microseismicity around rupture area of the 1944 tonankai earthquake from ocean bottom seismograph observations abstract along the nankai trough southwestern japan the philippine sea plate psp is subducting beneath the eurasian plate and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100 200 years the most recent large thrust event in the eastern nankai trough off kii peninsula was the 1944 tonankai earthquake in this region current seismicity is very low and hypocenters are not determined accurately by the land seismic network we conducted microseismicity observations around the rupture area of the 1944 tonankai earthquake using ocean bottom seismographs obss hypocenters were determined using a 2 d seismic velocity structure model based on an airgun obs seismic survey results obtained show that the seismicity was relatively active near the trough axis these earthquakes may relate to deformation of the subducting philippine sea plate on the other hand microseismicity in the rupture area of the 1944 tonankai earthquake was very low this low level seismic activity in the co seismic rupture area of the 1944 tonankai earthquake likely relates to a single large asperity off kii peninsula

new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 moore g f taira a klaus a becker l boeckel b cragg b a dean a fergusson c l henry p hirano s hisamitsu t et al 2001 new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 geochemistry geophysics geosystems 2 article no 2001gc000166

platinum group elements in tulameen coal british columbia canada nickel sulfide fire assay neutron activation analysis and lead fire assay direct current plasma spectroscopy were used to determine platinum group elements pge and gold in 30 channel samples and 23 fault related samples from the blakeburn opencast mine south central british columbia canada in the majority of the channel samples concentrations of os ir ru rh pd and re were below detection pt in the coal ranged from below detection to 170 ppb and au from below detection to 38 ppb concentrations of all the pge with the exception of ru were on average higher in the coals than in the sediments in the deformed coal samples os ru rh and re were typically below detection anomalous values of pt up to 10 000 ppb were determined in three deformed coal samples and pd and ir were comparatively enriched in the majority of deformed coal samples up to 6 100 and 22 ppb respectively compared with channel samples on the basis of sem edx and comparisons with ash and organic matter contents it is likely that pt and au are present in the coal as discrete particles precipitated from circulating ground waters on contact with the organic rich reducing and low ph environment of the coal pt and au distributions appear to be related to sources of the elements in ground water i e interbedded volcanic units and the tulameen ultramafic complex for pt and interbedded clastic sedimentary units for au enhanced concentrations of pt and pd occur in coal samples associated with two faults

seismicity in the incoming subducting philippine sea plate off the kii peninsula central nankai trough 1 along the nankai trough southwestern japan the philippine sea plate phs is subducting beneath the eurasian plate and large interplate earthquakes have occurred repeatedly the latest large thrust earthquake at the central nankai trough off the kii peninsula was the 1944 tonankai earthquake mw 8 2 although a previous ocean bottom seismograph obs experiment from november 2001 to february 2002 showed seismicity around the rupture area of the 1944 tonankai earthquake there were large uncertainties regarding hypocenter locations especially with respect to depth we conducted another obs experiment to obtain accurate locations of earthquakes and their focal mechanisms the new results show earthquake clusters at the trough axis within the oceanic crust the clusters were located near faults cutting the oceanic crust and thus the earthquakes may have occurred along preexisting faults in the incoming phs seismicity in the subducting phs is characterized by seismic quiescence in the oceanic crust of the coseismic rupture area of the 1944 tonankai earthquake the change in seismicity in the oceanic crust can be explained by a change in the stress in the incoming subducting phs the stress in the oceanic crust of the phs changes from trough normal tension at the trough axis caused by the bending of the phs to downdip compression beneath the kii peninsula the seismicity in the subduction zone likely relates to the balance of stress in the incoming and the subducting plate as well as to coupling between the subducting and overriding plates

a fast robust method for detecting and characterizing azimuthal anisotropy with marine ps converted waves and its application to the west svalbard continental slope summary a method for using ps converted waves to detect and characterize azimuthal seismic anisotropy is developed and applied to four component ocean bottom seismometer data acquired near the base of the continental slope west of svalbard the method determines the distribution of coherent signal vibrating perpendicular to the sagittal plane the transverse direction and employs an optimized rotation technique to separate the effects of anisotropy from other contributions to the seismic amplitude the magnitude of the ps wave transverse amplitude when normalized by its radial counterpart is shown to correspond to the degree of ellipticity of particle motions and is used to constrain the anisotropic symmetry class in the near surface sediments results indicate the presence of orthorhombic or horizontal transversely isotropic symmetry that is consistent with the presence of near vertical aligned cracks from its orientation the observed anisotropy is associated with 1 gravitational downslope stresses in the sediment sequence which dips by approximately two degrees to the sw and 2 tectonic stresses associated with the segmented spreading centres of the northern knipovich ridge which spread in the nw se direction aligned cracks are likely to influence the local distribution of gas hydrate and free gas in these sediments an investigation of the effect of seismic noise on optimized rotation shows that the vectorial combination of independent geophone records has variable sensitivity to source related noise this sensitivity peaks where the sagittal azimuth bisects the deployment orientation of the geophones on the seabed a method to reduce the sensitivity to directional noise is developed and shown to improve the transverse amplitude plots this approach to noise reduction may be used to improve the quality of seismic images derived from multicomponent data

