alaska event 5 was the best documented by radar satellite imagery and deposit mapping we use the new eulerian tephra transport model ash3d to simulate transport and deposition of event 5 tephra at distances up to 350 km the eruption which started at about 1230 utc on 23 march 2009 sent a plume from the vent elevation estimated at 2 3 0 1 km above sea level or a s l to about 16 2 km above sea level in 5 min the plume was a few kilometers higher than would be expected for the estimated average mass eruption rate and atmospheric conditions possibly due to release of most of the eruptive mass in the first half of the 20 minute event the eruption injected tephra into a wind field of high shear with weak easterly winds below 3 km elevation strong southerly winds at 6 10 km and weak westerlies above 16 km model simulations in this wind field predicted development of a northward migrating inverted v shaped cloud with a southwest trending arm at a few kilometers elevation which was not visible in ir satellite images due to cloud cover and a southeast trending arm at 10 km elevation that was clearly visible simulations also predicted a deposit distribution that strongly depended on plume height a plume height below 15 km predicted ash deposits that were located west of those mapped whereas good agreement was reached with a modeled plume height of 15 18 km field sampling of the deposit found it to contain abundant tephra aggregates which accelerated the removal of tephra from the atmosphere we were able to reasonably approximate the effect of aggregation on the deposit mass distribution by two methods 1 adjusting the grain size distribution taking the erupted mass 2 isomass lines that was within a few tens of percent of mapped area however they under predicted deposit mass at very proximal 250 km locations modeled grain size distributions at sample locations are also generally coarser than observed the mismatch may result from a combination of limitations in field sampling approximations inherent in the model errors in the numerical wind field and aggregation of particles larger than 0 063 mm

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

fluid generation and distribution in the highest sediment input accretionary margin the makran fluids in subduction zones can influence seismogenic behaviour and prism morphology the eastern makran subduction zone offshore pakistan has a very thick incoming sediment section of up to 7 5 km providing a large potential fluid source to the accretionary prism a hydrate related bottom simulating reflector bsr zones of high amplitude reflectivity seafloor seep sites and reflective thrust faults are present across the accretionary prism indicating the presence of fluids and suggesting active fluid migration high amplitude free gas zones and seep sites are primarily associated with anticlinal hinge traps and fluids here appear to be sourced from shallow biogenic sources and migrate to the seafloor along minor normal faults there are no observed seep sites associated with the surface expression of the wedge thrust faults potentially due to burial of the surface trace by failure of the steep thrust ridge slopes thrust fault reflectivity is restricted to the upper 3 km of sediment and the deeper decollement is non reflective we interpret that fluids and overpressure are not common in the deeper stratigraphic section thermal modelling of sediments at the deformation front suggests that the deeper sediment section is relatively dewatered and not currently contributing to fluid expulsion in the makran accretionary prism

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

a synthesis of heat flow determinations and thermal modeling along the nankai trough japan 1 we review marine heat flow data along the nankai trough and show that observations 30 km seaward of the deformation front are 20 below conductive predictions 129 94 mw m 2 but consistent with the global heat flow average for oceanic crust of the same age 16 28 ma heat flow values 30 km seaward of the deformation front are generally 20 higher than conductive predictions this heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench 200 and 140 mw m 2 and steep landward declines to values of approximately 60 mw m 2 over distances of 65 and 50 km along the muroto and kumano transects respectively along the ashizuri transect the lack of heat flow data precludes a definitive interpretation we conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 c higher near the toe of the margin wedge and 50 100 c lower near the downdip limit of the seismogenic zone than estimated by purely conductive models

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

some remarks on the thermal nature of the double bsr abstract the numerical solution of the heat transfer problem which takes into account the latent heat stefan problem of the gas hydrate dissociation shows that the double bsr at the nankai trough site could have been caused by the upwards displacement of a previously deeper phase boundary base of the gas hydrate stability zone after an abrupt increase of the sea bottom temperature by approximately 4 c or more which happened about 10 ka ago the obtained conclusions are believed to be correct for other areas of the ocean as well where two or more bsrs have been observed in order to explain the nature of the phenomenon of the double bsr it is necessary to examine also other factors besides paleooceanographic ones for example lithological geochemical properties of sediments

a user friendly one dimensional model for wet volcanic plumes 1 this paper presents a user friendly graphically based numerical model of one dimensional steady state homogeneous volcanic plumes that calculates and plots profiles of upward velocity plume density radius temperature and other parameters as a function of height the model considers effects of water condensation and ice formation on plume dynamics as well as the effect of water added to the plume at the vent atmospheric conditions may be specified through input parameters of constant lapse rates and relative humidity or by loading profiles of actual atmospheric soundings to illustrate the utility of the model we compare calculations with field based estimates of plume height 9 km and eruption rate 4 105 kg s during a brief tephra eruption at mount st helens on 8 march 2005 results show that the atmospheric conditions on that day boosted plume height by 1 3 km over that in a standard dry atmosphere although the eruption temperature was unknown model calculations most closely match the observations for a temperature that is below magmatic but above 100 c