layer stripping of shear wave splitting in marine ps waves summary the properties of split s waves can be used to infer 1 the state of stress and strain in the earth 2 the directional dependence of hydraulic conductivity and 3 small changes in pore fluid pressure in the rock mass that occur in response to dynamic processes such as the earthquake cycle measurements of split s waves are particularly useful in shallow 1000 m subseabed marine sediments where s wave splitting from an azimuthal elastic anisotropy is typically produced by the presence of near vertical aligned cracks here we present a method of measuring small amounts of s wave splitting in marine p to s mode converted waves and illustrate the technique with data from an ocean bottom seismometer obs deployed on the west svalbard continental slope the analysis applies a modified version of the alford rotation and layer stripping technique developed for zero offset s wave sources and treats ps waves that undergo mode conversion at reflectors that are close to the seabed in comparison with the overlying water depth when the seismic record contains coherent signal on both the in plane and out of plane components the layer stripping technique is capable of decoupling the swave splitting from the effects of p wave velocity anisotropy and reflector dip that influence the downgoing p wave part of the ray path the amount of s wave splitting in the data is small however and we find that this causes a greater practical problem for the analysis than the known theoretical limitations of the layer stripping theory such as use of a finite offset source for the analysis of the example data we develop a number of procedures that are necessary to mitigate the low signal to noise levels these include using a wide range of shot receiver azimuths to generate data redundancy methods of identifying and rejecting poor measurements and a predictive layer stripping approach that minimizes the propagation of errors through the analysis that arise from scatter in the layer by layer results with the ps waves of the example data which have a dominant period about 30 ms we find the technique is capable of measuring s wave splitting to a precision of about 0 5 ms for 4 or 5 layers the number of layers successfully treated would increase if the amount of s wave splitting were larger than in these data for which the total cumulative s wave splitting was about 10 ms over a 450 m depth interval the orientation of the fast split s wave was measured with a precision of about 15 our results give an s wave velocity anisotropy of 1 2 per cent in the shallowest 25 30 m subseabed that implies the presence of a differential horizontal stress at or very close to the seabed the s wave splitting accumulated throughout the investigated section at a rate that was consistent with predictions made for a single set of parallel fluid filled cracks with crack density about 0 015 the fast s wave was found to be oriented at 75 15 for the uppermost 150 m before drifting clockwise to an azimuth of 190 210 the clockwise drift in the fast s wave polarization direction implies a transfer of dominance between one set of cracks in the near surface probably related to the slope of the seabed and a different set in deeper sediments of tectonic origin a zone of azimuthal isotropy where the mechanisms that produce the elastic anisotropy may cancel each other out occupies the interval between the two almost orthogonal sets of cracks from interpretation

micro seismicity around the seaward updip limit of the 1946 nankai earthquake dislocation area we observed micro seismicity around the seaward updip limit of the 1946 nankai earthquake dislocation area by using pop up type ocean bottom seismographs deployed for three months in 1998 at the subduction zone we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely we constructed 3 d p and s wave velocity structure models based on the results of airgun obs seismic surveys along the nankai trough we located 83 earthquakes using these 3 d models these hypocenters show two groups of seismicity one seismicity group locates in oceanic layer 3 within a subducting seamount these earthquakes may reflect the deformation of the seamount caused by its subduction the other seismicity group locates near the updip limit of the thermally modeled locked zone these hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust

a fault model of the 1946 nankaido earthquake derived from tsunami data abstract a fault model of the 1946 nankaido earthquake m 8 2 is determined by the use of tsunami records of uwajima shimotsu and hososhima which were located within or near the area of major coseismic crustal deformation synthetic tsunamis computed for various fault models are matched with the observed tsunamis to determine the fault parameters a low angle thrust model slightly revised from a previous model by ando is consistent with the observed tsunamis the duration of faulting is constrained as less than 10 min based upon the tsunami the fault is divided into an eastern and a western segment corresponding to areas associated with and without aftershocks respectively the fault area and dislocation for the western segment are 150 70 km2 and 6 m and those for the eastern segment are 150 70 km2 and 3 m respectively the total seismic moment is 4 7 1028 dyn cm significantly smaller than that obtained from a geodetic model by fitch and scholz but still larger than that of the seismic model by kanamori the discrepancy in seismic moment between the seismic and the present models ran2 could be interpreted in terms of a slow dislocation on the fault but this interpretation does not match the seismic intensity distribution and damage pattern and the slow slip model for the nankaido earthquake is rejected the discrepancy between the two seismic moments is considered insignificant within error involved in data and modeling assumptions if the revised geodetic model ran2 is modified the seismic moment required to explain the observed tsunamis would be reduced further by 30 if we consider the uncertainties involved in the fault model of kanamori and the fault finiteness effect affecting the amplitude of seismic waves the seismic moment required to interpret the seismic wave data could be increased possibly being more than twice that of kanamori thus the two seismic moments from the different data sets could be close to each other within allowable tolerance this implies that the rise time of the nankaido earthquake was short enough to generate short period seismic waves from both the western and the eastern fault segments