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

deep sea bottom simulating reflectors calibration of the base of the hydrate stability field as used for heat flow estimates ocean drilling program and deep sea drilling project downhole data from three areas the southwestern japan nankai margin the continental slope off peru and the blake bahama outer ridge provide temperature calibrations for bottom simulating reflectors bsr that mark the base of a clathrate hydrate stability field the inferred temperatures at bsrs provide an important reference for the mapping of geothermal gradient and heat flow from subduction zone accretionary sedimentary wedges the borehole results provide information on which stability field is applicable for the bsrs and thus calibrate the heat flow estimates while an ideal calibration has not been possible the bsr temperatures at the three sites in the temperature range 25 27 c have been estimated with uncertainties of 0 7 to 2 0 c the temperatures correspond closely to the laboratory dissociation temperatures for pure water pure methane hydrate at equivalent pressures no laboratory data are available for seawater salinity and methane at equivalent pressures but extrapolation from lower pressures gives temperatures 1 2 c lower which is just significantly different the data also could be explained by the stability curve for seawater salinity and methane with about 7 co2 or with a small amount of higher hydrocarbons but most hydrate samples that have been recovered by deep sea drilling have contained almost pure methane the uncertainties in the temperature at the bsr should contribute no more than 5 error in heat flow estimates from bsr depths if the pure water methane stability field is used

anomalously cold temperatures observed at the base of the gas hydrate stability zone on the u s atlantic passive margin in situ measurements to depths of 415 m below sea floor in methane hydrate bearing sediments on the u s atlantic passive margin indicate that temperatures at the bottom simulating reflector bsr are anomalously low by 0 5 2 9 c if the bsr marks the base of gas hydrate stability bghs several hypotheses may explain the occurrence of the bsr at inappropriate pressure temperature p t conditions 1 if the bsr does not mark the bghs then p t conditions need not be sufficient to dissociate gas hydrate at this depth 2 the bsr may lie at nonequilibrium p t conditions due to incomplete readjustment in response to upper pleistocene holocene climate change however the occurrence of the blake ridge bsr at an overly shallow depth cannot be easily explained by realistic combinations of pressure driven deepening sea level rise and temperature driven shoaling bottom water temperature changes 3 the properties of sediments or pore fluids may inhibit the stability of gas hydrate in particular capillary forces arising in the fine grained montmorillonite rich sediments of the blake ridge may lead to shoaling of the bsr in this setting

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

bottom simulating reflector dynamics at arctic thermogenic gas provinces an example from vestnesa ridge offshore west svalbard the vestnesa ridge comprises a 100 km long sediment drift located between the western continental slope of svalbard and the arctic mid ocean ridges it hosts a deep water 1000 m gas hydrate and associated seafloor seepage system near seafloor headspace gas compositions and its methane carbon isotopic signature along the ridge indicate a predominance of thermogenic gas sources feeding the system prediction of the base of the gas hydrate stability zone for theoretical pressure and temperature conditions and measured gas compositions results in an unusual underestimation of the observed bottom simulating reflector bsr depth the bsr is up to 60 m deeper than predicted for pure methane and measured gas compositions with 99 methane models for measured gas compositions with 4 higher order hydrocarbons result in a better bsr approximation however the bsr remains 20 m deeper than predicted in a region without active seepage a bsr deeper than predicted is primarily explained by unexpected spatial variations in the geothermal gradient and by larger amounts of thermogenic gas at the base of the gas hydrate stability zone hydrates containing higher order hydrocarbons form at greater depths and higher temperatures and contribute with larger amounts of carbons than pure methane hydrates in thermogenic provinces this may imply a significant upward revision up to 50 in the case of vestnesa ridge of the amount of carbon in gas hydrates

massive methane release triggered by seafloor erosion offshore southwestern japan vast amounts of methane hydrate exist beneath continental margins but whether this methane releases from sediment on a large scale and affects the oceans and atmosphere remains unclear analysis of newly acquired three dimensional seismic images and drilling data from a large gas hydrate province reveal a recently eroded v shaped depression the depression sharply cuts through a relic bottom simulating reflection bsr and hydrate laden sediments the shape of the relic bsr indicates that the seafloor depression was once a large anticline that has recently been eroded and released an estimated 1 51 10 11 m 3 of methane we hypothesize that erosion of the seafloor via bottom water currents unroofed buoyant hydrate laden sediments and subhydrate overpressured free gas zones beneath the anticline once triggered gas driven erosion created a positive feedback mechanism releasing gas and eroding hydrate bearing sediment we suggest that erosive currents in deep water methane hydrate provinces act as hair triggers destabilizing kilometer scale swaths of the seafloor where large concentrations of underlying overpressured methane exist our analysis suggests that kilometer scale degassing events are widespread and that deep water hydrate reservoirs can rapidly release methane in massive quantities

distribution of gas hydrates on continental margins by means of a mathematical envelope a method applied to the interpretation of 3d seismic data 1 a 3d seismic volume from the nankai trough accretionary wedge se japan is used to evaluate the subsurface distribution of gas hydrates as a function of structural and stratigraphic complexity variable heat flow patterns and the presence of subsurface fluid conduits eleven equations were modified for depth pressure and temperature modeled in 3d and compared with the distribution of bottom simulating reflections bsrs offshore nankai the results show that the equations produce overlapping and thus potentially consistent predictions for the distribution of bsrs leading us to propose the concept of a bsr stability envelope as a method to quantify the subsurface distribution of gas hydrates on continental margins in addition we show that the ratio r between shallow and deep bsrs of seven subenvelopes which are defined by bsr stability equations indicates local gas hydrate equilibrium conditions values of r 1 the majority of bsrs are located in warmer structural traps the method in this paper can be used to recognize any divergence between observed and theoretical depths of occurrence of bsrs on 3d or 4d time lapse seismic volumes in the nankai trough our results point out for equilibrium conditions in bsrs located away from the megasplay fault zone and major thrust faults this latter observation demonstrates the applicability of the method to a the recognition of subsurface fluid conduits and b the prediction of maximum and minimum depths of occurrence of gas hydrates on continental margins under distinct thermal and hydrologic conditions