the 1946 nankai earthquake and segmentation of the nankai trough abstract we perform a simple subevent analysis of seismic p waveforms from historical records of the 1946 nankai earthquake the results of this analysis establish that the earthquake rupture consisted of two main subevents one lying within the 1 day aftershock area off the kii peninsula while the other lies much farther to the west in the area of pronounced crustal deformation of western shikoku these conform to two segments of the nankai trough which have long been referred to in studies of historical earthquakes and geology of the nankai trough but to our knowledge have not previously been established as having distinct megathrust rupture behavior we show that these two subevents are bounded by pronounced features in the geometry of the subducting philippine sea plate psp a tear in the plate to the east beneath the kii peninsula and a subducting seamount farther to the west we speculate that these features may be associated with low strength parts of the upper and or lower plates that may be susceptible to permanent anelastic deformation which prevents the accumulation of the elastic strain energy necessary to sustain seismic rupture

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

changes in shear wave polarization azimuth with depth in cymric and railroad gap oil fields shear wave s wave polarization azimuths although consistent over large depth intervals changed abruptly and by large amount of various depths in nine component vertical seismic profiling vsp data from the cymric and railroad gap oil fields of the southwest san joaquin basin a simple layer stripping technique made it possible to follow the polarization changes and determine the s wave birefringence over successive depth intervals because the birefringence and polarization azimuth are related to in situ stresses and fracture information from such analysis could be important for reservoir development near offset vsp data from cymrix indicated that the subsurface could be appproximated roughly as two anisotropic layers the upper layer from the surface to 800 ft 240 m had vertical s wave birefringence as large was about 6 percent down to 1300 ft 400 m in the upper layer the polarization azimuth of the fast s wave was n 60 e while in the lower layer it was about n 10 e refinement of the laye

effective stress and pore pressure in the nankai accretionary prism off the muroto peninsula southwestern japan 1 we developed a theoretical method for predicting effective stress and pore pressure based on rock physics model we applied the method to reveal the pore pressure distribution within the nankai accretionary prism off southwestern japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement from the crack aspect ratio spectrum estimated from laboratory and well log data we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory by iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3d tomographic inversion we estimated in situ mean effective stress within the accretionary prism pore pressure is then the difference between the effective stress and the confining stress when we calculated pore pressure we considered compressive state of stress in the accretionary prism our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement we determined a normalized pore pressure ratio of 0 4 0 7 abnormal pore pressures develop in the under thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward the increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence and the pore pressure mean effective stress contrast at the decollement becomes smaller landward of the deformation front

seismic velocity anisotropy and fluid pressure in the barbados accretionary wedge from an offset vertical seismic profile with seabed sources 1 the state of compaction and fluid pressure in the barbados accretionary wedge near its toe at ocean drilling program site 949 were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array the model constrained by a three dimensional seismic survey and well logs shows 1 a velocity gradient of about 1 1 25 s 1 in the uppermost 180 230 m of the wedge 2 a zone of variable but no net change in velocity between 230 and 350 m depth 3 a low velocity zone 40 50 m thick just above the decollement at 391 m and 4 a displacement of the low velocity zone by thrust faults pore fluid pressure sections derived from p wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured the 160 m thick underconsolidated basal zone shows anisotropy which increases downward the lowest 40 50 m has velocity varying 1 azimuthally 3 being fastest in the direction of plate convergence and 2 in the vertical plane 2 5 horizontal faster than vertical after correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio does not exceed 0 9 and in the lowest 40 50 m of the basal zone is between 0 71 and 0 82 with fluid pressure hydrostatic lithostatic pressure hydrostatic between 0 5 and 0 65 in accordance with in situ measurements of fluid pressure in the decollement zone beneath these indicate that the accretionary wedge is stronger and less overpressured than was previously supposed

in situ stress magnitudes at the toe of the nankai trough accretionary prism offshore shikoku island japan quantifying the orientation and magnitude of tectonic stresses is essential toward understanding deformation and faulting in subduction zones however constraints on in situ horizontal stress magnitudes shmin and shmax are rare we estimate shmin and shmax at ocean drilling program site 808 at the toe of the nankai accretionary prism offshore japan using coupled constraints from 1 the width of wellbore breakouts together with estimates of rock strength and a model describing stress redistribution at the borehole wall and 2 limits on regional differential stress defined by failure on preexisting faults our analysis extends from 175 to 915 m below seafloor mbsf and spans the active frontal thrust for an upper bound on rock unconfined compressive strength ucs and assuming hydrostatic formation pore pressure shmin and shmax referenced to the seafloor increase from 6 5 mpa at 175 mbsf to 17 4 mpa at 915 mbsf with the stress state gradually transitioning from a thrust or strike slip faulting regime above 800 mbsf to a normal faulting regime below for cases with higher formation pore pressure horizontal stresses are slightly lower but follow a similar pattern we show that estimated shmin and shmax are strongly dependent on ucs breakout width and friction coefficient all of which are characterized by uncertainty our results suggest that the prism is near thrust failure in the upper 300 mbsf but far from failure below this may be reconciled with active thrusting if thrust faults are locally weaker than the surrounding rock or if shmax fluctuates during the seismic cycle

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

elemental and isotopic chloride geochemistry and fluid flow in the nankai trough 1 the first transect of pore fluid cl profiles across the deformation front of an accretionary prism and underthrust section nankai trough and the first cl isotopic profile in this environment are presented these demonstrate that there is channelized horizontal episodic flow through most of the section arcward of the deformation front and in the lowermost section seaward of the deformation front in the underthrust sediments channels appear to be continuous fluid velocities within the channels based on cl gradients are 13 5cm year the near balance of depth integrated flow and the subduction of pore fluid indicates that flow episodes are frequent 37cl values are lower than seawater throughout the section averaging 6 1 per mil this demonstrates fluid mineral exchange of cl and suggests that subducted solids carry enriched 37cl that is efficiently recycled from deeper levels in the subduction zone back into seawater and may be responsible for the seawater mantle 37cl fractionation

shear wave polarizations and subsurface stress directions at lost hills field 2 2 s wave data matrix accomplished by computationally rotating sources and receivers although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth considerable improvement in consistency resulted from analytically stripping off a thin near surface layer whose fast s wave polarization direction was about n 6 e s wave birefringence for vertical travel averaged 3 percent in two zones 200 700 ft and 1200 2100 ft 60 210 m and 370 640 m which had closely similar s wave polarizations between those zones the polarization direction changed and the birefringence magnitude was not well defined s wave polarizations from two concentric rings of offset vsps were consistent in azimuth with one another and with polarizations of the near offset vsp this consistency argues strongly for the robustness of the s wave polarization technique as applied in this area the s wave polarization pattern in offset data fits a model of vertical cracks striking n 55 e

shear wave velocities and anisotropy in the barbados accretionary complex s wave velocities at the base of the barbados accretionary wedge were determined from mode converted s waves in two vertical seismic profiles in ocean drilling program boreholes at hole 949c the profile extends from 92 to 397 m below seafloor mbsf ending 17 m into the decollement zone separating the wedge from the underthrust sediment on the subducting plate beneath at hole 948d the profile extends from 105 to 471 mbsf ending 19 m above the decollement zone s wave velocities are 303 549 m s at hole 949c and 453 679 m s at hole 948d the profiles show that the lowest 70 m of the accreted sediment just above the decollement has lower s wave velocity 379 26 m s at hole 949c and 453 11 m s at hole 948d and higher ratio of p wave to s wave velocity vp vs 4 59 0 31 at hole 949c and 3 99 0 10 at hole 948d than in the overlying layer this coincides with high clay content that has a high proportion of smectite shear wave splitting observed in records from the lowest 100 m of the accretionary wedge at both sites indicates little or no anisotropy in the low velocity zone a single measurement at hole 949c of the horizontal component of polarization of the leading split s wave from an ocean bottom shot is n68 e 20 at hole 949c split shear wave delays in the downward propagating s wave originating at a fault at 275 mbsf indicate strong anisotropy in the range 275 306 mbsf and little or no anisotropy in the range 306 380 mbsf the rate of increase of delay between 275 and 306 mbsf 0 52 0 16 ms m can be modeled by either a hudson cracked medium with crack density 0 186 0 054 or preferably a hornby model of shale with 44 of clay particles aligned within 5 of the symmetry plane this strong anisotropy is attributed to scaly fabric alignment of clay particles by pervasive shear in the fault zone which is observed in core the low anisotropy in the remainder of the section shows that there is very little overall alignment of pore space in the lower part of the accreted sediments supporting the hypothesis that the sediments are undercompacted rather than hydrofractured the low s wave velocities and high vp vs ratios indicate undercompaction and low effective stress despite the tectonic burial of the lowermost accreted sediments

western nankai trough seismogenic zone results from a wide angle ocean bottom seismic survey the nankai trough southwestern japan is recognized as a vigorous seismogenic zone with well studied historic earthquakes this paper presents results of a wide angle ocean bottom seismographs obs study at the western nankai trough seismogenic zone the obs data used were acquired on a profile 250 km long across the presumed coseismic slip zone of the 1946 nankaido earthquake ms 8 2 the main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake the crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge p wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities vp 5 0 5 6 km s and 6 6 6 8 km s in oceanic layers 2 and 3 respectively the maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with vp 3 4 4 6 km s in the deeper part the subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward 3 2 and 7 2 at 0 50 km and 50 100 km from the trough axis respectively the oceanic crust is smooth to the hypocenter zone down to 40 km depth beneath shikoku island our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust island arc crust contact zone even though there is large uncertainty about the seaward limit of the coseismic slip zone the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism

targeted 3 d prestack depth imaging at legs 190 196 odp drill sites nankai trough japan 1 target oriented 3 d prestack depth migration psdm tied to known depths of key reflections at three odp drill sites in the nankai trough provides an improved representation of the subsurface depths and p wave velocities around the drill holes the resulting velocity volume is representative of the in situ velocities and allows us to extend the 1 d logging and core velocity data from the drill sites to the entire seismic transect the psdm velocity gradients closely match those at the drill sites although the absolute values of the velocity curves are somewhat different the average psdm vertical velocity gradient above the decollement decreases from 0 99 s 1 seaward of the trench axis to 0 87 s 1 landward in the proto thrust and frontal thrust zone with a velocity inversion of about 50 150 m s at the decollement below the decollement the vertical gradient in the underthrusting section decreases from 0 97 s 1 in the basin to 0 72 s 1 beneath the frontal thrust the lateral velocity gradient in the underthrusting section between the basin and the deformation front is 0 02 s 1 corresponding to 10 thinning in the direction of subduction

p t conditions in the south wales coalfield evidence from coexisting hydrocarbon and aqueous fluid inclusions siderite nodules in the carboniferous coal measures of south wales contain cavities which are often infilled with quartz carbonates sulphides and hydrocarbons the quartz contains a mixture of hydrocarbon and aqueous fluid inclusions the aqueous fluid inclusions consist of a dilute brine 3 wt nacl equivalent and have homogenization temperatures in the range 97 212 oc mean 143 oc the hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons their homogenization temperatures are in the range 35 78 c mean 54 c it is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus a p t estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point homogenization temperatures assuming hydrocarbon saturation this method gives temperatures between 130 and 160 oc and pressures between 40 and 55 mpa the timing of mineralization is uncertain but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin i e in the upper carboniferous the hydrothermal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

thin layer tuned reflections from an anisotropic decollement zone 1 the decollement beneath the barbados accretionary complex is a gently dipping shear zone 10 40 m thick within which slippage occurs between the overlying sediments of the accretionary wedge and the underthrust sediments on the subducting tectonic plate the density and seismic velocity of the decollement derived from modeling of tuned seismic p wave reflections from its top and base are very low and indicate high porosity sustained by above hydrostatic pore pressure previous modeling assumed that the decollement is isotropic here we investigate whether neglecting anisotropy might bias the interpretation likely causes of anisotropy are aligned cracks and aligned pores and grains within the clays unrecognized anisotropy might cause misestimation of porosity and confusion of p s p mode converted waves with the tuned p wave reflections horizontal cracks within the decollement would cause lower vertical velocity than in an isotropic material with the same porosity this reduces the interpreted porosity but by only a few percent because crack density is limited by the observed finite strength of the decollement p s p mode converted waves would be caused by anisotropy due to nonhorizontal alignment within the decollement or in the accretionary wedge above appreciable p s p conversion requires anisotropy too great to be caused by aligned cracks alone a decollement of aligned anisotropic clay particles gives appreciable mode conversion when 50 of the clay particles are within 30 of perfect alignment reflections for incidence angles up to 26 from a strongly anisotropic decollement zone with horizontal alignment are indistinguishable from those from an isotropic zone

p wave and s wave velocity structure of northern cascadia margin gas hydrates summary single channel seismic and wide angle reflection data collected in september 2005 were analysed along a 2 d profile of 10 ocean bottom seismometers obss on the continental slope region off vancouver island near odp site 889 and iodp site u1327 the objectives were to determine the shallow p wave and s wave velocity structure associated with marine gas hydrates and to estimate the hydrate concentration and distribution in the sediment pore space combined traveltime inversion of single channel and obs data produced a p wave velocity model down to the depth of the bottom simulating reflector bsr at 230 5 m below the seafloor mbsf mean velocities which increased from 1 50 km s 1 at the seafloor to 1 88 km s 1 at the bsr are in good agreement with the sonic log data from sites 889 and u1327 the increase in p wave velocity of the hydrate bearing sediments relative to a background no hydrate velocity was utilized to estimate the hydrate concentration by using effective medium theory an average concentration of 13 per cent in the interval from 120 230 mbsf was estimated from the p wave velocity model lateral continuity of the model data confirms that these average hydrate concentrations are also found around the drillsites out to distances of a few kilometres forward modelling of s waves was carried out using the data from the obs horizontal components above the bsr s wave velocities are higher than a background velocity profile based on a rock physics model and on global averages for unconsolidated sediments this increase in velocity suggests that the hydrate is distributed as part of the load bearing matrix to increase the rigidity of the sediment

s wave velocities and anisotropy in sediments entering the nankai subduction zone offshore japan summary p and mode converted s waves from airgun shots recorded along a line of ocean bottom seismometers were used to construct a 2 d velocity section across the nankai trough and the toe of the accretionary complex offshore southeast japan the site was chosen because of plans to drill into the seismogenic zone of the main subduction thrust which has a history of m 8 earthquakes the section is constrained by well log data from three ocean drilling program boreholes s wave velocities of the sediments over the entire section down to 1 1 km below seafloor range from 107 to 915 m s 1 shear wave splitting indicates anisotropy in all the sediments except possibly the distal trench fill if interpreted conventionally as being due to vertical stress aligned cracks the anisotropy in both the trench and the toe of the accretionary wedge is consistent with crack orientation nw se parallel to the direction of maximum tectonic compression crack density is 0 01 0 03 consistent with closeness to fracture criticality

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

the late jurassic oblique collisional orogen of sw japan new structural data and synthesis the structural configuration of sw japan mainly reflects a late jurassic early cretaceous orogeny the region is divided into an inner belt and an outer belt on the japan sea and pacific ocean sides respectively by a strike slip fault the median tectonic line mtl both consist of a series of stacked nappes the inner belt is divided into a jurassic olistostrome known as the tanba zone and a hinterland area comprising continental triassic jurassic sediments the tanba zone is sliced into two units a lower one with late jurassic matrix and triassic early jurassic radiolarite olistoliths tectonically overthrust by an upper unit comprising an olistostrome with middle jurassic matrix and blocks which include late paleozoic limestone basic lava and radiolarite the tanba zone is overthrust by a paleozoic nappe complex derived from the hinterland the basal sole of the nappe corresponds to a peculiar unit called the ultra tanba zone in the chugoku area the hinterland is divided into an upper nappe the oga nappe formed by permo carboniferous limestone and permian clastic rocks and a lower one the sangun maizuru nappe formed by paleozoic high pressure hp metamorphics the sangun schists the permian maizuru olistostrome and the dismembered paleozoic yakuno ophiolites in the northern part of sw japan the tanba zone is in faulted contact with the circum hida and the hida zones the former is interpreted as the equivalent of the oga and sangun maizuru nappes of the chugoku domain crushed by post cretaceous tectonics the latter consists of paleozoic high temperature ht metamorphic rocks and late triassic early jurassic granite locally mylonitized and covered by early jurassic sandstone the outer belt is formed by a superficial nappe similar to the tanba zone thrust upon a deep domain characterized by a synmetamorphic ductile deformation the deep domain is divided into a lower unit the oboke unit formed by continental derived arenites and a green schist nappe consisting of oceanic sediments and resedimented ophiolites the green schist nappe overthrusts the oboke unit under synmetamorphic conditions with an eastward displacement the two belts are separated by the ryoke zone which corresponds to the southern part of the tanba zone affected by a cretaceous ht metamorphism and sharply cut by the mtl a geodynamic model is proposed for the jurassic orogeny of sw japan assuming that the evolution of the inner and outer belts are linked in the late triassic early jurassic sw japan is an active plate margin the upper plate or south china block consisted of the hinterland and the tanba belt a forearc basin the lower plate consisted of an oceanic area and the south japan continent the basic mechanism of the orogeny is ascribed to the oblique subduction and collision of the south japan continent

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

a survey of the jurassic system of china terrestrial jurassic rocks are widespread in china whereas marine jurassic rocks are confined to xizang tibet south qinghai south xinjiang and in limited parts of the sequence to west yunnan guangdong south hunan and east heilongjiang geographically occurrences of jurassic deposits may be divided into three regions and 13 districts i the xizang qinghai region with the south tethyan himalayan north tethyan himalayan lhasa xanza shiquanhe bangon dengqen karakorum tanggula and qamdo districts ii the coastal volcanic region of east china with the guangdong hunan fujian zhejiang heilongjiang and northeast china districts and iii the stable giant basin region of west china with the sichuan yunnan and northwest china districts these regions and districts are described briefly and preliminary biostratigraphic sequences and palaeogeography are presented the upper and lower boundaries of the jurassic system in china are briefly discussed the jurassic seas of china evidently were

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

three dimensional architecture of the nankai accretionary prism s imbricate thrust zone off cape muroto japan prism reconstruction via en echelon thrust propagation 1 a 9 km wide 92 km long three dimensional 3 d seismic reflection volume acquired off shikoku island japan images the seaward portion of the subduction of the philippine sea plate at the nankai trough and nankai accretionary prism detailed interpretation of the imbricate thrust and protothrust zones the portions of the prism between the deformation front and the first out of sequence thrust shows a high degree of variability in the thrust faults that all parallel the frontal thrust but are arranged in en echelon patterns along strike and frequently include complications such as piggyback faults and fault splays interestingly the sinuous seafloor morphology of the prism does not accurately reflect the en echelon 3 d architecture of the primary prism thrusts seafloor morphology appears to average across several thrusts along strike and is further modified by near surface thrust splays and backthrusts suggesting that care must be taken in interpreting seafloor relief in terms of lateral continuity or thrust fault geometry subduction of the kinan seamounts 20 km northeast of the center of the muroto 3 d volume generated a scallop shaped embayment in the prism the rebuilding process appears to influence the northeastern portion of the 3 d volume where a 625 m landward step in the position of the frontal thrust and numerous changes in prism architecture are observed these observations imply that accretionary prisms may reattain equilibrium following seamount subduction by lateral en echelon fault propagation into damaged zones that facilitate an increase accretion rate until a laterally continuous deformation front is reestablished

shear waves from three component ocean bottom seismographs off lofoten norway indicative of anisotropy in the lower crust summary data from four three component ocean bottom seismographs obs situated on a 145 km long profile on the rost high on the continental shelf off lofoten northern norway have been modelled using 2 d seismic ray tracing and a p and s wave velocity model along the profile is presented the s wave model has been obtained from a study of the high quality horizontal component data a vp vs ratio of 1 95 2 15 is found in the sedimentary layers on the rost high these high values are consistent with shaly sediments which are identified in dredged bedrock samples south of the rost high a considerably lower vp vs ratio ca 1 6 is estimated for the sediments which can be attributed in part to a higher degree of compaction in the deepest sediments and in part to a more sandy lithology in the upper and middle crystalline crust a vp vs ratio of 1 75 is obtained strong s reflections and p to s conversions at the moho are observed on all four instruments and the modelling of these arrivals indicates that the s wave velocity in the lower crust varies with angle of incidence from 3 5 km s 1 vp vs 1 95 at vertical incidence to 4 0 km s 1 vp vs 1 7 at 60 70d this angle of incidence dependent s wave anisotropy the order of 10 per cent is consistent with some horizontal or subhorizontal layering in the lower crust

the south wales coalfield low grade metamorphism in a foreland basin setting the south wales coalfield comprises namurian westphalian and stephanian sequences which developed in the external zone of the variscan orogenic belt sedimentation is thought to have been concentrated in a paralic basin related to crustal flexure in a foreland basin setting linked to variscan tectonic loading to the south it has long been recognized that coal rank varies across the coalfield from high volatile bituminous rank in the east and southeast to anthracite in the northwest this is confirmed here on the basis of comprehensive volatile matter and vitrinite reflectance data sets computer contoured to generate isovol and isoreflectance maps a steady increase in coalification with depth is also recorded clay mineral investigations show the presence of illite and kaolinite across the entire coalfield area while neither pyrophyllite nor mixed clays were detected illite crystallinity values from mudstones are in the range 1 20 0 44 2 vitrinite reflectance data indicate maturation temperatures in the range 123 290 c clay mineral and ic values suggest temperatures up to 230 c while previous descriptions of pyrophyllite from the coalfield would allow locally for even higher temperatures coalification appears to have occurred rapidly maximum 5 ma following deposition implying high heat flux during westphalian times this is also indicated by fluid inclusion evidence from quartz crystals in clay ironstone nodules the inference is of a high geothermal gradient c 50 c km 1 during westphalian times contrasting with an earlier gradient estimate of 20 37 c km 1 relatively high gradients have also been determined previously for the ruhr coalfield which occupies a similar tectonic setting in the external zone of the variscan orogenic belt such a gradient is at odds with the proposed foreland basin tectonic setting for the south wales coalfield in which low geothermal gradients would be anticipated

the slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model abstract a new model for the shape of the philippine sea plate psp subducting beneath southwest japan was constructed by combining marine seismic survey results and seismicity data this resulted in a plate model that is generally 3 5 km shallower than previous estimates and contains new details of plate boundary structure near the nankai trough axis we estimated the slip distribution of the 1946 nankai earthquake by inverting tsunami waveforms using the new plate boundary model and found that the large slip off cape muroto and seismic moment were reduced by about 10 from the previous estimate of tanioka and satake 2001a however we also found reason to suspect the tide gauge closest to the epicenter at shimotsu and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area we also find that our new rupture model correlates well with features of the subducting plate boundary such as a large subducting seamount off cape muroto kodaira et al 2000a this suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy or both

developing stress monitoring sites using cross hole seismology to stress forecast the times and magnitudes of future earthquakes abstract a new understanding of rockmass deformation suggests that changing stress in the crust modifies the geometry of the distributions of fluid saturated grain boundary cracks and pores pervading almost all rocks in the crust these stress aligned micro cracks cause the widely observed splitting of seismic shear waves which are sensitive to the details of the microcrack geometry this means that analysing shear wave splitting along appropriate ray paths above small earthquakes can monitor the build up of stress before large earthquakes this allowed the time and magnitude of an m 5 earthquake in iceland to be successfully stress forecast such forecasting using small earthquakes as the source of shear waves is possible only on those rare occasions when the comparatively severe restrictions on the geometry of source receiver and large earthquake allow shear wave splitting to be analysed along appropriate ray paths this paper suggests that in the absence of such pronounced local seismicity and optimum recording geometry the time and magnitude of future earthquakes can be estimated by analysing shear wave splitting in controlled source cross well seismology between three boreholes we suggest that stress forecasting at such stress monitoring sites smss is the best deterministic option for reliable forewarning of large earthquakes near any earthquake vulnerable township or vibration sensitive installation a preliminary stress monitoring site is currently being set up in northern iceland

vp vs ratio and shear wave splitting in the nankai trough seismogenic zone insights into effective stress pore pressure and sediment consolidation to estimate variation of stress state and sediment consolidation in the nankai plate subduction zone off southwest japan we measured the p wave to s wave velocity ratio vp vs and s wave splitting along the seismic line extending from the trench to the seismogenic zone for this purpose we used active source seismic data recorded by multicomponent ocean bottom seismometers obs because it is difficult to identify the ps converted reflection waveforms for each of the geological boundaries in this deep offshore region we focused on the more easily identified pps refracted waveforms that register the conversion of the up going p waves to s waves at the igneous crust surface we estimated the average vp vs ratio within the sedimentary section by using the time lag between the p refracted waves and pps converted waves this vp vs ratio changes abruptly at the trough axis i e the deformation front of the accretionary prism arguably because of compaction associated with the accretion process we observ

detailed coseismic slip distribution of the 1944 tonankai earthquake estimated from tsunami waveforms coseismic slip distribution on the fault plane of the 1944 tonankai earthquake is estimated from inversion of tsunami waveforms three improvements from a previous study satake 1993 are made these are 1 smaller subfaults are used to resolve detailed slip distribution 2 the sub faults fit better to the plate interface geometry and 3 finer and more accurate bathymetry data is used the inversion result shows that a maximum slip of about 3 m occurred on the plate interface off shima peninsula the total seismic moment is estimated to be 2 0 1021 nm mw 8 2 the result confirms that the 1944 tonankai earthquake did not rupture the plate interface beneath the tokai region and supports the existence of the seismic gap in the tokai region suggested by ishibashi 1981 the slip of about 1 5 m on the plate interface beneath atsumi peninsula northeast of the large slip area is necessary to explain the observed tsunami waveforms although no seismic moment release was estimated from strong motion data by kikuchi et al 1999 this may suggest that the rupture beneath atsumi peninsula was slow

estimation of gas hydrate concentration from multi component seismic data at sites on the continental margins of nw svalbard and the storegga region of norway high resolution seismic experiments employing arrays of closely spaced four component ocean bottom seismic recorders were conducted at a site off western svalbard and a site on the northern margin of the storegga slide off norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed data from p waves and from s waves generated by p s conversion on reflection were inverted for p and s wave velocity vp and vs using 3d travel time tomography 2d ray tracing inversion and 1d waveform inversion at the nw svalbard site positive vp anomalies above a sea bottom simulating reflector bsr indicate the presence of gas hydrate a zone containing free gas up to 150 m thick lying immediately beneath the bsr is indicated by a large reduction in vp without significant reduction in vs at the storegga site the lateral and vertical variation in vp and vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the bsr derivation of hydrate content from vp and vs was evaluated using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background velocity of the sediment host the error in the average vp of an interval of 20 m thickness is about 2 5 at 95 confidence and yields a resolution of hydrate concentration of about 3 if hydrate forms a connected framework or about 7 if it is both pore filling and framework forming at nw svalbard in a zone about 90 m thick above the bsr a biot theory based method predicts hydrate concentrations of up to 11 of pore space and an effective medium based method predicts concentrations of up to 6 if hydrate forms a connected framework or 12 if hydrate is both pore filling and framework forming at storegga hydrate concentrations of up to 10 or 20 were predicted depending on the hydrate model in a zone about 120 m thick above a bsr with seismic techniques alone we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer not only because of the uncertainty in the layer by layer variation in lithology but also because of the negative correlation in the errors of estimation of velocity between adjacent layers in this investigation an interval of about 20 m thickness equivalent to between 2 and 5 layers in the model used for waveform inversion was the smallest within which one could sensibly estimate the hydrate content if lithological layering much thinner than 20 m thickness controls hydrate content then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data